Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 206

∆φ CO contains two components, the code NCO bias and
the code loop filter velocity correction. The code NCO bias (see Figure 5.13) is the
phase increment per clock that accounts for the marching of time in the P code rep-
lica code generator. When applied to the P replica code generator, this is 10.23
Mchip/s. When applied to the C/A replica code generator, this is 1.023 Mchip/s. The
code E - L correlator spacing, D , requires a second output from the NCO to be 2/ D
faster than the spreading code chip rate to clock the 2-bit shift register. Algorithm
(5.29) assumes that the NCO produces the code generator clock. If the code genera-
tor clock is produced by dividing the NCO shift register clock by 2/ D , then algo-
rithm (5.29) and the NCO bias must be scaled accordingly. The code loop filter
velocity correction is the combination of carrier aiding and code loop filter output.
This combined output corrects the P replica code generator for Doppler (and a small
order effect due to changes in ionospheric delay) referenced to the P code chip rate.
Usually the code generator provides the divide-by-10 function required for the C/A
code generator if both P and C/A codes are generated. This is the correct factor for
the spreading code chip rate and the code Doppler/ionospheric delay components.

The earlier definition of

5.7.1.4 Obtaining a Measurement from the Code Accumulator
To obtain a measurement, the code accumulator must be propagated to the nearest
FTF( n ). This results in the set of measurements P i ( n ), X 1 i ( n ), and Z i ( n ) for SV i . When
converted to time units of seconds, the result is T Ti ( n ), the transmit time of SV i at the
receiver time epoch n . This is done very much like (5.28), except the time T is
replaced with the skew time, T s , and the code accumulator is not updated.

P temp = Pf +∆φ T

c co s

P i ( n ) = fractional part of P temp (chips)

X temp = ( X 1 + whole part of P temp )/15,345,000

X 1 i ( n ) = remainder of X temp (chips)

Z i ( n ) = remainder of [( Z + whole part of X temp )/403,200] (1.5 seconds) (5.29)

Note that (5.29) produces no error due to the measurement propagation process
for the code accumulator measurements because the code NCO is running at a con-
stant rate, ∆φ CO per clock, during the propagation interval. The following equation

converts the code accumulator measurements to the SV i transmit time, T Ti ( n ). Dou-
ble precision floating point computations are assumed.

[ ] ( )

() () () ()

Tn = Pn X n + 1 10 23 . × 10

6

+ Zn × 1 5 . (seconds) (5.30)

Ti i i i

5.7.1.5 Synchronizing the Code Accumulator to the C/A code and P Code
Generators
Synchronizing the code accumulator to the C/A code and P code generators is the
most complicated part of the pseudorange measurement process. This is because the
count sequences taking place in the code generator shift registers are PRN
sequences, while the count sequence taking place in the code accumulator is a linear
sequence. Fortunately, predictable reset timing events in the PRN shift registers per-