North Slope 2000: Flight Testing Military/Civil GPS Receivers Against North Warning System Radars

Victor Andreone, Ricardo Mediavilla and Collin Broughton

Abstract:	This paper describes testing performed for the Atmospheric Early Warning System (AEWS) program office, Hill AFB, UT, from January to August 2000. The project evaluated various military/civil aviation GPS navigation systems during exposure to inadvertent jamming by ground surveillance radars. The test program provided a means to obtain a quick and pragmatic answer to the question: Can GPS test units coexist with the North Warning System (NWS) radars? In developing the answer, the AEWS Program Office wanted an empirical correlation of GPS performance to Aeronautical Radionavigation Service (ARNS), Wide Area Augmentation System (WAAS) Minimum Operational Performance Standards (MOPS), and other theoretically derived interference values then in discussion. It was believed that such a test program could provide data to support, negate or otherwise temper suggested radar spectrum and operational modifications necessary for coexistence. SAIC and the 746th Test Squadron at Holloman AFB chose nine GPS systems with different interference susceptibilities for test. These systems included coarse/acquisition (C/A) commercial receivers (Garmin III, Globalmap 100, Ashtech Z 12) and precision (P)-code military receivers (Miniature Airborne GPS Receiver [MAGR], Precision Lightweight GPS Receiver [PLGR]). Additionally, these systems were operated either alone or aided by inertial systems and connected to Fixed Reception Pattern Antennas (FRPAs) or Controlled Reception Pattern Antennas (CRPAs). The test aircraft was the 46th Test Group C-12J twinturbo prop test bed used extensively in assessing navigation system performance for the Services. Two equipment racks housing the test samples and recording stations were installed in the aircraft, ground-checked, and flight-tested at Holloman AFB, NM, prior to ferrying to Elmendorf AFB, AK. The radar systems chosen to be potential interferers were two AN/FPS-117 long-range radars and one AN/FPS-124 short-range radar located on Alaska's North Slope. These radars form a part of the NWS extending from Alaska to Iceland and operated by the USAF AEWS program office. The 84 Radar Evaluation Squadron (84 RADES), Hill AFB, UT, provided support for coordination of the radars’ operation and recording data in time synchronization with the aircraft-recorded flight data. Additionally the 84 RADES provided Radar Coverage Prediction (RCP) plots and Advanced Refractive Effects Prediction (AREP) diagrams for the intended test area and test times. These computer programs allowed the design of a series of test flights at altitudes between 1,000 feet to 25,000 feet MSL, ranging from low altitude, single-radar overflights to high altitude, multiple-radar overflights. The data recorded during test flights were processed to remove format/scaling anomalies and to limit the data to the test tracks only. SAIC then analyzed the data to identify incidences of interference. Each incidence was evaluated in comparison to all of the other test samples, as well as the radar-indicated data and individual receiver channel data. The data logs include time figures of merit, position data in various coordinate systems, carrier-tonoise ratio, and jamming-to-signal ratio. The results indicated an absence of coexistence problems between radar and GPS systems at the top system level of operation. This would be the level at which the navigation function is considered as the critical parameter. However, single channels occasionally showed the presence of inadvertent jamming even though the composite solution, developed by averaging multiple channels, held solid. The receivers with greater functional robustness, predictably, showed less radar interference effects than the more basic systems. While no single series of tests can be inferred to be conclusive, the tests pointed out that single parameter interference standards could result in costly and possibly unwarranted fixes.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	912 - 923
Cite this article:	Andreone, Victor, Mediavilla, Ricardo, Broughton, Collin, "North Slope 2000: Flight Testing Military/Civil GPS Receivers Against North Warning System Radars," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 912-923.
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