Analyses de cas optiques et radar sur le terrain

Thayer, G. D.

  1. Introduction
  2. Présentation des données d'indices de réfraction radio
  3. Analyse d'incidents ovnis sélectionnés par classes
  4. Synthèse des résultats
  5. Conclusions et recommandations pour travaux futurs
    Références

Présentation des données d'indices de réfraction radio

2 méthodes de présentation des profils verticaux de réfraction radio sous forme graphique sont utilisées dans ce chapitre. Les 2 méthodes sont basées sur l'utilisation de la réfractivité radio, N, où :

N = (n - 1) x 106

l'indice de réfraction radio, n, étant toujours très proche de l'unité dans l'atmosphère. La valeur maximum de N qui est susceptible d'être rencontrée dans l'atmosphère ne dépasse pas bien 400 ; des valeurs proches de 500 pourraient occasionnellement être expérimentées au-dessus de la surface de la Mer Morte, à 1200 pieds sous le niveau de la mer, dans les mois d'été.

Une caractéristique de tous les profils verticaux de N est un décroissement général avec l'altitude ; des départs de tout profil donné d'un décroissement moyen avec l'altitude sont des caractéristiques significatives de propagation anormale d'ondes radio. Par conséquent les profils d'indice de réfraction illustrés pour beaucoup de cas d'ovnis dans la section suivante sont donnés en termes d'unités A [Bean, 1966a] où :

A(z) = N(z) + 313(1-e-0,14386z)

ici N(z) est le véritable profil de réfraction, fonction de l'altitude, z, en km, et le dernier terme représente le décroissement moyen avec l'altitude d'un profil de réfraction radio moyen :

N(z) = 313e-0,14386z

Le nombre 313 est une valeur moyenne de réfraction de surface. Un profil N qui n'est pas anormal, tracé sur un graphique avec A(z) en abscisse et z en ordonnée, apparaîtra comme une ligne verticale globalement droite, peut-être avec une légère inclinaison dans une direction ou l'autre. D'un autre côté, un profil N with strongly super-refractive or subrefractive display a marked zigzag character sur un tracé de A(z) par rapport à z. L'utilisations d'unités A permet une taille d'échelle plus généreuse pour l'abscisse que dans le cas de tracés avec l'unité N.

Ray tracings, calculés et tracés par ordinateur, sont illustrés pour quelques-uns des profils de réfraction. L'ordinateur calcule également le profil M, et le trace sur le même graphique que le ray tracing. Les unités M sont définies par :

M(z) = N(z) + z/a

Où "a" est le rayon de la Terre. Ceci est équivalent à ajouter 156,9 unités N par km au profil observé. Le gradient de ducting (voir chapitre 6-4) est de -156,9 N. km-1, any layer with such a gradient will be represented on an M(z) plot as a vertical line. Layers with dN/dz > -156,9 km-1, (not ducting) will show a trace slanting up to the right, whereas strong ducts with dN/dz < -156,9 km-1 will show a trace slanting up to the left. Hence the M-unit plot is very convenient for exposing the existence or non-existence of radio ducts in N(z) data.

Analyse d'incidents ovnis sélectionnés par classes

Dans les discussions qui suivent les incidents d'ovnis sont référencés par les numéros de cas qui leurs sont affectés dans les fichiers du projet ovni. La lettre fait référence à l'origine du cas : les cas indicés "B" proviennent des fichiers du projet Blue Book de l'USAF, ceux indicés "N" sont pour les cas fournis par le NICAP (Comité National d'Enquêtes pour les Phénomènes Aériens), les indices "C" font référence à des cas qui firent l'objet d'enquête par le personnel du Projet Colorado, et les indices "X" ont été attribués aux cas qui furent reçus après la date butoir pour être inclus dans les fichiers réguliers (i.e., après que l'analyse informatique de l'ensemble des cas de fichier du projet ait été déjà achevée). Les cas indicés "X" sont également identifiés par leur indice B-, N-, ou C-.

Classe 1 : Incidents ovni étant principalement des contacts visuels

Classe 1-A : Cas principalement visuels, semblables à une étoile

1321-B. Ceci est un bon exemple d'étoile mal identifiée combinée à un retour radar apparemment non lié aboutissant à produire un signalement d'ovni. L'incident eut lieu à la base aérienne de Finland (à 60 miles au NE de Duluth), Minnessota, avec une observation civile près de Grand Marais, Minnessota (à 50 miles au NE de la base de Finland) la nuit du 5 au 6 septembre 1966, entre 21 h 30 et 00 h 15 LST (03 h 30-06 h 15 GMT). Le temps était clair, sans plafond nuageux, la visibilité de plus de 15 miles ; a display d'aurore boréale était en cours. Applicable radio refractivity profile is shown in Fig. 1. Visual reports of a "white-red-green" object "moving but not leaving its general location" were received at Finland AFB about 2130 LST. An FPS-90 search radar was activated but there was "too much clutter to see anything in that area ..." At 2200 LST a return was detected; it "flitted around in range from 13 to 54 mi.., but always stayed on the 270° azimuth." A pair of F-89s was scrambled from Duluth AFB and searched the area at altitudes of 8,000-10,000 ft. The two aircraft "merged with blip, apparently wrong altitude, no airborne sighting"; the radar operators insisted the target was at 8,000-10,000 ft., the same altitude at which the scrambled aircraft were flying. The pilots reported that they "only observed what was interpreted to be a beacon reflection."

Figure 1: International Falls - Radio Refractivity Profile - International Falls, 6 Sep 1966
Fig 1

Available meteorological data show that the winds were southwesterly, 7 knots at the surface, and northerly (320° to 30° at 25 to 65 knots aloft. The closest available radiosonde data (international Falls 1200 GMT 0600 LST) 6 September, show a temperature inversion and strong humidity lapse through a layer extending from 1029-1259 m. above the surface. The gradient of radio refractivity through this layer averaged -114N/km (corrected for radiosonde sensor lag). This layer would be expected to show a significant partial reflection at radio frequencies. If the layer were present over Finland AEB at the same elevation, it could have produced false targets by partial reflection of real ground targets, which would have appeared to be at altitudes of from 8,300-9,800 feet because of the geometry of such reflected targets (sec. Section VI, Chapter 5). This agrees well with the reported "UFO" altitudes of 8000-10000 ft.

Anomalous propagation echoes are not usually confined to a single direction. There are three possible explanations in this case and in other similar cases: a single real object was being tracked; the radar operators were not looking for targets on other azimuths; the partially reflecting layer may have been anisotropic (i.e. displaying a preferred direction for strongest reflection). There is no direct physical evidence for the existence of such anisotropic layers, but no studies have been made to determine whether or not they might exist. Apparent anisotropy in radar AP returns has often been observed, although not usually over such a narrow azimuth range as was apparently the case at Finland AFB.

Regarding the visual reports submitted, the comment of the investigating officer at Finland AFB is of particular interest:

The next evening, at 2200 hours, the "white-red-green" object reappeared in the sky at exactly the same position it had appeared on 5 September. This officer observed it and determined it to be a star which was near the horizon and would settle beneath the horizon after midnight. It did appear to "sparkle" in red-green-white colors, but so do other stars which can be pointed out from this mountain top.

The officer refers to Rangoon Mountain, elevation 1,927 pieds, from which many of the visual observations were made.

The star that the officer saw was in all probability Lambda Scorpio (Shaula) a magnitude 1.7 star at -37° declination and 17 hr. 31 min. right ascension. It would have set at just about 1 h 30, 90th meridian time, if the horizon were unobstructed. An obstruction of only 4° would cause Lambda Scorpio to "set" at 1:15 a.m. CST; a 4° angle is equivalent to a 35 ft. tree or building at a distance of 500 ft. The southerly declination would indicate that the star was in the southwest, which is compatible with the visual reports that were submitted.

Additional meteorological effects may have been present in this case. In particular, the southwesterly surface winds present are quite likely to have advected relatively cool, moist air from nearby Lake Superior under the elevated warm, dry layer noted previously, thus tending to increase the strength of the inversion and associated humidity lapse. Some of the optical effects noticed by the observers in this instance, strong red-green scintillation, apparent stretching of the image into a somewhat oval shape, and the red fringe on the bottom, may have been due to strong and irregular local refraction effects in the inversion layer (or layers).

This UFO report seems to have resulted from a combination of an unusually scintillating star and false radar targets caused by AP from a strong elevated layer in the atmosphere. This pattern is found in a number of other cases.

Reports with elements similar to the preceding case are:

113-B. [*Case numbers referred to thusly are so listed in the project's files. ] Nemuro AF Detachment, Hokkaido, Japan, 7 February 1953, 2230 LST (1230 GMT). Weather was clear. Visual description fits a scintillating star (flashing red and green, later white with intermittent red and green flashes, then later steady white) rising in the east (only motion was slow gain in altitude, "[I believe] that the object did not move with respect to the stars in its vicinity"). CPS-5 radar painted a single pip at 85° azimuth, range 165 mi., which operator regarded as interference. Visual object was boresighted with radar antenna and azimuth read as 91°±2°. Elevation estimated as 15° initially (2230 LST). No stars brighter than magnitude 3 were in this azimuth between 0° and 30° elevation angle at that time. Blue Book file suggests Deneb or Regulus as likely objects, but their positions are far away from the sighted object. In view of two observers' comments that light "shown from beneath" object, it is very probable that they saw a lighted Pibal balloon, possibly launched from the Russian-held Kurile Islands to the east and northeast of Hokkaido (launch time 1200 GMT). The investigating officer noted the exceptionally good visibility prevalent in the area on clear nights.

1306-B. Edwards AFB, Kernville, Calif., 30 July 1967, 2217-2400 LST. Weather: clear, calm, warm (83°F). Two civilians reported observing one or two blue, star-like objects that appeared to circle, bob, and zigzag about a seemingly fixed star; these objects "instantly disappeared" about 1 hr. 45 min. after sighting. Edwards AFB RAPC0N radar picked up "something" at about 2230 LST "for several sweeps." Blip seemed to be moving south at about 50-60 mph. There is no apparent connection between the radar and visual reports. The visual UFO did not appear to move at 50-60 mph. Data, including weather data, on this report are insufficient to form an opinion. The most likely possibility seems to be that the visual LJFO consisted of the direct image plus one or two reflected images of the "fixed star" that the observer reported. What may have produced the reflected images remains conjectural. For example, a turbulent layer of air with strong temperature contrasts could produce images similar to those described by the witnesses. The instantaneous disappearance of the UFOs is consistent with an optical phenomenon. As for the radar "track", a blip appearing for only "a few sweeps" could be almost anything: noise, AP, or possibly a real target flying near the lower limits of the radar beam.

1212-B. Tillamook, Ore., 13-14 March 1967, 2230-0008 LST. Weather: clear with "stars plainly visible," some ground fog, thin broken cirriform clouds estimated at 10,000 ft., visibility 15 mi. This is a good example of some of the confusion that arises in reporting UFO incidents. Initial visual observer reports indicated object at about 45° to 50° elevation angle, yet when the Mt. Hebo radar station "contacted target" it was at 39 mi. range, 9,200 ft. height. This is an elevation angle of only about 2°. This inconsistency seems to have gone unnoticed in the Project Blue Book file on the case. The radar target, as plotted, stayed at 39 mi. range and slowly increased height to 11,200 ft., then shifted almost instantaneously to 48 mi. range. Subsequently the radar target slowly gained altitude and range, disappearing at 55 mi. and 14,000 ft. (still at about a 2° elevation angle). The azimuth varied between 332° and 341° during this time. Average apparent speed of the radar track was low: the first part of the track was at zero ground speed and a climb rate of about 100 ft/min, the second part of the track was at an average ground speed of about 16 mph. and a climb rate of about 100 ft/min. In between there is a jump of 9 mi. range in one minute, a speed of 540 mph. The characteristics of this radar track are suggestive of radar false targets or slow-moving AP echoes. The jump may be a point where one echo was lost, and another, different echo began coming in. This effect is apparently a frequent cause of very high reported speeds of UFOs (Borden, 1953). The visual reports are suggestive of either a scintillating star if the reported angle is higher than actual, or an aircraft. There was an electronic warfare aircraft "orbiting" at high altitude seaward of Tillamook at the time of the sighting, and it seems quite plausible that this was the visual UFO. However, this was discounted in the Blue Book report because the aircraft's position it did not check with the radar contact.

115-B. Carswell AFB (région de Fort Worth), Texas, 13 février 1953, 02 h 35 LST. Temps : clair avec visibilité illimitée ; couche d'inversion de température avec sharp humidity lapse à 3070 pieds d'altitude, elevated radio duct at 4240 pieds d'altitude. Applicable refractivity profile for 0300 LST shown in fig. 2. Des observateurs visuels virent une formation de 3 lumières brillantes qui effectuaient une série de manoeuvres suggérant un appareil avec des feux d'atterrissage effectuant plusieurs tonneaux puis grimpant rapidement et partant au loin. Les opérateurs tentèrent alors de répérer l'objet sur un radar APG 41, et après 2 mn environ ils brought in two apparently stationary targets on the correct azimuth. It seems likely that these returns were from ground objects seen via partial reflection from the strong elevated layers (gradients -154 and -311 km-1). The visual sighting was probably an aircraft.

237-B. Haneda AFB (Tokyo), Japon, 5-6 août 1952, 23 h 30-00 h 30 LST. Temps : exceptionellement beau, couverture nuageuse de 0,3 à environ 10 miles au nord et 10 miles au nord de la zone de contact, excellente visibilité, isolated patches of low clouds, Mt. Fuji (60 n. mi.) "clearly discernible," scattered thunderstorms in mountains northwest, temperature at Haneda 78° F, dew point 73° F. Observers saw a bright, round light (about 1 mrad arc) surrounded by an apparently dark field four times larger, the lower circumference of which tended to show some bright beading. It was low in the sky at about 30° -50° azimuth. Object appeared to fade twice, during which time it appeared as a dim point source. It disappeared, possibly becoming obscured by clouds, after about an hour. The sky at Haneda AFB was overcast by 01 h 00 LST. One of the visual observers noted that near the end of the sighting the object seemed somewhat higher in the sky and that the moon seemed proportionately higher in elevation. The pilot of a C-54 aircraft coming in for a landing was directed to observe the object and he replied that it looked like a brilliant star, and he dismissed the sighting as such.

Figure 2 - Radio Refractivity Profile - Carswell AFB, 13 Feb 1953
Dessin d'Adamski en 1952

Lorsque l'on demanda au contrôleur de la base aérienne de Shiroi de chercher la cible sur le radar GCI, il ne put rien trouver pendant 15 mn. Il indiqua : Il y avait 3 ou 4 blips on low beam but none I could definitely get a movement on or none I could get a reading on the RHI (range-height indicator) scope." A new controller taking over at 2345 LST "believed" he made radar contact with the object and an F-94 was scrambled. This officer stated: "The target was in a right orbit moving at varying speeds. It was impossible to estimate speed due to the short distances and times involved." By the time the F-94 arrived in the area of the "bogie," Shiroi GCI had lost radar contact; regaining contact at 0017 LST "on a starboard orbit in the same area as before." The F-94 was vectored in to the target, and at this point the timing becomes confused. The Shiroi controller states that the F-94 "reported contact at 0025 (LST) and reported losing contact at 0028 (LST)." The F-94 radar operator states: "At 0016 (LST) I picked up a radar contact at 10° port, 10° below, at 6,000 yd. The target was rapidly moving from port to starboard and a lock-on could not be accomplished. A turn to the starboard was instigated [sic] to intercept target which disappeared on scope in approximately 90 sec. No visual contact was made with the unidentified target." Shiroi GCI had lost the F-94 in ground clutter, and had also lost the target. It is not clear whether the GCI radar ever tracked the fast-moving target described by the F-94 crew. The maximum range of the F-94's radar is not given in the Blue Book report.

The F-94 pilot stated that the weather was very good with "exceptional visibility of 60-70 miles," yet this fast-moving UFO, obviously far exceeding the F-94's airspeed about 375 knots), was seen by neither the aircraft crew nor the observers on the ground at Shiroi GCI even though the UFO track crossed over very close to Shiroi GCI number four. There are many other inconsistencies in the report of the incident besides the timing and the lack of visual contact by the F-94 crew. The bright, quasi-stationary object sighted NE of Haneda AFB, and seen also from Tachikawa AFB (about 30 mi. west of Haneda AFB), should have been visible to the south of Shiroi AFB, but was never seen by any of a large number of persons there who attempted such observations. Also, at 0012 LST the object being tracked by GCI's CPS-l radar reportedly "broke into three smaller contacts maintaining an interval of about 1/4 mile." The blips on the CPS-l were described as small and relatively weak, but sharply defined.

2 choses semblent apparentes :

  1. the object seen at Haneda and Tachikawa AFB was much farther away than the observers realized;
  2. the visual UFO and the target tracked by radar were not the same.

The first statement is supported by the inability of the observers at Shiroi to see anything to the south; the second statement is supported by numerous inconsistencies between the visual and radar sightings. The two most important of these latter are:

  1. During times when the GCI radar could not find the target, the visual object was in about the same location as during those times when it could be found on radar;
  2. The visual object was seen for at least five min. after the time when the airborne radar on the F-94 indicated that the UFO had left the area at a speed well in excess of 300 mph.

The most likely light source to have produced the visual object is the star Capella (magnitude 0.2), which was 8° above horizon at 37° azimuth at 2400 LST. The precise nature of the optical propagation mechanism that would have produced such a strangely diffracted image as reported by the Haneda AFB observers must remain conjectural. Complete weather data are not available for this case, but it is known that the light SSE circulation of moist air from Tokyo Bay was overlain by a drier SW flow aloft. A sharp temperature inversion may have existed at the top of this moist layer, below which patches of fog or mist could collect. The observed diffraction pattern could have been produced by either

  1. interference effects associated with propagation within and near the top of an inversion, or
  2. a corona with a dark aureole produced by a mist of droplets of water of about 0.2 mm. diameter spaced at regular intervals as described by Minnaert (1954).

In either event, the phenomenon must he quite rare. The brightness of the image may have been due in part to "Raman brightening" of an image seen through an inversion layer.

Nor can exact nature of the radar propagation effects be evaluated, due to the lack of complete weather data. However, a substantial inference that the radar returns were of an anomalous propagation nature is derived from:

  1. the tendency for targets to disappear and reappear;
  2. the tendency for the target to break up into smaller targets;
  3. the apparent lack of correlation between the targets seen on the GCI and airborne radars;
  4. the radar invisibility of the target when visibility was "exceptionally good."

Singly, each of the above could be interpreted in a different light, but taken together they are quite suggestive of an anomalous propagation cause.

In summary, it appears that the most probable causes of this UFO report are an optical effect on a bright light source that produced the visual sighting and unusual radar propagation effects that produced the apparent UFO tracks on radar.

104-B. Goose AFB, Labrador, 15 December 1952, 1915-1940 Local Mean Solar Time. Weather: clear and visibility unlimited (30 mi.). The crews of an F-94B fighter and a T-33 jet trainer saw a bright red and white object at 27° azimuth while flying at 14,000 ft. The aircraft attempted an intercept at 375 knots indicated air speed, but could not close on the UFO. After 25 min. of reported chase, although the aircraft had covered a distance of only about 20 mi. (about 3.5 min. at 350 knots ground speed) the object faded and disappeared. During the chase, the radar operator in the F-94B had a momentary lock-on to an unknown target at about the correct azimuth for the UFO. Since this was so brief, it was felt (by Air Intelligence, presumably) that the set had malfunctioned. No GCI contact was made.

The official Air Force explanation for this UFO incident is that the aircraft were chasing Venus which was setting about the time of the sighting, and that the radar "target" was simply a malfunction. It seems likely that this explanation is essentially correct. However, it is unlikely that experienced pilots would have chased a normal-appearing setting Vénus. It is more probable that the image of Vénus was distorted by some optical effect, possibly a slight superior mirage, and that loss of the mirage-effect (or the interposing of a cloud layer) caused the image to fade away. All items of the account may be explained by this hypothesis, including the report that the object had "no definite size or shape," as the image would no doubt be somewhat "smeared" by imperfections in the mirage-producing surface. The small-angle requirement of a mirage is satisfied since the pilots reported the object seemed to stay at the same level as the aircraft, regardless of altitude changes that they made (another indication of great distance).

14-N. Ce dossier consiste en fait en 2 cas semblables signalés par un pilote des Capital Airlines ayant 17 ans et 3 000 000 miles à son actif. Le 1er cas eut lieu au-dessus du centre de l'Alabama la nuti du 14 novembre 1956 ; le 2nd cas fut la nuit du 30 août 1957, au-dessus de Chesapeake Bay près de Norfolk, en Virginie.

La 1ère observation eut lieu à environ 60 miles au NNE de Mobile, en Alabama, lors d'un vol de New York à Mobile dans un Viscount à haute altitude, probablement autour des 25 000 pieds. C'était par une nuit sans Lune, étoilée et il y avait an occasionally broken undercast. L'objet vu fut décrit comme une lumière bleue-blanche intense d'environ 1/10ème de la taille de la Lune (~3' arc) et environ 7 ou 8 fis aussi brillante que Vénus à sa magnitude la plus brillante. Il apparût d'abord à 22 h 10 LST at the upper left of the Viscount's windshield falling towards the right and decelerating rapidly as a normal meteor would. Pilot and co-pilot both took it to be an unusually brilliant météore. Cependant, ce "météore" did not burn out as expected, but "abruptly halted directly in front of us and began to hover motionless." The aircraft at this time was over Jackson, Ala. and had descended to 10,000 ft. The pilot contacted Bates Field control tower in Mobile and asked if they could see the object which he described to them as "a brilliant white light bulb." They could not see it. The pilot then asked Bates to contact nearby Brookley AFB to see if they could plot the object on radar. He never learned what the result of this request had been. The object began maneuvering "darting hither and yon, rising and falling in undulating flight, making sharper turns than any known aircraft, sometimes changing direction 90° in an instant -- the color remained constant, -- and the object did not grow or lessen in size. "After a "half minute or so" of this maneuvering, the object suddenly became motionless again. Again, the object "began another series of crazy gyrations, lazy eights, square chandelles, all the while weaving through the air with a sort of rhythmic, undulating cadence." Following this last exhibition, the object "shot out over the Gulf of Mexico, rising at the most breath-taking angle and at such a fantastic speed that it diminished rapidly to a pinpoint and was swallowed up in the night."

The whole incident took about 2 mn. The pilot remembers noting that the time was 2212 EST. The object appeared to be at the same distance from the aircraft, which was flying a little faster than 300 mph. during the entire episode.

Le 2nd incident signalé par ce pilote, le 30 août 1957, Chesapeake Bay report, occurred as he was flying another Capital Airlines Viscount at 12,000 ft. approaching Norfolk, Va. There was a Northeast Airlines DC-6 flying at 20,000 ft. "directly above" the Viscount. In this case, the object "was brilliant; it flew fast and then abruptly halted 20 mi. in front of us at 60,000 ft. altitude." The Northeast pilot looked for the object on radar and "could get no return on his screen with the antenna straight ahead but when tilted upward 15° he got an excellent blip right where I told him to look for the object."

This object "dissolved right in front of my eyes, and the crew above lost it from the scope at the same time. They said it just faded away. This sighting covered "several minutes."

These two similar sightings are very difficult to account for. The first sighting over Alabama has most of the characteristics of an optical mirage: an object at about the same altitude seeming to "pace" the aircraft, the meanderings being easily accountable for as normal "image wander." However, there are two aspects that negate this hypothesis:

  1. the manner of appearance and disappearance of the UFO is inconsistent with the geometry of a mirage; the high angle of appearance at the top of the windshield is particularly damaging in this regard;

  2. there was no known natural or astronomical object in the proper direction to have caused such a mirage. Venus, the only astronomical object of sufficient brightness, was west of the sun that date; Saturn had set 4 hr. 30 mm. earlier, and there was not even a first magnitude star near 190° -210° azimuth, 0° elevation angle.

The second sighting is equally difficult to explain as a mirage, which seems to be the only admissable natural explanation in view of the pilot's experience as an observer. The reasons are twofold:

  1. the apparent angle at which the object was observed is incompatible with a mirage;
  2. there was apparently a radar return obtained from the object which is incompatible with the hypothesis that it was an astronomical object, the most likely mirage-producer.

The pilot stated that the Northeast DC-6 flying at 20,000 ft. "painted" the UFO at 15° elevation and a range of 20 mi. This would place the UFO at about 48,500 ft., the pilots estimate of 60,000 ft. apparently being in error. Presumably then, the elevation angle as viewed from the Capital Viscount was about 19°. It is very unlikely that any temperature inversion sufficient to produce a mirage would be tilted at such an angle. For a near-horizontal layer to have produced such an image (plus the radar return) by partial reflection of a ground-based object seems equally unlikely. The largest optical partial reflection that such a layer might produce at an angle of 19° would be about 10-14 as bright as the object reflected (see Section VI, Chapter 4). This is a decrease of 35 magnitudes. Such a dim object would be ordinarily invisible to the unaided eye.

In summary, these two cases must be considered as unknowns.

1065-B. Charleston, S. C., 16 January 1967, 1810 LST. The observational data in this case are insufficient to determine a probable cause for the sighting. A civilian "walked out of his house and saw" two round objects. He estimated that they were about 30° above the horizon. They appeared to be "silver and blue, with a red ring." These objects were alternately side by side and one above the other, and a beam of light issued "from the tail end." The observer does not state how he knew which was the "tail end," or even at what azimuth he saw the objects. They "vanished in place," still at 30° elevation. After the Charleston AFB was notified of the sighting, some unidentified returns were picked up on an MPS-14 search radar. An investigating officer later determined that these returns were spurious. The case file states:

[The officer] called [8 March 1967] to provide additional information in regard to the radar sighting. [The officer] was informed by the Charleston AFB that the radar paints were not of UFOs. A check of the equipment was made and it was learned that the individual monitoring the radar set had the "gain" [control] on the height finder turned up to the "high" position. This caused the appearance of a lot of interference on the radar scope. Personnel at Charleston AFB determined the paints on the radar to be this interference. The personnel turned the gain on high again and picked up more "UFOs". When the gain was turned down the UFOs disappeared.

There apparently were no radar UFOs in this case. The residue is a visual sighting by a single observer with insufficient data for evaluation. What the observer saw could conceivably have been

  1. a mirage with direct and reflected images of a planet (Jupiter w'as at 68° azimuth, 5° elevation) or a bright star,
  2. an aircraft, or
  3. a genuine unknown (i.e., a possible ETI object). There is no real evidence either for or against any of these possibilities.

Class I-B: Primarily visual, meteor-like cases.

1323-B. Sault Saint Marie AFB, Mich., 18 September 1966, 0100 LST. Weather: clear, calm. There is a very brief Blue Book file on this incident. Two sergeants of the 753rd Radar Squadron saw a bright light, ellliptical in shape and apparently multicolored of unsaturated hues, which appeared low over the treetops to the SE and moved in a straight line toward the west, disappearing instantaneously in the WSW. Duration of this sighting was 2-5 sec. The report states that the object was also tracked by a long-range AN/FPS-90 heightfinder with azimuth, range, and altitude available on request. Since this information is not included in the folder, no firm conclusion may be reached as to the probable cause of the radar sighting or even as to whether or not the radar and visual objects were correlated.

The general visual appearance, brightness range, motion and mode of disappearance are all compatible with the hypothesis that the object was a large meteor. Some large meteors display even more unusual appearance than this report. If it was a meteor, the radar may have actually tracked it; radar tracks of large meteors are not unknown. Of course, the radar track may have been spurious, or may have indicated that the object was unnatural. The tracking data would be required to settle the point.

The radio refractivity profile for 0600 LST, shown if Fig. 3 indicates that an intense super-refractive layer existed within the first 372 m. (1220 ft.) above the surface. This profile is conducive to the formation of AP echoes on ground-based radar, so there is some possibility that the observed radar data in this UFO incident may have been spurious. This case would seem to merit further investigation.

1206-N. Edmonton, Alberta, 6 April 1967, 2125-2200 LST. Weather: very clear, cool, temperature about 35°F, little or no wind at surface, stars bright, no moon. Observers state that a bright object appeared in the NNW low on the horizon, moving fast, appeared to hover, and then disappeared. The night before, a whitish object like a normal star only much larger had appeared in the same place (NNW). A Pacific Western Airlines pilot independently reported chasing a UFO whose position was relayed to him by GCA radar from Edmonton International Airport. This UFO appeared to move somewhat erratically, was seen only briefly by the pilot as a reddish-orange lighted effect, and did not travel the same course as the visual object described above.

The general atmospheric conditions prevailing during this sighting were conducive to AP. The description of the GCA radar track is suggestive of AP (quasi-stationary target appearing to jump in position), and the description of the UFO of 5 April is suggestive of the diffracted image of a star seen through a sharp temperature inversion. In the absence of detailed meteorological data, the most probable conclusion seems to be that the primary sighting was a meteor and that no genuine UFO case exists here. However, this case also might merit a more intensive investigation.

Figure 3 - Profil de réfractivité radio - Sault Saint Marie, 18 Septembre 1966
Fig 3

1207-B. Paris (Texas), 7 mars 1967, 16 h 45 LST. Weather: clear, visibility 15 mi. This is an unconfirmed report by a single observer who could not even be reached for verification of the report by members of this project staff. He claimed to have seen two lights that made a 90° turn at high speed, appeared to separate and come back together again and then went straight up. Speed varied from fast to slow to fast, in excess of known aircraft speed. The last statement is the witness's interpretation. He stated that radar at Paris AFB had tracked this UFO, but all military radar installations in the area disclaim any UFO tracks that night. It seems probable that the visual sighting was either an aircraft, whose sound was not heard by the witness for some reason, or a pair of meteors on close, nearly parallel paths. The quick dimming of a meteor burning out may be interpreted as a 90° turn with sudden acceleration away from the observer of a nearly-constant light source, which then seems to disappear in the distance.

Classe I-C : Principalement visuel, lumière floue ou lueur

15-B. Blackhawk et Rapid City (Dakota du Sud) et Bismarck (Dakota du Nord), 5-6 Août 1953, 20 h 05-02 h 50 (LST). Météo : Temps clair, visibilité excellente, conditions stables, inversions de température et radio surface ducts prevalent. Voir Fig. 4. La nuit était sombre et sans Lune.

L'incident initial dans cette chaîne d'observations d'ovnis fut l'observation par un observateur du GOC d'une lumière rouge luisante stationnaire à 20 h 05 (LST) près de Blackhawk (Dakota du Sud). Cette lumière commença rapidement à se déplacer à 30 ° sur la droite, shot straight up, puis se déplaça sur la gauche, revenant à sa position d'origine. Un companion pensa qu'il s'agissait juste de la lumière rouge de la tour (un feu d'avertissement sur une tour de transmission FM normalement simplement visible de leur localisation). Le signalement fut relayé au Centre de Filtrage de Rapid City, et 3 aviateurs du site radar furent envoyé dehors pour chercher à voir l'ovni. Ils virent ce qui était sans aucun doute un météore, à en juger par leur description. Lorsqu'il fut informé de la nouvelle observation, l'opérateur radar commença à chercher des cibles non identifiées. Il en trouva de nombreuses.

Figure 4 - Profil de réfractivité radio - Bismark (Dakota du Nord), 5 Août 1963
Fig 3

Au cours des 4 heures suivantes un grand nombre de blips non identifiés apparurent sur le radar de Rapid City. Nombre d'entres eux étaient transitoires, des blips en déplacement avec une durée de vie plutôt courte, généralement perdus dans le bruit de fond au sol. Un chasseur F-84 fut dirigé sur un blip stationnaire près de Blackhawk, et le pilote "pris en chasse" un ovni qu'il trouva sur le lieu sur une direction de 320 ° M sans le rattraper. Le F-84 pourchassait probablement une étoile, dans ce cas Pollux (mag. 1,2) qui se trouve au la position exacte (335 ° d'azimut vrai, près de l'horizon).

Lorsque le poste GOC de Blackhawk appela pour dire que l'objet d'origine était revenu pour la 3ème fois, 1 autre F-84 fut envoyé sur le signalement visuel, aucun contact radar ne pouvant être établi. Le pilote fit un contact visuel et headed out on un vecteur de 360 ° magnétique (~15 ° vrai). A ce moment le radar repéra ce qui était apparemment un écho fantôme, c'est-à-dire, qui "accompagnait" l'appareil, toujours sur le côté éloigné du radar. Le chasseur en l'occurrence pourchaissait probablement une autre étoile, dont l'image aurait pu avoir été déformée. Le signalement du pilote selon lequel l'ovni visuel "l'accompagnait" semble avoir renforcé la croyance de l'opérateur radar qu'il suivait effectivement un ovni, et non un écho fantôme. L'étoile dans ce cas pourrait très bien avoir été Mirfak (magnitude 1,9), qui, at 20 h 40 (LST), était à l'azimut 15 ° et 5 à 7 ° d'angle d'élévation environ. Le 2nd pilote, interrogé par le Dr. Hynek, indiqua avoir pensé qu'il pourchassait une étoile, bien qu'il y ait eu certains aspects de l'aparence de l'objet qui l'aient dérangé. Il indiqua également que le verrouillage radar de tir, qu'il avait signalé par radio durant la chasse, était du à un mauvais fonctionnement de l'équipement, et que la visée radar de tir continua de mal fonctionner lors de son retour à la base. Cet équipement ne fut jamais vérifié par la suite au sujet de ces défaillances (i.e., ni avant ni pendant l'enquête officielle de la FA sur l'incident).

Les observations de Bismarck (Dakota du Nord) commençèrent lorsque le Centre de Filtrage de Bismarck fut alerté de la présence d'ovnis par Rapid City. A 23 h 42 (LST) le sergent en poste là et plusieurs observateurs volontaires sortirent sur le toi et repérèrent rapidement 4 objets. Les descriptions de ces objets par les divers observateurs furent cohérentes avec l'hypothèse selon laquelle il s'agissait d'étoiles, bien que certaines anomalies apparentes aient amené les premiers enquêteurs de la FA à déduire par des triangulations brutes que les objets observés devaient être proches. Il apparaît maintenant que l'ensemble des 4 objets étaient des étoiles vues à travers une couche d'inversion de température. Les observateurs indiquèrent que les objets ressemblaient à des étoiles, mais que leur mouvement apparent et leurs changements de couleur semblaient exclure cette possibilité.

La synthèse du Dr. Hynek de la nature probable des 4 objets de Bismarck est éclairante :

L'objet #1, qui était bas sur horizon à l'Ouest et qui disparut entre minuit et 01 h 00 était l'étoile Arcturus observée à travers une inversion de surface. Arcturus était bas sur l'horizon dans l'Ouest et s'est couchée approximativement à 12 h 20 (LST) à 289 ° d'azimut.

L'objet #2 -- était l'étoile Capella observée à travers une inversion de surface. A 00 h 11 (CST) Capella était à un azimut de 40 ° et 15 ° d'élévation... [et] à 02 h 00 (CST) [elle] était à 53 ° d'azimut et 30 ° d'élévation, ce qui s'accorde avec les positions données par [les 2 témoins].

Les objets #3 et #4 étaient, avec un haut degré de probabilité, la planète Jupiter et l'étoile Betelgeuse, observées à travers une inversion de surface. Jupiter avait une magnitude stellaire ... de -1,7 [et était] basse sur l'horizon Est à approximativement 92 ° d'azimut. Betelgeuse ... était également basse sur l'horizon de l'Est et approximativement à 81 ° d'azimut.

La déclaration d'un des témoins de Bismark comprend les commentaires suivants :

...ils semblaient plus brillants que la plupart des étoiles et par moment semblaient prendre une teinte mat plutôt bleuâtre.

Ils semblaient se déplacer dans les cieux, mais à un rythme plutôt lent et à moins qu'une personne ne fixe sa tête contre un objet immobile pour éliminer le mouvement de la tête il était difficile de dire s'ils se déplaçaient.

Celui à l'Ouest disparut finalement sous l'horizon et celui au Nord-Est sembla se fondre graduellement avec le reste des étoiles jusqu'à ne plus être visible.

Figure 5 - Profil de réfractivité radio - Rapid City, 5 Août 1953
Fig 3

La dernière déclaration est typique de la description donnée par les témoins ayant apparemment observé une étoile brillante à travers une couche d'inversion. It would seem to be circumstantial evidence of the diffraction-brightening predicted by Raman for propagation along an inversion layer (voir section 6 chapitre 4). Cependant, il existe une explication alternative à une simple diffractive blurring or smearing de l'image d'une étoile, by spreading the available light over a larger area of the eye's retina, may cause a psychological illusion of brightening of the object.

Figure 6 - Profil de réfractivité radio - Rapid City, 6 Août 1953
Fig 3

Les conditions météorologiques étaient généralement favorables à une propagation anormale sur les 2 endroits. Le profil de réfractivité pour Rapid City à 20 h 00 (LST) le 5 Août monre une inversion de température épaisse de 0,5 ° C au-dessus d'une couche épaisse de 109 m, bien que le gradient de réfractivité résultant soit de seulement -77 km-1 (fig. 5). Le profil de 08 h 00 (LST) profile (fig. 6) montre a pronounced elevated duct entre 833 et 1007 m avec un gradient de -297 km-1 ; une inversion élevée de 3,2 ° est signalée à travers cette couche. Une forte couche d'inversion evidently formed during the night et fut "remontée" au niveau de 833 m par le réchauffement solaire après le level du Soleil vers 05 h 00 (LST).

Le profil de Bismarck pour 21 h 00 (LST) le 5 Août (fig. 4) montre une inversion de température de 1,2° C entre la surface et le niveau de 109 m, la couche résultante formant une radio duct avec un gradient de réfractivité de -182 km-1. On remarquera que les observations de Bismarck montrent plus d'indices d'effets optiques d'une couche d'inversion que les observations de Rapid City.

En résumé, les observations de Rapid City-Bismarck semblent avoir été provoquées par une combinaison de :

  1. étoiles vues à travers une couche d'inversion,
  2. au moins 1 météore,
  3. des échos AP sur un radar GCI, et
  4. de possibles échos fantômes sur le radar GCI et le mauvais fonctionnement d'un radar de visée embarqué (bien que l'officier commandant du détachement de Rapid City ait par la suite été sceptique sur le fait qu'il y ait même eut en fait un écho fantôme présent sur le radar GCI).

Cas 5 [(cases referred to thusly are found in Section IV.]. Louisianna-Texas (Ft. Worth) area, 19 September 1957, sometime between midnight and 0300 LST.

Figure 7 - Profil de réfractivité radio - Fort Worth, 19 Septembre 1957
Fig 3

The weather was clear. The radio refractive index profiles for Ft. Worth, for 1730 and 0530 LST, 18-19 September 1957, are shown in Figs. 7 and 8. The aircraft was flying at an altitude between 30,000 and 35,000 ft. as recalled 10 years later by the witnesses involved. There was a slight temperature inversion at an altitude of 34,000 ft., which may have been associated with a jet stream to the north.

Figure 8 - Profil de réfractivité radio - Fort Worth, 20 Septembre 1957
Fig 3

Il existe une possibilité qu'une inversion de température très fine et intense ait été présente cette nuit-la au-dessus de certaines zones localisées à une altitude d'environ 34000 pieds, a layer capable of giving strong reflections at both radar and optical frequencies. There are many aspects of the visual appearance of the UFO that are strongly suggestive of optical phenomena: the bright, white light without apparent substance, the turning on and off "like throwing a switch," the amorphous red glow without "any shape or anything of this nature." The radio refractivity profile for the time of the sighting, with several strong super-refractive layers, is conducive to the formation of radar AP echoes. The description of the GCl radar targets is suggestive of AP phenomena:

All of a sudden they would lose it, or something. They had it and then they didn't, they weren't sure. There was a lot of confusion involved in it. They'd give you these headings to fly. It would appear to just -- they had maybe a hovering -- capability and then it would just be in a different location in no time at all.

This type of behavior is typical of moving AP targets. The elevated duct shown on the Fort Worth profiles is very thick, and seems fully capable of causing these effects.

In summary, it is possible to account for the major details of the sighting through three hypotheses:

  1. The UFO at 30,000 to 35,000 ft. may have been a combined radio-optical mirage of another aircraft, at great distance, flying just below a thin inversion layer which was also just above the B-47's flight path. This aircraft would have had to have
    1. displayed landing lights which were turned off (creating the first sighting),
    2. been equipped with 2800 MHz radar, and
    3. displayed a red running light (causing the red glow).
  2. The GCI UFOs were AP echoes.
  3. The last "red glow" at "15,000 feet" may have been a ground source, which became obscured or was turned off as the aircraft approached.

Il y a de nombreux aspects inexpliqués à cette observation, cependant, e une solution telle que donnée ci-dessus, bien que possible, ne semble pas hautement probable. Une des caractéristiques les plus dérangeantes du rapport est l'insistance de l'opérateur radar, faisant référence aux radar au sol et aériens, que ...this would all happen simultaneously. Whenever we'd lose it, we'd all lose it. There were no "buts" about it, it went off. Another unexplained aspect is the large range of distances, bearing angles, and to some extent, altitudes covered by the UFO. The radar operator's comment that the return "had all the characteristics of -- a ground site -- CPS6B," indicates that an airborne radar source is unlikely due to the large power requirements. There remains the possibility that the "red glow" was the mirage of Oklahoma City which was in about the right direction for the original "red glow" and presumably had a CPS6B radar installation, but subsequent direction and location changes would seem to rule out this possibility and the grazing angle at the elevated inversion layer would be too large for a normal mirage to take place.

In view of these considerations, and the fact that additional information on this incident is not available, no tenable conclusion can be reached. From a propagation standpoint, this sighting must be tentatively classified as an unknown.

Classe 1-D: Principalement visuel, apparence diverse : semblable à un vallon, un avion, etc.

Au-dessus du Labrador, 30 juin 1954, 21:05-21:27 LST. Météo : (à 19 000 pieds) clair, avec une couche brisée de stratocumulus en-dessous, visibilité excellente. Aucun contact radar ne fut fait durant cet incident. Une synthèse du récit de 1ère main de son expérience par le pilote :

J'étais aux commandes d'un Boeing Strato cruiser de la BOAC en route de New York pour Londres via Goose Bay Labrador (escale de carburant). Peu après être passé au-dessus de Seven Islands à 19 000 pieds, Véritable Vitesse de l'Air 230 noeuds, mon copilote et moi-même prirent conscience de quelque chose se déplaçant along off our port beam à une altitude inférieure à une distance de peut-être 5 miles, à l'intérieur et à l'extérieur d'une couche brisée de nuages strato cumulus. Alors que nous regardions, ces objets montèrent au-dessus du nuage et nous pouvions maintenant en voir clairement 1 grand et 6 petits. Alors que nous volions vers Goose Bay le grand objet commença à changer de forme et le plus petit à se déplacer relativement au plus grand...

Nous informèrent Goose Bay que nous avions quelque chose de bizarre en vue et ils s'arrangèrent pour envoyer un chasseur (F-94 ?) sur nous. Plus tard je changeais de fréquence radio pour contacter ce chasseur ; le pilote me dit qu'il m'avait en vue sur le radar se rapprochant face à moi à 20 miles. A ce moment les petits objets semblaient entrer dans le grand, puis le gros se rétrécit. Je donnais une description de cela au chasseur et a bearing des objets par rapport à moi. Je dû alors revenir sur la fréquence de Goose pour autorisation de descente. Je ne sais pas si le chasseur a vu quelque chose, car il n'avait pas atterri lorsque j'ai quitté Goose pour Londres.

La description de l'ovni dans ce cas, un objet opaque, sombre et "comme de la gelée", changeant constamment de forme, suggère une cause optique. Très peu de données météorologiques sont disponibles pour cette partie du monde à la date en question, de sorte que la présence de mécanismes significatives de propagation optique ne peut être confirmée ni exclue. Néanmoins, certains faits dans le cas suggèrent fortement un phénomène de mirage optique :

  1. L'ovni était toujours situé dans les quelques degrés d'un plan horizontal contenant l'appareil, satisfaisant ainsi la nécessité d'un angle réduit ;
  2. L'appareil volait à une altitude stable de 19000 pieds pendant les 85 miles nautiques sur lesquels l'ovni apparut "accompagner" l'appareil, et l'avion maintenait donc une relation constante avec toute couche atmosphérique à une altitude fixe ;
  3. L'ovni sombre fut vu devant un fond de ciel brillant dans 5° à 20 ° du soleil couchant ; des images prartiquement identiques, montrant un comportement "semblable à une méduse" pourrait être courramment observé partout où des mirages sont observés avec un fort contraste lumineux présent. La réflexion de la Lune sur une eau doucement ondulante présente un comportement assez semblable.

Il y a une forte suggestion que l'ovni dans ce cas ait été un mirage : une réflexion du terrain sombre en-dessous vu against le ciel brillant, "argenté" à gauche du soleil couchant. La couche réfléchissante serait une inversion de température aigüe et fine située à une altitude juste au-dessus de celle de l'appareil naviguant. La plupart des faits dans cet incident peuvent être expliqués par cette hypothèse. La nature sombre et opaque de l'image vient du contraste de luminosité et du phénomène de "réflexion totale". L'arrangement du grand objet et des petits en une ligne fine juste au-dessus de la trajectoire de vol de l'appareil, ainsi que la manière de disparaître, sont proportionné avec un mirage. A mesure que la couche de production du mirage s'affaiblit (avec la distance) ou que l'angle de visualisation décroît (l'appareil entâmait-il sa descente à ce moment ?), le mirage semble diminuer jusqu'à un certain point et dispparaître. Ce type de mirage est qualifié de mirage supérieur et a souvent été signalé au-dessus de l'océan (voir section 6, chapitre 4).

La principale difficulté avec cette explication, outre le fait d'avoir à faire l'hypothèse de l'existence de la couche produisant le mirage, est la façon d'expliquer l'anisotropie du mirage. Une anisotropie de cette sorte, i.e., un mirage limité à certains azimuts de visualisation, est courant dans des mirages terrestres lorsqu'ils sont vus d'un lieu unique. Mais une couche de mirage à travers laquelle une image réfléchie pourrait être vue seulement dans une direction constante principale (plus quelques petites images "satellites") sur une distance de 85 miles nautiques est assez inhabituelle.

Reste ici la mince possibilité que l'appareil lui-même ait produit la couche de mirage par intensification (par la compression induite par l'onde de choc du passage de l'appareil dans l'air) d'une couche barely subcritical, i.e., une dans laquelle le gradient de température est juste un petit peu inférieur à la valeur requise pour produire un mirage. Cette hypothèse satisferait la nécessité directionnelle de l'observation, mais l'arrangement résultant d'hypothèses est trop spéculatif pour former une solution acceptable à l'incident.

Cette observation inhabituelle devrait par conséquent être affectée à la catégorie d'un phénomène naturel presque certain, qui est si rare qu'il n'a apparemment jamais été signalé auparavant ni depuis.

Figure 9a - Profil de réfractivité radio - Spokane, 10 Décembre 1952
Fig 3

304-B. Odessa (Washington), 10 décembre 1952, 19 h 15 (LST). Météo : clair au-dessus du plafond nuageux à 3000 pieds ; appareil à 26000-27000 pieds. 2 pilotes dans un appareil F-94 observèrent un grand objet rond et blanc, plus grand que tout type d'avion connu. Une lumière dim reddish-white seemed to come from 2 fenêtres. It appeared to be able to "reverse direction almost instantly," and did a chandelle in front of the aircraft. After this the object appeared to rush toward the aircraft head-on and then would "suddenly stop and be pulling off." The pilot banked away to avoid an apparently imminent collision, and lost visual contact. Fifteen minutes later the aircraft radar picked up something which the crew assumed was the UFO, although there is no evidence that it was. The object was reported to be moving generally from west to east at 75 knots. It was never sighted.

This sighting has been described as a mirage of Venus, although the reported 75 knot speed and 270° direction of motion is in contradiction to this hypothesis. The general description of the object as well as the reported motion is suggestive of a weather balloon. However, the peculiar reversals of direction, although they could have been illusory, and particularly the loss of visual contact are at odds with the balloon hypothesis.

The radiosonde profile for Spokane, 1900 LST, is shown in Fig.9 and is inconclusive. The tropopause, where the sharpest temperature inversions are likely, is at about 30,500 ft. above sea level, too high to have produced a mirage visible à 26 000-27 000 pieds.

The closeness of the timing between the radiosonde release at 1900 LST and the sighting at 1915 LST suggests that the F-94 crew may have seen a lighted pibal balloon. The description given, including the two dimly-lit "windows," is typical of the description of a pibal balloon by those not familiar with weather instrumentation. Such a balloon would rise to at least 17,000 ft. in 15 min., and the reported motion, 270° at 75 knots, is in excellent agreement with the upper winds at the highest level plotted for the Spokane profile: 280° at 66 knots at 18,000 ft.

Figure 9b - Profil de réfractivité radio - Spokane, 10 Septembre 1952
Fig 3

19-X. [361-B]. Kirtland AFB, Albuquerque, N.M., 4 Nov. 1957, 2245-2305 LST. Weather: scattered clouds with high overcast, visibility good, thunder-storms and rain showers in vicinity, light rain over airfield. Observers in the CAA (now FAA) control tower saw an unidentified dark object with a white light underneath, about the "shape of an automobile on end," that crossed the field at about 1500 ft. and circled as if to come in for a landing on the E-W runway. This unidentified object appeared to reverse direction at low altitude, while out of sight of the observers behind some buildings, and climbed suddenly to about 200-300 ft., heading away from the field on a 120° course. Then it went into a steep climb and disappeared into the overcast.

The Air Force view is that this UFO was a small, powerful private aircraft, flying without flight plan, that became confused and attempted a landing at the wrong airport. The pilot apparently realized his error when he saw a brightly-lit restricted area, which was at the point where the object reversed direction. The radar blip was described by the operator as a "perfectly normal aircraft return," and the radar track showed no characteristics that would have been beyond the capabilities of the more powerful private aircraft available at the time. There seems to be no reason to doubt the accuracy of this analysis.

1482-N. About 15 mi. east of Utica, N. Y., 23 June 1955, 1215-1245 LST. Weather: overcast at 4,000 ft., visibility good below. Reported by the co-pilot of a Mohawk Airlines DC-3. They were cruising at 3,000 ft. at 160 knots, when he noticed an object passing approximately 500 ft. above at an angle of about 70° (20° from vertical). It was moving at "great speed." The body was "light gray, almost round, with a center line .... Beneath the line there were several (at least four) windows which emitted a bright blue-green light. It was not rotating but went straight." The pilot also saw this UFO; they watched it for several miles. As the distance between the DC-3 and the UFO increased, the lights "seemed to change color slightly from greenish to bluish or vice versa. A few minutes after it went out of sight, two other aircraft (one, a Colonial DC-3, the other I did not catch the number) reported that they saw it and wondered if anyone else had seen it. The Albany control tower also reported that they had seen an object go by on Victor-2 [airway]. As we approached Albany, we overheard that Boston radar had also tracked an object along Victor-2, passing Boston and still eastbound."

The pilot and co-pilot computed the "speed" of the UFO at 4,500-4,800 mph. from the times of contact near Utica and at Boston. There are a number of inconsistencies in this report, aside from the most obvious one: the absence of a devastating sonic boom, which should be generated by a 150 ft. ellipsoidal object travelling at Mach 6 or better in level flight at 3,500 ft. It does seem likely that the Boston GCA report was coincidental and involved a different object.

The residue is a most intriguing report, that must certainly be classed as an unknown pending further study, which it certainly deserves. Statements from some. of the other witnesses involved would help in analyzing the event, and should prove useful even 13 years after the fact. It does appear that this sighting defies explanation by conventional means.

10-X. [371-B.] Continental Divide, N. M., 26 janvier 1953, 21 h 15-22 h 00 LST. Météo : couverture nuageuse légère et élevée, nuages bas épars, très bonne visibilité. Un aviateur basé au 769ème escadron AC&W at Continental Divide (elevation 7,500 ft.) observed a "bright reddish-white object" about 10 mi. west of the radar site and approximately 2,000 ft. above the terrain. The radar subsequently painted a strong, steady return at 9 mi. range and about 2500-7500 pieds au-dessus de lasurface. This object passed behind a nearby hill and reappeared, heading north at about 10-15 mph. Radar track confirmed this. The object then moved to the west at 12-15 mph to a point 18 mi. west of the radar site. It then turned north for about 10 mi., and subsequently turned back on a heading of 128° inbound to the station. Radar and visual contact was lost near the area where the object was first detected. Before disappearing, the object seemed to shrink in size and fade in color to a dull red.

Figure 10 - Carte météo synoptique - Graphe de pression constante, 300 mb, M. S. I. de près de 30 000 pieds - 27 janvier 1953 (03 h 00 Z) Image des USA recadrée pour n'inclure que les données météo
Fig 3

There seems to be little doubt in this case that the visual and radar contacts were in fact of the same object. The obvious interpretation is that the object seen and tracked on radar was a weather balloon, a lighted pibal used for obtaining data on upper winds. This explanation was considered and rejected by Air Force investigators for 2 reasons:

  1. The sighting occurred 1 hr. 15 mm. after the scheduled release of the Winslow, Ariz. pibal, the only one that seemed likely to have showed up in the sighting area, and the balloon ought to have burst by then, since they generally burst at 30 000 pieds, une altitude que the Winslow pibal should have reached 25 min. after launch;
  2. The reported direction of movement was, at least part of the time, directly opposite to the reported upper winds as derived from the Albuquerque radiosonde flight. These winds were reported from the Ouest entre 10000 et 30000 pieds.

Actually, neither of these two reasons is sufficient to discount the balloon theory. In the first place, weather balloons are often released later than the scheduled time, and this possibility was apparently not checked. In the second place, pibal balloons are often known to leak and consequently to rise at a much slower rate than normal. Often they have so little buoyancy that they may be caught in local updrafts or downdrafts. These leaking balloons are usually carried away by the horizontal wind flow at such a rate that they are lost from sight of the observing station before they reach burst altitude. The pibal data from Winslow, Ariz. for 0300 GMT 27 January 1953, (2000 LST 26 January) is listed as "missing" above the 500 mb level (about 19,000 ft. m.s.l.), which is a strong indication that the balloon may have been leaking. It is therefore entirely conceivable that the Winslow pibal balloon could have been in the vicinity of Gallup, N. M. (west of the radar site) at 2115 LST on the night in question.

The problem of the observed direction of movement cannot be completely resolved, because it depends largely on an analysis of mesoscale winds in the lower atmosphere, that is, on a scale smaller than ordinarily analyzed n synoptic weather maps. The synoptic maps for 2000 LST 26 January 1953, for the 700 mb (about 10,000 ft.), 500 mb (about 19,000 ft.), and 300 mb (about 27,000 ft.) levels are shown in Figs. 10 and 11.

Figure 11 - Carte météo synoptique - Graphe de pression constante, 500 mb, M. S. I. de près de 19000 pieds - 27 Janvier 1953 (0300 Z) [Image des USA recadrée pour n'inclure que les données météo]

Although the general windflow in the Arizona-New Mexico area for at least the 700 and 500 mb maps is from the west, there are indications of a secondary mesoscale circulation somewhere in the vicinity of the Arizona-New Mexico border, which is embedded in the general trough overlying the southwestern states. Especially significant are the winds at the 700 and 500 mb levels at Tucson and at Phoenix, mainly at the 500 mb level, which show evidence of a mesoscale cyclonic circulation in the area.

In view of the general meteorological situation at the time, a quite likely explanation for the Continental Divide sighting is as follows: The Winslow pibal balloon, which was leaking, was carried away to the east, probably sinking slowly as it went, and was lost from view of the Winslow weather station. Upon reaching the general vicinity of Gallup, N. M. the leaking balloon was probably caught up in a local cyclonic vortex and updraft, which, being instigated by the mesoscale cyclonic flow in the region may have formed on the windward side of the range of low mountains forming the Divide in that area. This would have caused the balloon to be carried toward the north, slowly rising, as first observed. This would be followed in sequence by a turn to the west, and ultimately, upon reaching a somewhat higher level, a turn toward the southeast again as the balloon became caught in the more general flow from the west and northwest prevailing at middle levels in the atmosphere.

This hypothesis fits the details of the observations rather well, and considering the lack of additional information or data pertaining to this incident, the UFO should probably be tentatively identified as a weather balloon.

321-B. Niagara Falls, N. Y., 25 July 1957, 0025 LST. Weather: clear, excellent visibility. Observers saw a "circular brilliant white object with pale green smaller lights around its perimeter." Object appeared to move slowly at nearly constant altitude, and then went into a "fast, steep climb," disappearing in about 5-8 min. The object was tracked on a CPS-6B radar for about 3 min. moving from SW to NE, in agreement with prevailing winds in the area.

The rate of climb could not have been very great, or the object would not have remained in sight for 5 à 8 mn. The official AF view is that the object was a lighted balloon, and in the absence of other data or a more complete file on the case, there seems to be no more likely explanation.

Classe 2 : Incidents ovni étant principalement des contacts radar, avec ou sans observations visuelles

Classe 2-A : Principalement radar, avec retours radar de nature semblable à une PA : retours flous, vagues ou erratiques, retours multiples, retours sporadiques, etc.

1211-B. McChord AFB, Seattle, Wash., 2 October 1959, 0020-0320 LST. Weather: clear, fog moved in at 0150 LST after initial sighting, wind from 100 at 10 knots (approx.). Radar at McChord AFB picked up a total of five or more unidentified tracks between 0020 to 0320 LST. These targets appeared to be at elevation angles of about l0° -20° and azimuths of l70° -l90°. The range would change from 4,000 yd. to 8,000 yd., and the flight patterns were described as "erratic;" returns would occasionally appear in pairs. The radar blips were described as "weak." Data on the vertical beam width and the antenna pattern characteristics of the radar are lacking.

Visual observers were apparently told to go outside and look for an UFO at about 10° elevation and 190° azimuth. They found one - "round," "the size of a quarter" (distance not specified), "white and blue flickering light," a rather good description of a scintillating star. There was a second magnitude star at precisely the correct azimuth (190°) at the time, although the elevation angle would have been only about 1° or so. A sharp temperature inversion, with mist trapped below it, could have easily produced the effect of larger size as well as increased the apparent elevation angle by about 1°. Even trained observers consistently over-estimate the elevation angle of objects near the horizon., as in the "moon illusion" (the apparent increase in size of the rising moon).

When "last seen," at about 0150 LST, the object was reported to be about 20° elevation and 170° azimuth. At that time another bright star (0.7 magnitude fainter than the first one) was located at about 172° azimuth and about 10° elevation, values commensurate with the apparent visual position (again, assuming over-estimate of elevation angle). Near the horizon these were the only two stars of third magnitude or greater in that part of the sky at that time.

The description of the radar targets, weak, erratic blips, together with the reported formation of a low-level fog (that hindered visual observations after 0150 LST), suggests the presence of a shallow temperature inversion-humidity trap that was producing AP echoes on the radar set. The UFO report states that temperature inversions were "prevalent" in the area.

In summary, this UFO incident appears to have been caused by radar AP echoes and associated visual star sightings, both observed at small angles through a surface temperature inversion-humidity trap layer.

103-B. Gulf of Mexico, off Louisiana coast (28° N 92° W), 6 December 1952, 0525-0535 LST (1125 GMT). Weather: clear, dry, light winds, visibility excellent, full moon. The radio refractivity profile for Burwood, La., about 175 mi. NE of location of sighting, for 0900 LST is shown in Fig. 12; a very strong super-refractive layer is shown on this profile over a height interval extending from the surface to 456 m. (1,500 ft.). A sharp temperature inversion existed at the top of this layer. As an aircraft was returning to Galveston, Tex. at 20,000 ft. burn-off flares from oil refineries became visible. The radar was activated on 100 mi. range to check for the Louisiana coastline. The range to the nearest point on the coastline was about 89 mi. and assuming standard propagation conditions, the range to the radar horizon should have been on the order of 140 mi. Surprisingly, the coastline could not be seen on the radarscope. Instead a number of unusual echoes were observed. Initially there were four moving an a course of 120° true azimuth. These blips moved at apparent speeds of over 5,000 mph., coming within 15-20 mi. of the aircraft's position. Eventually they disappeared from the scope. The radar set was calibrated, but more blips appeared still moving SE across the scope.

Visual observations consisted of one or two blue-white flashes, one of which, as viewed from the waist blister, appeared to pass under a wing of the aircraft. All of these may have been above the horizon, since the wingtip would appear well above the horizon as viewed from this position. The observers stated that the flashes "did not alter course whatsoever." These visual sightings were probably Geminid meteors; the wing operations officer stated: "Visual sightings are indecisive and of little confirmatory value."

One of the radar witnesses stated: "One object came directly towards the center of the scope and then disappeared." After 10 min. of radar observation, a group of the blips merged into a half-inch curved arc about 30 mi. from the aircraft at 320° relative azimuth and proceeded across and off the scope at a computed speed of over 9,000 mph. After this, no more unidentified returns were noted on the radar.

Figure 12 - Profil de réfractivité radio - Burwood (LA) - 6 Décembre 1952
Fig 3

The radar returns obtained in this incident were probably caused by the deep super-refractive layer near the surface shown in Fig. 12.

That this layer was present at the time and in the area is indicated by the failure of the aircraft radar to detect the Louisiana coastline even though burn-off flares on the shore were visible to the unaided eye. The layer was probably slightly stronger at the time of the incident, thus constituting a thick radio duct. A transmitter located above a radio duct and emitting a high enough frequency to be affected, as the radar undoubtedly was, does not excite propagation within the duct. This implies that the coastline below the duct would not be visible to the radar located above the duct.

The strange moving targets seen on the radar were probably caused by imperfections in the atmospheric layer forming the radio duct, allowing the radio energy to enter the ducting layer at various points. This would create sporadic ground returns. The returns may have been caused by a series of gravity waves running along the ducting layer in a SE direction; this is a phenomenon which is at present only poorly understood. In any event, spurious radar images have often been noted under propagation conditions of this sort, often moving at apparent speeds of from tens to thousands of miles per hour.

In summary, it seems most likely that the cause of this sighting can be assigned to radar AP, for which there is meteorological evidence, and meteors.

7-C. White Sands Missile Range, N. M., 2 March 1967, 1025-1132 LST. Weather: apparently clear (few meteorological data are available). A single witness at the summit of highway 70 over the Sacramento Mountains (Apache Summit, 9,000 ft. elevation) reported seeing "silvery specks" passing overhead from north to south. The witness called Holloman AFB, and range surveillance radar was requested to look for the objects. Two aircraft were scrambled, but neither reported a sighting, although they searched the area where the UFOs were reported.

Two radars were in operation. Both tracked a number of targets, most of which were stationary and so intermittent in nature as to prevent lock-on (see Case 16). Significantly, none of the radar targets was behaving in the manner described by this witness (i.e., moving steadily south at high altitude). Therefore, this incident is considered to be primarily a radar contact.

The probable nature of each of the three types of radar contact made is examined below.

  1. The stationary, intermittent targets. Most of these can be identified with terrain features, peaks or ridges, that would normally be just below the radar's line of sight. If the atmospheric conditions were such as to render these points just barely detectable by the radars, they would probably appear as intermittent, stationary targets of the type described.
  2. The object at 25,000 ft. that "drifted east three or four miles in about 10 minutes" was apparently moving with the prevailing upper winds from the west; it may have been a weather balloon, or some similar device.
  3. The circular track executed by the Holloman radar was interpreted by the radar engineers on the base as being a noise track. This seems quite likely, despite some apparent discrepancies noted in the report. If this track represented a real target, it is strange that the Elephant Mountain radar never picked it up, in spite of the fact that the apparent track passed within about 6.5 mi. of the second radar's location.

190-N. Detroit, Mich., March 1953, about 1000 to 1100 LST (exact date and time unknown). Weather: "perfectly clear." A USAF pilot and a radar operator, flying in an F-94B fighter on a practice training mission, were directed by GCI radar at Selfridge AFB to intercept some unknown targets which appeared to be over downtown Detroit. The pilot and radar operator looked in that direction and saw "tiny specks in the sky, which appeared to look like a ragged formation of aircraft."

The aircraft at this time was about 30 mi. NW of downtown Detroit, and the targets "appeared to be over the city's central section." The pilot turned the aircraft to an intercept course. During this time, perhaps "three or four minutes," the objects were visible to the pilot as "a ragged formation traveling slowly in a westward direction;" the objects appeared to be "a little lower than our aircraft." The pilot started his intercept run under full military power, without afterburner, at approximately 500 mph.

The pilot recalls thinking several times that details of the unknowns, like wings, tails, etc. should have "popped out" as they approached, so that identification could be made, but they did not. The ground radar had both the F-94B and the unknowns "painted as good, strong targets." The unknowns could still not be identified, but "seemed to get a little larger all the time."

The F-94B's radar operator began to get returns and "thought he was picking up the targets." The pilot looked at his instruments to see if he could "inch out a little more speed without going into after-burner," and when he looked up again "every last one" of the objects was gone. The pilot asked GCI where the UFOs were, and was told they were still there, "loud and clear." They continued to fly headings given by GCI right into the center of the targets, flying and turning in "every direction," but there was nothing in sight. The pilot states: "Gradually the targets disappeared from ground radar after we had been amongst them for 3 ou 4 mn." The F-94B then returned to base.

Since the exact date of this sighting is unknown, no applicable meteorological data are available. Any explanation of this incident must therefore remain speculative in nature. If the UFOs are considered to have been material objects, then they would have had to have shifted position some tens of miles in the 2 à 4 s while the pilot was looking down at his instruments. This does not explain why they continued to appear on the ground radar. The only admissable hypothesis would seem to be that they became invisible as the fighter approached, but this does not account for the fact that they could not be picked up on airborne radar while the aircraft was searching the area.

There is one hypothesis that seems to fit all of the observed facts: that the "ragged formation" was actually an inferior mirage (see Section VI, Chapter 4). The angular conditions are satisfied: the objects appeared "slightly below the level of the aircraft," and reflections of the sky above the horizon would seem dark when seen projected against the hazy sky directly over the city. A layer of heated air, trapped temporarily below a cooler layer by a stable vertical wind shear, could produce a wavy interface that would reflect the sky in a few spots. This phenomenon is quite similar to the familiar road mirage. Like, a road mirage it suddenly disappears when one gets too close and the viewing angle becomes either too large or too small.

If the warm air below, the source of which would presumably have been the downtown area of Detroit, were also considerably moister than the cooler air above as is quite probable, then the radio refractive index would decrease quite suddenly across the inter-face. This would tend to produce anomalous propagation effects, including false echoes, on radar, and would explain why ground radar could continue tracking the unknowns when the pilot and airborne radar operator could no longer see them. The airborne radar, being immersed in the layer would probably not receive AP echoes of any duration other than, perhaps, occasional random blips.

After the aircraft had thoroughly mixed the opposing air currents by flying repeatedly through the interface as it searched for the targets, the ground radar returns would gradually fade away. This corresponds to what was actually observed.

In summary, without the data to make a more definitive evaluation of this case, the most likely cause seems to be a combined radio-optical mirage as described above. If so, this is another example of a natural phenomenon so rare that it is seldom observed: for a 0.25° critical mirage angle, the temperature contrast required is on the order of 10° or 15°C in the space of about 1 cm.

Washington, D.C. (see Appendix L) 19-20 and 26-27 July 1952.

Figure 13 - Profil de réfractivité radio - Silver Hill (MD) - 19 Juillet 1952

Weather: mostly clear, a few scattered clouds, visibility 10 to 15 mi., temperature 76° to 87°F, dewpoint 61° to 72°F, surface winds from SE, light, near surface, from 300° to 320° aloft, light. Radio refractive index profiles are shown in Figs. 13, 14, and 15, in Md., at an elevation of 88 m. (289 ft.) above sea level. There are a tremendous number of reports of UFOs observed on these two nights. In most instances visual observers, especially in scrambled aircraft, were unable to see targets indicated on ground radar, or to make airborne radar contact. Ground radar observers were often able to find a return in the general area of reported visual contacts, especially in the case of ground visual reports where only an azimuth was given. A few excerpts from typical reports during these incidents are given below:

Control tower operator, Andrews AFB, 0100 to 0500 EST, 20 July 1952:

An airman became excited during the conversation and suddenly yelled "there goes one." I saw a falling star go from overhead a short distance south and burn out. About two minutes later (the airman) said, "There's another one;

did you see the orange glow to the south?" I said I thought I saw it, but he pointed south and I had been looking south-west. I went up on the roof---and watched the sky in all directions. In the meantime Washington Center was reporting targets on their radar screen over Andrews. Andrews Approach Control observed nothing.

[The airman] was in the tower talking on the phone and interphones. He was watching a star and telling various people that it was moving up and descending rapidly and going from left to right, and [another airman] and I, listening to him from the roof, believed we saw it move too. Such is the power of suggestion.

This star was to the east slightly to the left of and above the rotating beacon. [The airman] reported the star as two miles east of Andrews and at an altitude of 2,000 ft.

A short time later, approximately 0200 hours, I saw a falling star go from overhead to the north. A few minutes later another went in the same direction. They faded and went out within two seconds. The sky was full of stars, the Milky Way was bright, and I was surprised that we did not see more falling stars.

All night Washington Center was reporting objects near or over Andrews, but Andrews Approach Control could see nothing, however they could see the various aircraft reported so their [radar] screen was apparently in good operation.

At 0500 hours Washington Center called me and reported an unknown object five miles southeast of Andrews field. I looked and saw nothing, That was the last report I heard.

Figure 14 - Profil de réfractivité radio - Silver Hill (MD) - 25 Juillet 1955

A USAF Captain at Andrews AFB radar center:

At about 0200 EST Washington Center advised that their radar had a target five miles east of Andrews Field. Andrews tower reported seeing a light, which changed color, and said it was moving towards Andrews. I went outside as no target appeared on Andrews radar and saw a light as reported by the tower. It was between 10° and 15° above the horizon and seemed to change color, from red to orange to green to red again. It seemed to float, but at times to dip suddenly and appear to lose altitude. It did not have the appearance of any star I have ever observed before. At the time of observation there was a star due east of my position. Its brilliance was approximately the same as the object and it appeared at about the same angle, 10° to 157deg; above the horizon. The star did not change color or have any apparent movement. I estimated the object to be between three and four miles east of Andrews Field at approximately 2,000 ft. During the next hour very few reports were received from Washington Center. [According to Washington Center's account, however, the 0200 EST object was seen on radar to pass over Andrews and fade out to the southwest of Andrews -- G. D. T.] At approximately 0300 EST I again went outside to look at the object. At this time both the star and the object had increased elevation by about 10°. [The azimuth would have also increased about 10°, so that the observed change was apparently equal to the sidereal rate, 15° of right ascension per hour -- G. D. T.] The object had ceased to have any apparent movement, but still appeared to be changing color. On the basis of the second observation, I believe the unidentified object was a star.

The account of the airman referred to by the Andrews AFB control tower operator:

Airman [X] called the tower and reported he had seen objects in the air around Andrews; while we were discussing them he advised me to look to the south immediately. When I looked there was an object which appeared to be like an orange ball of fire, trailing a tail; it appeared to be about two miles south and one half mile east of the Andrews Range [station]. It was very bright and definite, and unlike anything I had ever seen before. The position of something like that is hard to determine accurately. It made kind of a circular movement, and then took off at an unbelievable speed; it disappeared in a split second. This took place around 0005 EST. Seconds later, I saw another one, same description as the one before; it made an arc-like pattern and then disappeared. I only saw each object for about a second. The second one was over the Andrews Range; the direction appeared to be southerly.

Figure 15 - Profil de réfractivité radio - Silver Hill (MD) - 26 Juillet 1952

The account of a staff sergeant at Andrews AFB follows. He was apparently describing the same object that the radar center Captain had observed.

Later on we spotted what seemed to be a star north-east of the field, which was in the general direction of Baltimore. It was about tree top level from where I was watching. It was very bright but not the same color (as some apparent meteors). This was a bluish silver. It was very erratic in motion; it moved up from side to side. Its motion was very fast. Three times I saw a red object leave the silver object at a high rate of speed and move east out of sight. At this time I had to service a C-47 and lost sight of it for the night. The time was about 0330.

The visual sightings in these incidents seem to be either meteors, apparently quite numerous at the time, or stars, but a few descriptions are not adequate to make an identification and hence may represent unknowns.

The radar tracks reported, at various times, from Washington National Airport, Andrews AFB, and Bolling AFB are generally not correlated with each other, with airborne radar/visual observations, or with ground visual reports, except in a very general way, e.g., a star sighted on the azimuth supplied by the radar track.

An investigation of the radar tracks reported by Borden and Vickers (1953) is very informative. The authors observed, on the night of 13-14 August 1952, radar tracks very similar to those described in the 19-20 and 25-27 July incidents. The targets appeared to move with the upper winds at various levels at twice the observed wind speed, suggesting that they were ground returns seen by partial reflections from moving atmospheric layers of relatively small horizontal extent (i.e., patches of local intensification of a general super-refractive stratum). Borden and Vickers state:

The almost simultaneous appearance of the first moving targets with the [stationary] ground returns, [the latter] signifying the beginning of the temperature inversion, suggested that the target display was perhaps caused by some effects existing in or near the inversion layers.

The authors also relate similar target patterns observed during testing of a new radar at Indianapolis in November, 1952. They state:

Targets were larger, stronger, and more numerous than those observed by the writers during the Washington observations. At times the clutter made it difficult to keep track of actual aircraft targets on the scope.

In all major respects this report (Borden, 1953) is an excellent analysis of the probable radar situation during the July 1952, Washington sightings.

The atmospheric conditions in existence at the times of these UFO incidents, as shown in Figs. 13, 14, and 15, are rather peculiar. Refractivity profile for 19 July 2200 LST shows a surface inversion of l.7° C (3.1° F) but the resulting refractivity gradient is only -81 km-1, about twice the "standard" value. There is a rather unusual subrefractive layer at 3833 to 4389 m. produced by overlying moist air. Relative humidity drops from 84% at surface to 20% at base of this layer, then climbs to 70% at top of the layer. A number of significant levels are missing from this profile, which is common in 1952 Silver Hill profiles, but even so it is indicative of unusual atmospheric conditions. The radar sightings were made between 2340 LST and 0540 LST (July 20), and the atmospheric stratification was no doubt more strongly developed by that time. In addition, Silver Hill is at an elevation of 88 m. (289 ft.) above MSL, whereas Washington National Airport is at an elevation of only 13 m. (43 ft.). The intervening 75 m. is precisely that part of the atmosphere in which some of the most spectacular super-refractive and ducting layers would be expected to develop. Indeed, records for 1945-1950, during which radiosonde upper-air soundings were launched from Washington National Airport, reveal a much stronger tendency for the formation of anomalous propagation conditions than the Silver Hill data.

The profiles for 25 July and 26 July, 2200 LST are more complete than the 19 July profile, although some significant levels were noted as missing from the 26 July profile. Otherwise, the foregoing comments apply to these profiles as well. The 25 July profile shows a super- refractive surface layer and a strong elevated duct; there is a 4.6°C (8.3° F) temperature inversion through the elevated duct. It is perhaps significant that unidentified radar targets began appearing at 2030 LST on 25 July. The 26 July profile has a l.2° C (2.2°F) surface inversion without a humidity lapse sufficient to cause super-refraction; however, a 0.9° C inversion between 1115 and 1275 m. is associated with a sharp humidity drop and a resulting elevated duct with a gradient of -167 km-1. This elevated layer is quite strong enough to produce AP effects on radar. Unidentified radar targets began appearing at 2050 LST on 26 July and continued until after midnight.

In summary, the following statements appear to be correct:

  1. The atmospheric conditions during the period 19-20 and 25-27 July, 1952, in the Washington, D. C., area, were conducive to anomalous propagation of radar signals;
  2. The unidentified radar returns obtained during these incidents were most likely the result of anomalous propagation (AP);
  3. The visual objects were, with one or two possible exceptions, identifiable as most probably meteors and scintillating stars.

Wichita. Kans. area, 2 August 1965, "early morning hours" up to "shortly after 0600" LST.

Figure 16 - Profil de réfractivité radio - Topeka - 2 Août 1965

Weather: clear, temperature 61° F to 70°F, wind at surface: light from WSW. This is classed as primarily radar since the bulk of the reports were from radar and the first visual object was never described. The refractivity profiles for Topeka, Kans. and Oklahoma City, Okla are shown in Figs. 16 and 17.

During the early morning hours of 2 August 1965, the Wichita Weather Bureau Airport Station was contacted by the dispatcher of the Sedgwick County Sheriffs Department with regard to an object sighted in the sky near Wellington, Kans. (25 mi. south of Wichita). The radar operator, Mr. John S. Shockley observed what appeared to be an aircraft target near Udall, Kans., 15 mi. northeast of Wellington. This target moved northward at 40 to 50 mph.

During the next hour and a half several of these targets were observed on the radar scope over central Kansas moving slowly northward occasionally remaining stationary, or moving about erratically.

Figure 17 - Profil de réfractivité radio - Oklahoma City- 2 Août 1965

Mr. Shockley checked with the Wichita Radar Approach Control, however they were not able to observe a target simultaneously, with the exception of one aircraft south of McConnell Air Force Base near Wichita.

Later, a target was observed about seven miles NNW of Wellington, Kans., moving slowly southward. The Wellington Police Department was contacted and two officers went three miles west of the city, to see if they could observe anything. The target passed about one mile west of the city as observed on radar. The officers did not observe it until it was southwest of the city. They described it as a greenish-blue light that moved slowly away from them.

The dispatcher called again, with a report that two officers at Caldwell, Kans. (35 mi. south of Wichita) had sighted an object near the ground east of the city. A target was observed about two miles northwest of the city that moved northward and disappeared.

At daybreak, the dispatcher reported that the Wellington officers had an object in sight east of the city. Radar indicated a target in that area moving southward about 45 mph. Four or five people stopped their cars and watched the object with the officers. It was described as an egg-shaped object about the size of three automobiles, made of a highly polished silver metal.

Shortly after 0600C, a target was observed five miles north of Wellington moving southward. The target moved directly over the city to a point ten miles south of the city where it disappeared. The officers in Wellington were contacted but were able to observe absolutely nothing in the sky overhead during that time.

The radar was operated in long pulse, at 50 mi. range, with STC off. The targets were coherent and appeared from six to nine thousand feet on the RHI scope during the early morning and about four or five thousand feet later in the morning.

The descriptions of most of the visual objects in this sighting are too cursory to allow for any reasonable conjecture as to the real nature of the objects. One of the objects, described as "a greenish-blue light that moved slowly away," may have been a star.

In most instances the radar targets did not seem directly related to the visual UFOs. This is characteristic of radar anomalous propagation returns.

The refractivity profiles both show highly refractive surface layers, with a 6.7° C (12.1° F) surface inversion at Topeka and a 9.7°C (17.5°F) surface inversion at Oklahoma City. In addition, the Topeka profile shows a strong elevated layer at 2720 m. with a 0.6°C inversion. The temperature inversion at Oklahoma City produced a surface layer having an optical refractivity gradient (at 5570Å of -101 km-1; this layer would extend the theoretical optical horizon for the eye of an observer 2 m. above the surface of a smooth earth from the normal value of 5.6 km. (9 mi.) to 8.5 km. (about 14 mi.). Such inversions can produce many strange effects, including the visibility of objects normally well below the horizon.

In summary, since the atmospheric conditions were conducive to anomalous radar propagation, and the radar targets displayed AP-like characteristics, this incident may probably be classified as consisting of radar false targets, with associated optical sightings that may have been enhanced by a strong temperature inversion at the surface.

Class II-B. Primarily radar, returns mostly single, sharp, aircraft-like blips, behaving in a continuous manner (i.e., no sudden jumps, etc.).

19-B. Walesville-Westmorland (New York), 1-2 juillet 1954, 1105-1127 LST. Météo : temps apparemment clair. Les signalements du 1er juillet 1954 arrivèrent au Dépot AF de Rome (New York), à propos d'un ovni ayant l'apparence d'un ballon. L'officier en charge dit croire qu'il s'agissait d'un ballon partiellement dégonflé, et que s'il était encore là le jour suivant, il déclencherait une enquête.

A 11:05 LST le 2 juillet 1954, le F-94C 51-13559 décolla pour une mission d'entraînement de routine. Le GCI demanda à l'appareil de change sa mission pour intercepter un appareil inconnu à 10 000 pieds. Le pilote identifia un appareil C-47 par son marquage de queue, et il lui fut alors demandé de vérifier un second appareil non-identifié qui se était à basse altitude et apparently letting down pour atterrir à la base aérienne de Griffith. Le rapport de la FA indique :

Alors que le pilote commençait une descente, il remarqua que le température du cockpit augmenta brusquement. L'augmentation de température amena le pilote à scruter les instruments. La lumière d'alarme incendie était allumée et le pilote informa l'observateur radar de ce fait. La lumière d'alarme incendie resta allumée après que les gaz aient été mis sur "ralenti" et donc moteur fut arrêté et l'équipage s'éjecta avec succès.

L'appareil s'écrasa à "l'Intersection de Walesville" et fut détruit. L'appareil toucha une maison et une automobile, blessant mortellement 4 personnes.

Le récit ci-dessus est tiré du rapport d'accident officiel de l'USAF ("Résumé des circonstances"). Il n'existe pas de dossier Blue Book parce qu'aucun ovni ne fut impliqué.

Conclusion :

  1. Le premier objet était probablement un ballon;
  2. Il n'y avait aucun ovni dans le cas de l'accident de l'appareil.

93-B. Wright-Patterson AFB, Ohio, August 1952, 1050-1113 LST. Weather: scattered clouds at 25,000 ft. This case, occurring almost over Project Blue Book's home base, is a very good example of confusion or contradictory evidence tending to obscure the true nature of a UFO incident.

Figure 18 - Bellefontaine 1 - Wright-Patterson AFB - Août 1952 - Graphe de course/vitesse de l'ovni

At 1051 LST an unidentified radar track appeared 20 mi. NNW of Wright Patterson AFB on the 664th AC&W Squadron's GCI radar at Bellefontaine. The radar operator stated that the course was 240° at 400 knots. Elsewhere the report states 450 knots; how he determined this is not made clear. Two F-86 aircraft from the 97th Fighter-Interceptor Squadron, Wright-Patterson AFB, were vectored in and made visual contact at 1055 LST. Fighters stayed with the object until 1113 LST. The F-86s climbed to 48,000 ft., fell off, and made a second climb. One aircraft had airborne radar activated and received a "weak" return. The object was described as "silver in color, round in shape," and its altitude was estimated as 60,000-70,000 ft. The object appeared on the radar gunsight film as a "fuzzy, small image ... with discernible motion ... that could be any darn thing."

In this incident it is apparent that

  1. the UFO was a real object, and
  2. the visual and radar sightings (both ground and airborne) were of the same object.

All of the evidence points to a weather balloon except for the 400-450 knot speed, and the 240° flight path, which is against the prevailing upper winds. Known aircraft were ruled out because of the altitude. A U-2 would "fit," but the first one was not flown until 1955, and the visual appearance was all wrong. The radar returns eliminated astronomical objects, mirage was ruled out because of the high angles, and the sighting occurred "above the weather." The conclusion was: unknown.

Figure 18 - Bellefontaine 2 - Wright-Patterson AFB - Août 1952 - Graphe de course/vitesse de l'ovni

However, buried deep in the report was the radar operator's note that "At the time it was dropped (1113 LST) object was five miles northwest of Springfield, Ohio." This allows the UFO's course to be plotted on a map; Figs 18 and 19, shows such a map plot. It is readily apparent from this that the UFO's true heading was about 111° at an average speed of only 44 knots. Apparently no one thought to make this simple check. Since the highest reported winds from the radiosonde launched at Dayton at 1000 LST were 260°/31 knots at 50,000 ft. and 270°/33 knots at 55,000 ft. the plotted track of the UFO is consistent with the observed upper winds. The blip was first "painted" at a 240° azimuth, which may explain where that quantity originated in the UFO movement report.

Conclusion: almost certainly a weather balloon. Note that the winds reported for the Wright-Patterson AFB 1000 LST show winds blowing first from the east, then from the SSE, ultimately from the west at higher altitudes. These winds were blowing in such a manner that it is conceivable that Wright-Patterson's own radiosonde balloon may have been the UFO in this incident.

76-B. Near Charleston, W. Va., 4 May 1966, 0340 LST. Weather: Severe thunderstorms in area. Pilot of a Braniff Airlines Boeing 707 flying at 33,000 ft. observed on his left side what appeared to be a fast-flying aircraft with landing lights. Braniff's airborne radar recorded this unknown. Pilot requested the radar operator at Charleston sector of Indianapolis ARTC to look for traffic at his 8:30 or 9:00 position, and the radar picked up a track in this position. Return made a sweeping turn and disappeared off scope to the southwest.

An American Airlines pilot flying 20 mi. behind the Braniff plane saw the object. It appeared to him to be a normal aircraft with landing lights. This pilot stated he had often seen such aircraft with lights during AF refueling missions.

Estimated speed of the unknown was 750-800 mph. No unusual maneuvers were performed or any that were beyond known military aircraft capabilities at the time. AF explanation is that the unknown was an aircraft with landing lights on. This is consistent with the reported facts.

Cas 2. Lakenheath (Angleterre), 13-14 août 1956, 22 h 30-03 h 30 LST. Météo : globalement claire jusqu'à 0300 LST le 14 (pour les détails voir la section 4).

La probabilité qu'une propagation anormale de signaux radar puisse avoir été impliquée dans ce cas semble être faible. 1 ou 2 détails suggèrent une PA, en particulier la disparition rapportée de la première piste alors que l'ovni semblait survoler le radar GCA de Bentwaters. A nouveau cela doit être soupesé avec la déclaration du contrôleur de Lakenheath selon laquelle il y avait peu ou pas de traffic ou de cibles sur l'écran, ce qui ne suggère pas du tout des conditions de PA, et le comportement de la cible près de Lakenheath — apparemment continu et suivi sans difficulté. Le "tailing" du chasseur de la RAF, à lui seul, semble indique la possibilité d'une image fantôme, mais ceci n'est pas en accord avec le rapport selon lequel l'ovni arrêta de suivre le chasseur, alors que ce dernier retournait à sa base, et partit dans une direction différente. Les opérateurs radar furent apparemment prudents pour calculer la vitesse de l'ovni d'après les distances et heures estimées, et les vitesses furent rapportées comme étant cohérentes de course en course, entre les épisodes stationnaires. Ce comportement serait relativement cohérent avec les réflexions depuis des couches atmosphériques — mais pas dans tant de directions différentes.

Un mirage visuel à Bentwaters semble être hors de question en raison des observations combinées au sol et en l'air ; le pilote de C47 a apparemment vu l'ovni sous lui. Les objets visuels ne semblent pas avoir été des météores ; les déclarations d'observateurs selon lesquelles les météores étaient nombreux impliquent qu'ils étaient capables de différencier l'ovni de météores.

En résumé, il s'agit du cas plus intriguant et inhabituel des dossiers radar-visuels. Le comportement apparemment rationnel, intelligent de l'ovni suggère un appareillage mécanique d'origine inconnue comme l'explication la plus probable de cette observation. Cependant, au regard de la faillibilité inévitable des témoins, des explications plus conventionnelles de ce signalement ne peuvent être totalement exclues.

Kincheloe AFB, Sault Saint Marie, Michigan, 11-12 septembre 1967, 22 h 00-23 h 30 LST. Météo : claire, plafond illimité, visibilité illimitée (au-delà de 20 miles), pas d'orages dans cette région, vent de surface de 140°/4 noeuds, en haut de 240°-270°/15-35 noeuds. Le profil de réfractivité radio de Sault Saint Marie pour l'heure la plus applicable est montré en figure 21.

This is a good example of moving radar targets that cannot be seen visually, where there is a "forbidden cone" over the radar site. Some of the returns were even seen to approach within 5-15 mi. of the radar and disappear, apparently subsequently reappearing on the other side of the radar scope at about the same range that they disappeared. This sort of behavior is symptomatic of AP-echoes.

The meteorological data tend to confirm this interpretation. The refractivity profile shown in Fig. 21 displays three peculiarities: a strong subrefractive layer at the surface, a strong elevated duct at 325-520 m. (about 1100-1700 ft.) and a super-refractive layer at 1070-1360 m. (about 3500-4500 ft.). A ray-tracing is shown for this profile in Fig. 20. The ray shows noticeable changes in curvature as it passes through the different layers, an indication that strong partial reflections would be expected. With this profile, moving AP-echoes, produced in the manner described by Borden and Vickers (1953), could be expected to appear at apparent heights of between 2,000-3,000 ft. and 7,000-9,000 ft. No height information was supplied with this report, so the calculation above cannot be verified.

In summary, it appears that this is a case of observations of moving AP-echoes produced by unusually well stratified atmospheric conditions.

Figure 20 - Kincheloe AFB (Sault Saint Marie), 9 Novembre 1967 - Diagramme ray-trace

156-B. Gulf of Mexico, Coast Guard Cutter "Sebago," 25"47'N 89° 24'W, 5 November 1957, 0510-1537 LST. Météo  : not given, but apparently some clouds in area. The most applicable radio refractivity data available are for Key West, Fla. 0600 and 1800 LST, 5 November 1957. They are shown in Figs. 22 and 23. One visual and three radar objects were included in this case. The ship's heading was 23° true. The first contact was a radar blip picked up at 0510 LST at 290° true azimuth, 14 miles. It moved south, approached the ship within 2 mi., and returned north along ship's port side. Contact was lost at 0514 LST. Average speed of this UFO was calculated as 250 mph. At 0516 LST a new blip was picked up at 188°, 22 mi.; this target departed at a computed 650 mph., disappearing at 0516 LST at 190°, 55 mi. The third radar target was acquired at 0520 LST at 350°, 7 mi.; it appeared to be stationary. While the third radar target was being watched on the scope, a visual object was observed for about 3 sec. at 0521 LST travelling from south to north at about 31° elevation between 270° and 310° azimuth. The third radar target remained stationary for about 1 min. and then slowly moved to the northeast, finally accelerating rapidly and moving off scope at 15°, 175 mi.

Figure 21 - Profil de réfractivité radio - Kincheloe AFB (Sault Saint Marie), 12 Septembre 1967

The visual object was described as "like a brilliant planet;" it was undoubtedly a meteor, and in any event obviously was unrelated to radar target number three, the only radar target visible at the same time.

The radar targets were, with the possible exception of the first one, erratic and unpredictable in their movements. The second and third radar blips appeared suddenly, well within the normal pick-up range of the ship's radar. These two blips were probably caused by anomalous propagation. The two Key West profiles, although taken at some distance from the ship's position, are indicative of rather unusual atmospheric conditions in the area. Indeed, the 1800 LST profile is probably one of the most unusual radio refractive index profiles that has ever been observed. The atmospheric structure was apparently one of alternating very wet and very dry layers. Patterns of this sort are often very stable in these subtropical latitudes, and tend to extend in rather homogeneous form over large horizontal distances. The ray-tracing of this profile, Fig. 23a, shows even greater changes in ray curvature. Strong partial reflections should be expected under these conditions.

Figure 22 - Profil de réfractivité radio - Key West - 5 Novembre 1957, 18 h 00 (CST)

The first radar target behaved generally like an aircraft, and the AF investigators were of the opinion that it was an aircraft, probably from Eglin AFB to the north.

In summary, the weight of evidence points toward anomalous propagation as the cause of the radar echoes, the first possibly being an aircraft. The visual object was apparently a meteor.

Coincidentally, the ship, SS Hampton Roads, at 27° 50'N 91° 12'W sighted a round, glowing object high in the sky that faded as darkness approached at 1740-1750 LST. This object appeared to move with the upper winds. AF investigators concluded that it was in all probability a weather balloon.

Figure 23 - Profil de réfractivité radio - Key West - 5 Novembre 1957, 06 h 00 (CST)

101-B. Canal Zone, 25 November 1952, 1806-2349 LST. Weather: generally clear, a few scattered clouds, ceiling and visibility unlimited, visibility at 2,000 ft. was 50 mi. Radio refractivity profiles for Balboa, 1000 and 2200 LST 25 November 1952, are shown in Figs. 24 and 25. Two unidentified objects were tracked by gun-laying radar during the period 1806-2349 LST. These objects, never present simultaneously, could have represented two tracks of the same object. The radar returns were described as "firm and consistent," and the objects were said to maneuver in a "conventional manner" at an average speed of 275 knots. Apparently the track speeds were as high as 720-960 mph. at times. Two B-26s, a B-17, and a PBM were scrambled but no radar or visual contact could be made with the unknowns. The UFOs were not spotted from the ground, with the exception of a single report that an officer saw, low in the sky, an "elongated yellow glow" giving a soft light like a candle. It moved quickly, disappearing in the west in about 3 sec. There were scattered clouds. It seems possible that this was the sighting of a meteor seen through thin clouds producing the soft, yellow-glow effect. In any event, the description does not correspond with the simultaneous radar track of the first UFO.

Figure 23a - Profil M Key West, 5 Novembre 1957 - Diagramme ray-trace

With visibility of 50 mi. it seems strange that the scrambled aircraft could not sight either of the UFOs. The Air Force report comments:

It is believed that due to radar units being slightly off calibration and due to delay in communication, interceptors did chase their own tail or were sent to intercept themselves.

It is also believed that the majority of the radar plots were legitimate unidentified objects.

The preparing officer knows of no object which flies at 275 knots, that could remain in the Canal Zone area for nearly six hours, maneuver from 1000 through 28,000 feet altitude, make no sound, and evade interception.

In fact, it is difficult to imagine any material object that could accomplish all these feats. The strange radar tracks were probably the product of anomalous propagation conditions, an hypothesis that would account for the facts above. The atmospheric conditions were certainly favorable for AP, as can be seen from the A-profiles in Figs 24 and 25. However, there are two considerations that argue against this hypothesis.

  1. The targets tracked behaved in a more rational, continuous manner, and covered a greater altitude range, than AP echoes of the type usually observed;
  2. If they were AP echoes, should these targets have appeared at not only 1806-2349 LST but around 1000 LST when the profile was obviously more favorable for AP than the 2200 LST profile?
Figure 24 - Canal Zone 1 - Profil de réfractivité radio - Balboa (Canal de Panama) - 25 Novembre 1952, 22 h 00 (LST)

Despite these two contradictions to the AP hypothesis, the lack of any visual corroboration of the two UFOs makes any other hypothesis even more difficult to accept. This case therefore seems to fall, albeit inconclusively, into the classification of probable AP radar returns.

Cas 21. Colorado Springs, Colorado, 13 mai 1967, 15 h 40 LST (16 h 40 MDT). Météo : couvert, froid, averses éparses et averses de neige (graupel) dans le secteur, vents orientés nord d'environ 30 miles/h, rafales à 40 miles/h, visibilité air — plus de 15 miles (l'aéroport de Colorado Springs n'est pas limité par l'horizon ; des visibilités de 100 miles sont communément rapportées les jours clairs). Ceci est un cas uniquement radar, et est d'un intérêt particulier parce que l'ovni ne pouvait être vu, lorsqu'il y avait toute indication qu'il aurait dû être vu (voir section 4).

Du moment où l'ovni a été repéré pour la 1ère fois au moment ou le vol Braniff a atterri sur la piste 35, la piste de l'ovni s'est comportée comme un écho fantôme, peut-être un retour du sol réfléchi depuis l'appareil. Ceci est indiqué par le fait que le blip de l'ovni apparût au double de la distance du blip de Braniff, et au même azimut, bien que l'angle d'élévation semble avoir été différent. Lorsque Braniff atterrit, cependant, la situation changea radicalement. Le blip de l'ovni tira vers la droite (l'est) et passa au-dessus de l'aéroport à une altitude indiquée d'environ 200 pieds. Comme indiqué par la FAA, ceci est précisément la procédure correcte pour un appareil en approche, ou un pratiquant une approche ILS mais n'ayant en fait pas l'intention d'atterrir. Bien que la trajectoire de l'ovni soit passée à moins de 1,5 mile de la tour de contrôle, et que le personnel présent ait été alerté de la situation, l'ovni n'était pas visible, même aux jumelles. Un vol de Continental Airlines, qui fut suivi 3 à 4 miles derrière l'ovni au 1er contact, et qui volait dans la même direction, ne le vit jamais non plus.

Figure 25 - Canal Zone 2 - Profil de refractivité radio, Balboa (Canal de Panama), 25 novembre 1952, 10 h 00 LST

Les antennes transmettrices des radar PAR comme ASR sont situées à l'est de la piste 35, et elles sont à environ 1000 pieds éloignées de la ligne SO-NE. Un écho fantôme semble donc être exclu par au moins les considérations suivantes :

  1. Un écho fantôme, qu'il soit direct ou indirect, ne sera normalement pas indiqué à une hauteur de 200 pieds alors que le producteur de l'écho est au sol, comme c'était le cas ici ;
  2. Un fantôme direct est toujousr au même azimut que la cible en mouvement, et un écho indirect est au même azimut que le réflecteur fixe impliqué (voir Section 6, chapitre 5). Si un fantôme indirect avait été impliqué ici, l'écho fantôme serait donc toujours apparu bien à l'est du Braniff, pas au même azimut.

Les caractéristiques de vol au radar de l'ovni dans ce cas sont toutes compatibles avec l'hypothèse que l'inconnu était un jet de la série century (F100, F104, etc.), bien que rien n'ai jamais été vu ou entendu.

Ceci doit rester comme l'un des cas radar les plus intriguants connus, et aucune conclusion n'est possible à ce jour. Il semble inconcevable qu'un écho de propagation anormale se comporte de la manière décrite, en particulier concernant les changements d'altitude signalés, même si une PA était probable à ce moment. Au regard de la situation météorologique, il semblerait qu'une PA était plutôt improbable. A côté de cela, quelle est la probabilité qu'un retour de PA n'apparaisse qu'une seule fois, et à ce moment semble exécuter une parfaite approche d'entraînement ILS ?

Cas 35. Vandenberg AFB, Lompoc, Californie, 6-7 octobre 1967, 19 h 00-01 h 30 LST. Météo : clair, bonne visibilité, fortes inversions de température près de la surface causées par une advection of very warm (80°-90°F), dry air over the cool ocean surface (water temperature 58°-59°F). This sighting begins with an apparent mirage (of a ship probably 60 mi. beyond the normal horizon) and continues with a very large number of unknown targets that were found on tracking radars which were being used in a search mode (they normally are not used in this way). The project case file contains a good analysis of the probable nature of the radar targets, some of which were apparently birds and some apparently ships tracked at 80 mi. ranges as well as other AP-like returns that may have been associated with local intensification of the ducting layer. The nature of the visual objects is not as clear, although at least two of them appear to have been superior mirages of ships beyond the normal horizon. There were possibly some meteor sightings involved.

The meteorological conditions were quite interesting. The warm, dry air was apparently quite close to the water surface, at least in places. Data from Vandenberg and San Nicholas island indicate that in places the inversion was no thicker than about 90 m. (10 mb pressure difference). The contrast that may have existed can be calculated from these data:

  A ou près de la surface de la mer A 90 m ou moins
Pression 1004 mb 994 mb
Temperature °F 58 °F 90 °F
Temperature °C 14 °C 32 °C
Temperature °K 287 °K 305 °K
Optical N (5570A) 275 (ppm) 256 (ppm)

The optical refractive index gradient that may have existed at the time was therefore on the order of -210 ppm. km-1, or a somewhat greater negative value, depending upon the thickness chosen for the layer. The value above is computed as (256-275) /0.090, based on the 90 m. maximum thickness assumed. Since the critical value of the gradient for a superior mirage is -157 ppm. km-1, it is quite apparent that the conditions required for the formation of extended superior mirages were most likely present on the date in question. The only problem with this explanation is the reported elevation angle of 10°, but as pointed out in the conclusions to this chapter such estimates by visual observers are invariably over-estimated by a large factor.

In summary, the conclusions arrived at by the investigators in this case seem to be adequately supported by the meteorological data available.

The sighting reported for 12 October 1967, 0025 LST, seems to be a classic example of the description of a scintillating, wandering star image seen through a strong inversion layer. Note particularly the estimated ratio of vertical and horizontal movements. Two very bright stars would have been close to the horizon at this time: Altair, magnitude 0.9, would have been at 277° azimuth and about 4° elevation angle; Vega, magnitude 0.1, would have been at about 313° azimuth and about 12° elevation angle. Of the two, Altair seems the more likely target because of the smaller elevation angle; the observers gave no estimate of either azimuth or elevation angle.

Synthèse des résultats

Une synthèse des résultats de cette investigation est donnée en tableau 1.

Le lecteur devrait noter que l'affectation de cas dans la catégorie de cause de PA probable pourrait avoir été faite sur la base du seul témoignage observationnel. C'est-à-dire, qu'il n'y avait aucun cas où les données météorologiques disponibles tendaient à nier l'hypothèse de propagation anormale, amenant ainsi le cas à être affecté à une autre catégorie. Par conséquent, un examen des données météorologiques disponibles pour les 19 cas de PA probable est en ordre.

  1. Every one of the 19 cases is associated with clear or nearly clear weather. In 15 cases weather is described as "clear and visibility unlimited" (CAVU), in many of these "exceptional visibility" is noted; in four cases the weather is "generally clear," with some scattered clouds, or a "high, thin broken" condition (usually meaning cirriform clouds). Such weather is indicative of stable atmospheric conditions that are favorable for the formation of layered, stratified refractive index profiles, i.e., they are conducive to anomalous propagation effects. The a priori probability of such a result, from a truly random sample of dates-times-places is roughly on the order of one chance in 200,000 (assuming that the probability of clear weather is roughly 0.5 in any single case).
Tableau 1 - Fréquence d'occurrence des causes d'ovnis les plus probables
Classe Explication la plus probable Total classe
Propagation anormale Appareillage de fabrication humaine Inconnu Pas ovni
I-A 6 1 2 0 9
I-B 2 1 0 0 3
I-C 1 0 1 0 2
I-D 0 4 2 0 6
Toute classe I 9 6 5 0 19
II-A 6 0 0 0 6
II-B 4 2 2 1 9
Toute classe II 10 2 2 1 15
Toutes classes 19 8 7 1 35
  1. Of the 19 cases, all but two occur during the night. Although AP often occurs during the daytime, the nighttime hours are generally more favorable, and tend to greatly increase the a priori probability of encountering AP.
  2. In the 11 cases for which pertinent meteorological data are available, in every case the refractive index profile is favorable, to a greater or lesser degree, for the presence of anomalous propagation effects. The weakest case, the data for Silver Hill, 19 July 1952, (see p. 47), where inadequacies in the data were pointed out, has a near-super-refractive surface layer (gradient -81 ppm. km-1 and an elevated subrefractive layer. Of the remaining 11 profiles, seven showing ducting gradients (-157 ppm, km-1 or greater negative value) and four show super-refractive gradients (-100 to -157 ppm. km-1). Since the a priori probability of the occurrence of such profiles is on the order of 0.25 (Bean, 1966b), the a priori probability of this result, given a truly random sample, is on the order of one in 106.

En synthèse globale de ces résultats, as they pertain to anomalous propagation of radio or optical waves, il semble que là où les données observationnelles ont désigné une propagation anormale comme étant la cause probable de l'incident ovni, les données météorologiques sont considérablement en faveur de la plausibilité de l'hypothèse de PA. Que ce résultat puisse avoir été que coincidental a été montré comme étant seulement de probabilité éloignée.

Conclusions et recommandations pour travail ultérieur

The following conclusions can be stated as a result of the investigation reported in this chapter:

  1. Anomalous Propagation (AP) effects are probably responsible for a large number of UFO reports in cases involving radar and visual sightings.
  2. There are two common patterns that are evidenced in radar-visual cases involving anomalous propagation effects:
    1. Unusual AP radar targets are detected, and visual observers are instructed where to look for apparent UFOs and usually "find" them in the form of a star or other convenient object.
    2. Unusual optical effects cause visual observers to report UFOs and radar operators are directed where to look for them. As above, they usually "find" them, most often in the form of intermittent AP echoes, occasionally of the unusual moving variety.
  3. In radar-visual UFO sightings there is a pronounced tendency for observers to assume that radar and visual targets are correlated, often despite glaring discrepancies in the reported positions. There is a perhaps related tendency to accept radar information without checking it as carefully as the observer might normally do; hence errors are promulgated such as, direction of UFO movement confused with the azimuth at which it was observed on the radar scope, and UFO speed reported that is grossly at variance with plotted positions at times (both of these effects are well illustrated in Case 93-B).
  4. There is a general tendency among even experienced visual observers to grossly over-estimate small elevation angles. Minnaert (1954) states that the average "moon illusion" involves a factor of 2.5-3.5. The results of the present investigation imply that objects at elevation angles as small as 1° are estimated to be at angles larger than the true value by at least this factor or more. Interestingly, all of the elevation angles reported of visual objects in the cases examined in this chapter, not a single one is reported to be less than 10°. The fact that radar may subsequently "see" the UFOs at angles of only 1° to 4° seems not to bother the visual observers at all; in fact when the visual observers report apparent height-range, these values often turn out to be equivalent to elevation angles of only a degree or two. There seems to be a sort of "quantum effect" at work here, where an object must be either "on the horizon" (i.e., at 0° or at an elevation of greater than 10°.
  5. There are apparently some very unusual propagation effects, rarely encountered or reported, that occur under atmospheric conditions so rare that they may constitute unknown phenomena; if so, they deserve study. This seems to be the only conclusion one can reasonably reach from examination of some of the strangest cases (e.g., 190-N, 5 and 21).
  6. There is a small, but significant, residue of cases from the radar-visual files (i.e., 1482-N,Case 2) that have no plausible explanation as propagation phenomena and/or misinterpreted man-made objects.

A number of recommendations for future UFO investigative procedures are indicated by the results of this chapter:

  1. In any investigation of a UFO report, extremely careful efforts should be made to determine the correct azimuth and elevation angles of any visual or radar objects, by "post mortem" re-creation of sightings if necessary. This information is probably more useful in analysis of the case than the description of the objects or targets.
  2. Reported speeds and directions of UFOs, especially of radar UFOs, should be carefully checked (again, "post mortem" if necessary) and cross-checked for validity. This information is also often critical for subsequent analysis.
  3. Every effort should be made to get the most comprehensive and applicable meteorological data available for an UFO incident as quickly as possible. Many types of weather data are not retained permanently, and it is difficult or impossible to retrieve the appropriate data for a sighting months or years after the fact. Copies of original radiosonde recordings should be obtained for the closest sites, since these may be analyzed in more detail than that routinely practiced by weather bureaus for synoptic purposes. It should be emphasized that, for example, a nighttime profile is usually more germane to a nighttime sighting than is a daytime profile. For example, if an UFO incident occurs at 2100 or 2200 LST, an 0600 LST (next day) raob will generally be more pertinent to the propagation conditions involved than will an 1800 LST raob. The converse is also true.
  4. Any field team investigating UFO reports and seeking to explore all radio/optical propagation aspects of the sighting (a highly desirable goal), should be equipped with the following personnel as a minimum:
    1. An expert on the unusual aspects of electromagnetic wave propagation, at both radio and optical wave lengths;
    2. An expert in the interpretation and theory of radar targets, who is acquainted with all types of anomalous propagation and other spurious radar returns;
    3. An expert with wide experience in the physiology and psychology of human eyesight, and familiarity with optical illusory effects, etc.;
    4. A meteorologist, with specialized experience in micro-meteorology-climatology, mesoscale meteorology, and atmospheric physics.

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