Ch 22. Robust Positioning in the Presence of Multipath and NLOS GNSS Signals
Gary McGraw, Paul Groves, Benjamin Ashman
Chapter Overview:
Multipath interference and non-line-of-sight (NLOS) reception are phenomena that all GNSS receivers must contend with, and for many applications they are the dominant error sources. This chapter explains how these errors arise and provides a survey of the multitude of techniques available to mitigate them.
Multipath mitigation techniques include antenna siting to avoid multipath; antenna types that enhance direct signals and attenuate reflected signals, particularly for fixed sites; adaptive antenna array processing; correlation signal processing; measurement processing techniques like carrier smoothing; navigation processing to de-weight or exclude measurements impacted by multipath; and post-processing and modelling techniques that provide estimates to correct multipath errors. Applicability of these techniques to different GNSS receiver types varies greatly, with mobile phones being especially constrained.
NLOS reception is an additional challenge faced by many applications, especially for users in urban environments. Many receiver multipath mitigation techniques, including antenna and signal processing approaches, do not address NLOS reception. However, navigation processing techniques that help to de-weight or exclude multipath can also be adapted to mitigate NLOS reception. Furthermore, 3D mapping-aided techniques, such as shadow matching, are an example of how signal propagation modeling can be applied to actually use NLOS reception and signal blocking as sources of positioning information.
The chapter has the following sections:
22.1 Introduction
22.2 Characteristics of Reflected Signals, NLOS Reception, and Multipath Errors
22.3 Receiver-Based Multipath Mitigation.
22.4 Carrier Smoothed Code
22.5 Real-time Navigation Processor-based NLOS and Multipath Mitigation
22.6 Post-processing Techniques for Multipath Mitigation
22.7 3D Mapping-Aided GNSS
22.8 Summary
References
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Fig. 22.1: Multipath interference and NLOS reception.
Multipath interference occurs when a signal is received via multiple paths. Within the GNSS community, it is commonplace to classify NLOS reception as multipath. However, the two effects are not the same; their error characteristics are quite different. As a reflected path is always longer than the direct path, NLOS reception always results in a positive ranging error that is independent of the signal and receiver design. By contrast, the coherent nature of multipath interference can produce both positive and negative ranging errors and these vary with the signal and receiver designs.
Fig. 22.6(b): Code multipath error envelopes for different code types.
The maximum positive and negative code pseudorange errors as a function of multipath delay for BPSK(1), BPSK(10), and BOC(1,1) code types. Receiver front end band-limiting with a baseband-equivalent 10 MHz low pass filter is modeled. The simulation assumed a multipath-to-direct amplitude ratio of a=0.2 and a correlator spacing of 1/20 of a BPSK(1) code chip.