Global Positioning System Reference
In-Depth Information
Carrier signal
1 ms
Generated every ms phase must be continuous
The same carrier frequency is used for 1 second
C/A code
One ms of C/A code used for 20 ms
FIGURE 11.7
Generation of carrier and C/A code signals.
three local codes: the early, the prompt, and the late code. This subject will be
further discussed in Section 11.9.
The generation of the local signal from Equation (11.3) can be summarized
as follows: The carrier phase
θ
k
(n)
is updated every ms, and the phase must
be continuous at 1 ms interval. The C/A code
C
k
is updated every 20 ms, and
the carrier frequency
f
k
is updated every second. This operation is illustrated in
Figure 11.7.
Once the prompt local code is generated, it is multiplied by 1 ms input sig-
nal point by point to obtain 5000 product values. Since the frequencies of the
local and input signals are very close, the 5000 product values have approxi-
mately the same values. Figure 11.8 shows the operation on 3 ms of data, which
are divided into segments
A
,
B
,and
C
. In this figure the product values from
each ms are divided into two portions. The portion donated by
a
is the part
before the initial C/A code phase, and the part after the initial C/A code phase
is designated as
b
. Because the initial C/A code phase is used to determine
the pseudorange, it is desirable to put 1 ms of data together, starting from an
initial C/A code phase. To accomplish this goal, the portions from two adja-
cent ms are grouped together to form a new set of segments (shown in the
figure as 1 and 2). For example, the
b
portion of
A
and the a portion of
B
are grouped together and designated as 1. The
b
portion of
B
and a portion
of
C
are grouped together and designated as 2. Each segment contains 5000
points. These 5000 points are then summed into one point, which is the peak
correlation output from one ms of data and the locally generated code. Thus,
after these operations, 1 ms of input data (5000 points) only generates one out-
put data point. Let us reference this output point as the compressed output. The
compressed output is obtained from 5000 input data starting from an initial C/A
code point. The compressed output is usually obtained from two consecutive
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