Kadena AFB, Okinawa, October 1951October 1951

On October 3, 1951, at 13:27 (1:27 p.m.) Zulu time, a radar operator at a U.S. Air Force ground control station near Kadena Air Base, Okinawa, detected an unusual radar echo on the high-beam scope of an AN/CPS-1 search radar. The object appeared at a bearing of 010 degrees, about 90 statute miles from the station.

The radar operator who first detected the object described a large, sausage-shaped blip that was twice as wide as it was long. He estimated its dimensions at roughly two miles wide by four miles long. Notably, the long axis of the blip always stayed at a right angle to the object's direction of movement. The echo was not particularly bright, but it was solid and easy to see. After tracking the object on the high beam, a second radar operator picked up the same target on the low-beam long-range scope at a bearing of 355 degrees, about 200 miles away. This operator described the echo as even larger, estimated at two and a half to three miles wide and six to eight miles long, with the same perpendicular orientation to its flight path.

The object's ground speed was estimated at roughly 2,500 to 3,000 miles per hour based on the radar plots. It changed heading dramatically, from approximately 265 degrees to 355 degrees, a shift of about ninety degrees, within just 15 seconds. The speed remained remarkably constant throughout the sighting. No visual confirmation of the object was attempted or reported. Weather conditions at the time were generally clear, with no precipitation echoes on the radar scope.

Investigators interviewed three experienced radar operators and found them all reliable and well-trained. All three agreed on the basic facts of the sighting. The radar equipment was functioning normally, and no interference or equipment malfunction was detected. Other radar sites in the area, though physically closer to the object's track, did not detect it. Only the AN/CPS-1 at Site 51 captured the target on its high and low beams.

Analysis of the case generated differing interpretations. An Air Force analyst argued that the sausage shape and the constant velocity could be explained by a formation of multiple aircraft flying in wide array. A Philco Technical Representative offered a different theory, suggesting that the target was likely a man-made missile equipped with a radar beacon transponder operating in the S-band frequency. He based this conclusion on the object's ability to maintain constant velocity while executing a sharp ninety-degree turn, characteristics he considered incompatible with natural objects subject only to gravity, air resistance, and initial momentum. He also noted that a directional radar echo (one that changes appearance with the target's heading) would be more consistent with an emitted signal from a beacon than a reflected radar return.

Air Force officials requested calibration studies to test competing theories. A memorandum from Headquarters, Twentieth Air Force noted that analysis was still pending and that special radar equipment calibration would be required before final conclusions could be reached. The file indicates this calibration was not yet complete when the report was finalized.

The Air Force's official evaluation listed on the cover sheet was "unknown," meaning the object remained unidentified. The full case file of 91 pages is reproduced below as held by the National Archives.