Investigation of Multipath Mitigation Methods for High Precision GNSS Applications

A. Broumandan, Y. Zhang, A. Schultz

Abstract:	Multipath propagation causes major deficiencies to satellite based positioning and navigation systems. It is one of the major sources of accuracy degradation and a dominant issue for high precision Global Navigation Satellite System (GNSS) applications. Multipath propagation results in biased and corrupted GNSS measurements, which leads to inaccurate position estimates. Consequently, multipath mitigation techniques are essential for high accuracy positioning using pseudorange and carrier phase observations. Numerous methods have been proposed to alleviate the effects of multipath on measurements. The most popular multipath mitigation algorithms are narrow correlators (Van Dierendonck et al., 1992), double delta correlators (Irsigler and Eissfeller, 2003), and the multipath estimating delay locked loop (MEDLL) (Townsend et al., 2000). Double delta correlators represent a family of special discriminators that employs two pairs of correlators rather than one to estimate the code phase of line of sight (LOS) signal by compensating for the multipath influence significantly. NovAtel’s Pulse Aperture Correlator are implemented based on the concept of double delta correlators. These correlator-based multipath mitigation techniques have been used extensively in most high grade GNSS receivers. Although the implementation of these enhanced techniques reduces the multipath bias and the measurement noise on code pseudorange measurements as compared to the standard early-late correlator, the performance of these techniques is still limited in multipath environment. These algorithms, and many others, are based on the code phase difference between the LOS and a reflected signal. They do not take into account the difference in the carrier phase and other signal parameters of the received signals. Although the multipath effect can be reduced by all aforementioned multipath mitigation techniques, multipath error still affects the measurements. In another approach, the multipath affected signals would be detected and removed from the final position solution (Mubarak and Dempster 2010). This rejection may cause a significant effect on dilution of precision in some cases, however this rejection may be justifiable due to emerging new satellite positioning systems. With an increasing number of GNSS satellites becoming available and new signals being proposed, there may be around 50 satellites signals available at a given location by a multi-GNSS receiver. Since the number of satellites required for a navigation solution is lower than this, a GNSS receiver can afford to disregard satellite signals that are affected by multipath propagation. Mubarak and Dempster (2010) have suggested a method to detect multipath using a proposed parameter known as early late phase (ELP). This method is essentially based on measuring the phase difference between early and late correlators. This research shows that positioning error caused by multipath can be reduced by excluding the satellites detected as experiencing multipath from the navigation solution. However, this may cause a significant increase in dilution of precision in some cases. Schipper et al. (1999) have proposed another multipath detection method which is based on carrier phase and pseudorange measurements. A code-carrier measurement difference is then generated between the code pseudorange and the carrier phase measurements. This code carrier differences is then analyzed to determine if multipath components have affected the measurements or not. In order to reduce the multipath effect on measurements and improve the accuracy of the position solution in multipath environments (e.g. urban environments where the number of signal reflections could be larger than two and/or some of the signal reflections might be stronger than LOS signals), this paper analyses an improvement in different multipath mitigation techniques based on reducing the early and late correlator spacing to one sample. The multipath error improvement is characterized in measurement and position solution levels for a high precision GNSS receiver. It is well known that the multipath has the largest bias on measurements when it is in phase or out of phase with respect to the LOS signal. Due to the satellite/receiver motion, multipath induced bias on measurements has a periodic pattern whose frequency depends on the satellite-receiver dynamic where for a static user and reflector case can vary from 0.01 Hz to a few Hz (Sleewaegen 1997). This observation suggests that multipath propagation does not always induce a significant bias on measurements. In addition, due to satellite-receiver geometry and reflectors’ location the measurements from different satellites are not affected by multipath simultaneously or in the same magnitude. Considering these facts, one can implement a signal quality monitoring system by monitoring the singal amplitude variation and reject the one with highest multipath error. The proposed method will be analyzed based on a theoretical point of view as well as extensive measurements in multipath environments. The experimental measurements are based on receiving and analyzing live GPS signals in different multipath environments including urban canyon. A high performance wideband RF front-end designed in Nexteq Navigation is exploited to down-convert GPS raw signals to an intermediate frequency. The correlation and signal tracking processes are implemented in a Field-Programmable Gate Array (FPGA) based platform. The correlator spacing and the number of correlators in each channel are adjustable to accommodate different multipath mitigation techniques. Herein, the performance of the proposed method will be compared with a receiver utilizing narrow correlator, double delta and Combination of Squared Correlators (CSC). The narrow correlator method has been selected for comparison since they are the widely implemented in high precision receivers. The double delta correlator method has been chosen since it has of the best theoretical performance in mitigating multipath among all multipath mitigation techniques (Broumandan & Lin 2008). The performance of the proposed method will be compared with the aforementioned methods in measurement and navigation domains for high precision GNSS application. In addition, the developed system and method will be compared against a high precision commercial receiver utilizing the double delta correlator in position and measurement domains. The initial results reveal that the proposed method is able to detect and mitigate the affected satellites signals and improve the positioning performance. ? References: Broumandan, A., and T. Lin (2008) “Performance of GNSS Time of Arrival Estimation Techniques in Multipath Environments,” Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation. Falletti, E., B. Motella, M. Troglia Gamba (2012) “Multipath Mitigation in SoL Environments Using a Combination of Squared Correlators,” Pproceedings of the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation. Irsigler, M., and B. Eissfeller (2003) “Comparison of Multipath Mitigation Techniques with Consideration of Future Signal Structures,” Proceedings of the 16th International Technical Meeting of the Satellite Division of the U.S. Institute of Navigation, Portland, September 9-12, pp. 2584-2592. Mubarak, O. M. and A. G. Dempster (2010) “Exclusion of Multipath-Affected Satellites Using Early Late Phase,” Journal of Global Positioning System, Vol. 9, No.2 :145-155, DOI: 10.5081/jgps.9.2.145 Schipper, B. W., S. I. Sheikh, and L.C. Vallot (1999) “GPS Multipath Detection Method and System,” US Patent, 29 Jun, US005917445A. Sleewaegen, J., M. (1997) “Multipath Mitigation, Benefits from using the Signal-to-Noise Ratio,” Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation. Townsend B., J. Wiebe, and A. Jakab (2000) “Results and analysis of using the MEDLL receiver as a multipath meter,” Proceedings of the U.S. ION National Technical Meeting, Anaheim, California, January 26-29, pp. 73-79. Van Dierendonck A. J., P. Fenton, and T. Ford (1992) “Theory and performance of narrow correlator spacing in a GPS receiver,” Journal of the U.S. Institute of Navigation, Vol. 39, No. 3, pp. 265-283.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3221 - 3230
Cite this article:	Broumandan, A., Zhang, Y., Schultz, A., "Investigation of Multipath Mitigation Methods for High Precision GNSS Applications," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3221-3230.
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