Provincetown, Mass., Sept. 21, 1950 Case

Excerpt from NARCAP TR-6 Report from Dec, 2002 (pages 21-24)

4. DATE: September 21, 1950     TIME: unknown    CLASS: R ground radar

LOCATION:                        SOURCES: Hynek (1978) 139



RADAR DURATION: unspecified EVALUATIONS: Blue Book - unknown

PRECIS: An MIT radar observer and 2 colleagues conducting a weather radar project under contract to the US Signal Corps were tracking Air Force F-86s from a radar site at Provincetown. The following is from a report made to Major Turtle, Staff Weather Officers, 33rd Fighter Wing, Otis AFB, Mass.:

An exceedingly puzzling event occurred during the 3rd run when the planes were heading northeast at 30,000 feet. We picked up another plane in the radar beam travelling about due north on a converging course towards the F-86s. It was moving very rapidly and I told the pilots about it, its range and direction from them. The echo caught up with, passed, and then crossed the course of the 86s, suddenly went into a very tight (for the speed) turn to the right, headed back toward Boston and passed directly over our flight. (Perhaps went under.) The sketch [unavailable] represents, as closely as we can remember, the relative positions of the two planes. Two other observers were with me at the time and we have checked over the facts rather closely. The pilots will undoubtedly recall the incident. They said they didn't see anything which is not too surprising considering the speed of the object and the fact that it may have passed several thousand feet above or below them and still looked like coincidence to the radar. Figuring conservatively, the speed of the object was approximately 1200 MPH, and the centrifugal force exerted on the ship during the turn amounted to something more than five g's. It gave an excellent radar echo which could not be mistaken for anything else and in all respects except for the velocity seemed a normal radar target. It passed out of the beam while we continued to track our flight, but we focused on it again for a few seconds shortly after it was rapidly approaching Boston.... It was very evidently an interception of some sort on our flight, but what? The turn was utterly fantastic .... A few rough calculations concerning control surfaces, angles, etc., only adds to the puzzle that this object must have been entirely unconventional in many and basic respects. Perhaps the thing that bothers me the most is that it gave a very good radar echo, which implies irregular surfaces and comparatively large size, large enough so the pilots might have had a good chance to see it. .. It seems highly probable that I may be poking into something that is none of my business, but on the other hand, it may be something that the Air Force would like to know about if it doesn't already.. ..

NOTES: The description seems to be of a PPI display with no height finder. It appears that the aircraft altitude cited came from pilot reports.

This kind of target behavior - a single discrete target, presentation identical to that of an aircraft, making a continuous track at very high speed with a midcourse turn onto a markedly different azimuth - is not at all typical of anomalous propagation. Partial reflection from moving waves on an elevated inversion could generate fairly good spot targets, but the speed of such tracks is 2 x wind speed with some consistent relationship to wind direction: in this case the target was too fast by an order of magnitude and the rough geography of the account is sufficient to establish a change of heading too great to comfortably equate with winds. Sporadic ground echoes due to supeirefractivity can present the illusion of fast targets on any heading, but the probability of a random mechanism generating a coherent track of any length is very low, as is the probability that a number of sporadic returns from very different ground reflectors over a very wide area, each with a very different propagation history, could present a) consistently and b) as "very good" spot targets. Neither CAT nor any other possible atmospheric inhomogeneity seems appropriate to the target presentation, speed or behavior.

Radio frequency interference or spurious internal signals are equally improbable explanations of a target which a) "in all respects" except speed had the presentation of an aircraft target (requiring discrete pulse trains with a systematic reference to the PRF and scan rate of the receiver, such as signals from another similar radar), and b) made a complex PPI track with no obvious geometric relationship to the scope center (implying a source with no systemmatic reference to the scan-rate of the receiver). A so-called spot target such as an aircraft is displayed on a PPI as a short arc composed of the integrated spot returns from a number of radar pulses on adjacent scope traces, whose non-radial movements bear a very complex relationship to a changing set of trace radii; and a repeating noise pulse or pulse-train (even neglecting the complexity of the target arc) which displayed as a track comprising at least one straight sector with a significant vector tangential to the scope radius, plus a "very tight turn", would in itself imply a very complex and fortuitous pattern of changing pulse-repetition frequencies. Such varying cyclicity is not typical of any pulsed emitter in the radar band, and the likelihood of random noise generating such a target is obviously vanishingly small.

Birds, insects, balloons and other wind borne objects are ruled out for reasons including speeds, headings and presentation. Multiple trip returns from aircraft beyond the unambiguous range are not helpful in this case, since moving targets so detected will be displayed at spuriously slow speed proportional to their tangential vector, and the displayed speed is already a problem; and AP ground returns detected in this way are subject to the same objections already discussed.

The date of the incident rules out the possibility of multiple-trip returns from an artificial satellite. But one possibility which deserves to be considered is multiple-trip returns from meteor-wake ionisation: a large meteor on a tangential heading to the receiver at low elevation might be detected by a sensitive search radar circa 1950, although neither the peak output nor the frequency are known. The track of such a meteor might be considerably distorted and appear to execute an approach towards the site and a hyperbolic turn onto a receding heading. Due to the rate of ion recombination the trail will be scanned essentially as a spot target. But there are objections to this hypothesis: 1) the geography and headings cited cannot be made to fit such a trajectory; 2) a meteor trail is unlikely to yield an "excellent echo" indistinguishable from that of an aircraft even within the unambiguous range of a radar operating at optimum frequency, still less at ranges displayed by multiple trip; 3) displayed speed of 1200 mph is far too low even for this mechanism; 4) the duration of the track (sufficient to allow the operator time to contact the F-86s and supply range and heading of the target before it even approached them) is clearly too great for a meteor which would only be within the earth's atmosphere for a few seconds.

A "ghost" reflection is on the face of it the most plausible explanation. Quantitatively speaking, such apparent "interceptions" by high-speed targets are quite typical of ghosts due to radar energy being scattered from the aircraft to an efficient secondary reflector and back again. The most efficient secondary reflectors are large ground structures, such as metal roofs, and corner-reflectors such as empty trucks and metal fencing. The displayed range of a ghost will exceed that of the primary reflector by an amount equal to the distance between the two reflectors, and the ghost will always appear on the azimuth of the primary reflector. Since the angular rate of the ghost equals that of the primary reflector but at greater displayed range it can appear to move at much higher speeds. In this way, an aircraft flying over such a reflector might be "intercepted" by a ghost which approaches at high speed, merges with or paces the aircraft target briefly and then accelerates away again. In the present case, however, the aircraft were flying at 30,000' so that any ghost due to a ground reflector could not be displayed closer than about 5.7 miles from the F-86s.

For aircraft and ghost to coincide on the PPI the secondary reflector would have to be another nearby aircraft. Since there were apparently two F-86s the possibility arises of reflection from one to the other, but the target was initially painted at considerable range from the two F-86s (far enough that it could be seen closing at 1200 mph whilst the radar operator talked to the pilots about it) whilst the aircraft were evidently flying together on the same heading and both at 30,000. Later the target "passed out of the beam while we continued to track our flight." Finally, it can be shown that if the F-86s were on a NE heading and the "intercepting" ghost was on a N heading then the reflection geometry requires the F-86s to be SE of the radar site, in which case a ghost could not possibly be displayed turning "back toward Boston" (NW of Provincetown) as this would place it at closer displayed range than the primary reflector (an F-86).

Another unreported aircraft would be required as a secondary reflector, but the coincidence of ghost and F-86 echoes would still demand that it flew by in close physical proximity, which raises the questions of why it was not seen and, more importantly, why it was not itself painted by the radar, being (ex hypothesi) an efficient enough reflector to generate a "very good" ghost echo by secondary scattering.

The only possible explanation would seem to be a ghost due in no way the F86s but entirely to another unreported aircraft and a secondary ground reflector. The collocation of the ghost and the F-86s on the PPI would thus be completely coincidental, and a remote primary reflector would explain why it was not seen visually by the pilots. The aircraft would probably have been a fast jet for its ghost to achieve 1200 mph and would be required to maintain a very favourable aspect with an efficient ground reflector for a period evidently of some minutes, initially producing a ghost track on an essentially straight N heading at 1200 mph with a sufficient duration to permit confirmation of its speed and heading over a number of sweeps followed by radio contact with the F-86 pilots, then turning abruptly as the culpable aircraft flew by the ground reflector. This trajectory simply must represent a ground track of some tens of miles, all the while presenting an "extremely good" ghost return. This is inherently somewhat improbable, and at no time is the presence of the responsible aircraft mentioned despite the fact that when its ghost track turned abruptly onto a heading for Boston it would itself have been displayed on-scope very close to it, and thus very close to the F-86s, its relationship to its ghost then, if at no other time, becoming at least noteworthy - if not actually understood by the operators.

In summary, although there is no visual corroboration of the target in this case and only one radar instrument was involved, the target as described is not easily explained. However, in the absence of scope photographs a ghost echo remains a possible, albeit a not very satisfactory, interpretation.

STATUS: Insufficient information