Reconstruction of the Satellite Ephemeris from Time-spaced Snippets

Peter J. Duffett-Smith, Anthony R. Pratt

Abstract:	Satellite receivers incorporated into cellular telephones often need assistance to operate successfully. This is because of the weak satellite signal environments usually encountered wherever a cellular telephone is used, for example inside a building, or when the phone is carried around inside a bag or pocket in an undefined orientation with respect to the sky. There have been several papers at recent ION meetings about the Enhanced GPS method (EGPS) by which the signals from the cell towers themselves can be used to provide accurate time assistance, sufficient to span many minutes so that the satellite receiver always operates in re-acquisition mode [1,2]. It has been shown how the combination of GPS and GSM, for example, can maintain an autonomous integrated navigation solution throughout the idle-time when the phone is not being used to make a call such that its position is known within about 50 m at any time [3]. In a further extension of the idea, the inexpensive crystal oscillator in the cell phone can be trained to behave as if controlled by an atomic standard [4]. Cellular telephones have small batteries, so that a commercial product incorporating a satellite receiver requires the receiver to be off most of the time in order to conserve battery power. Best operation, as noted above, is in short bursts between long periods during which the navigation solution is maintained using the cellular signals. Yet the receiving of satellite ephemeris data, essential for accurate positioning, may take tens of seconds of continuous operation, and therefore is often sent on request to the cellular telephone from a GPS server in the network rather than attempting direct reception and decoding. This limits the use to networks having such assistance available, and usually requires subscription by the user to the assistance service (often known as A-GPS). With EGPS, the availability of fine time assistance at the micro-second level opens the possibility of reconstruction of the satellite ephemeris from short bursts of data spaced minutes apart. We show that there are limitations in this method which have a minimum duration requirement which result from the parity algorithms used in legacy GPS signals (C/A code on L1). Providing that the rules are satisfied, the full ephemeris information can be assembled over a period of time without the requirement for continuous operation. This can result is significant power savings. An added bonus is that an extension to the method can result is improved bit error rate (BER) under conditions of low carrier-to-noise levels. We provide a methodology to support this process in conjunction with fine time aiding using the cellular network signals. These methods are practical for use in cellular telephones, freeing the user from the need to subscribe to an A-GPS service, and enabling fully-autonomous operation.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	1867 - 1875
Cite this article:	Duffett-Smith, Peter J., Pratt, Anthony R., "Reconstruction of the Satellite Ephemeris from Time-spaced Snippets," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 1867-1875.
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