Global Positioning System Reference
In-Depth Information
2. Multipath propagation
Multipath propagation occurs mostly due to reflected GNSS signals from surfaces (such
as buildings, metal surfaces etc.) near the receiver, resulting in one or more secondary
propagation paths. These secondary path signals, which are superimposed on the desired
direct path signal, always have a longer propagation time and can significantly distort the
amplitude and phase of the direct path signal. This eventually leads to a deformation in the
correlation function as shown in Fig.
1, where a direct LOS signal is added constructively
Impact of multipath on cor
relation shape
1.4
LOS Signal
NLOS Signal
Combined Signal
1.2
1
0.8
0.6
0.4
0.2
0
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
Code Delay [chips]
Fig. 1. Received correlation function in two path static channel model, path delays: [0 0.5]
chips, path amplitudes: [0 -3] dB, in-phase combination.
with an in-phase (i.e., 0
◦
phase difference), delayed (0.5 chips delayed) and attenuated
(-3 dB attenuated) version of it to form a compound signal. The deformed correlation
shape introduces an error bias in the pseudorange measurement that resulted in a degraded
positioning performance.
In severe multipath environments like those in dense urban areas, it may be possible that
the LOS signal is obstructed completely and only the reflected signals are present. These
multipath effects on the code phase measurements are most crucial, and the multipath error
can reach up to a few tens of meters, or a couple of hundred at most Gleason & Egziabher
(2009). Moreover, unlike other error sources, multipath cannot be reduced through differential
processing, since it decorrelates spatially very rapidly. All these issues are the main driving
factors for the research conducted in the context of this thesis striving for an optimum
correlation-based multipath mitigation technique in terms of mitigation performance as well
as implementation complexity.
3. Influence of signal and receiver parameters on multipath error
The way multipath affects the tracking and navigation performance of a receiver depends on
a number of signal and receiver parameters. Among them, the most influential parameters
are:
•
Type of signal modulation,
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