Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 203

transmit time corresponding to the phase state of the code NCO and the replica
PRN code generator state after each code NCO update. It does this by discrete inte-
gration of every code phase increment over the interval of time since the last NCO
update and adds this number to the code accumulator. The combination of the rep-
lica code generator state (integer code state) and the code NCO state (fraction code
state) is the replica code state. Since the code phase states of the PRN code generator
are pseudorandom, it would be impractical to read the code phase state of the PRN
code generator and then attempt to convert this nonlinear code state into a linear
GPS time state, say, by a table lookup.
There are too many possible code states, especially for the P code generator. A
very practical way to maintain the GPS time in a GPS receiver is to use a separate
code accumulator in the GPS receiver baseband process and to synchronize this
accumulator to the replica PRN code generator phase state. Figure 5.30 (derived
from the code tracking loop of Figure 5.13) illustrates the high-level block diagram
relationship between the replica PRN code generator and the code accumulator
(which is not included in Figure 5.13) in the code tracking loop of one GPS receiver
channel.
A typical GPS navigation measurement incorporation rate is once per second. A
typical GPS receiver FTF for scheduling measurements is 20 ms, which is the same
as the 50-Hz navigation message data period. The receiver baseband process sched-
ule for updating the code and carrier NCOs is usually some integer subset of the

Code loop
discriminator

In-phase (I) and quadra-phase (Q) signals
Code phase
increment per clock

Error signals

Numerical
controlled
oscillator

E, P, L
replica
code

1 chip per
NCO overflow

Code loop
filter

Code generator

Reset signals

Clock f

c

Code setter

Receiver hardware

Synchronization signals

Receiver baseband software

Code accumulator
(code phase offset)

Transmit time

Figure 5.30 Relationship between PRN code generator and code accumulator.