Modernizing GPS Autonomous Navigation with Anchor Capability

J.A. Rajan, P. Brodie, H. Rawicz

Abstract:	GPS Block IIR satellite payloads provide a number of innovations to the GPS system. One of these, Autonomous Navigation or AutoNav, provides the ability for the GPS satellites to self-navigate by interactively using data from the rest of the constellation. AutoNav was originally conceived as a technique to ensure survivability of the GPS system. This capability has been demonstrated at ITT and the Aerospace Corporation via years of simulations, ground testing, and background on-orbit testing. The GPS IIR satellites have also successfully demonstrated on-orbit payload re-programmability, essential for on-orbit improvements to the system. This paper discusses potential enhancements to AutoNav to substantially improve GPS survivability performance. In particular, we describe the concept of "anchoring" the GPS satellite constellations to precisely surveyed locations on the surface of the earth, such as Monitor Stations. This concept referred to as "AutoNav with Anchor" (patented [6]), provides significant enhancement to position and timing performance. The Anchor concept significantly enhances and improves the performance of extended (survivable) operations. The operational concept for implementing this capability heavily leverages existing system assets, thereby reducing the cost of implementation. Anchor Stations measure GPS pseudo ranges, delta ranges, and clock errors, to all visible satellites. These measurement residuals are piped back to the satellites for incorporation in their AutoNav Kalman Filter. In addition to increased performance, an additional effect of this technique is to simplify the amount of upload necessary to maintain highly accurate navigation (for normal, as well as for extended navigation). This reduces both life cycle cost and operational cost of the system. The Anchor concept also allows the use of highly precise ground-based references such as Hydrogen Maser clocks, as part of the satellite's Kalman Filter navigation computation (instead of solely through ground based uploads), to further enhance constellation performance. System level simulation results are provided demonstrating performance enhancements that are feasible using "AutoNav with Anchor". The paper also quantifies further performance enhancements that are obtained by using Multiple Anchors. The AutoNav with Anchor capability can be achieved directly by means of on-orbit software upgrades to the GPS IIR/IIR-M satellites. The concept similarly can be incorporated into the GPS IIF satellites.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1534 - 1542
Cite this article:	Rajan, J.A., Brodie, P., Rawicz, H., "Modernizing GPS Autonomous Navigation with Anchor Capability," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1534-1542.
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