GPS Overview | Appendix G | |
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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
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.
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