Carrier and Code Tracking - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 89

PRN code

Incoming
signal

Carrier loop
discriminator

Carrier loop
filter

NCO carrier
generator

FIGURE 7.6. Basic GPS receiver tracking loop block diagram.

quite fast. It can be seen that the noise on the tracking frequency is much smaller
than with a noise bandwidth of 60 Hz. In the third case where the noise bandwidth
is 10 Hz, the tracking loop is not fast enough to reach the real frequency before
a phase shift occurs. Therefore, it is not likely to converge to the proper value.
A large noise bandwidth implies that the tracking loop quickly locks to the real
frequency but has a relatively large frequency noise in the locked state. A smaller
noise bandwidth implies that it can take some time before the tracking loop can
be locked to the frequency, but after the lock the frequency is stable. Some imple-
mentations split the PLL into two filters, often called pull-in and tracking filters.
For land applications, a typical value for noise bandwidth is about 20 Hz.

7.4 Carrier Tracking

To demodulate the navigation data successfully an exact carrier wave replica has
to be generated. To track a carrier wave signal, phase lock loops (PLL) or fre-
quency lock loops (FLL) are often used.
Figure 7.6 shows a basic block diagram for a phase lock loop. The two first
multiplications wipe off the carrier and the PRN code of the input signal. To wipe
off the PRN code, the I p output from the early-late code tracking loop described
above is used. The loop discriminator block is used to find the phase error on
the local carrier wave replica. The output of the discriminator, which is the phase
error (or a function of the phase error), is then filtered and used as a feedback to
the numerically controlled oscillator (NCO), which adjusts the frequency of the
local carrier wave. In this way the local carrier wave could be an almost precise
replica of the input signal carrier wave.
The problem with using an ordinary PLL is that it is sensitive to 180 ◦ phase
shifts. Due to navigation bit transitions, a PLL used in a GPS receiver has to be
insensitive to 180 ◦ phase shifts.
Figure 7.7 shows a Costas loop. One property of this loop is that it is insensitive
for 180 ◦ phase shifts and hereby a Costas loop is insensitive for phase transitions
due to navigation bits. This is the reason for using this carrier tracking loop in
GPS receivers. The Costas loop in Figure 7.7 contains two multiplications. The
first multiplication is the product between the input signal and the local carrier
wave and the second multiplication is between a 90 ◦ phase-shifted carrier wave