GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 189

FIGURE 9.3 Fine time obtained from BASS method.

reports these fine times every 10 ms and also reports the beginning of the C/A
code as discussed in Section 8.12. These fine times are used to estimate the
pseudorange. Even the fine time calculated from 10 ms of data is somewhat
noisy and if an individual point is used to calculate the user position, the accuracy
may not be very good. A smoothing method such as the least mean square fit
should be used to find the fine time from a relatively long record of data such
as many tens of milliseconds of data points. These methods should improve the
accuracy of the fine time, which should provide better accuracy in the calculated
user position.

9.3 CONVERTING TRACKING OUTPUTS TO NAVIGATION DATA

The next step is to change the output data (every 20 ms) as shown in Figures 9.1
and 9.2 into

+ −

1 (or 0) values. There are several ways to accomplish
this. One common way is to find the difference between adjacent millisecond
outputs. If the difference is beyond a certain threshold, there is a data transition.
For the conventional tracking program, the threshold is usually obtained from
the minimum anticipated amplitude of the output. Since strong and weak signals
produce different amplitudes as shown in Figure 9.1, the minimum values should

1and