Cycle Slip State Variables Enhance Tight Coupling of a GPS-INS

John Lukesh

Abstract:	My paper for last year’s meeting [1] introduced the GPS cycle slip state variable and its utility when used with a loosely coupled GPS-INS system. Now I have extended the concept for use with tightly coupled systems. The primary advantage is in recovery of carrier tracking after an extended period of jamming. As before, this work is being done using simulation programs written in MATLAB. My simulated GPS/INS system uses a Kalman filter to estimate user position and other variables needed in generating the feedback commands to the carrier NCO and the code generator which in turn maintain code and carrier lock. Conventional tracking loops are not involved. Because of this, performance is optimal. Also, as is true for any properly implemented system in which a Kalman filter replaces a classical control mechanization, the “loop gains” are automatic fallouts of simply defining the system error process models for the Kalman filter. Easy. Easy to set up in the first place and easy to modify for different conditions (different IMUs for example). The feedback command to the carrier NCO involves two variables that are affected by the Kalman filter estimation: the estimated pseudorange to the particular satellite and the carrier error phase compensation, which includes the cycle slip error estimate for that satellite. I confirmed that the control of the Kalman filter estimates of position error and carrier phase slip error both to the nonlinear system does what I want, which is to provide for significant (more than 1 cycle) corrections to position error without letting the carrier tracking error signal used with the Kalman filter get out of the linear range. This was done with the current version of my program set with conditions chosen to result in a multiple cycle position error after jamming. After jamming, the filter, using both carrier and code observations, had to take out more than 2 meters of position error (equivalent to about 10 carrier cycles). It did this gracefully. The cycle slip state variable does its job. This paper gives the GPS-INS system models and the Kalman filter design. Simulation results are given with particular emphasis on recovery after jamming.
Published in:	Proceedings of the 2000 National Technical Meeting of The Institute of Navigation January 26 - 28, 2000 Pacific Hotel Disneyland Anaheim, CA
Pages:	106 - 114
Cite this article:	Lukesh, John, "Cycle Slip State Variables Enhance Tight Coupling of a GPS-INS," Proceedings of the 2000 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2000, pp. 106-114.
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