GNSS Receiver Operation Overview - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 67

Code tracking

Incoming
signal

Navigation
data extraction

Pseudorange
calculation

Acquisition

Carrier
tracking

FIGURE 5.1. One receiver channel. The acquisition gives rough estimates of signal param-
eters. These parameters are refined by the two tracking blocks. After tracking, the naviga-
tion data can be extracted and pseudoranges can be computed.

An acquisition search through all possible satellites is quite time-consuming. That
is, in fact, the reason why a warm start is preferred if possible.

5.1.1 Acquisition
The purpose of acquisition is to identify all satellites visible to the user. If a satel-
lite is visible, the acquisition must determine the following two properties of the
signal:

Frequency The frequency of the signal from a specific satellite can differ from
its nominal value. In case of downconversion, the nominal frequency of the
GPS signal on L1 corresponds to the IF. However, the signals are affected
by the relative motion of the satellite, causing a Doppler effect. The Doppler
frequency shift can—in the case of maximum velocity of the satellite com-
bined with a very high user velocity—approach values as high as 10 kHz;
see Tsui (2000), page 39. For a stationary receiver on Earth, the Doppler
frequency shift will never exceed 5 kHz.

Code Phase The code phase denotes the point in the current data block where the
C/A code begins. If a data block of 1 ms is examined, the data include an
entire C/A code and thus one beginning of a C/A code.

Many different methods have been used: they are all in one way or another
based on the GPS signal properties. The C/A code correlation properties are es-
pecially important; see Section 2.3.4.
The received signal s is a combination of signals from all n visible satellites

s 1 s 2 s n s ( t ) = ( t ) + ( t ) +···+ ( t ). (5.1)

When acquiring satellite k , the incoming signal s is multiplied with the local
generated C/A code corresponding to the satellite k . The cross correlation be-
tween C/A codes for different satellites implies that signals from other satellites
are nearly removed by this procedure. To avoid removing the desired signal com-
ponent, the locally generated C/A code must be properly aligned in time, that is,
have the correct code phase.