This report describes the investigation of a type of unidentified moving target which has been observed recently in considerable numbers on the viewing screens of air traffic control radar equipment operated by the Civil Aeronautics Administration. This investigation was conducted by means of interviews with personnel concerned, by study and correlation of official records, and by first-hand observation of numerous targets on the Washington Microwave-Early-Warning (MEW) radar and on the Indianapolis ASR-2 radar.
It was determined that targets which are known to operating personnel by various terminologies such as "ghosts," "angels," or "pixies" do not represent new phenomena; nor are they peculiar to the Washington area. Correlation of controllers' reports with United States Weather Bureau records indicated that a surface temperature inversion was almost always noted when such targets appeared on the radar.
Firsthand observation in the tracking and subsequent motion analysis of 80 of these unidentified targets indicated that a large number of these were actually secondary reflections of the radar beam. Apparently these reflections were produced by isolated refracting areas which traveled with the wind at or near the temperature inversion levels.
Although the exact size, shape, and composition of these isolated areas are not known, it is believed that they may be atmospheric eddies produced by a shearing action of dissimilar air strata. It appears possible that such eddies may refract and focus the radar energy with a lens effect to produce small concentrations of ground return with sufficient intensity to show up on the radar display. It is also believed that the correlation of the appearance of these radar targets with visual reports of so-called "flying saucers" is due to the strong probability that both effects are caused primarily by abrupt temperature inversions.
Such radar targets are usually easy to recognize because of their generally weak return and slow ground speed. Unfortunately, radar returns from small helicopters sometimes present these some characteristics. Spurious targets of this type can become a nuisance under busy traffic conditions, particularly in localities where helicopter operations are prevalent.