Global Positioning System Reference
In-Depth Information
in order to determine accurately the shape of the multipath corrupted correlation function.
According to Townsend et al. (1995), MEDLL provides superior long delay multipath
mitigation performance compared to nEML at the cost of multi-correlator based tracking
structure.
A new technique to mitigate multipath by means of correlator reference waveform was
proposed in Weill (1997). This technique, referred to as Second Derivative correlator, generates
a signal correlation function which has a much narrower width than a standard correlation
function, and is therefore capable of mitigating multipath errors over a much wider range
of secondary path delays. The narrowing of the correlation function is accomplished by
using a specially designed code reference waveform (i.e. the negative of the second order
derivative of correlation function) instead of the ideal code waveform used in almost all
existing receivers. However, this new technique reduces the multipath errors at the expense of
a moderate decrease in the effective Signal-to-Noise Ratio (SNR) due to the effect of narrowing
the correlation function. A similar strategy, named as Slope Differential (SD), is based on the
second order derivative of the correlation function Lee et al. (2006). It is shown in Lee et al.
(2006) that this technique has better multipath performance than nEML and Strobe Correlator.
However, the performance measure was solely based on the theoretical MEE curves, thus its
potential benefit in more realistic multipath environment is still an open issue.
A completely different approach to mitigate multipath error is used in NovAtel's recently
developed Vision Correlator Fenton & Jones (2005). The Vision Correlator (VC) is based
on the concept of Multipath Mitigation Technique (MMT) developed in Weill (2002). It can
provide a significant improvement in detecting and removing multipath signals as compared
to other standard multipath resistant code tracking algorithms (for example, PAC of NovAtel).
However, VC has the shortcoming that it requires a reference function shape to be used to fit
the incoming data with the direct path and the secondary path reference signals. The reference
function generation has to be accomplished a-priori, and it must incorporate the issues related
to Radio Frequency (RF) distortions introduced by the front-end.
Several advanced multipath mitigation techniques were also proposed in Bhuiyan (2011),
Granados et al. (2005), Granados & Rubio (2000), Lohan et al. (2006). These techniques, in
general, offer better tracking performance than the traditional DLL at a cost of increased
complexity. However, the performance of these techniques have not yet been evaluated in
more realistic multipath channel model with real GNSS signals.
The rest of this chapter is organized as follows. Multipath propagation phenomena is
described first, followed by a description on the influence of signal and receiver parameters
on multipath error. The following section provides an elaborate description on different
multipath mitigation techniques starting from the conventional state-of-the-art techniques to
the relatively complex advanced multipath mitigation techniques. An extensive literature
review on the contemporary research on multipath mitigation techniques are also provided.
The performance evaluations of some of the multipath mitigation techniques are shown in
Section 6 via multipath error envelopes and also via simulations in multipath fading channel
model. Finally, some general conclusions are drawn based on the discussions provided in
earlier sections.
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