Oiseaux et insectes

Roy H. Blackmer, Jr.R.J. Allen, R.T. CollisC. Herold, R. I. Presnell, 1968

Il est possible que la première observation d'un écho radar venu d'un oiseau ait été faite par R. M. Page s1Page, R. M.: "Radar Goes to Sea," (Unpublished report of early tests of the XAF radar by the U.S. Naval ResearchLaboratory. Described by Bonham and Blake 1956, (1939) du Laboratoire de Recherche Navale en février 1939. Elle fut faite avec un radar expérimental de 200 MHz (le XAF) sur le USS New York près de Puerto Rico. Les échos d'oiseaux, tels que rapportés par Lack et Varley s2Lack, D. and G.C. Varley. "Detection of Birds by Radar," Nature, Vol. 156, (1945) , furent obtenus sur un dispositif radar de garde-côtes de 10 cm près de Dover au cours de 1941. Des vérifications visuelles confirmèrent ces 2 premières détections par radar comme étant les retours d'oiseaux individuels. De nombreuses observations d'oiseaux par radar ont été faites depuis, en particulier de migrations d'oiseaux comme cela est mis en évidence dans une bibliographie compilée par Myres s3Myres, M.T. "Technical Details of Radar Equipment Detecting Birds, and a Bibliography of Papers Reporting the Observation of Birds with Radar," Field Note No. 9, The Associate Committee on Bird Hazards to Aircraft, National Research Council of Canada, Ottawa, (1964). listant 89 articles, et un texte écrit par Eastwood s4Eastwood, F. Radar Ornithology, Methuen and Co., Ltd., 11 New Fetter Lane, London, 1967.. Les sections de coupe radar (sigma) ont été mesurées sur des oiseaux en position fixe suspendus dans une bride non-réfléchissante et d'oiseaux en vol. Les valeurs obtenues, montrées dans les tableaux 2 et 3, varient avec les espèces, aspect et longueur d'onde radar.

En raison de la variation de la puissance 4ème inverse avec la distance, un oiseau à faible distance dans le faisceau principal peut donner un écho radar comparable en intensité à celui d'un appareil dans le faisceau principal à longue distance. Par exemple, si un pigeon avec une section de coupe radar broadside de 100 cm2 volant au sein du faisceau radar à une distance de 10 miles, il produirait un signal aussi fort qu'un avion à réaction avec une valeur de 106 cm2 (100 m2) volant au sein du faisceau radar principal à une distance de 100 miles. Cependant, si l'appareil volait dans un lobe de côté de 40 dB moins puissant que le faisceau principal dans lequel l'oiseau vole les 2 produiraient des signaux d'intensité égale à même distance. Si le lobe de côté était 30 dB de moins, un oiseau dans le faisceau principal à 10 miles aurait l'air d'un appareil à 17,8 miles, et si le lobe de côté était de 20 dB de moins, l'oiseau à 10 miless aurait l'air d'un appareil à 31.6 miles.

Théoriquement la distance maximale de détection telle que dictée par la quantité de signal radar retournée par des oiseaux peut être calculée. Cependant, la vérification n'est pas aisée à cause de la difficulté de repérer un oiseau et établir qu'il correspond à un blip particulier sur un écran radar. Ceci est particulièrement difficile en présence de sea clutter comme cela a été expérimenté lors d'une expérience menée par Allen et Ligda s5 Allen, R.J. and M.G.H. Ligda. "Services for Bird Counter Study and Design," Final Report, Contract DA 42-00 7 AMC-306 (Y), SRI, Menlo Park, (1966). au SRI. Lors d'une expérience menée par Konrad s6Konrad, T.G., J.J. Hicks, and E.B. Dobson. "Radar Characteristics of Birds in Flight," Science, Vol. 159, (1968), 274-280., des oiseaux individuels furent lâchés depuis un appareil volant au-dessus de l'eau à 5500 pieds-6000 pieds à 8-10 miles marins des radars. Après séparation de l'appareil et des oiseaux sur l'écran radar, chaque oiseau individuel fut automatiquement suivi sur des périodes allant jusqu'à , de sorte que la cible observée fut catégoriquement identifiée comme étant un oiseau. Des floppées d'oiseaux ont été détectées à des distances d'au moins 51 miles marins comme cela est rapporté par Eastwood et Rider s7Eastwood, E. and G.C. Rider. "Some Radar Measurements of the Altitude of Bird Flight," British Birds, Vol. 58, (1965), 393-420..

Tableau 2 - Résumé des données de section de coupe radar des oiseaux s8s27Konrad, T.G., J.J. Hicks, and E.B. Dobson. "Radar Characteristics of Birds in Flight," Science, Vol. 159, (1968), 274-280.
Oiseau Bande radar Points at point/sec Section de coupe radar moyenne (cm2) Section de coupe radar médiane (cm2) Root-mean-square fluctuations in Cross-section (cm2) Mean-to-Median Ratio, p
Quiscale X 230 16 6,5 24 2,4
S 230 27 13 31 2,2
UHF-VV* 230 0.73 0,58 0,6 1,3
UHF-VH† 230 0.37 0,15 0,7
Quiscale X 116 15 7,2 21 2,1
S 116 23 11 32 2,2
UHF-VV* 116 0.41 0,32 0,5 1,3
UHF-VH† 116 0.03 0,015 0,04

Moineau X 129 1.9 1,0 2 1,9
S 129 15 11 11 1,4
UHF-VV* 129 0.025 0,015 0,02 1,3
UHF-VH† 129
Moineau X 233 1,3 0,60 2 2,2
S 233 12 11 5 1,1
UHF-VV* 233 0.020 0,02 0,01 1,1
UHF-VH† 233
Pigeon X 160 15 6,4 28 2,3
S 160 80 32 140 2,5
UHF-VV* 160 11 8,0 7,0 1,3
UHF-VH† 160 1,2 0,7 1,4

*VV, Transmit vertical polarization and receive vertical polarization.

†VH, Transmit vertical polarization and receive cross-polarized or horizontal component.

Tableau 3 - Variation de section de coupe radar avec aspect s10s28Konrad, T.G., J.J. Hicks, and E.B. Dobson. "Radar Characteristics of Birds in Flight," Science, Vol. 159, (1968), 274-280.
Bande radar Aspect* Sigma de section de coupe (cm2)
Etourneau sansonnet (Sturnus vulgaris) X Tête 1,8
X Broadside 25,0
X Queue 1,3
Pigeon biset (Columba livia) X Tête 1,1
X Broadside 100
X Queue 1,0
Moineau domestique (Passer domesticus) X Tête 0,25
X Broadside 7,0
X Queue 0,18
Corbeau freux (Corvus frugilegus) X Broadside 250
Buzzard de Turquie X Inconnu 25 à 250
Canard et poulet UHF† Tête 600
UHF† Queue 24

*For the cross-section measurements of the starling, pigeon, sparrow, and rook, the birds were suspended from a tower with their wings folded; the radar elevation angle was 18°. Measurements of the turkey buzzard were made when the bird was in flight; measurements of the duck and chicken were made when the birds were standing or squatting.

†400 megacycles.

Figure 4 : Distribution des altitudes moyennes des oiseaux sur l'année s12Eastwood, E. and G.C. Rider. "Some Radar Measurements of the Altitude of Bird Flight," British Birds, Vol. 58, (1965), 393-420.
Figure 4 : Distribution des altitudes moyennes des oiseaux sur l'année s12Eastwood, E. and G.C. Rider. "Some Radar Measurements of the Altitude of Bird Flight," British Birds, Vol. 58, (1965), 393-420.

Très peu d'oiseaux volent au-dessus de 13000 pieds ; la plupart volent en-dessous de 5000 pieds. Dans une étude menée par Farrari (1966) des rapports de l'USAF de collisions oiseau-appareils durant 1965, 27 % de l'ensemble des collisions avaient lieu à moins de 100 pieds. 28 % entre 100 pieds et 2000 pieds, 21 % entre 2000 pieds et 3000 pieds, et les 24 % au-dessus de 3000 pieds. Si l'on peut supposer que la probabilité d'une collision oiseau-appareil est tout aussi probable à toutes les altitudes (ce qui pourrait ne pas être complètement valide en raison de l'ascension et de la descente) this should be somewhat of a representative figure of the height of flight for birds. There was one reported bird-aircraft strike at 17000 pieds and a few sightings above 20000 pieds, however the number of birds flying at these altitudes appears to be extremely small.

Eastwood et Rider s13Eastwood, E. and G.C. Rider. "Some Radar Measurements of the Altitude of Bird Flight," British Birds, Vol. 58, (1965), 393-420. ont rapporté une analyse assez complete de l'altitude de vol de divers oiseaux observés au radar au Laboratoire de Recherche Marconi en Angleterre. Leurs conclusions se sont accordées de très près avec ce qui précède ; près de 90 % de l'ensemble des oiseaux étaient sous les 5000 pieds. Les oiseaux volent plus haut de nuit et pendant les périodes de migration de printemps et d'automne. Un tracé de la distribution moyenne d'altitude au cours de l'année est montré en figure 4. L'ensemble de ces chiffres est probablement applicable en tant qu'altitude au-dessus du terrain général ; i.e., à 5000 pieds au-dessus du niveau moyen de la mer, 90 % des oiseaux voleraient à des altitudes inférieures à 10000 pieds m.s.l. La quantité de couverture nuageuse affecte également la hauteur à laquelle volent les oiseaux. Des diagrammes inclus par Eastwood et Rider s14Eastwood, E. and G.C. Rider. "Some Radar Measurements of the Altitude of Bird Flight," British Birds, Vol. 58, (1965), 393-420. indiquent clairement a marked tendency for higher mean altitudes to be flown in the presence of complete cloud cover.

Target airspeed is another means for identifying a bird. It can be obtained vectorially from a knowledge of the wind velocity and the radar-measured target velocity. Houghton s15Houghton, F. "Detection, Recognition and Identification of Birds on Radar," Proc. Eleventh Wx. Radar Conf., (1964), 14-21. determined the airspeed of a limited sampling of the birds by visually identifying each through a telescope aimed by tracking radar Fig. 5. In all cases the wind speeds were less than 5 knots. Target air speed cannot invariably distinguish between a helicopter, a slow moving aircraft and a bird flying in a high wind without precise knowledge of the wind at the bird altitude.

Figure 5 : Diagramme de vitesse aérienne des oiseaux s16Houghton, F. "Detection, Recognition and Identification of Birds on Radar," Proc. Eleventh Wx. Radar Conf., (1964), 14-21.
Figure 5 : Diagramme de vitesse aérienne des oiseaux s16Houghton, F. "Detection, Recognition and Identification of Birds on Radar," Proc. Eleventh Wx. Radar Conf., (1964), 14-21.

Flocks of birds sometimes produce rings on a radar scope which expand from a number of fixed points. These have been called "ring angels" and were first attributed to birds by Ligda s17Ligda, M.G.H. "Radar Observations of Blackbird Flights," Texas J. of Science, Vol. 10, (1958), 225-265. . Visual confirming observations were lacking at that time. Later, Eastwood, Isted and Rider s18Eastwood E., G.A. Isted and G.C. Rider. "Radar Ring Angels and the Roosting Behaviour of Starlings," Proc. Royal Soc., London, Vol. 156, (1962), 242-267. verified that radar ring angels were definitely caused by the dispersal of starlings (Sturnus vulgaris) from their roosts at sunrise. After several radar scope observations were studied, it became possible to pinpoint the centers of the rings and the approximate locations of the roosts. A number of observers equipped with radio telephones were stationed at each location and signaled the precise moment of emergence of the successive flocks of starlings from the roost under observations. These data were correlated with the radar scope presentations to confirm definitely the generation of ring angels by birds. The mean air speed of starlings leaving the roost was measured as 37 knots.

Under some conditions, slow-moving ring echoes may be produced by the rise of a temperature inversion layer in the early morning hours after sunrise. Sea-breeze fronts have occasionally been seen on radar as a line, and at other times as a boundary between scattered and concentrated signal returns as shown by Eastwood s19Eastwood, F. Radar Ornithology, Methuen and Co., Ltd., 11 New Fetter Lane, London, 1967.. How much of the line produced is due to the meteorological effects and how much by birds and insects is still a matter for speculation. However, Eastwood s20Eastwood, F. Radar Ornithology, Methuen and Co., Ltd., 11 New Fetter Lane, London, 1967. cites reports by glider pilots sharing upcurrents with birds taking advantages of the lift provided. This and some limited study of the characteristics of the radar scope signals, produce some indication as to the validity of the bird theory.

Some studies have been made on target signal fluctuation and other signature analysis techniques in connection with birds s21Eastwood, F. Radar Ornithology, Methuen and Co., Ltd., 11 New Fetter Lane, London, 1967. and even with insects s22Glover, K.M., K.R. Hardy, C.R. Landry, and T. Konrad. "Radar Characteristics of Known Insects in Free Flight," Proc., Twelfth Wx. Radar Conf., Norman, Oklahoma, (1966), 254-258. . Some of the signal characteristics have been attributed to aspect of the target and others to wing motion. There is ample evidence that insects are to be found in the atmosphere well above the surface. Apart from flying insects, creatures such as spiders can become airborne on strands of gossamer and be borne aloft in convective air currents. Glick s23Glick, P.A., "The Distribution of Insects, Spiders and Mites in the Air," Tech. Bull. No. 673, U.S. Dept. of Agriculture, (1939) reports in considerable detail the results of collecting insects from aircraft over the southern U.S. and Mexico. He found concentrations of insects of the order 1 per 2 cubic kilometers in the layer between 1000 pieds et 4000 pieds above the ground, with more widely spaced encounters up to four or five times the latter height. Although more recent data do not appear to have been collected, it is common for sailplane pilots to experience many types of insects impinging on the canopy or the leading edges of the wings at altitudes exceeding 10000 pieds au-dessus du terrain. Less commonly, birds feeding on insects carried aloft by thermals are observed at similar altitudes.

The radar cross-sections (sigma) of the various insects listed in Table 4 (measured at wavelenths of 3.2 cm.) range from 0.01 cm2 to 1.22 cm2 for all but the locust which has a maximum sigma value of 9.6 cm2. The ability of any given radar system to detect radar cross sections of these low values is a function of its design, its current performance, and the ability of the operator. Ultra-sensitive radar systems such as the MIT Lincoln Laboratory radars at Wallops Island, Va. have reported minimum detectable cross-sections at 10 km. of 6x10-4 cm2 for the X-band, 2.5x10-5 cm2 for the S-band, and 3.4x10-5 cm2 for the UHF radars (Hardy, 1966). The X-band radar is 2 orders more sensitive than required to detect the listed insects at a range of 10 km. and probably is functioning close to the limit of detectability. The majority of other radar systems in general use today are less sensitive. Some are not able to detect insects in the lower range of a values. Tabulation of a large number of radar system characteristics has been published in classified documents by RAND. Major radar parameters for some airborne sets are listed in an article by Senn and Hiser s24Senn, H.V. and H.W. Hiser. "Major Radar Parameters for Airborne Weather Reconnaissance," Proc. Tenth Wx. Radar Conf., Washington, D.C. (available from the AMS, Boston, Mass.), (1963), 341-347..

Insects are commonly found at surprisingly high altitudes. Swarms of butterflies and other insects are found in summer on 14000 pieds. mountain peaks in the Rockies. A few insects have been reported at over 25 000 pieds. altitudes in the Himalayas.

Verification of insects as causing a particular blip on a radar scope is even more difficult than birds. Flowever, this was accomplished as reported by Glover, et al s25Hardy, K.R., D. Atlas, and K.M. Glover. "Multiwavelength Backscatter From the Clear Atmosphere," J. Geophys. Res., 71, No. 6, (1966), 1537-1552.. Single insects were released from an aircraft and tracked by radar at altitudes from 1,6 à 3,0 km et à des ranges jusqu'à 18 km. Experiments of this sort and other studies involving clear atmosphere probing with high-power radars (Atlas, 1966; Hardy, 1966 and 1968) have led to valid conclusions that most of the dot echoes are caused by insects or birds.

Tableau 4 - Résumé des données de section de coupe radar d'insectes mesurées à 3,2 cm s26[de Hajousky et al, 1966]
Insecte Longueur du corps (mm) Diamètre du corps (mm) sigma L (cm2) sigma tau (cm2)
Diptère Mouche Range Crane, Timpula Simplex 13 1 0,30 0.02
Mouche verte, Lucilia Ceasar 9 3 0,25 0.10
Hyménoptère Abeille domestique, Apis Mellifera 13 6 1,00 0.30
California Harvester Ant, Pogonomyrmex Californicus 13 6 0,04 0.02
Coléoptère Convergent Lady Beetle, Hippodamia Convergens 5 3 0,02 0.01
Twelve-spotted Cucumber Beetle, Diabratica Duodecimpunctata 8 4 0,14 0.05
Lépidoptère Noctuelle des graminées, Cirphis Unipuncta 14 4 1,22 0.12
Alfalfa Caterpillar Butterfly, Colias Eurytheme 14 1,5 0,65 0.02
Orthoptère Blue Winged Locust, Trimeratropic Dyanipennis 20 4 9,60 0.96
Aranedia Araignée (non identifiée) 5 3,5 0,10 0.06

Attention has been given by Browning (1966) to the determination of the velocity characteristics of some clear-air dot angels. A 5.42 cm. pulse Doppler radar with a 1° beam elevated at 30° and rotating at 4 rpm was used in the study. A series of radar soundings spaced about half to one hour apart were obtained at 500 ft. altitude intervals up to 3000 ft. using range-gating techniques. Temperature, humidity and wind data were collected simultaneously with the radar soundings.

Three kinds of angel population were distinguished according to their mean deviation from the swarm velocity, their average vertical motion, their maximum relative velocities and their sigma values. Atmospheric inhomogeneities or the presence of plant seeds appeared to be ruled out because of the small back-scattering cross-sections of individual angels (less than approximately 0.1 cm2), their discreteness in space and velocity, their often quite large mean deviations (up to 4 m sec-1) from a uniform velocity, and the fact that the only major upward velocities occurred after sunset, at a time when the lapse rate was becoming increasingly stable. The same data suggest insects as the likeliest cause.