Probing the Clear Atmosphere with Radar

Ever since they were placed in use for aircraft detection, radars have detected "angels." An "angel" is a radar reflection from a location in the atmosphere that does not contain a known discrete target. Such reflections have been variously attributed to birds or insects too distant to see by eye, or to local fluctuations in the refractive index of the air. For quite some time heated scientific arguments have taken place in attempts to explain these observations with only limited success. During the past year, in a series of experiments sponsored by the Air Force and the National Aeronautics and Space Administration, the Applied Physics Laboratory has made great inroads toward a scientific explanation of radar reflections from the clear atmosphere, i.e., an atmosphere in which no substance or target is visible by eye. In essence it was found that birds, insects, and refractive index fluctuations are all detectable by radar, if the radar has the proper characteristics. In the summer of 1965, the multi-wavelength radar facility constructed at Wallops Island, Virginia, by the Lincoln Laboratories for a re-entry physics program was put into use by APL for the study of atmospheric phenomena principally aimed toward the detection of clear air turbulence. The basic radar system is composed of three coordinated radars at 3.2, 10.7, and 71.5 cm wavelengths with powers of 1, 3, and 6 megawatts and