A High Anti-Jam GPS-Based Navigator

Donald Gustafson, John Dowdle and Karl Flueckiger

Abstract:	Current GPS-based navigation system performance is marginal in high interference environments in both civilian and military applications. This paper presents a new approach to solving these problems using a design that may be implemented at low cost in software in existing and future GPS receivers. The design approach includes GPS-only navigation capability; however, other sensors may be employed, such as inertial instruments (gyros and accelerometers), radars, altimeters, etc. A multidimensional state is estimated recursively in real time using raw measurement data, which includes (1) in-phase and quadrature data from a bank of correlators, and (2) raw measurements from other sensors. The data are processed in a single processor that produces the desired navigation system outputs (e.g., position, velocity, time, and attitude). In addition, the GPS measurements may be used to calibrate correlated errors in the other sensors. Traditional systems are not optimal at high jammer-to-signal (J/S) ratios as a consequence of modular design, use of traditional fixed-gain or gain-scheduled tracking loops, and use of artificial moding logic. In contrast, the approach described in this paper employs a nonlinear filter that operates efficiently at all J/S levels. The design incorporates traditional code loop functions. Filter gains continuously adapt to changes in the J/S environment, and the error covariance propagation is driven directly by measurements to enhance robustness under high jamming conditions. Extended-range correlation may be optionally included to increase the code tracking loss-of-lock threshold under high jamming scenarios. Computational complexity is comparable to an extended Kalman filter. The system has been tested via simulation and in the laboratory using a Nortel GPS simulator and Plessey receiver. Results indicate a consistent improvement in C/A code tracking of at least 15 dB in wideband anti-jam capability relative to traditional designs. This improvement can be traced to several factors: (1) elimination of moding logic, resulting in seamless operation at all values of J/S, (2) continuous adaptive estimation of J/S using measurements from the full correlator bank, and (3) effective handling of measurement nonlinearities. This technology is currently being developed for use in next generation deeply integrated INS/GPS navigation systems employing microelectromechanical system (MEMS) sensors with a near-term goal of a single package consuming less than three Watts in a volume of three cubic inches.
Published in:	Proceedings of the 2000 National Technical Meeting of The Institute of Navigation January 26 - 28, 2000 Pacific Hotel Disneyland Anaheim, CA
Pages:	495 - 503
Cite this article:	Gustafson, Donald, Dowdle, John, Flueckiger, Karl, "A High Anti-Jam GPS-Based Navigator," Proceedings of the 2000 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2000, pp. 495-503.
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