Global Positioning System Reference
In-Depth Information
TABLE 12.3 Additional Satellites Generating Data
Code Delay
Chips
First 10 Chips
C/A Octal
PRN
120
145
0671
121
175
0536
122
52
1510
123
21
1545
124
237
0160
125
235
0701
126
886
0013
127
657
1060
128
634
0245
129
762
0527
130
355
1436
131
1012
1226
132
176
1257
133
603
0046
134
130
1071
135
359
0561
136
595
1037
137
68
0770
138
386
1327
to measure first the true sampling frequency of a receiver. One way to measure
the sampling frequency is to use a frequency counter in the laboratory. However,
this instrument may not be available to some users.
The WAAS uses geo-stationary satellites to receive data measured from many
ground stations, and it transmits information to GPS users for position correction.
Because the WAAS satellites are geo-stationary, the Doppler frequency caused
by their motion is very small, typically on the order of a few tens of Hz. Thus the
signal transmitted by the WAAS can be used to calibrate the sampling frequency
in a GPS receiver. The WAAS signal frequency is at 1575.42 MHz. The sam-
pling frequency of a C/A code GPS receiver is in the neighborhood of 5 MHz.
The ratio of these two frequencies is about 300; thus 10 Hz inaccuracy in the
WAAS frequency will be translated to about 0.03 Hz (10/300). The accuracy
of the sampling frequency measured through this approach should be less than
1 Hz. However, the WAAS signal can be rather weak for users in certain areas.
The noise in the received signal can be the dominant factor in determining the
sampling frequency.
The C/A codes of the WAAS satellite can be generated through the same
approach as discussed in Section 5.6. The only information needed here is the
code delay chips in Table 5.3. The additional satellites are listed in Table 12.3
(
1
)
.
These satellites are incorporated in the cacode.m program at the end of
Chapter 11. The only satellite can be found in the received data (collected in
Dayton, Ohio) is PRN 122 with a delay of 52.
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