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GPS Overview Appendix G
SUPERSTAR II User Manual Rev 3 73
present to some degree, due to real world conditions.
When a GPS multipath signal converges at the GPS antenna, there are two primary problems that occur:
1. a multiple signal with amplitude and phase shifting, and
2. a multiple signal with differing ranges.
When a direct signal and multipath signal are intercepted by the GPS antenna, the two signals will sum
according to the phase and amplitude of each. This summation of signals causes the composite to vary greatly
in amplitude, depending on the degree of phase shift between the direct signal versus the multipath signal. If
the multipath signal lags the direct path signal by less than 90° the composite signal will increase in amplitude
(relative to the direct signal, depending on the degree of phase shift between 0° and 90°). As well, if the
multipath signal lags the direct path signal by greater than 90° but less than 270° the composite signal will
decrease in amplitude. Depending on the relative amplitude of the multipath signal (or signals), the composite
signal being processed by the receiver correlator may experience substantial amplitude variations. A worst case
scenario is when the multipath signal experiences a lag of 180° and is near the same strength as the direct path
signal – this will cause the multipath signal to almost completely cancel out the direct path signal, resulting in
loss of satellite phase lock or even code lock.
Because a multipath signal travels a greater distance to arrive at the GPS antenna, the two C/A code
correlations are, by varying degrees, displaced in time, which in turn causes distortion in the correlation peak
and thus ambiguity errors in the pseudorange (and carrier phase, if applicable) measurements.
As mentioned in previous paragraphs, it is possible that the received multipath signal has greater amplitude
than the direct path signal. In such a situation the multipath signal becomes the dominant signal and receiver
pseudorange errors become significant due to dominant multipath biases and may exceed 150 meters. For
single point pseudorange positioning, these occasional levels of error may be tolerable, as the accuracy
expectations are at the 5 meter CEP level (using standard correlator). However, for pseudorange single
differencing DGPS users, the accuracy expectations are at the 1 meter CEP level (with no multipath).
Obviously, multipath biases now become a major consideration in trying to achieve the best possible
pseudorange measurements and position accuracy.
If a differential base station is subject to significant multipath conditions, this in turn will bias the range
corrections transmitted to the differential rover receiver. And in turn, if the rover receiver also experiences a
high level of multipath, the rover receiver position solutions will be significantly biased by multipath from both
stations. Thus, when the best possible position solutions are required, multipath is certainly a phenomenon that
requires serious consideration.
G.4.3 Hardware Solutions For Multipath Reduction
A few options exist by which GPS users may reduce the level of multipath reception. Among these include:
antenna site selection, special antenna design, and ground plane options.
G.4.3.1 Antenna Site Selection
Multipath reception is basically a condition caused by environmental circumstances. Some of these conditions
you may have a choice about and some you may not.
Many GPS reception problems can be reduced, to some degree, by careful antenna site selection. Of primary
importance is to place the antenna so that unobstructed line-of-sight reception is possible from horizon to
horizon and at all bearings and elevation angles from the antenna. This is, of course, the ideal situation, which
may not be possible under actual operating conditions.
Try to place the antenna as far as possible from obvious reflective objects, especially reflective objects that are
above the antenna’s radiation pattern horizon. Close-in reflections will be stronger, and typically have a shorter
propagation delay allowing for auto correlation of signals with a propagation delay of less than one C/A code
chip (300 meters).