Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 216

C / N 0 drops below the bit synchronization design threshold. This causes serious navi-
gation integrity problems for C/A code receivers under conditions of signal attenua-
tion or RF interference. This problem is compounded if there is no design provision
to adapt the bit synchronization process for poor C / N 0 conditions or for the naviga-
tion process to check for bit synchronization errors.

5.7.2 Delta Pseudorange
The definition of delta pseudorange to SV i is as follows.

() ( ) ( ) ( )

∆ρ n = ρ n + J − ρ n − K m (5.32)

i i i

where:

ρ i ( n + J ) = pseudorange at J FTF epochs later than FTF( n ) (m)

ρ i ( n − K ) = pseudorange at K FTF epochs earlier than FTF( n ) (m)

J = 0 or K depending on design preferences (dimensionless)

Even though (5.32) implies that delta pseudorange could be derived from the
code tracking loop, the result would be a very noisy measurement. This differential
measurement will be more than two orders of magnitude less noisy if taken from the
carrier tracking loop operated as a PLL. If the carrier loop is operated as a FLL, the
measurement is taken the same way, but the FLL is only about 100 times less noisy
than code measurements.
The carrier Doppler phase measurements are extracted by the receiver baseband
process from the carrier tracking loop using a carrier accumulator. It is more simple
but similar to using the code accumulator to extract transmit time measurements
from the code tracking loop. The carrier accumulator consists of the integer cycle
count, N CA , and the fractional cycle count,

Φ CA , of the carrier Doppler phase

measurement.
The carrier accumulator is updated after each carrier loop output to the carrier
NCO using the following algorithm (remembering that the equal sign means that the
right-hand side replaces the value on the left-hand side):

ΦΦ+ = f ∆Φ T

temp CA c CA

( )

Φ = fractional part of Φ cycles (5.33)

CA temp

( ) ( )

N = N last value + integer part of Φ cycles

C A CA temp

where:

Φ temp = temporary Φ CA register

f c = carrier NCO clock frequency (Hz)

∆Φ CA = carrier NCO carrier Doppler phase increment per clock cycle

= carrier loop filter velocity correction + velocity aiding (if any)

T = time between carrier NCO updates (seconds)