Global Positioning System Reference
In-Depth Information
Carrier and Code Tracking
7.1
Motivation
The acquisition provides only rough estimates of the frequency and code phase
parameters. The main purpose of tracking is to refine these values, keep track, and
demodulate the navigation data from the specific satellite (and provide an estimate
of the pseudorange). A basic demodulation scheme is shown in Figure 7.1.
The figure shows the scheme used to demodulate the input signal to obtain the
navigation message. First, the input signal is multiplied with a carrier replica. This
is done to wipe off the carrier wave from the signal. In the next step, the signal
is multiplied with a code replica, and the output of this multiplication gives the
navigation message. So the tracking module has to generate two replicas, one for
the carrier and one for the code, to perfectly track and demodulate the signal of
one satellite. In the following, a detailed description of the demodulation scheme
is conducted.
7.2
Demodulation
Let
f
L1
and
f
L2
be the carrier frequencies of L1 and L2 for the signal transmitted
from satellite
k
with powers
P
C
,
P
PL1
,and
P
PL2
for C/A or P code. The C/A code
sequence is
C
k
and the P(Y) code sequence is
P
k
(
)
(
)
t
t
. If the navigation data
sequence is called
D
k
(
)
t
, the total signal is given as
2
P
C
C
k
2
P
PL1
P
k
s
k
D
k
D
k
(
t
)
=
(
t
)
(
t
)
cos
(
2
π
f
L1
t
)
+
(
t
)
(
t
)
sin
(
2
π
f
L1
t
)
2
P
PL2
P
k
D
k
+
(
t
)
(
t
)
sin
(
2
π
f
L2
t
).
(7.1)
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