Observations radar d'ovnis

Edward U. Condon

The public became generally aware of radar at the end of World War II when the story of its important use in that war was told, after having been kept secret for some 12 years. A good non-technical account of this development is given in R. M. Page, The Origin of Radar (1962).

Le mot radar est un acronyme pour RAdio Detection And Ranging. Basically, most radar systems operate in the following way. A transmitter sends out short pulses of electromagnetic energy at regular intervals. These are sent out through an antenna designed to radiate a narrow beam within a small angle of its main direction. This beam of pulses travels outward at the speed of light. If it encounters an obstacle, which may be a metallic object like an airplane, a rain storm, or a bird or a flock of birds, it is partially scattered in all directions from the obstacle. In particular a part of the beam is scattered back toward the transmitter. When it arrives back at the transmitter it is received and indicated or displayed in various ways, depending on the special purpose for which the system was designed. By the fact of there being a returned signal at all, the function of detection is accomplished. By the time delay involved between the transmission of the outgoing signal and the return of the back-scattered signal, the distance of the scattering object is inferred, thus accomplishing the function of ranging.

To get a beam of sufficiently narrow distribution in angle as to enable inferring from what direction the scattered signal was returned, the antenna must have a diameter of the order of ten times the wavelength of the radio waves which it uses.

In the period since 1945 the technology has had an enormous development so that nowadays there are elaborate networks of land and shipbased radar systems, as well as radar systems carried by most airplanes, which have become vitally necessary to the safe operation of civil and military aircraft. In addition to the use of radar in connection with navigation, it has become a valuable tool in meteorological work in that distant rain storms can be detected by radar. Also the trails of ionized air left by meteors can be detected and studied by radar, providing for the first time the means for observing meteors in the daytime.

There are many popular misconceptions about radar. It is important at the outset to realize that the returned, radar signal does not give a a sharply focussed image or picture of the obstacle that has been detected. What one gets when it is displayed on a cathode-ray screen is simply a diffuse blob of light indicating that something is there, in the direction the antenna is pointed (with some exceptions) and at the distance indicated by the time delay between transmission and reception of the back-scattered pulse. Of course, a large airplane gives a more intense signal than a flock of small birds at the same range, and skilled operators learn to make valid inferences about the nature of the object detected from other things that they know about the general situation together with the magnitude of the returned signal.

It is important also to recognize that the propagation of the outgoing and the back-scattered pulses is ordinarily assumed to be rectilinear and at the normal speed of light. But the actual propagation is affected by temperature and humidity difference in the air path along which the radio pulse travels. This can give rise to anomalous propagation that is analogous to but in detail not identical with the effects which give rise to mirages in the propagation of light through such an atmosphere. Usually the radar set operator does not know enough about the actual atmospheric conditions to make allowance for effects of this kind and, if they happen to be pronounced, can be led to make erroneous decisions. Another point is that, although the antenna sends out most of its energy in a single narrow beam, small amounts of energy go out in several other directions, known as sidelobes, so that a large or a nearby object in the direction of a sidelobe can give rise to a received signal that is indistinguishable from a small or distant object in the direction of the main beam.

The overall radar system is a rather complicated set of electronic equipment which can malfunction in various ways giving rise to internally generated signals which the operator will tend to regard as reflections made by outside obstacles which are in reality not there.

Usually the returned radar signals are displayed on the screen of a cathode ray tube and observed visually by the operator. On this account, subjective judgments of the operator enter into the final determination of what is seen, how it is interpreted and how it is reported. The data obtained from radar systems are thus not as completely objective as is often assumed. In some few instances subjectiveness is somewhat reduced by the fact that the cathode ray screen is photographed, but even when this is done there is a subjective element introduced at the stage where a human observer has to interpret the photograph of the radar screen.

Radar operators do report unidentified targets from time to time and so there exists a category of UFO cases in which the unidentified flying object was seen on a radar screen. In a few cases there is a close correlation between an unknown thing in the sky seen visually and something also displayed on radar.

However in view of the many difficulties associated with unambiguous interpretation of all blobs of light on a radar screen it does not follow directly and easily that the radar reports support or "prove" that UFOs exist as moving vehicles scattering the radio pulses as would a metallic object. Le projet du Colorado a engagé les services de l'Institut de Recherche de Stanford pour réaliser une étude générale du fonctionnement des systèmes radar du point de vue de la relation de leurs indications aux ovnis. L'étude qui fut menée résultat en la production du chapitre 5 de la section 6, par le Dr. Roy H. Blackmer, Jr. et son associé, R. J. Allen, R. T. S. Collis, C. Herold et R. I. Presnell.

Les études des signalements radar d'ovnis spécifiques et leur interprétation sont présentées dans le chapitre 5 de la section 3 par Gordon Thayer. Thayer est un spécialiste de la propagation radio de l'équipe de l'Environmental Science Services Administration à Boulder. Dans ce chapitre, Thayer présente une analyse détaillée de quelques 35 cas, certains étant visuels, d'autres radar, et certains les 2. Les phénomènes optiques et radar sont traités ensemble en raison de la similarité des problèmes de propagation d'onde impliqués.

Dans sa synthèse des résultats il indique : ...il n'y a pas eu de cas où les données météorologiques disponibles tendaient à réfuter l'hypothèse de propagation anomale... Cependant, Thayer avance que des données météorologiques adéquates pour une interprétation approfondie manque souvent de sorte que beaucoup de données observationnelles de ce type seraient nécessaires pour pouvoir traiter une plus grande proportion de l'ensemble des cas radar d'ovnis signalés.

Au regard de l'importance du radar to the safe operation de tous appareils, il est essentiel que des recherches plus poussées soient menées pour acquérir une connaissance aussi précise que possible de la propagation anormale des signaux radar. Cependant, il est considéré que cela peut être mieux réalisé en s'attaquant directement au problème plutôt qu'au travers d'une investigation détaillée du domaine dans les cas d'ovnis.