Autonomous GNSS Data Extraction in High Signal Attenuation Conditions

Anthony R. Pratt and Charles R. Offer

Abstract:	The reception of GNSS signals under conditions of high attenuation in the transmission path has been well studied over the last few years. A significant remaining problem is associated with the demodulation of the data signal containing, inter alia, time and ephemeris parameters. This has been partially solved by the pragmatic approach of providing this information through a communications link which is sometimes available to the GNSS receiver. This technique has been subject to intense study for the cellular wireless communication applications and this has produced a large number of patents covering innovative aspects to the various solutions. There are two key aspects to performing data demodulation at low signal levels. The first of these is to determine from the signals the epoch at which data bit transitions may occur. This information is useful even if data demodulation cannot be made as it permits a near 2.5dB improvement in receiver sensitivity through extended data transition-free coherent integration periods. The second aspect to data demodulation at low signal levels is the possibility to perform data integration to improve the bit error rate. The design of the new L1C data format has been specifically focused on providing for this eventuality. However, the same technique has not been successfully applied to other GNSS signals. The paper examines two aspects of the necessary algorithms to improve data sensitivity. Simple methods are analysed to improve data bit transition detection to level close to the threshold sensitivity of high sensitivity GPS receivers. Two methods of improving the BER in data extraction are examined based on a maximum likelihood approach using known characteristics of the GPS data structure. These show that worthwhile improvements can be made in the data extraction threshold without using an associated communications system for either data bit transition timing or as a source of satellite data.
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:	1884 - 1893
Cite this article:	Pratt, Anthony R., Offer, Charles R., "Autonomous GNSS Data Extraction in High Signal Attenuation Conditions," 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. 1884-1893.
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