Design of an Adaptive Vector-tracking Loop for Reliable Positioning in Harsh Environment

S.F. Syed Dardin, V. Calmettes, B. Priot, J-Y. Tourneret

Abstract:	Navigation in GNSS denied environment is a great challenge for GNSS receivers because of possible interference and signal blockage. When high localisation performance is expected complementary sensors, such as inertial sensors, odometers are often used. They provide a navigation solution with an error which drifts over time, and a GNSS receiver which provides absolute measurements is often required to achieve sensor calibration. When a commercial off-the-shelf receiver is used, pseudoranges (PRs) are often considered and lead to an inconsistent position estimator in presence of multipath (MP). Several approaches propose a statistical measurement model for processing PR measurements in constrained environment (in [1] the PR error is assumed to follow a Gaussian distribution whose mean and variance are arbitrary). However, performance levels are dependent on the used receiver. Moreover the integrity requirements are not satisfied as these techniques do not take into account the way the PRs are estimated inside the receiver. The aim of this paper is to present a GNSS receiver able to mitigate multipath effects in order to provide reliable PR measurements. Such a receiver could be efficiently used to calibrate any dead reckoning sensor, or to operate in a standalone mode. The proposed receiver is based on basic vector architecture as in [2] or [3]. The main drawback of a vector-based receiver is that any tracking error in one channel can potentially adversely affect other channels, and thus lead to an increasing degradation in the navigation performance. The main objective of this study is to propose specific processing to improve tracking loop performance and to investigate the robustness, the availability and the reliability of the designed receiver. The core of the receiver is the navigation system (NS) which is based on an extended Kalman filter. This filter uses a cinematic model of the vehicle (an IMU could also be used to improve the system model) and observations to provide an estimator of the vehicle position and velocity, as well as the covariance matrix of this estimator. In the framework of this study, measurements consist of PRs obtained from efficient GNSS signal tracking techniques. The major significance of this work is that an adaptive vector tracking scheme (associated with specific processing which takes advantage of the global navigation solution) allows 1) each GNSS tracking channel to provide a reliable estimate of PR measurement in presence of MP, or 2) any contaminated PR measurement to be detected. This study considers slow fading channels for which multipath can be classified as coherent or non-coherent depending on the value of the Doppler frequency with respect to that of the direct path (DP). Different techniques have been proposed to mitigate multipath impact within the receiver tracking loop. The most efficient methods use high resolution time-frequency decomposition [4], or channel deconvolution [5]. However, these methods still exhibit a high computational complexity. Here the tracking loop is improved by considering 1) a phase-locked loop (PLL) assisted by a frequency-locked loop (FLL) which uses an FFT-based frequency discriminator and 2) the knowledge of the Doppler frequency distribution deduced from the vehicle velocity estimated by the NS, for decorrelating non coherent MP in the frequency domain. The major idea of the proposed algorithm is to explore the MP spectrum from the FFT output and to manage tracking loop bandwidth and integration time depending on the lowest value of the multipath frequencies. This adaptive processing allow the loop to track efficiently the direct path, i.e., to accurately estimate its phase and frequency in presence of non-coherent multipath. Consequently a conventional coherent delay-locked loop (DLL) can be used to achieve DP delay estimation. Indeed non-coherent multipath mitigation is performed through the coherent integration. In case of coherent MPs, the errors of the delay discriminator have to be considered and multipath decorrelation (which needs to estimate DP and MP delay) is complex [6]. In this study, a bank of correlators is used to implement a multiple double delta structure [7] (with different chip spacings). Taking advantage of the knowledge of the a priori delay distribution deduced from the vehicle location mean and variance computed by the NS, this structure allows the receiver 1) to detect MP, 2) to reduce the delay estimator error and 3) to verify the integrity of the delay discriminator output data. Moreover, in presence of contaminated discriminator data, the NS outputs are used to aid the code tracking loop to achieve code alignment, which allows the FLL-assisted PLL to provide a relevant estimate of the DP Doppler frequency. The proposed adaptive vector-based receiver (GPS L1 C/A signal is considered) is implemented to carry out simulations for different multipath fading channels. Improvements resulting from this architecture are discussed. The analysis conducted in this paper will address with much attention the PR measurement reliability, the PLL robustness and the receiver performance. [1] M. Spangenberg, V. Calmettes, O. Julien, J.-Y. Tourneret and G. Duchâteau, “Detection of variance changes and mean value jumps in measurement noise for multipath mitigation in urban navigation”, Journal of the institute of navigation, vol. 57, no. 1, pp. 35-52, April 2010. [2] M. Petovello and G. Lachapelle, “Comparison of vector-based software receiver implementations with application to ultra-tight GPS/INS integration”, ION GNSS 2006. [3] H. Won, D. Dötterböck, B. Eissfeller, “Performance Comparison of Different Forms of Kalman Filter Approach for a Vector-Based GNSS Signal Tracking Loop”, Journal of the Institute of Navigation 2010, Vol 7, N° 3, pp 189-199. [4] N. Sokhandan, A. Broumandan, J. T. Curran and G. Lachapelle, “High resolution delay estimation for urban GNSS vehicular navigation”, ION GNSS 2012. [5] K. Drag?nas and K. Borr, “Multipath Mitigation Based on Deconvolution”, Journal of Global Positioning Systems, Vol. 10, No. 1, pp. 79-88, 2011. [6] M. Spangenberg, V. Heiries, A. Giremus, V. Calmettes, “Multi-Channel Extended Kalman Filter for Tracking BOC modulated signals in the presence of multipath”, ION GNSS 2005. [7] G. A. McGraw and M. S. Braasch, “GNSS multipath mitigation using gated and high resolution correlator concepts”, in Proceedings of the he ION NTM ’99.
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:	3548 - 3559
Cite this article:	Dardin, S.F. Syed, Calmettes, V., Priot, B., Tourneret, J-Y., "Design of an Adaptive Vector-tracking Loop for Reliable Positioning in Harsh Environment," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3548-3559.
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