Global Positioning System Reference
In-Depth Information
One can consider that the input data and the local data are on the surfaces of the
two cylinders. The local data is rotated 5,000 times to match the input data. In
other words, one cylinder rotates against the other one. At each step, all 5,000
input data are multiplied by the 5,000 local data point by point and results are
summed together. It takes 5,000 steps to cover all the possible combinations of
the input and the local code. The highest amplitude will be recorded. There are
21 pairs of cylinders (not shown). The highest amplitude from the 21 different
frequency components is the desired value, if it also crosses a certain threshold.
Computer program (p7 1) will show the above operation with fine frequency
information in Section 7.13. The determination and setting a specific threshold
are not included in this program, but the results are plotted in time and frequency
domain. One can determine the results from the plots.
7.9 MODIFIED ACQUISITION BY CIRCULAR CORRELATION
(
4
)
This method is the same as the one above. The only difference is that the length
of the FFT is reduced to half. In step 3 of the circular correlation method in the
previous section the local code
l
si
(
n
) is generated. Since the
l
si
(
n
)isacomplex
quantity, the spectrum is asymmetrical as shown in Figure 7.5. From this figure
it is obvious that the information is contained in the first-half spectrum lines. The
FIGURE 7.5
Spectrum of the locally generated signal.
Search WWH ::
Custom Search