Acquisition of Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 266

TABLE 10.5 Correct Frequency Readings in 100 Runs

Data Length (ms) ms Summed C/N 0 (dB)

25 24 23

240 1 59 39 19.5

240 2 74.5 56 40.5

240 4 82 67 58

360 1 89.5 65.5 45

360 2 98.5 90 63

360 4 100 94 85.5

480 1 99 93 75.5

480 2 100 99.5 95.5

480 4 100 100 99

600 1 100 99 91

600 2 100 100 98

600 4 100 100 100

slightly degraded because blocks containing navigation data will reduce coherent
processing gain.
It appears that with a data length of 480 ms and after summing 2 ms together,
the result is satisfactory. There are a maximum of 24-phase transitions in 480 ms
of data. If 2 ms summed together, there are 240 outputs. Twenty-four navigation
phase transitions can only affect 10% of the data, and there is still enough data to
obtain the correct frequency. If 10 ms are summed together, there are 48 outputs,
and 24-phase transitions can affect 50% of the data. Thus summing of many ms
of data may not be advisable.

10.17 FIRST NAVIGATION PHASE TRANSITION DETERMINATION

In a conventional GPS receiver the information, such as carrier frequency and
initial C/A code phase obtained from the acquisition, can be used to track a
signal. The navigation data phase transition can be found through the tracking
program. For a weak signal it is difficult to determine the exact location of the
navigation data transition from the tracking program. In order to process the data
efficiently, the exact location of the navigation data phase transition is needed
such that one can start to process the data every 20 ms. Thus in this section the
determination of the location of the navigation phase transition is discussed. In
general, it is easier to determine whether there is an existence of data transition
than to determine the exact location of a data transition.
The basic approach to determine the location of the data transition is as fol-
lows: After finding the fine carrier frequency, as discussed in the previous section,
the C/A code and carrier frequency can been stripped off from the input signal
and 1 ms of data is summed into one point. Forty ms of reference data (40 points)
can be generated to correlate with 40 ms of input data to find whether a data