Long-Term Extended Ephemeris Prediction for Mobile Devices

Peter Stacey and Marek Ziebart

Abstract:	GNSS receivers in difficult signal reception areas can find it difficult to acquire the broadcast ephemeris from the satellites and hence determine a position fix. One method to alleviate this is to provide a satellite ephemeris to the receiver to use whilst waiting for the broadcast ephemeris to be received. In this paper a technique to produce extended ephemerides of GPS satellites for mobile devices will be presented. This solution only utilises previously acquired broadcast ephemeris parameters. This is an example of self-assisted GPS, in which an estimated ephemeris can be generated locally on a mobile device rather than provided by an external source. The GPS receiver in the mobile device can then utilise this estimate of the satellite position whilst waiting for the broadcast ephemeris to be acquired, which can take many minutes when performing a warm start in challenging signal reception environments. The time-to-first-fix is therefore reduced. The software takes the form of an orbit prediction algorithm as well as an orbit determination algorithm. Results are shown for these algorithms working on a desktop computer and on a mobile telephone. The orbit prediction algorithm utilises a suite of force models and a numerical integrator to produce an estimated ephemeris. The satellite orbit errors in the geocentric radial direction are 3 m (RMS) after 1 day, 9 m (RMS) after 3 days, and 21 m (RMS) after 7 days. An orbit determination algorithm is also presented that modifies the estimate of the satellite position at the start of the orbit prediction arc; orbit prediction from these modified initial conditions gives a modest improvement in the geocentric radial direction, but large improvements in the along-track direction. The orbit determination algorithm can be used to help the orbit prediction algorithm if the along-track error is seen to be too large.
Published in:	Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011) September 20 - 23, 2011 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	3235 - 3244
Cite this article:	Stacey, Peter, Ziebart, Marek, "Long-Term Extended Ephemeris Prediction for Mobile Devices," Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 3235-3244.
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