Advanced Techniques for Processing GPS Carrier Measurements in Space Applications

Sasha Draganov and Lin Haas

Abstract:	This paper presents advanced GPS-based algorithms, which improve the navigation accuracy of an orbiting spacecraft and dramatically shorten convergence times. This is achieved by processing time-differenced Accumulated Delta-Range (ADR) measurements in a Kalman filter in addition to pseudorange measurements. The paper shows that lower navigation errors can be achieved if ADR measurements used in a single filter update are separated by a relatively long interval, e.g. 100 seconds. The difficulty in using time-differenced ADR measurements is that they are applicable at different time epochs, whereas a typical Kalman filter application requires measurements applicable at the time of the state vector update. This difficulty is overcome by a mathematical formulation which accurately maps an ADR difference into an instantaneous measurement. This mapping utilizes the orbit propagation model in deriving the measurement equations. The potential of the new algorithm is demonstrated in a software simulation of GPS-based navigation of a geostationary spacecraft. For a geostationary spacecraft, the problem of having an accurate velocity estimate is especially acute. The simulation scenario limits tracking of GPS satellites to those which have a line-of-sight and where the predicted link budget is sufficient for acquisition and tracking. The estimated state includes the user position, velocity, and user clock states. The propagation model neglects atmospheric drag and solar radiation pressure effects, but includes the gravitational effects of the sun and moon. Modeled measurement error sources include receiver noise and GPS satellite clock and ephemeris errors. The noise level for each measurement is scaled to account for different levels of the signal-tonoise ratio at the receiver, which in turn depends on the distance and mutual orientation of the GPS and geostationary satellites. The simulation shows that this new ADR processing technique improves the navigation accuracy by an order of magnitude and markedly shortens filter convergence time.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	2035 - 2043
Cite this article:	Draganov, Sasha, Haas, Lin, "Advanced Techniques for Processing GPS Carrier Measurements in Space Applications," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 2035-2043.
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