Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 202

particular receive time epoch (epoch n ) in the GPS receiver. Every epoch in the PRN
code that is transmitted by SV i is precisely aligned to the GPS time of week as main-
tained inside SV i 's time-keeping hardware. When this transmitted PRN code reaches
the user GPS receiver, which is successfully correlating a replica PRN code with it,
the phase offset of the replica code with respect to the beginning of the GPS week
represents the transmit time of SV i . The first thing that the navigation process needs
for position measurement incorporation is the SV i transmit time corrected to true
GPS time. It is therefore logical that the natural measurement ( SV i transmit time)
should be sent to the navigation process (along with the receive time) and not the
pseudorange measurement (along with the receive time). This is because the SV i
transmit time is lost after this artificial computation is performed. This forces the
navigation process into a wasteful iterative process of computing the SV i transmit
time. Highly sophisticated GPS receivers implement vector tracking of the SVs
instead of scalar tracking described herein. This overcomes this problem because
either the raw I and Q measurements or the discriminator outputs are sent to the
navigation process as measurements for Kalman filtering by the navigation process.
Thus, the navigation process dynamically changes the noise bandwidth of the
tracking loops in an optimal manner.
Typically, the GPS receiver will take a set of measurements at the same receive
time epoch. This is why the receive time is not identified with any particular SV PRN
number in (5.27). When the GPS receiver schedules a set of measurements, it does
this based on its own internal clock, which contains a bias error with respect to true
GPS time. Eventually the navigation process learns this bias error as a byproduct of
the GPS navigation solution. The SV transmit time also contains a bias error with
respect to true GPS time, although the control segment ensures that this is main-
tained at less than 1 ms. This correction is transmitted to the receiver by SV i as clock
correction parameters via the navigation message. However, neither of these correc-
tions is included in the pseudorange measurement of (5.27). These corrections and
others are determined and applied by the navigation process.
5.7.1.1 Pseudorange Measurement
From (5.27), it can be concluded that if the receiver baseband process can extract the
SV transmit time from the code tracking loop, then it can provide a pseudorange
measurement. The precise transmit time measurement for SV i is equivalent to its
code phase offset with respect to the beginning of the GPS week. There is a
one-to-one relationship between the SV i replica code phase and the GPS time. Thus,
for every fractional and integer chip advancement in the code phase of the PRN code
generator since the initial (reset) state at the beginning of the week, there is a corre-
sponding fractional and integer chip advancement in the GPS time. The fractional
and integer chip codephase will hereafter be called the code state. The receiver base-
band process time keeper, which contains the GPS time corresponding to this code
state, will hereafter be called the code accumulator.
The replica code state corresponds to the receiver's best estimate of the SV trans-
mit time. The receiver baseband process knows the code state because it sets the ini-
tial states during the search process and keeps track of the changes in the code state
thereafter. The receiver baseband code tracking loop process keeps track of the GPS