An Autodop Algorithm for the Block IIR AUTONAV Capability

Alison Brown

Abstract:	The Block IIR GPS satellites presently under development are designed to include an Autonomous Navigation capability (AUTONAV) to reduce the work load of the Control Segment and to improve the survivability of the GPS system. AUTONAV operates using an on-board ephemeris and clock estimation algorithm updated periodically with cross-link ranging measurements from other satellites. The Intersatellite Transfer System (ITS) of the NUDET system hosted on board the NAVSTAR satellites is used to provide the cross-link ranges. Each satellite makes pseudo-range measurements to other visible satellites from timing data broadcast on the ITS link. On the next ITS data link cycle, the pseudo-range measurements, ephemeris and clock data, and position and clock error covariance data are broadcast across the ITS link to all adjacent satellites for use in their AUTONAV solution. The AUTONAV algorithm on board the satellites is similar in many ways to the navigation algorithm in a GPS receiver. GPS receivers navigate by processing four or more pseudo-range and delta-range measurements to determine position and velocity and the receiver's clock errors. The AUTONAV algorithm processes the cross-link pseudo-ranges to determine the satellite's ephemeris parameters. As shown in the paper, there is a unique transformation that can be performed between the six Keplerian orbital parameters estimated in the AUTONAV filter, and the six position and velocity components which also describe the satellite's motion. Since the AUTONAV algorithm uses cross-link ranges to determine the satellite's ephemeris solution, its performance is sensitive to the satellite geometry as well as the accuracy of the range measurements. With conventional GPS navigation, the scale factor between the range measurement accuracy and the navigation performance is presented by the Dilution of Precision (DOP) figures of merit. DOP figures of merit can also be computed for the AUTONAV solution geometry provided by the cross-link range measurements. An AUTODOP algorithm is presented in this paper which computes the DOP figures of merit for the Block IIR satellite constellation. The conventional DOP algorithm is used to provide a simple yet effective method of determining the accuracy. of the GPS navigation service. The AUTODOP algorithm described in this paper will similarly provide a useful method of determining the accuracy of the AUTONAV solution. This algorithm will have applications in analyzing different Block IIR satellite constellations and in planning the Block IIR launch schedule. By computing the cross-link ranging DOPs, the optimum phasing of the minimum number of Block IIR satellites can be determined which provides sufficient geometry for AUTONAV to be operational. This paper describes the operation of the AUTODOP algorithm and compares the DOPs computed for the Block IIR constellation with the simulated performance of the AUTONAV algorithm.
Published in:	Proceedings of the 2nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1989) September 27 - 29, 1989 The Broadmoor Hotel Colorado Spring, CO
Pages:	291 - 293
Cite this article:	Brown, Alison, "An Autodop Algorithm for the Block IIR AUTONAV Capability," Proceedings of the 2nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1989), Colorado Spring, CO, September 1989, pp. 291-293.
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