Multipath Corrections for a GPS Receiver

Charles R. Cahn, Mangesh M. Chansarkar

Abstract:	In a GPS receiver, multipath causes an error in the meas-urement of the pseudorange because it affects the delay-lock tracking loop, which functions to equalize the corre-lation powers at early and late time positions. Typically, the multipath component of the received signal is mod-eled as a delayed replica of the line-of-sight (LOS) signal with an arbitrary carrier phase shift relative to the LOS signal. The effect of multipath is to distort the correlation function in the receiver. If the multipath delay is more than about 1.5 PN chip, the multipath component has no effect on the delay-lock tracking of the LOS signal, be-cause the PN processing gain effectively discriminates against the multipath. However, there is still a possibility of erroneously tracking on the delayed correlation peak due to a multipath component. With the SiRF multiple-correlator receiver, such a condition of erroneous tracking can be detected by the existence of significant correlation values at taps early with respect to punctual. The errone-ous tracking can then be eliminated by sufficiently ad-vancing the tracking position until the early correlation values no longer exist. If the multipath delay is less than 1.5 PN chip, there may not be resolvable correlation peaks, and the multipath component will create a PN code tracking error in the delay-lock loop as a consequence of the assymetrical distortion of the correlation function, which is distorted based on the multipath component. A computational cor-rection to the pseudorange can be made proportional to the distortion. A computer study of the behavior of this computational correction demonstrates that a significant improvement in the pseudorange error is achievable par-ticularly for large lagging errors provided that the mul-tipath component is weaker than the LOS signal, as is typically the case. The correlation values include a base-line due to noise which must removed to do the computa-tional correction properly. In the SiRF multiple-correlator receiver, this can be done easily by measuring the base-line due to noise on a correlation tap sufficiently early with respect to punctual that there is guaranteed to be zero correlation with the received GPS LOS signal. Compari-son is made to the use of a narrowed early-late correlator spacing to reduce the error due to multipath.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	551 - 557
Cite this article:	Cahn, Charles R., Chansarkar, Mangesh M., "Multipath Corrections for a GPS Receiver," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 551-557.
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