Comparison of Multipath Mitigation Techniques with Consideration of Future Signal Structures

Code and carrier multipath is still a major issue for high-precision GNSS applications. While many system-inherent errors or errors resulting from the signal propagation characteristics can be modeled, calibrated or eliminated by use of special observation methods (e.g. by differencing code and/or carrier observations), multipath errors are not correlated between two receivers (even if the receivers are separated by only a few meters) and will thus not cancel out by differencing observations. Multipath is thus the dominating error source for many GNSS applications. In order to minimize the influence of multipath signals, several approaches can be taken into account. Among them are the use of special multipath limiting antennas (e.g. choke-ring or multi-beam antennas), post processing techniques to reduce carrier multipath (e.g. multi-antenna spatial processing [7]) or carrier smoothing to reduce code multipath. However, the most important multipath mitigation approaches are receiver-internal correlation techniques. This paper provides a survey of current receiver-internal correlation techniques that are able to minimize code and/or carrier multipath. The mitigation techniques discussed in this paper are the Narrow CorrelatorTM, the Double-Delta correlation techniques, the Early/Late Slope Technique (ELS) and the Early1/Early2 Tracker. While all of these techniques are able to reduce code multipath, only a few of them can also be used to mitigate carrier multipath. These correlation techniques are first introduced with respect to their basic functionality and implementation. In a second step, the performance of these multipath mitigation concepts is illustrated in the form of multipath error envelopes. Assuming a dedicated multipath scenario with one multipath component and a constant signal attenuation factor, this approach allows an intuitive comparison of the aforementioned mitigation techniques. In addition, the performance analysis for these mitigation techniques will also be carried out for different signal types. For this purpose, not only the current GPS signals are considered, but also signals of the modernized GPS and the future Galileo system (namely BOC signals). The main purpose of this paper is to summarize the most important efforts of (receiver-internal) multipath mitigation, to compare the performance of these approaches with respect to each other and to compare their performance when using different types of signals.