Tracking GPS Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 168

FIGURE 8.3 Code and carrier tracking loops.
The acquisition program determines the beginning of the C/A code. The code
loop generates early and late C/A codes and these two codes are the C/A code
time shifted typically by approximately one-half-chip time of 0.489 us ( 1 / 2 ×
1 . 023 × 10 6 ) or less. The early and late codes correlate with the input C/A codes
to produce two outputs. Each output passes through a moving average filter and
the output of the filter is squared. The two squared outputs are compared to
generate a control signal to adjust the rate of the locally generated C/A code to
match the C/A code of the input signal. The locally generated C/A code is the
prompt C/A code and this signal is used to strip the C/A code from the digitized
input signal.
The carrier frequency loop receives a cw signal phase modulated only by
the navigation data as the C/A code is stripped off from the input signal. The
acquisition program determines the initial value of the carrier frequency. The
voltage-controlled oscillator (VCO) generates a carrier frequency according to
the value obtained from the acquisition program. This signal is divided into two
paths: a direct one and one with a 90-degree phase shift. These two signals are
correlated with the input signal. The outputs of the correlators are filtered and
their phases are compared against each other through an arctangent comparator.
The arctangent operation is insensitive to the phase transition caused by the