Global Positioning System Reference
In-Depth Information
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Multipath Mitigation Techniques for
Satellite-Based Positioning Applications
Mohammad Zahidul H. Bhuiyan and Elena Simona Lohan
Department of Communications Engineering, Tampere University of Technology
Finland
1. Introduction
The ever-growing public interest on location and positioning services has originated a
demand for a high performance Global Navigation Satellite System (GNSS), such as the
Global Positioning System (GPS) or the future European satellite navigation system, Galileo.
The performance of GNSS is subject to several errors, such as ionosphere delay, troposphere
delay, receiver noise and multipath. Among all these errors, multipath is the main limiting
factor in precision-oriented GNSS applications. The reception of multipath creates a bias
into the time delay estimate of the Delay Lock Loop (DLL) of a conventional navigation
receiver, which eventually leads to an error in the receiver's position estimate. In order to
mitigate the multipath influence on navigation receivers, the multipath problem has been
approached from several directions. Among them, the use of special multipath limiting
antennas (i.e., choke ring or multi-beam antennas), the post-processing techniques to reduce
carrier multipath, the carrier smoothing to reduce code multipath, and the code tracking
techniques based on receiver internal correlation function are the most prominent approaches.
In this chapter, the discussion is mainly focused on the correlation-based multipath mitigation
techniques at the receiver side; since the correlation-based multipath mitigation approach is
by far the most convenient and popular way to deal with multipath error for a stand-alone
GNSS receiver. The classical correlation-based code tracking structure used in GNSS is
based on a feedback delay estimator and is implemented via a feedback loop. The most
known feedback delay estimator is the Early-Minus-Late (EML) DLL technique, where two
correlators spaced at one chip from each other are used in the receiver in order to form a
discriminator function, whose zero crossings determine the path delays of the received signal
Baltersee et al. (2001), Bischoff et al. (2002), Chen & Davisson (1994), Fine & Wilson (1999),
Fock et al. (2001), Laxton (1996). The classical EML fails to cope with multipath propagation
Dierendonck et al. (1992), Simon et al. (1994). Therefore, several enhanced EML-based
techniques have been introduced in the literature during the last two decades in order to
mitigate the impact of multipath, especially in closely spaced path scenarios. One class of
these enhanced EML techniques is based on the idea of narrowing the spacing between the
early and late correlators, i.e., narrow EML (nEML) or narrow correlator Dierendonck et al.
(1992), Irsigler & Eissfeller (2003), McGraw & Braasch (1999). The choice of correlator spacing
depends on the receiver's available front-end bandwidth along with the associated sampling
frequency Betz & Kolodziejski (2000). Correlator spacings in the range of 0.05 to 0.2 chips are
commercially available for nEML based GPS receivers Braasch (2001).
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