C-2
GG Surveyor GPS+GLONASS Reference Manual
In a real situation, multipath is usually a combination of many reflections, all with
different delays and different power. Real-life multipath is often described as either
close-in multipath or far multipath. Close-in multipath occurs when the reflecting
surface is close to the satellite antenna direct line, and the delay is small; usually,
these reflections come from a surface near the antenna, for example, an antenna on a
tripod on the ground would pick up close-in multipath from reflections off the ground
below and around the tripod. Choke-ring antennas are probably the best cure for
close-in multipath. Correlator-based multipath techniques, as shown in Figure C.1,
are all bad at rejecting very close-in multipath mitigation.
Very close-in multipath causes only a small change in the ideal correlation function,
so it is almost impossible for the correlator-base multipath integration to determine
the error. Far multipath can cause very large errors if a good multipath mitigation
technique is not used.
Figure C.1: Relative Performance of Multipath Mitigation Techniques
0
50
100
150
200
250
300
350
400
-100
-80
-60
-40
-20
0
20
40
60
80
100
Multipath Delay (meters)
Tracking Error (meters)
Multi ath Error Envelo es
Generic Standard
1
Correlator
Generic Narrow
2
Correlator
Ashtech
Edge Correlator™
Ashtech
Strobe Correlator™
Multipath:
Amplitude -3 dBs
Phase 0,180 deg.
BW = 10.5 MHz
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