German Aerospace Center (DLR)
Institute of Communications and Navigation
Department Communications Systems
Broadband Systems Group

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Multipath Mitigation for GNSS Recievers

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Within global navigation satellite systems (GNSS), such as the Global Positioning System (GPS) or the future European satellite navigation system Galileo, the user position is determined based upon the code division multiplex access (CDMA) navigation signals received from different satellites using the time-ofarrival (TOA) method. A major error source for positioning comes from multipath, the reception of additional signal replica due to reflections caused by the receiver environment. The reception of multipath introduces a bias into the time delay estimate of the delay lock loop (DLL) of a conventional navigation receiver, which finally leads to a bias in the receiver’s position estimate. Multipath is today still one of the most crucial problems in GNSS, as the error is caused locally and can not be corrected through the use of correction data, which is provided by reference receiver stations or networks. In particular in urban environments multipath can cause a heavy degradation, since the line-of-sight (LOS) path can be affected simultaneously by blockage and shadowing, which increases the impact of the multipath significantly.

Media:

• Video showing a particle filter coping with the adverse propagation in an urban canyon [AVI]
• Go for the urban canyon in Google Earth [KMZ]

Algorithms for Multipath Mitigation:

The advances in the development of signal processing techniques for multipath mitigation have led to a continual improvement of performance, whereas basically two major approaches can be distinguished: The class of techniques that actually mitigate the effect of multipath by modifications of the antenna pattern (either by means of hardware design or with signal processing techniques) or by aligning the more or less traditional receiver components (e.g. the early/late correlator) and the class of multipath estimation techniques, which treat multipath (in particular the delay of the paths) as something to be estimated from the received signal, so that its effects can be trivially removed at a later processing stage. Most of the conventional mitigation techniques are in some way aligning the discriminator/timing error detector (TED) of the DLL to the signal received in the multipath environment. Well-known examples of this category are amongst others the Narrow Correlator and the Strobe Correlator. For the estimation techniques static and dynamic approaches can be distinguished, according to the underlying assumption of the channel dynamics. Examples for static multipath estimation are those belonging to the family of maximum likelihood (ML) estimators, where the probably best-known technique is the multipath estimating delay lock loop (MEDLL). Dynamic algorithms for estimation of time-varying synchronization parameters in spread spectrum systems have been suggested in the field of communications using the extended Kalman filter as well as the sequential Monte Carlo approach. For navigation systems estimators based on sequential importance sampling (SIS) methods (particle filtering) have been considered for static and dynamic scenarios. Aside from the aforementioned mitigation techniques various other concepts have been shown to improve the multipath mitigation performance in navigation systems, including the use of antenna arrays and array processing techniques for multipath and interference mitigation and concepts which are based on the exploitation of the mutual dependencies of the signal delays through the common position parameters like the vector DLL (VDLL), including the position domain ML estimator and joint positioning and mitigation tracking algorithms.

Channel recorded in an urban environment	Classification of multipath mitigation algorithms for GNSS receivers

A very promising approach to reduce the impact of multiapth is the use of sequential estimation techniques. In difference to the ML methods, where independent estimates are obtained for successive observation intervals, sequential apporaches allow to make explicit use of the channel’s temporal or spatial correlation. The following figures illustrate the benefit of a novel detect-and-track partcile filter algorithm, which was developed by DLR:

Urban pedestrian channel at low elevation: Heavy multipath and LOS blockage and shadowing.	Detection and tracking of two simultaneous echoes using a particle filter	Keeping track of a shadowed LOS during heavy multipath with a particle filter

Closely related to our work in the field of multipath mitigation are our activities on measuring, characterizing and modeling the satellite navigation channel in multipath prone environments (see Channel Modeling). The outcome of these activities serves as a sound basis for the development of novel mitigation algorithms such as the particle filtering methods.

Presentations:

• Joint CNES/DLR presentation on the simulation of advanced channel models at Navitec 2008 [PDF]
• Presentation on Bayesian filtering for multipath mitigation at ION GNSS 2007 [PDF]
• Presentation on maximum likelihood estimation for multipath mitigation at ION GNSS 2006 [PDF]

Papers:

You can follow the ELIB link to our internal electronic library where you can download the paper as PDF and retrieve citation information, e.g. for BibTeX.

• Bayesian Filtering and Sequential Monte Carlo Methods

• B. Krach, R. Weigel
  Markovian Channel Modeling for Multipath Mitigation in Navigation Receivers [ELIB]
  Proc. 3rd European Conference on Antennas and Propagation (EuCAP 2009),
  Berlin, Germany, Mar. 2009


• M. Lentmaier, B. Krach, P. Robertson
  Bayesian Time Delay Estimation of GNSS Signals in Dynamic Multipath Environments [ELIB]
  International Journal of Navigation and Observation (IJNO), Volume 2008
  Special Issue on Future GNSS Signals, Apr. 2008
  Article ID 372651, 11 pages, doi:10.1155/2008/372651


• B. Krach, M. Lentmaier, P. Robertson
  Joint Bayesian Positioning and Multipath Mitigation in GNSS [ELIB]
  Proc. 2008 IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2008),
  Las Vegas, Nevada, USA, Mar./Apr. 2008


• B. Krach, M. Lentmaier, P. Robertson
  Bayesian Detection and Tracking for Joint Positioning and Multipath Mitigation in GNSS [ELIB]
  Proc. 5th Workshop on Positioning, Navigation and Communication 2008 (WPNC 2008),
  Hannover, Germany, Mar. 2008


• M. Lentmaier, B. Krach, P. Robertson, T. Thiasiriphet
  Dynamic Multipath Estimation by Sequential Monte Carlo Methods [ELIB]
  Proc. 20th International Technical Meeting of the Institute of Navigation Satellite Division (ION GNSS 2007),
  Fort Worth, Texas, USA, Sep. 2007



• Maximum Likelihood Estimation

• B. Krach, M. Lentmaier
  Efficient Soft-Output GNSS Signal Parameter Estimation using Signal Compression Techniques [ELIB]
  Proc. 3rd ESA Workshop on Satellite Navigation User Equipment Technologies (NAVITEC 2006),
  Noordwijk, Netherlands, Dec. 2006


• M. Lentmaier, B. Krach
  Maximum Likelihood Multipath Estimation in Comparison with Conventional Delay Lock Loops [ELIB]
  Proc. 19th International Technical Meeting of the Institute of Navigation Satellite Division (ION GNSS 2006), pp. 1742-1751
  Fort Worth, Texas, USA, Sep. 2006



• Simulation of Advanced Multipath Channels

• B. Krach, G. Artaud
  Performance Assessment of Navigation Signals in Realistic Multipath Environments [ELIB]
  Proc. 4th ESA Workshop on Satellite Navigation User Equipment Technologies (NAVITEC 2008),
  Noordwijk, Netherlands, Dec. 2008


• F. Schubert, B. Krach
  Simulation of High-Realistic Multipath Environments: Developments and Applications [ELIB]
  Proc. 2nd EuNavTec Positions Congress 2008 (POSITIONs 2008),
  Dresden, Germany, Oct. 2008


• M. Lentmaier, B. Krach, T. Jost, A. Lehner, A. Steingass
  Assessment of Multipath in Aeronautical Environments [ELIB]
  Proc. 11th European Navigation Conference (ENC GNSS 2007),
  Geneva, Switzerland, May 2007