Global Positioning System Reference
In-Depth Information
20-ms coherent integration
10
4
4.5
×
4
3.5
3
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
60
70
80
90
100
(a) 20 ms coherent integration.
20-ms coherent integration summed 10 times
10
5
×
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
10
20
30
40
50
60
70
80
90
100
(b) 20 ms coherent integration followed by 10 noncoherent
FIGURE 12.26
Correlation peaks of reflected channel at 10,000 ft.
the correlation peak of the reflected signal can be seen clearly after 10 noncoher-
ent integrations. The time difference between these peaks can be related to the
measured distance
d
BC
in Figure 12.25b. In the actual measurements,
the positions of the peaks (direct and reflected signals) were calculated from the
fine time resolution method, as discussed in Section 8.11.
Sometimes, the second channel cannot obtain a distinct correlation peak from
the direct signal. Then, the correlation peak of the direct signal obtained from the
first channel is used as the reference because it is prominent and easy to identify.
If the correlation peak of channel 1 is used as reference, the distance between
the upper and the lower antenna must be taken into consideration.
The amplitude of the correlation peak does not decrease rapidly with an altitude
increase because the refection is from the earth surface rather than from a point
source. The reflection from the earth surface does not have the
R
4
relation, where
R
is path length from the source to the receiver. The
R
4
relation is often used
in a radar equation where the target is usually considered as a point source.
This passive altimeter method allows one to find altitudes higher than 10,000 ft.
=
AB
+
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