Tightly Coupled Attltude Determination Using GPS Carrier Phase Observables

Gene Howell and Wang Tang

Abstract:	The synergism of integrating an inertial navigation system (INS) with the Global Positioning System (GPS) is well known. The GPS provides position and velocity information to the INS to enhance the INS navigation accuracy, while the INS aids the GPS with velocity information to increase its anti-jamming capability by narrowing the tracking loop bandwidth. The standard GPS/INS integration approach is to implement a Kalman filter to process measurements and estimate errors in both systems. The integration can be “tightly coupled”, whereby the GPS pseudoranges are the measurements to the filter, or “loosely coupled”, where 3-dimensional position is first found by the GPS receiver, then used as measurements to the filter. Although GPS was designed primarily as a positioning and timing system, recent signal processing advances have allowed the phase of the GPS carrier to be measured very accurately. Using the carrier phase observable, the distance between two receivers can be determined precisely, as can the orientation of the baseline. A number of techniques have been proposed for attitude determination using the carrier phase observable, and simulation and test results have heen obtakd. TheauitudedetcmdhalgolithmstypicayI6ndthe three Euler angles of yaw, pitch, and roll. The subsequent integration is similar in nature to the loosely coupled filter using 3-dimensional po&iou as measurements since the GPS observables are pre- processed before being used by the integrating filter. However, by using the carrier phase directly, the integrating filter becomes tightly coupled in attitude. Two major benefits are gained by integrating GPS with the INS in this way. 1) the need to perform gyrocompassing, which normally takes from 5 to 30 minutes, is eliminated; 2) the need to excite heading error by maneuvering to cause level accelerations (s-turn) for in-flight alignment is eliminated. This paper describes an INS/GPS Kalman Biter tightly coupled in attitude. An overview of the algorithms is described and the measurement equation is given. Simulation results are presented. Questions relating to initialization of the system and observabiity of attitude errors are addressed.
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:	1759 - 1766
Cite this article:	Howell, Gene, Tang, Wang, "Tightly Coupled Attltude Determination Using GPS Carrier Phase Observables," 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. 1759-1766.
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