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Overview
Multipath occurs when GPS signals arrive at the receiver after being reflected off some
object. The reflected signals always travel a longer path length than the direct signal.
This leads to measurement errors in the receiver which is trying to measure the direct path
length to the satellite. The techniques for rejecting the reflected signals are know as
multipath mitigation.
The GG Surveyor implements two types of correlators for multipath mitigation: Edge
Correlator™ and Strobe Correlator™. Both these correlators improve multipath
mitigation over the traditional correlator schemes with standard (1-chip) correlator
spacing and narrow (1/10 chip) correlator spacing.
The Edge Correlator is standard with all products from the GG family. The performance
of an Edge correlator is slightly better than a narrow correlator with 1/10 chip spacing.
The Strobe Correlator (patent pending) implements a significantly different scheme than
any prior multipath mitigation scheme. The result is a multipath mitigation as good as the
best known techniques, but without the need for banks of correlators closely associated
with high-quality multipath mitigation techniques.
A detailed description of Edge and Strobe Correlation is given in Garin, van Diggelen, and
Rousseau (1996).
Evaluating Correlator Performance
Theoretical analysis of the different multipath mitigation techniques is a straightforward
analysis of how much error hypothetical multipath signals would cause. A plot of
multipath mitigation performance is made by assuming a reflected signal with a certain
power (usually half the power of the direct signal) and a certain delay. The induced error
on the range measurement is then calculated and plotted. Figure B.1 shows the errors
induced by a multipath signal half the strength of the direct signal. The x-axis shows the
multipath delay, which is the extra distance that the reflected signal travels compared to
the direct signal. The y-axis shows the induced range error caused by a multipath signal
with the indicated delay.
From this figure, you can see that typical narrow correlator performance and Edge
Correlator performance are similar, while Strobe Correlator performance is much better,
almost totally cancelling any multipath with a delay of more than 37m.
Multipath Mitigation
C-1
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