Performance analysis of GNSS multipath mitigation using antenna arrays

The Journal of Global Positioning Systems Vagle et al. The Journal of Global Positioning Systems (2016) 14:4 DOI 10.1186/s41445-016-0004-6 ORIGINAL ARTICLE Open Access Performance analysis of GNSS multipath mitigation using antenna arrays Niranjana Vagle* , Ali Broumandan, Ali Jafarnia-Jahromi and Gérard Lachapelle Abstract Multipath affects the shape of the correlation function and results in biased pseudorange measurements and erroneous navigation solutions. Antenna array processing, which uses signal spatial characteristics, is an effective method to mitigate various types of interference signals. However, the performance of most of the distortionless beamforming techniques degrades in multipath conditions due to the correlation between the desired Line of Sight (LOS) signal and multipath signals. This paper characterizes the performance of different beamforming techniques to mitigate multipath signals through the processing and analysis of simulated and actual data. The main novelty is the investigation of multipath mitigation performance of practically realizable antenna array-based GNSS receivers when the beamforming process is completely integrated into the tracking module after de-spreading. Beamforming techniques such as Delay And Sum (DAS) beamforming, Minimum Power Distortionless Response (MPDR) with and without spatial smoothing are considered. A novel multi-antenna simulator test-bed is developed to generate multipath signals for a multi-antenna platform. A software multi-antenna GPS receiver incorporating different beamforming techniques is then developed to generate pseudorange measurements and position solutions. Carrier-to-Noise ratio (C/N0), pseudorange errors and position solutions before and after beamforming are compared to show the effectiveness of different beamforming techniques to mitigate multipath. Results with simulated and actual GPS signals show improved performance using the MPDR beamformer with spatial smoothing. The utilization of spatial processing results in a pseudorange error reduction of up to 60 % and a position error reduction of up to 30 %. Keywords: GPS, Multipath, Beamforming, MPDR, Software simulator Introduction Although modern GNSS receivers provide high accuracy positioning and navigation solutions in open sky conditions, multipath remains a major error source in many environments. Multipath results in a distorted correlation function that is used to estimate delays and pseudoranges. This results in erroneous navigation solutions. Multipath also leads to incorrect ambiguity resolution affecting carrier phase positioning. If the multipath pseudorange error becomes large, the initial position solution is biased and the carrier phase ambiguity search space can be enlarged, resulting in longer ambiguity resolution time (Joosten et al. 2002). Long-delay code multipath caused by distant reflectors can be mitigated using currently available advanced correlator techniques such as * Correspondence: Plan Group, Department of Geomatics Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB T2N1N4, Canada Narrow Correlator™ (Dierendonck et al. 1992), Multipath Estimating Delay Locked Loop (MEDLL) (Van Nee et al. 1994) and Edge Correlator (Garlin et al. 1996) to name a few. However, multipath due to nearby reflectors is still a major problem for correlator-based techniques. Antenna array processing, a signal processing scheme that exploits the signal spatial features, is proven to be effective in mitigating different types of interference. Even though antenna array processing is well studied for wireless communication systems, the application of these techniques to GNSS differ from those systems. For instance, in most wireless communication systems, increasing the signal to noise ratio to reduce bit error rate is the main focus; for GNSS the focus is to improve time-delay estimation to improve estimated position accuracy. The effectiveness of different beamforming techniques for GNSS applications was studied in (Fern’andezPrades et al. 2016; Gupta et al. 2016; Broumandan et al. 2016; Cuntz et al. 2016; Amin et al. 2016; Daneshmand et al. © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Vagle et al. The Journal of Global Positioning Systems (2016) 14:4 2014; Arribas et al. 2014; Egea et al. 2014; Kalyanaraman and Braasch 2007; Kalyanaraman and Braasch 2010). Most of the distortionless beamforming techniques are developed with the assumption that there is no correlation between desired and interference signals. However, performance of these beamforming techniques degrades in multipath interference because there is a high degree of correlation between desired and multipath signals (Van Trees 2002). The effectiveness of antenna arrays to mitigate multipath interference has been studied through different robust beamforming techniques in GNSS applications (Brown 2000; Fu et al. 2003; SecoGranados et al. 2005; Sahmoudi and Amin 2007; Konovaltsev et al. 2007; Vicario et al. 2010; Fern’andezPrades et al. 2011; Daneshmand et al. 2013a; ManosasCaballu et al. 2013; Rougerie et al. 2011; Rougerie et al. 2012; Lee and Hsiao 2008). Sahmoudi and Amin (2007) used adaptive beamforming and high resolution direction finding methods to improve robustness against multipath and electronic interference. Vicario et al. (2010) analyzed robust beamforming techniques for Galileo ground stations and shown a reduction of tracking errors by 47 %. Fernández-Prades et al. (2011) studied the inherent capability of different eigen beamforming techniques to mitigate multipath through simulations. Some of these techniques assume either a linear array or a large planar array which is not however feasible for practical applications. Efficient maximum likelihood techniques to mitigate multipath are not practical for many applications due to their high computational burden. Even though the results from the previous research have shown that effective multipath mitigation is possible, the performance of antenna array based GNSS receivers in terms of time-delay estimation and position accuracy has not been analyzed extensively. Such performance is therefore assessed herein in terms of measurement and position accuracy through different beamforming techniques. The focus is on short-range multipath signals with specular reflections. As GNSS signals are below the noise level before the correlation process, spatial processing to mitigate multipath signals is mostly performed after the de-spreading process (i.e., correlation and Doppler removal) (Arr (...truncated)