Global Positioning System Reference
In-Depth Information
beginning of subframe 1 is 196. The corresponding beginning of the C/A code is
9803828. However, this point does not align with the beginning of subframe 1. In
order to align with the beginning of subframe 1, 7 ms will be added. These 7 ms
come from the first navigation data point at 7 ms shown at the top of the figure.
Since each millisecond contains 5,000 digitized data, 5,000 must be multiplied by
this 7 ms to obtain the beginning of subframe 1 in terms of the digitized input data
points. Thus, the beginning of subframe 1 is at 9803828
+
7
×
5000
=
9838828.
In Figure 9.6c the first phase transition is at 10 ms and the data are padded with
11 points at the beginning. The beginning of subframe 1 is at the 100th navigation
data point. The beginning of the C/A code is aligned with the beginning of
subframe 1 at 197. The corresponding beginning of the C/A code is 9850115.
In Figure 9.6d the first navigation data point is at 17 ms and the data are
padded with 4 points at the beginning. The beginning of the subframe 1 is at the
99th navigation data point. The beginning of the C/A code in front of subframe
1 is at 195. The corresponding beginning of the C/A code with index of 195
is 9752661. However, in order to align with the beginning of subframe 1, 7 ms
will be added. This 7 ms comes from the first navigation data point at 17 ms.
Since the beginning of the C/A code has a time resolution of 10 ms, 10 ms are
subtracted from the 17 ms to obtain 7 ms. The final value is 9752661
+
7
×
5000
=
9787661.
From the above discussion, one can see that it takes two steps to find the
beginning of subframe 1 in terms of the actual digitized input data points. The
first step is to find the index of the beginning of the C/A code just before subframe
1. The second step is to find the time between the desired beginning of the C/A
code to the beginning of subframe 1. The first step can be accomplished through
the following equation:
ind
=
2
(sfb
1
−
2
)
+
integer(nav
1
/
10
)
(
9
.
1
)
where
ind
is the index of the desired beginning of the C/A code;
sfb
1isthe
beginning of subframe 1;
nav
1 is the first navigation data point and
integer
means takes the integer part of the result.
The second step is to find the difference in milliseconds (
difms
), which can
be written as
difms
=
rem(nav
1
/
10
)
(
9
.
2
)
where
rem
means to take the remainder of the value in the parenthesis. The
desired input point corresponding to the beginning of subframe 1 can be written as
dat
=
bca(ind)
+
difms
×
5000
(
9
.
3
)
where
dat
is the digitized input data point;
bca
is the beginning of the C/A code.
Let us use these three equations to find the desired values in Figure 9.6. The
results are listed in Table 9.2.
The satellite are designated as a, b, c, and d instead of a real satellite number
because the information in satellite c is artificially created to illustrate a special
Search WWH ::
Custom Search