Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 204

FTF, such as 20, 10, 5, 2, or 1 ms. Assuming that the FTF is 20 ms, the receiver mea-
surement process maintains a monotone counter (call it the FTF counter) in 20-ms
increments derived from the receiver's reference oscillator. The FTF counter is set to
zero at power up, counts up, rolls over, counts up, and so on. Assuming that the nav-
igation measurement incorporation rate is 1 Hz, the navigation process will sched-
ule measurements to be extracted from the code and carrier tracking loops every
fiftieth FTF (i.e., based on the receiver's time epochs). When the receiver baseband
process extracts the measurements from the code and carrier tracking loops, it time
tags the measurements with the FTF count. The navigation process assigns and
maintains a GPS receive time corresponding to the FTF count. The receive time ini-
tialization can be the first SV's transmit time plus a nominal propagation time of,
say, 76 ms if the navigation process does not know the GPS time accurately. This
will set the initial receive time accurate to within 20 ms.
When a pseudorange measurement is scheduled on FTF( n ), the receiver base-
band code tracking loop process extracts the SV i transmit time from its code accu-
mulator and propagates this time forward to FTF( n ). The result is the SV i transmit
time with a measurement resolution of 2 - N of a code chip, where N is the number of
bits in the code NCO adder. If the code NCO adder uses a 32-bit register, this mea-
surement resolution is less than a quarter of a nanochip, which makes the code mea-
surement quantization noise negligible. The receiver baseband process can compute
the pseudorange measurement from the SV i transmit time using (5.27) and time tag
the measurement with FTF( n ) before sending the result to the navigation process.
However, the navigation process needs the (corrected) SV i transmit time to compute
the location of SV i when it transmitted the measurement. Hence, the best measure-
ment to send to the GPS navigation process is the (uncorrected) SV transmit time,
along with the FTF time tag. The navigation process applies the clock correction
(including relativity correction), uses the corrected SV i transmit time to compute the
SV i position, then computes the pseudorange plus other corrections before incorpo-
ration of the measurement. (Satellite clock and relativistic corrections are discussed
in Sections 7.2.1 and 7.2.3, respectively.)
5.7.1.2 Measurement Time Skew
Figure 5.4 illustrates the bit sync phase skew, T s , which exists between the SV data
transition boundaries and the receiver 20-ms clock epochs (i.e., the FTFs). The con-
trol segment ensures that every SV transmits every epoch within 1 ms of true GPS
time (i.e., the SV clocks are aligned to within 1 ms of true GPS time). Therefore, all
of the SV data transition boundaries are approximately aligned to true GPS time at
transmit time. However, at the GPS receiver the SV data transition boundaries are,
in general, skewed with respect to each other and with respect to the receiver's FTF
boundary. This is because the SVs are at different ranges with respect to the user
GPS receiver antenna phase center. The user GPS receiver must adjust the phases of
its integrate and dump boundaries in order to avoid integrating across the SV data
bit transition boundaries. The time skew, T s , is different for each SV being tracked,
and it changes with time because the range to the SVs change with time. Therefore,
the epochs from each replica code generator, such as the C/A code 1-ms epochs, are
skewed with respect to each other and to the FTF. As a result, the integrate and