GP1020
2.
Record the sign of accumulated data on the prompt arm for
each data bit period of 20 ms, with filtering to reduce the effect of
noise on the signal. Note that there is a sign ambiguity in the
demodulation process in that it is not possible to tell which data
bits are ‘0’s and which are ‘1’s from the signal itself. This
ambiguity will be resolved at a later stage when the full Naviga-
tion Message is interpreted.
Space Vehicle Navigation Message, which contains information on
the GPS system time for the transmission of the current subframe;
this is equal to term t
2
.
The time information in the navigation message allows the
receiver time to be initialised with a resolution of 20 milliseconds (one
data bit period) but with knowledge of the precision to much better
than one C/A code chip – a little less than 1 microsecond. As the time-
of-flight from the satellite to the receiver is in the region of 60 to 80
milliseconds an improved first guess for local time could include an
allowance for this delay to reduce the iteration time later.
By using the data to time-tag the TIC, along with the values of the
Epoch counter, the Code generator phase, and the Code clock
phase it is possible to measure the time of the SV signal in local
apparent time. This gives the value of t
1
needed for the pseudorange
measurement. The pseudorange can now be computed as t
1
2t
2
.
The error present in the time setting is the initial value of the user
clock bias, with an allowance for the various counter phases. Once
a Navigation Solution has been found the clock error is precisely
known and may be used for future pseudorange calculations.
Because the receiver clock drifts with time, the clock bias changes
with time and must be tracked by the Navigation software.
PSEUDORANGE MEASUREMENT
The measurement data registers provide the raw data neces-
sary to compute the pseudorange. This raw data is a sample, at
a given instant set by the GP1020 TIC, of the 20 ms and 1 ms
epoch counters, the C/A code phase counter and the code DCO
phase. By definition, the pseudorange is expressed in time units
and is equal to the satellite-to-receiver propagation delay plus
the user clock bias. The user clock bias is first estimated (blind
guessed is more likely with a cold start, but iteration then takes longer)
and then obtained as a by-product of the navigation solution. The
pseudorange is equal to the user’s apparent local time of reception
of the signal (t
1
) minus the GPS real time of transmission (t
2
).
With the demodulated data, the software has access to the
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