C/A Code Pseudoranging Accuracy - How Good Can It Get?

Lawrence Weill

Abstract:	By now it is common knowledge that the achievable accuracy in measuring pseudorange using the C/A code has surpassed what was in the minds of the original designers of the Global Positioning System. Recently, at least one and possibly several manufacturers of GPS receivers seem to have achieved submeter-level and even decimeter-level C/A code pseudorange measurement accuracies in their equipment. There are significant implications if these accuracies are, in fact, valid. For example, centimeter level, real-time differential positioning might be made possible with an Ll -only receiver. In such a receiver, the L2 carrier phase information normally used to mitigate Ll carrier phase ambiguities could be replaced by the combination of the highly accurate C/A code positioning information in conjunction with the use of redundant satellites. How far can one go in the quest for better C/A code pseudoranging accuracy? If we knew that the above-mentioned receivers were essentially at a theoretical accuracy limit, costly and futile efforts at further development could be avoided by GPS equipment designers. On the other hand, experience has shown that if substantial improvement is known to be theoretically possible, the theory involved often suggests the signal processing techniques needed to approach the performance limit. In this paper, results using modern estimation theory techniques are presented to establish a theoretical limit on ranging accuracy using a spread-spectrum code. A useful by-product is the determination of a code tracking architecture to achieve the performance promised by theory. Comparisons will also be given of the theoretically optimum performance and that achieved with typical schemes implemented in today’s modern receivers.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	133 - 141
Cite this article:	Weill, Lawrence, "C/A Code Pseudoranging Accuracy - How Good Can It Get?," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 133-141.
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