GNSS Receiver Operation Overview - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 69

FIGURE 5.3. Acquisition plot for PRN 19. Signals originating from PRN 19 are clearly
not present in the received signal as there is no sign of a peak in the acquisition plot.
The accuracy of the final value of the code phase is connected to the accuracy of
the pseudorange computed later on. The tracking contains two parts, code tracking
and carrier frequency/phase tracking:
Code tracking The code tracking is most often implemented as a delay lock
loop (DLL) where three local codes (replicas) are generated and correlated
with the incoming signal. These three replicas are referred to as the early,
prompt, and late replica, respectively. The three codes are often separated
by a half-chip length.
Carrier frequency/phase tracking The other part of the tracking is the carrier
wave tracking. This tracking can be done in two ways: either by tracking
the phase of the signal or by tracking the frequency.
The tracking is running continuously to follow the changes in frequency as a
function of time. If the receiver loses track of a satellite, a new acquisition must
be performed for that particular satellite.
5.1.3 Navigation Data Extraction
When the signals are properly tracked, the C/A code and the carrier wave can
be removed from the signal, only leaving the navigation data bits. The value of a
data bit is found by integrating over a navigation bit period of 20 ms. After reading
about 30 s of data, the beginning of a subframe must be found in order to find the
time when the data was transmitted from the satellite.
When the time of transmission is found, the ephemeris data for the satellite
must be decoded. This is used later on to compute the position of the satellite at
the time of transmission.