Global Positioning System Reference
In-Depth Information
8
6
4
2
0
−2
−4
−6
−8
0
5
10
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20
25
30
Time [s]
FIGURE 8.2. Correlation between 33 s of navigation data and the 8-bit preamble. The
peaks indicate the location of the beginning of a subframe.
when the preamble is located and
8 when an inverted preamble is located. An
example of the correlation between a navigation data sequence and the preamble
can be seen in Figure 8.2.
As seen from Figure 8.2, the correlation function gives maximum correlation
of 8 several times in this 33-s-long sequence. It should give only six maximum
correlation values as the sequence should contain six subframes. In addition to the
large number of maximum correlation values, it also gives a minimum correlation
value of
−
8 several times. As mentioned earlier, this means that an inverted in-
stance of the preamble has been located. The method for distinguishing which of
the maximum correlation values that really is a beginning of a subframe includes
the determination of the delay between consecutive maximum correlation values.
Only if the delay between the maximum correlation values is exactly 6 s and the
parity checks do not fail is the beginning of a subframe indicated.
When the correct preambles are located, the data from each subframe can be
extracted. If the correlation shows that the preamble is inverted, the entire naviga-
tion sequence must be inverted.
Due to the Doppler effect the length of the navigation bit can deviate from the
exact value of 20 ms. Over a short time this length difference even may accumu-
late to a significant value. Therefore, a better solution is to look for a preamble in
the original 1000-sps output from the tracking. The algorithm remains the same,
but each bit in the reference preamble pattern is converted to 20 values (samples).
Now the correlation peak will have a maximum value of 8
−
160 instead of
8. Simultaneously, this modified algorithm also finds bit transition time.
×
20
=
8.2.2 Extracting the Navigation Data
Every correct preamble marks the beginning of a navigation data subframe. Each
of the subframes contains 300 bits divided into 10 30-bit words. The structure of
the first two words of a subframe is shown in Figure 8.3.
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