Analysis of Pseudolite Code Interference Effects for Aircraft Precision Approaches

Gary A. McGraw

Abstract:	There is considerable research activity in the area of using the Global Positioning System (GPS) for air- craft precision approaches and landings. One GPS augmentation which is being considered for enhancing basic GPS accuracy and integrity are ground-based GPS pseudo-satellites (pseudolites). One issue as- sociated with pseudolites is the problem of interfer- ence with the GPS C/A-code (clear/acquisition-code) when the aircraft is close to the pseudolite, due to the greater relative signal strength of the pseudolite over the GPS satellites. A technique which has been proposed for mitigating the code interference prob- lem is to offset the pseudolite broadcast frequency from the nominal GPS frequency by the C/A-code chipping rate of 1.023 MHz. This frequency offset shifts the pseudolite code spectrum into the first null of the local code spectrum. An analysis has been performed to examine the code cross-correlation be- tween a GPS signal and an offset pseudolite signal. The analysis results show that the reduction in code cross-correlation achieved by the proposed offset of the pseudolite carrier is much less than has been claimed. In the worst-case only about 3.6 dB of addi- tional code cross-correlation attenuation is achieved beyond what is obtained by using two different GPS C/A-code sequences at the same carrier frequency. However the large code doppler induced by the carrier offset effectively transforms the code cross correlation to noise. Other techniques are then required to deal with pseudolite interference of GPS signals.
Published in:	Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994) June 6 - 8, 1994 Antlers Doubletree Hotel Colorado Springs, CO
Pages:	433 - 437
Cite this article:	McGraw, Gary A., "Analysis of Pseudolite Code Interference Effects for Aircraft Precision Approaches," Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994), Colorado Springs, CO, June 1994, pp. 433-437.
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