Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -9
To recover the data from spread spectrum signal at the receiver, the energy spread over
a wide bandwidth must be correlated or de-spread into a narrow bandwidth by frequency
and code shifting. Fig. 6-11 shows the criticality of correlation in terms of recovering the
signal.
Fig. 6-11 GPS Received Signal
Each GPS satellite transmits a unique PRN code. The receiver first demodulates received
BPSK signal to extract the satellite PRN code overlaid with Nav Data. The receiver then
generates a local copy of the PRN code and then shifts the timing of the local code by
1 bit, relative to a receiver time mark, until all 1,023 bits of the local code are in phase
(correlated), with the PRN code received from the satellite. A modulo 2 addition process
is used to recover the Nav Data.
If all 1,023 bit shifts have been tried without achieving correlation, the receiver local
oscillator frequency is offset to the next value and the process is repeated. The reason
for this frequency offset, is because satellites and receivers are in relative motion to one
another and hence Doppler shift in the transmitted carrier frequency occurs. The
transmitted signals can be shifted by up to +/- 5000 Hz at the point of reception.
The determination of the signal travel time and data recovery therefore requires not only
correlation with all possible codes at all possible phase shifts, but also identification of
the correct phase carrier frequency.
In the case of a receiver cold start, where the receiver does not have a current almanac
loaded, every PRN would be tried until an SV is found in view, after which the almanac
can be downloaded and used to determine which SV’s to acquire next. Because the
search for the first SV may take some time, GPS receiver cold starts take appreciably
longer than warm starts, where the GPS receiver has a current almanac stored in
memory.