Test Results of the Joint FAA/DOD Investigation of GPS Interference

This paper describes the test results of the joint FAA and DOD effi to identify the source of interference to civil GPS receivers, also known as the “wormhole”, observed in October 1994 just north of St. Louis, MO. The test team consisted of engineers and aviators fi-om the FAA, Naval Air Warfare Center (NAWC), DOD Joint Spectrum Center (JSC), Transport Canada Aviation (TCA), MITRE, and McDonnell-Douglas Aerospace (MDA). Initially the investigation focused on television transmissions as the possible cause of GPS interference. The third harmonic at 1575.75 MHz generated Iiorn television signals at 525.25 MHz (UHF Channel 23) was suggested as the source of interference to the civil GPS signal at 1575.42 MHz. Giving credibility to this speculation was the proximity of Channel 23 to the “wormhole” observed in St. Louis. Flight tests were conducted where theoretical analysis aC TV transmissions indicated sufficient signal strength may exist to affect GPS operation. In addition, flights were coordinated with MDA in St. Louis in order to measure the’effect of outdoor range antenna pattern emissions on GPS operation. These tests answered several technical questions regarding the potential for GPS interference and solved the “wormhole” mystery. In addition, the ef%ct of electromagnetic polarization has not been widely discussed in the interference literature. This paper presents the results of the laboratory and flight tests which measured GPS antenna performance in flight, illustrating the phenomenology associated with the circular and linear polarization responses.