Tracking Error Correction Algorithm in case of Quasi-Stationary C/A Code Interference

Laurent Lestarquit and Stephane Collet

Abstract:	Because C/A Gold code don’t have a high cross- correlation isolation, mutual code interference can cause false acquisition, C/No degradation and tracking error. There has been several studies on false acquisition and C/No degradation, but there has been very limited studies on tracking error, on which this paper will focus. The reason tracking error due to mutual C/A Gold code interference has been so little studied is that it has a very low occurrence for common GPS applications, because the relative Doppler has to be lower than the receiver code loop noise bandwidth (less than 1 Hz typically). For common GPS application, this value is very low compared to the Doppler distribution of the GPS signal due to the satellite motion, ranging from – 4,5 kHz to 4,5 kHz for a standing user. That's why tracking error due to mutual C/A code interference are very rare for common GPS application. However, there are 2 new positionning applications for which C/A signal will coexist with zero relative Doppler (quasi-stationary signals) : - The ranging signal broadcasted from GEO's (WAAS or EGNOS), when two or more GEO satellites are in common view, with a standing receiver. This is because a geostationary satellite remains at the same point in space relative to the earth. - The Ranging Per Pseudolite (RPP) experiment, conducted by CNES, in which the STENTOR geostationary satellite will use a GPS receiver to track 3 C/A signals coming from pseudolite on the ground. When the relative Doppler between two GPS signal is close to zero, the incomming C/A code are quasi-stationary, their relative phases have a slow variation, and the consequence is a slowly evolving, multipath like, tracking error. With equal power in the interfering signals, the tracking error can reach up to 18 meters. In case the interfering signal power is 10 dB higher, due for example to the receiver antenna gain pattern, the tracking error can even reach 55 meters. Note that both signal interfere and cause errors to one another. When the relative Doppler increases, but remains under the code loop bandwidth, the tracking error will be filtered in part. Because all the interfering signals are tracked on independant receiver channels, it is possible to identify the tracking error and correct it. The purpose of this paper is a correction algorithm that can be applied to the raw measurements. The receivers gives the relative code and phase offset, the relative Doppler and the relative power of both the useful and the interfering signals. Knowing the code cross-correlation function, it’s possible to predict the code and phase measurement error, and thus to correct it. If the relative Doppler is slightly different from zero, the filtering effect of the loop filters has to be taken into account. This correction technique has been tested on measurement collected using scenarios on a GSS GPS simulator and a receiver.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	2323 - 2329
Cite this article:	Lestarquit, Laurent, Collet, Stephane, "Tracking Error Correction Algorithm in case of Quasi-Stationary C/A Code Interference," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 2323-2329.
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