Maintaining GPS Positioning in Steep Turns Using Two Antennas

David Lawrence, H. Stewart Cobb, Clark Cohen, Jock Christie, J. David Powell, and Bradford Parkinson

Abstract:	Satellite availability studies commonly assume a fixed elevation mask angle. However, as an aircraft banks, relatively high satellites can be masked from the field of view of the GPS antenna. Not only are fewer satellites visible, but those that are in view are clustered in one section of the sky. This geometry typically leads to high PDOP. During steep turns, this effect can limit the availability of GPS and WAAS satellites, especially at high latitudes. However, if a second GPS antenna is installed on the aircraft, satellites masked from the main antenna may still be used. This paper presents experimental results of kinematic GPS positioning using two antennas. The Integrity Beacon Landing System (IBLS) developed at Stanford uses a bottom GPS antenna to acquire pseudolite signals. Attitude is provided to account for the moment arm from the top antenna to the bottom antenna. With a bottom antenna and attitude already available, the IBLS test aircraft required few changes to demonstrate GPS positioning using multiple antennas. A nine channel Trimble receiver with an RF section dedicated to each of the two antennas was used for these tests. Each channel could switch between the top and bottom antennas as required to track the desired satellite. The switches were performed using attitude information provided by a separate GPS attitude receiver. Satellites were handed off from one antenna to the other in real time as the aircraft attitude changed. Experiments were first performed using a model aircraft on the ground. Flight tests were then performed on a Piper Dakota already equipped for IBLS testing. The results of these tests show that multiple GPS antennas can be used effectively to improve the availability of GPS positioning.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1451 - 1459
Cite this article:	Lawrence, David, Cobb, H. Stewart, Cohen, Clark, Christie, Jock, Powell, J. David, Parkinson, Bradford, "Maintaining GPS Positioning in Steep Turns Using Two Antennas," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1451-1459.
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