Acquisition of GPS C/A Code Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 131

the tracking program. If the receiver is working in real time, the tracking program
will work on data currently collected by the receiver. Therefore, there is a time
elapse between the data used for acquisition and the data being tracked. If the
acquisition is slow, the time elapse is long and the information passed to the
tracking program obtained from old data might be out-of-date. In other words,
the receiver may not be able to track the signal. If the software receiver does
not operate in real time, the acquisition time is not critical because the tracking
program can process stored data. It is desirable to build a real-time receiver; thus,
the speed of the acquisition is very important.
7.3 MAXIMUM DATA LENGTH FOR ACQUISITION
Before the discussion of the actual acquisition methods, let us find out the length
of the data used to perform the acquisition. The longer the data record used the
higher the signal-to-noise ratio that can be achieved. Using a long data record
requires increased time of calculation or more complicated chip design if the
acquisition is accomplished in hardware. There are two factors that can limit
the length of the data record. The first one is whether there is a navigation data
transition in the data. The second one is the Doppler effect on the C/A code.
Theoretically, if there is a navigation data transition, the transition will spread
the spectrum and the output will no longer be a cw signal. The spectrum spread
will degrade the acquisition result. Since navigation data is 20 ms or 20 C/A
code long, the maximum data record for acquisition should be 10 ms. However,
data longer than 10 ms are also used for acquisition, which will be discussed in
Sections 12.5 and 12.8. The reasoning is as follows. In 20 ms of data at most
there can be only one data transition. If one takes the first 10 ms of data and
there is a data transition, the next 10 ms will not have one.
In actual acquisition, even if there is a phase transition caused by a navigation
data in the input data, the spectrum spreading is not very wide. For example, if
10 ms of data are used for acquisition and there is a phase transition at 5 ms,
the width of the peak spectrum is about 400 Hz (2/(5 × 10 − 3 )). This peak usu-
ally can be detected, therefore, the beginning of the C/A code can be found.
However, under this condition the carrier frequency is suppressed. Carrier fre-
quency suppression is well known in bi-phase shift keying (BPSK) signal. In
order to simplify the discussion let us assume that there is no navigation data
phase transition in the input data. The following discussion will be based on
this assumption.
Since the C/A code is 1 ms long, it is reasonable to perform the acquisition on
at least 1 ms of data. Even if only one millisecond of data is used for acquisition,
there is a possibility that a navigation data phase transition may occur in the data
set. If there is a data transition in this set of data, the next 1 ms of data will not
have a data transition. Therefore, in order to guarantee there is no data transition
in the data, one should take two consecutive data sets to perform acquisition.
This data length is up to a maximum of 10 ms. If one takes two consecutive
10 ms of data to perform acquisition, it is guaranteed that in one data set there is