Acquisition - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 71

Acquisition

The present and the following chapters are based on signals recorded according
to parameters described in Section 4.3. The theory can be applied similarly to
records with a different selection of parameters.

6.1 Motivation

The purpose of acquisition is to determine visible satellites and coarse values of
carrier frequency and code phase of the satellite signals.
The satellites are differentiated by the 32 different PRN sequences. The sec-
ond parameter, code phase, is the time alignment of the PRN code in the current
block of data. It is necessary to know the code phase in order to generate a local
PRN code that is perfectly aligned with the incoming code. Only when this is the
case, the incoming code can be removed from the signal. PRN codes have high
correlation only for zero lag. That is, the two signals must be perfectly aligned to
remove the incoming code. The third parameter is the carrier frequency, which in
case of downconversion corresponds to the IF. The IF should be known from the
L1 carrier frequency of 1575.42 MHz and from the mixers in the downconverter.
However, the frequency can deviate from the expected value. The line-of-sight
velocity of the satellite (with respect to the receiver) causes a Doppler effect re-
sulting in a higher or lower frequency. In the worst case, the frequency can deviate
up to 10 kHz. It is important to know the frequency of the signal to be able to

generate a local carrier signal. This signal is used to remove the incoming carrier
from the signal. In most cases it is sufficient to search the frequencies such that
the maximum error will be less than or equal to 500 Hz; see Akos (1997).

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