GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 190

be used as the threshold. For the BASS method the threshold is at ± π /2 as

discussed in Section 8.9.
From these transitions, it is easy to change the tracking results into navigation
data. The navigation data transition points must correspond to individual points
in the collected input data, which have a time resolution of 200 ns. This time
resolution can be used to find the relative time difference between different
satellites. The following steps can be applied to accomplish this goal. This method
represents only one way to solve the problem and is by no means the optimum
one. This method is presented because it might be easier to understand. The
following steps are used to convert phase transition to navigation data:

1. Find all the navigation data transitions. The beginning of the first navigation
data should be within the first 20 ms of output data because the navigation data are
20 ms long. However, there might not be a phase transition within 20 ms of data.
The first phase transition can be used to find the beginning of the first navigation
data. The first phase transition detected in the output data is the beginning of the
navigation data. If the first phase transition is within the first 20 ms of data, this
point is also the beginning of the first navigation data. If the first phase transition
occurs at a later time, a multiple number of 20 ms should be subtracted from
it. The remainder is the beginning of the first navigation data. For simplicity
let us just call it the first navigation data point instead of the beginning of the
first navigation data. This information will be stored and used to find the coarse
pseudorange discussed in Section 9.6. The first navigation data point can be
padded with data points of the same sign to make the first navigation data point
always occur at 21 ms. This approach creates one navigation data point at the
beginning of the data from partially obtained information. For example, if the
first phase transition occurs at 97 ms, by subtracting 80 ms from this value, the
first navigation data point occurs at 17 ms. These 17 ms of data are padded with
4 ms of data of the same sign to make the first navigation data 20 ms long.
This process makes the first navigation data point at 21 ms. This operation also
changes the rest of the beginnings of the navigation data by 4 ms. Thus, the
navigation data points occur at 21, 41, 61, and so on.
Figure 9.4 illustrates the above example. The upper part of Figure 9.4 shows
the output data from the tracking program and the bottom part shows the result
padded with additional data. The adjusted first navigation data point at 21 ms is
stored. If the first phase transition occurs at 40 ms, by subtracting 40, the adjusted
first navigation data point occurs at 0 ms. Twenty-one ms of data with either +

or can be added in front of the first navigation data point to make it occur

at 21 ms.
2. Once the navigation data points are determined, the validity of these tran-
sitions must be checked. These navigation data points must be separated by
multiples of 20 ms. If these navigation data points do not occur at a multiple of
20 ms, the data contain errors and should be discarded.
3. After the navigation data points pass the validity check, these outputs are
converted into navigation data. Every 20 outputs (or 20 ms) convert into one

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