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A new multipath detection and mitigation approach for pseudolite systems

: Kurz, Oliver

Institute of Navigation -ION-, Satellite Division, Washington/DC:
26th International Technical Meeting of The Satellite Division of the Institute of Navigation, ION GNSS 2013. Proceedings : September 16 - 20, 2013, Nashville Convention Center, Nashville, Tennessee
Manassas/Va.: ION, 2013
Institute of Navigation, Satellite Division (ION GNSS International Technical Meeting) <26, 2013, Nashville/Tenn.>
Fraunhofer IIS ()

This paper discusses the evaluation of a new multipath detection and mitigation concept developed by the Fraunhofer in the frame of the iGOing project. This project aims at developing a pseudolite system that provides a precise positioning service even in harsh indoor environments. Previous studies involving pseudolite broadcasting GPS L1 C/A like signals have shown how degraded the indoor RF environment is and particularly how severe the multipath effects are. Therefore, the main goal of the iGOing project is to evaluate the benefits that can be brought to the system 1) by modifying the characteristics of the broadcast signal (e.g. using structure similar to that of the new Galileo E6 CS signal or using a different carrier frequency) and 2) by introducing new multipath detection and mitigation algorithms in order to improve produce cleaner measurements and more accurate position estimates. This paper focuses on the latter aspects. The idea is to combine two different and well tested modules in order to reach a new way to fight the effects of multipath. The first module is an advanced RAIM algorithm that operates at the PVT level and enables the detection of biased measurements and the second module is a multi-tap correlator (MTC) that operates at the baseband processing level and enables high-resolution monitoring of the correlation process. The idea is to then to feed the outputs of the RAIM algorithm to the tracking unit so that it is informed about the presence of a potentially corrupted measurement. With this information the tracking unit can make use of the MTC to closely monitor the suspicious channel. In the case of a positive multipath detection in the channel, the system can switch to a specific specialized discriminator to mitigate the effects of the multipath or, if enough measurements are present, completely discard the faulty measurement. Using the very versatile architecture of the MTC module it is possible to configure both the number (up to 32) of desired complex correlator output taps and their position on the correlation function. As suggested in previous studies, it will then possible to efficiently detect the presence of multipath by judiciously combining the correlator outputs and comparing the result with a threshold that indicates whether or not the combined correlation peak observations exceed their corresponding noise level. If this threshold is exceeded, it is assumed that the incoming signal is affected by noise and multipaths. With up to 32 complex correlator output taps, the set of potential metrics is extremely large. It can however be reduced using the multipath-to-noise ratio and the degree of correlation criterions. In other word, the best candidates are the metric 1) whose behavior in presence of multipath vastly diverged from their behavior in presence of noise only and 2) which are the less correlated with other possible metrics. Once the presence of a multipath is confirmed, a specialized discriminator will be used to mitigate the effects of the multipath. In particular, the benefits the Narrow Correlator (NC), High-Resolution Correlator (HRC), Early-Late Slope (ELS) or S Curve Shaping approach will be thoroughly assessed and compared. To validate the overall multipath detection and mitigation approach proposed here and confirm its applicability to indoor pseudolite scenarios, extensive test campaigns will be conducted. The concept will be tested in three phases: First, in a GPS L1 C/A-based urban canyon scenario, then in an indoor scenario using pseudolites broadcasting GPS L1 C/A like signals, and finally in an indoor scenario using pseudolites broadcasting Galileo E6 CS like signals. Using this incremental approach it will be possible to efficiently fine-tune the algorithm and to properly account for both the unique characteristic of multipath in indoor environments and their dependence on signal frequency and modulation.