Project Blue Book file, RB-47, July 17, 1957

Read from the scan by GLM-OCR; expect the odd misread word.

west-bound leg, the three Elint (ECM) operators would check their equipment against the numerous ground radars and military communications facilities in that region.

## THE FIRST INCIDENT:

This event involved ONLY some unusual behavior of the AN/ALA-6 Elint equipment operated by ECM Monitor #2, Frank B. McClure. There are no reports of any visual UFO sightings.

McClure's current recollection is that the incident occurred as the RB-47 was approaching the coast, near Biloxi, Miss. The contemporary account by Piwetz disagrees saying that it occurred "approximately at Meridian." Because McClure did not notify any other crew members of the anomalous behavior of his equipment at the time, it is not possible to check other crew members for their recollections. However, for reasons to be discussed shortly, it is believed that McClure's recollection is correct and that the incident occurred near Biloxi.

As the RB-47 neared the coast, McClure decided to check the operation of his ALA-6 equipment, knowing that the aircraft was approaching air defense ground radars against which it could be operated. The system involved included the ALA-6 direction-finder, an associated AN/APR-9 Elint receiver and an AN/ALA-5 pulse analyzer.

This Elint system is designed to locate the position of ground radar and to measure its signal characteristics. By taking several bearings to the source of a radar signal at different, known locations along the airplane's flight path, the position of the radar can be determined by triangulation. The ALA-6 bearings are displayed relative to the airplane's fore-aft axis.

The ALA-6 can be fitted with several different types of antennas, depending on the frequency of the ground radars involved. (See: ALA-6 Handbook of Operating Instructions, T.O. 12P3-2ALA6, dated 1 May 1954:) During this mission, the RB-47 was using a Type AS-656 antenna system that covered the frequency range of 1,000 mc. (L-band) to 5,000 mc. (C-band). It consists of TWO nearly identical parabolic antenna dishes which are mounted back-to-back. The antenna assembly rotates at 150 rpm or 300 rpm, selectat by the operator.

To illustrate how the ALA-6 functions to measure bearing to a ground radar, which is important to understanding this UFO case, only ONE of the two back-back antennas will be considered for the moment. When the Elint aircraft is within receiving range of a ground radar, and when the radar antenna is illuminating the airplane, the ALA-6 antenna will receive the radar signal. (Because of the comparatively high scan rate of the ALA antenna relative to the ground radar, the ALA-6 is assured of receiving ground radar pulses whenever the airplane is illuminated by the radar.)

The signals received are fed to the APR-9 Elint receiver for detection/amplification for subsequent display on a small cathode-ray-tube (CRT) for the Elint operator. To display the bearing of the source of the received signal relative to the airplane's for aft axis, the antenna assembly contains a synchro-resolver whose rotor is geared to the antenna drive mechanism. The rotor winding of this synchro-resolver is excited from the output of the APR-9 receiver. And its sine/cosine output windings are connected to the deflection amplifiers and then to the vertical/horizontal deflection plates of the ALA-6 CRT display. (See Fig. 3.)

At such times as the airplane is illuminated by a radar, the ALA-6 will display the situation somewhat as shown in Fig. 1. (See next page.) The photo shown is an idealized version copied from the ALA-6 operator's manual.