Multipath Mitigation by Voting Channel Impulse Response in Navigation Domain with High-sensitivity GNSS Receivers

Z. He, M. Petovello

Abstract:	With the emergence of multiple GNSS constellation capability in GNSS receivers, there is an increasing demand for more accurate location based services (LBS). However, multipath induced errors are still one of the most significant error sources for many GNSS applications and hinder more accurate LBS for either pedestrian or vehicular users. Multipath mitigation problems have been studied for decades and can be tackled in several aspects. Among them, the most popular ones are the use of special multipath-resistant antennas such as chokering antenna, and post-processing techniques that are based on correlator outputs. For example, a standard GNSS receiver commonly uses an early-minus-late (EML) discriminator to estimate the path delay of the incoming signal. In order to improve the resolvability of the direct and multipath signal delays, well-known narrow correlator can enhance the receiver’s resolvability of multipath given a reasonably set front-end bandwidth and associated sampling frequency. Variants of narrow correlator techniques are the so-called double-delta technique that uses more than early, prompt and late correlators to estimate the delay, strobe correlator technique, pulse aperture correlator (PAC), multipath estimating delay lock loop (MEDLL) to name a few. As for high-sensitivity applications, multiple correlators can be used in order to suitably resolve the multipath with reasonable front-end bandwidth, sampling frequency according to the multipath statistics. For example, inverse filter and inverse correlation methods have proven to be effective. Due to the fact that above mentioned techniques is based on correlator outputs, the basic problem is actually de-convolution. The observed correlator outputs are the convolution sum of the filtered correlation function with the multipath propagation channel impulse response (CIR). The shape of the filtered correlation function is often limited by signal modulation scheme (such as BPSK and BOC), transmitter/front-end filtering, and local oscillator quality etc. Many of the abovementioned techniques are intended to increase the slope of the new ‘correlation function’, which will benefit the resolvability of multipath. In recognizing that current multipath mitigation methods are all based on single channel operation (i.e., they operate temporally) , it is inevitable that either the low signal SNR or the dominance of multipath will cause difficulties to estimate multipath and result in inaccurate observations. This paper explores the benefits of the position domain or navigation domain receiver architecture to resolve multipath, since it actually uses all channel information simultaneously (i.e., it operates spatially). First, single satellite CIRs (i.e., multipath parameters) are estimated by standard de-convolution methods. Even though this might not be highly accurate due to the limitation of the hardware or the multipath statistics, this is nevertheless a reasonable approach in a practical sense. Then, the estimated CIRs are projected onto position domain. In this way, single CIRs are voting in the position domain. Two metrics are used for direct and multipath signal estimation in position domain: the magnitude of the summed CIRs and the total occurrences of CIR within a region. Second, the spatial correlation function is derived and the relationship between the spatial correlation and the standard single correlation function is given. Initial experimental results show that the slope of the satellite’s new ‘correlation function’ represented in navigation domain is increased as compared to conventional scalar receivers, which indicates better resolvability of multipath. This matches the theoretical development that has recently been completed. The gain depends on the geometry of the satellites and the SNR of each satellite signal. Finally, the third metric, spatial channel impulse responses (SCIR) are estimated from multiple spatial correlators (such as in east, north and up axis) and are used to resolve the multipath with respect to direct signal in the position domain. The effectiveness of either one or a combination of these three metrics will be discussed with different scenarios. Comparison will be made between the maximum likelihood estimated and these de-convolution methods used in temporal, spatial domain. Initial results demonstrate that the resolvability of multipath by using spatial de-convolution is improved and that by using the metrics defined above, the reliability of the navigation solution and signal tracking is also improved. More real data will be collected in typical multipath environments such as urban canyon and foliage. Field test results will be provided in order to validate the algorithms proposed herein.
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:	2749 - 2758
Cite this article:	He, Z., Petovello, M., "Multipath Mitigation by Voting Channel Impulse Response in Navigation Domain with High-sensitivity GNSS Receivers," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2749-2758.
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