Correcting GNSS Multipath Errors Using a 3D Surface Model and Particle Filter

T. Suzuki, N. Kubo

Abstract:	Currently, the availability of global navigation satellite systems (GNSSs) is anticipated to improve because of the presence of various positioning satellites. However, because of the serious impact of multipath signals such as reflection and the diffraction signals caused by geographic features in urban environments, such improvements in the availability of satellite positioning do not necessarily also facilitate high precision positioning. The multipath error effect is highly dependent on the shape and geometry of geographic features (such as tall buildings) near a GNSS receiver and therefore cannot be addressed by differential GNSS techniques that attempt to remove most of the errors in GNSS positioning. Various practical and popular signal correlator techniques can also help mitigate multipath errors. However, when an antenna cannot receive a direct signal, these techniques do not provide satisfactory results because they presume that the antenna principally receives both direct and multipath signals. Urban environments include many locations where non-line-of-sight (NLOS) satellite signals are obstructed by buildings, making it difficult to determine and mitigate such errors from the signals alone. At the ION GNSS 2012 conference, we proposed a novel GNSS positioning technique that can be used in multipath environments [1]. The proposed technique attempts to estimate a user’s position, without pseudoranges containing multipath errors, by comparing GNSS signal strengths that have been simulated using 3D surface models of urban canyon environments. The GNSS signal strength is calculated by the multipath propagation based on a ray-tracing technique. Our results showed that the proposed technique worked well in a real urban canyon environment. In this paper, we further improve this technique. Rather than comparing the signal strengths to estimate user position, we directly correct the observed pseudorange containing multipath errors using the simulated signal delay and strength from the 3D surface model. The multipath error in the observed pseudorange depends on a signal correlator design that is implemented in GNSS receivers. Therefore, to correct the multipath errors using the simulated signal delay and strength, it is necessary to modify the signal correlator of GNSS receivers. Consumer GNSS receivers cannot be used for this purpose, so we use a GNSS software receiver to realize the proposed techniques in this paper. A serious barrier to using multipath simulation techniques to improve real-world positioning accuracy stems from the fact that a precise LOS vector and signal obstruction cannot be calculated when the user position is unknown because ray-tracing results are highly dependent on the user’s position. In other words, to simulate the true multipath error at the user’s position, it is necessary to accurately predetermine that position before simulation. We solve this problem by using a particle filter that provides a numerical approximation of the estimated states of a set of weighted random samples, called particles. Differing user position estimates are represented by these particles. We then evaluate the likelihood of each particle by comparing observed and simulated pseudoranges. Our algorithm is executed as follows: (i) User position estimates are created as particles; the state vectors of these particles are composed of random 2D positions, x and y, with the altitude of each particle calculated from the 3D terrain data. The particles are created at initial positions with a specified distribution, and the mean position of the particles is then determined using a position solution obtained by conventional single-point positioning. The particles created in this initial step are distributed over a large region. (ii) The LOS vector can be computed at each particle position using the satellite positions obtained from broadcast GNSS ephemeris data. We determine the signal refraction or diffraction point on the 3D surface using the relationship between the LOS and 3D model to estimate the signal delay and strength. To reduce complexity and computational load, this paper does not assess multipath signals with several reflections and diffractions. The multipath error at each particle location can be simulated using the estimated signal delay and strength, as well as the configuration of the signal correlator. (iii) For each particle, the simulated pseudorange, which includes simulated multipath errors, is compared with the observed real pseudorange. Particles with simulated pseudoranges close or equal to the actual pseudorange are considered to be the closest to the true position based on a probabilistic model of pseudorange matching. (iv) Finally, the weight of each particle is updated based on its likelihood, and all the particles are resampled based on their new weights. Particles that are sufficiently close to the true position survive this resampling step, and the average position of the remaining particles is taken as the final estimated user position. Using this technique, we can estimate the user position even if the observed pseudorange has a large multipath error or the GNSS receiver receives only NLOS (reflected or diffracted multipath) satellite signals. To confirm the effectiveness of the proposed technique, a positioning test was performed in a real-world urban canyon environment. We set up ground control points, which were predetermined high-accuracy positions, to compare the positioning accuracy. These results show that the proposed technique is effective and offers increased positioning accuracy within urban canyon environments that suffer from large reflection and diffraction multipath errors in GNSS signals. [1] Taro Suzuki, Nobuaki Kubo, GNSS Positioning with Multipath Simulation using 3D Surface Model in Urban Canyon, Proc. of ION GNSS 2012, pp.438-447, 2012.
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:	1583 - 1595
Cite this article:	Suzuki, T., Kubo, N., "Correcting GNSS Multipath Errors Using a 3D Surface Model and Particle Filter," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1583-1595.
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