It will be noted from Table I that many more unidentified targets are picked up by the Washington ARTC Center than by the Washington Airport Traffic Control Tower. This may be explained by the fact that the center is equipped with a MEW radar, while the tower is equipped with an airport surveillance radar, Type ASR-1. The most significant differences between the two types of equipment are listed in the following:
The almost simultaneous appearance of the first moving targets with the ground returns, 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.
| Altitude (MSL) |
Direction (Degrees) |
Velocity (Knots) |
|---|---|---|
| Surface | 170 | 5 |
| 1000 | 180 | 24 |
| 2000 | 190 | 26 |
| 3000 | 210 | 24 |
| 4000 | 210 | 23 |
| 5000 | 220 | 20 |
| 6000 | 220 | 16 |
| 7000 | 220 | 18 |
| 8000 | 220 | 17 |
| 9000 | 220 | 13 |
| 10000 | 240 | 12 |
| 11000 | 270 | 11 |
| 12000 | 270 | 13 |
| 13000 | 260 | 17 |
| 14000 | 260 | 21 |
| 15000 | 260 | 25 |
| 16000 | 270 | 25 |
| 17000 | 270 | 23 |
| 18000 | 270 | 22 |
| 19000 | 270 | 21 |
| 20000 | 260 | 20 |
| 21000 | 270 | 22 |
| 22000 | 280 | 24 |
| 23000 | 290 | 26 |
| 24000 | 280 | 26 |
| 25000 | 290 | 26 |
| 26000 | 300 | 30 |
| 27000 | 300 | 34 |
| 28000 | 300 | 38 |
| 29000 | 290 | 38 |
| 30000 | 290 | 36 |
| 31000 | 300 | 35 |
| 32000 | 300 | 35 |
| 33000 | 310 | 34 |
| 34000 | 310 | 40 |
| 35000 | 300 | 47 |
| 36000 | 300 | 49 |
| 37000 | 300 | 50 |
| 38000 | 300 | 48 |
| 39000 | 310 | 42 |
| 40000 | 320 | 38 |
| 41000 | 300 | 43 |
| 42000 | 300 | 53 |
| 43000 | 300 | 67 |
| 44000 | 310 | 69 |
| 45000 | 310 | 60 |
| Altitude (MSL) |
Direction (Degrees) |
Velocity (Knots) |
|---|---|---|
| Surface | Calm | 0 |
| 1000 | Calm | 0 |
| 2000 | 350 | 12 |
| 3000 | 340 | 12 |
| 4000 | 320 | 14 |
| 5000 | 320 | 16 |
| 6000 | 300 | 18 |
| 7000 | 300 | 20 |
| 8000 | 310 | 20 |
| 9000 | 310 | 22 |
| 10000 | 300 | 26 |
| 11000 | 290 | 28 |
| 12000 | 290 | 29 |
| 13000 | 300 | 30 |
| 14000 | 300 | 28 |
| 15000 | 290 | 29 |
| 16000 | 300 | 29 |
| 17000 | 300 | 29 |
| 18000 | 300 | 30 |
| 19000 | 300 | 32 |
| 20000 | 300 | 38 |
| 21000 | 290 | 38 |
| 22000 | 280 | 43 |
| 23000 | 280 | 48 |
| 24000 | 280 | 50 |
| 25000 | 270 | 52 |
| 26000 | 280 | 57 |
| 27000 | 270 | 61 |
| 28000 | 270 | 54 |
| 29000 | 270 | 55 |
| 30000 | 200 | 62 |
| 31000 | 270 | 63 |
| 32000 | 280 | 73 |
| 33000 | 280 | 84 |
It will be noted in Figs. 1, 2, and Fig. 3 that all targets observed in the first period were moving from the north or northwest. In Fig. 6 all targets were moving from the south or southwest, and in Fig. 7 all were moving from the west or northwest. The definite directional trend in each case eliminated the possibility that the unidentified targets were surface vehicles such as trains, trucks, automobiles, or boats. Had this been the case, some vehicles would have been moving in the reverse directions. In each case, target directions corresponded with the wind directions reported aloft. This fact suggested that whatever was producing the targets as being carried by the wind, the next step of the analysis was to determine, if possible, the altitude of the objects which produced the radar targets. Since the radar actually measures slant range which could in some cases be almost directly overhead from the high-beam MEW antenna, the minimum range of each target was used to determine the absolute maximum altitude of the object producing the target.
| Tower Radar | Center Radar | |
|---|---|---|
| Type | ASR-1 | MEW |
| Frequency | S-band | S-band |
| Pulse-repetition frequency | 1,000 | 900 |
| Pulse rate | 0.5 microsecond | 1 microsecond |
| Vertical coverage | 6,000 feet at 6 miles | 12,000 feet at 3 miles |
| Scan Rate | 28 per minute | 6 per minute |
| Display scopes | 12DP7 | 12DP7 and VG2 |
| Power output | 200 kilowatts | 400 kilowatts |
For example, a target which came within five nautical miles of the radar antenna could not be above an altitude of five nautical miles, or 30,400 feet. With the use of the slant-range principle, the absolute maximum altitude of each target was determined and is listed in Table V. When attempting later to determine the probable altitude of each target by studying the winds aloft, it was useful to have these maximum altitude figures to eliminate the necessity for consideration of higher altitude levels.
Since winds aloft can vary considerably during the period of a few hours, it was decided to use in this analysis only data on targets which were under observation during the periods from one hour before to one hour after the observations of the local winds aloft. These targets are listed in Table V.
During the observation period on the night of August 13-14, all targets on a southerly heading had ground speeds of at least 24 knots. The only reported winds with a southerly heading had a velocity of only 12 knots. These were winds at the 2,000- and 3,000-foot levels. Targets on a southeasterly heading had a speed range of 32 to 48 knots. However, the only winds on this heading were from 14 knots at 4,000 feet to 38 knots at 20,000 feet.
During the August 15-16 observations, targets on a north or northeasterly heading had speeds of 35 to 42 knots. The only reported winds moving in this direction ranged between 5 and 26 knots from the surface up to 9,000 feet. Targets on easterly headings had speeds from 22 to 45 knots. The only reported winds moving in this direction had speeds of from 10 to 24 knots between 10,000 and 25,000 feet.
In Fig. 9 and Fig. 10, the directions and velocities of the winds aloft are plotted on a polar projection diagram together with the directions and velocities of the observed targets. Agreement between the directions of the winds and the directions of the targets is apparent.
| Date Aug. 1952 | Starting Time EST | Direction (Degrees) | Target Speed (Knots) | Reflector Speed (1/2 Target Speed) | Absolute Maximum Altitude (Based on Minimum Slant Range) | Probable Altitude (Based on Winds Aloft) |
|---|---|---|---|---|---|---|
| 13 | 2159 | 005 | 28 | 14 | 63000 | 2000 |
| 2201 | 360 | 24 | 12 | 75000 | 2000 | |
| 2229 | 310 | 33 | 16.5 | 23000 | 8000 | |
| 2240 | 300 | 46 | 23 | 30000 | 9000 | |
| 2242 | 325 | 48 | 24 | 33000 | 9000 | |
| 2259 | 010 | 31 | 15.5 | 31000 | 2000 | |
| 2303 | 330 | 42 | 21 | 36000 | 8000 | |
| 2330 | 340 | 39 | 19.5 | 23000 | 5000 | |
| 2330 | 305 | 39 | 19.5 | 24000 | 8000 | |
| 2331 | 315 | 39 | 19.5 | 35000 | 8000 | |
| 2332 | 315 | 36 | 18 | 23000 | 8000 | |
| 2345 | 310 | 38 | 19 | 19000 | 8000 | |
| 2347 | 310 | 42 | 21 | 43000 | 8000 | |
| 2349 | 290 | 39 | 19.5 | 35000 | 7000 | |
| 2356 | 300 | 42 | 21 | 37000 | 7000 | |
| 2355 | 350 | 36 | 18 | 83000 | 2000 | |
| 15 | 2213 | 260 | 45 | 22.5 | 34000 | 14000 |
| 2226 | 225 | 35 | 17.5 | 24000 | 900 | |
| 2230 | 250 | 28 | 14 | 37000 | 10500 | |
| 2238 | 185 | 36 | 18 | 29000 | 900 | |
| 2240 | 210 | 42 | 21 | 18000 | 4500 | |
| 2353 | 275 | 23 | 11.5 | 29000 | 10500 n1This target could also have been a direct radar return from an object floating with the wind at 15000 to 17000 feet mean sea level |
One of the theoretically possible causes of the unidentified targets was the delayed pulse or second-time-around effect inherent in the radar method of time measurement. With a second-time-around effect, objects beyond the normal sweep range of a radar can be displayed on the scope because of reception of an echo pulse elicited not by the transmitted pulse which triggers the range sweep but by the preceding transmitted pulse. The apparent velocity of the target on the radar is no greater than and normally less than the velocity of the object producing the return. The heading of the radar target would not necessarily be parallel to the heading of the object unless the object was on a course radial to the radar antenna. These effects are illustrated in Fig. 11.
If we assume then that an object producing a second-time-around radar target was being carried by the wind, the apparent velocity of the target would be no greater than the wind velocity. However, the analysis of the targets listed in Table V showed that they were actually moving at speeds approximately double the wind velocities reported for the directions involved. This fact eliminated the possibility that the targets were being produced by the second-time-around effect.
When the target velocities plotted in Fig. 9 and Fig. 10 were halved, those plotted points clustered very closely around the wind plots. Further investigation of the doubled-speed effect indicated that this effect could be produced if the original radar beam were reflected downward to give a ground return, as shown in Fig. 12. If we assume that some sort of horizontal reflector was present aloft and that the angle of reflection equaled the angle of incidence of the radar beam, any horizontal movement of the reflector would produce a movement twice as great in the image being received on the radar scope. Furthermore.- the apparent motion of the image would be parallel to the motion of the reflector, as illustrated in Fig. 13.
When the observed target velocities were divided by two, the target motions corresponded closely to the reported wind directions and velocities at certain altitude levels. In nearly all of these cases the altitude levels, which are listed as probable altitudes in Table V, were at or adjacent to the temperature inversion levels.
With only one exception, no targets were seen moving at the speed and heading of the reported wind at any altitude. This suggested that the reflecting areas, which were capable of bending the radar beam, were nevertheless not of sufficient density to produce direct returns on the radar scope. Thus, it appeared likely that the reflection effect was being produced by the atmosphere itself. If this were the case, it would probably be a refraction rather than a reflection which was involved. This effect is shown in Fig. 14.
The uniformly small size of the observed targets as well as the relatively low frequency of their occurrences suggested that the conditions producing this effect were extremely localized and decidedly critical. Although the exact nature of the discontinuity is not known, one possible explanation might be that it is an eddy in the atmosphere. Such eddies may be produced by the shearing effect of dissimilar air masses moving at different speeds and headings at or near the inversion boundary. They might under certain conditions produce bulges in the inversion layer, concentrating and directing the radar energy with a lens effect to produce a return signal strong enough to show up on the radar scope. The relatively short paths of some of the radar targets before their fadeout might be attributed to the dissipation of these eddies in the stratified air mass.
Intermediate speed checks on numerous targets indicated that individual velocities remained quite steady during the observation period. It became possible to predict with accuracy the progress of specific targets from minute to minute. There was no evidence of hovering or of sudden increases in speed by any target. It is believed that previous reports of sudden accelerations of targets to supersonic velocities were due to a controller's transfer of identity from a faded target to another target which was just appearing on a different section of the scope.
It would be unwise to assume that all unidentified slow-moving radar targets are caused by refraction of radar energy. Small rain clouds produce much the same appearance on the scope. Other targets could be direct returns from bird formations, balloons, or debris carried aloft by convection or tornadoes. It has recently been reported that more than 4,000 balloons are released in the United States every day by Government and civilian research organizations s1 Many Potential "Saucers,", Science News Letter, Vol. 62, No. 7, Aug. 16, 1952, p. 106. A recent analysis of more than 1,000 visual reports of unidentified flying objects by the Air Technical Intelligence Center at Wright-Patterson Air Force Base indicates that 21.3 per cent of these may be attributed to balloons s2"Unidentified Aerial Objects Receive Careful Analysis by Air Force Experts," The Aircraft Flash, published by Department of the Air Force, Air Defense Command, Vol. 1, No. 4, Jan. 1953, p. 4 .
Examination of the logs of the Washington ARTC Center indicates that there is considerable correlation between the appearance of unidentified targets on the radar scope and the receipt of numerous visual reports of flying saucers. It should be noted that abrupt temperature inversions aloft can refract light in much the same way as radar waves and produce mirage effects. In a standard reference work on meteorology s3 Humphreys, W. J., "Physics of the Air," McGraw-Hill Publishing Company, New York, 1940. Humphreys reports that a temperature inversion (near the surface) of 1 deg. C per meter bends down a light ray into an arc whose radius is 0.16 that of the earth; an inversion of 10 deg. C per meter gives an arc radius of 0.016 that of the earth, or approximately 60 miles. This effect makes it possible for an observer to see in the sky the sun or some other bright light that is actually well below the observer's horizon. On rare occasions, multiple images of the same object may be visible. It is believed that many visual sightings of flying saucers can be explained by this phenomenon.