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referred as full correlation) the two codes can block .each other out and the GPS-data
remains and can be further processed. This method is called dispreading.
Figure 13
Every satellite has its own unique PRN-code so that the GPS receiver can distinguish
the signals from various satellites. GPS receiver is able to generate 32 PRN-codes.
Until now so many satellites have not been launched.
When the GPS receiver has to start up it doesn't know which GPS signal is from
which satellite. Therefore it tries to lock with the 32 known PRN-codes one by one. If
one code locks then the information of one satellite can be decoded. This information
also contains data about other satellites and the rest can soon be received too.
The main reason for using PRN codes in the GPS system is that the PRN code
enlarges the unambiguous measurement range. One must keep in mind that after 1023
bits the code is repeated. It is case that GPS-receiver is aware it is ‘looking’ at the
right code and not at its predecessor or successor. Looking at the wrong code gives a
navigation error of 300 km (corresponding to the code length of 1 millisecond).
Autocorrelation :
The ideal GPS receiver would have an infinitely wide receiver BW which would
allow the receiver to capture 100% of the GPS spread spectrum signal. The
normalized autocorrelation function for an infinitely wide BW is generally illustrated
as shown in figure below.
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