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This Project Blue Book file, released through The Black Vault, covers the RB-47 encounter of 17 July 1957 over Mississippi, Louisiana, Texas and Oklahoma. It includes the record card, which lists the sighting as identified as American Airlines flight 655, and Air Defense Command correspondence that carried the sighting as unknown. There is also Major Lewis D. Chase's observer data sheet and an electronic analysis finding it difficult to conclude nothing was present. James McDonald's reconstructed account is included, along with a CAA letter about airliner incidents.
Read from the scan by GLM-OCR; expect the odd misread word.
Because the top of the ALA-6 display corresponds to a bearing of $ 0^{\circ} $ , the radar shown in Fig. 1 is dead-ahead of the airplane. If the center of the displayed pattern were at the 3 o'clock position, then the radar would be abean of the airplane, on its right-hand side.
As the aircraft flies along, unless the radar is dead-ahead or astern, the displayed bearing to the signal source will move DOWN-SCOPE, providing the aircraft is in straight/level flight. If the airplane is turning toward the radar, then the bearing displayed will move UP-SCOPE. (See Fig. 2.)
As the RB-47 approached the coast in the pre-dawn hours of July 17, ECM Monitor #2 (McClure) decided to check out his equipment. Beginning at the high end of the APR-9 frequency range (5,000 mc.), McClure worked down until he found an S-band signal. The signal had all the characteristics of a CPS-6B air defense radar, one of several models then in use. (A slightly later version of the CPS-6B, known as the AN/FPS-10, had identical signal characteristics.)
It is important to note that McClure did not write down the specific frequency or other signal characteristics at this time, so we are not certain of the precise frequency of the S-band signal involved in this first incident.
The S-band signal even showed that the source was a scanning one, typical of radar. This is manifested by the brief appearance, then disappearance, of the signal bearing display on the ALA-6. As a ground radar antenna begins to illuminate the Elint airplane, the signal is briefly displayed. Then, as the radar antenna rotates to where it no longer illuminates the airplane, the signal displayed slowly disappears as the CRT phosphor de cays.
McClure was not surprised to see a signal with the characteristics of a CPS-6B, as he explained to me, for he knew that such a radar was installed near Biloxi, near the RB-47 position. This radar was used in conjunction with an Air Force training school for ECM operators.
NORMAL OPERATION of AN/ALA-6 Elint Direction-Finder
The signal was "mighty strong," according to McClure, as would be expected if it came from the CPS-6B radar at Biloxi. However, the bearing to the signal displayed on the ALA-6 showed the radar to be on the RIGHT-HAND SIDE of the RB-47, at roughly the 5 o'clock position. The Biloxi radar would be on the LEFT-HAND SIDE of the aircraft. The bearing displayed on the ALA-6 would place the unknown radar in the Gulf, and McClure knew there were no such shipboard radars.
As the RB-47 proceeded toward Meridian, McClure noted another anomalous characteristic. Instead of the displayed bearing moving down-scope, it moved up-scope. McClure, knowing that such behavior could occur if the airplane were in a turn, says he called the pilot on the intercom to ask if he was maneuvering. The pilot replied that he was not. (This would confirm the fact that the incident happened near the coast rather than at Meridian, as Piwetz reports, because the airplane did turn west at Meridian.) Document, cited by the archive. The PDF is mirrored here; the original link is above. The text was read from the page images by GLM-OCR; expect the odd misread word. 57 pages are in the text index: search them above, or from the library's search.