Acquisition of GPS C/A Code Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 144

millisecond as the original C/A code. If there are two phase shifts by the naviga-
tion data, the only regions where the original C/A code and the delayed C/A code
have a different code are shown in Figure 7.6b. The rest of the regions generate
the same “new code.” If there are 5,000 data points per C/A code, delaying two
data points only degrades the performance by 2/5,000, caused by a navigation
transition. Therefore, using this acquisition method one does not need to check
for two consecutive data sets to guarantee that there is no phase shift by the
navigation data in one of the sets. One can choose a longer data record and the
result will improve the correlation output.
Experimental results indicate that 1 ms of data are not enough to find any
satellite. The minimum data length appears to be 5 ms. Sometimes, one single
delay time of 400 ns is not enough to find a desired signal. It may take several
delay times, such as 0.4, 0.8, and 1.2 us, together to find the signals. A few weak
signals that can be found by the circular correlation method cannot be found by
this delay and multiplication method. With limited results, it appears that this
method is not suitable to find weak signals.
7.11 NONCOHERENT INTEGRATION
Sometimes using 1 ms of data through the circular correlation method cannot
detect a weak signal. Longer data records are needed to acquire a weak signal.
As mentioned in Sections 7.4 and 7.5, an increase in the data length can require
many more operations. One way to process more data is through noncoherent
integration. For example, if 2 ms of data are used, the data can be divided into
two 1 ms blocks. Each 1 ms of data are processed separately and the results
are summed together. This operation basically doubles the number of operations,
except for the addition in the last stage. The improvement obtained by this method
is less than with the coherent method but there are fewer operations. The detailed
discussion can be found in Sections 10.16 and 10.17.
Another advantage of this latter method is the ability to keep performing acqui-
sition on successive 1 ms of data and summing the results. The final result can
be compared with a certain threshold. Whenever the result crosses the threshold,
the signal is found. A maximum data length can be chosen. If a signal cannot
be found within the maximum data length, this process will automatically stop.
Using this approach, strong signals can be found from 1 ms of data, but weak sig-
nals will take a longer data length. If there is phase shift caused by the navigation
data, only the 1 ms of data with the phase shift will be affected. Therefore, the
phase shift will have minimum impact on this approach. Conventional hardware
receivers often use this approach.
7.12 COHERENT PROCESSING OF A LONG RECORD OF DATA ( 6 )
This section presents the concept of processing a long record of data coherently
with fewer operations. The details are easy to implement, thus they will not