Autocorrelation in GPS

[senmeis]

Hi,

In GPS there are CA code and M code. As far as I understand it takes only a few ms to synchronize with CA code but it takes very long (maybe days) to synchronize with M code even if you know the M code because you have to wait for the start of this M code sequence. Is it correct?

 

[Baluncore]

Is it correct?

No. You do not need to wait for a week because you can calculate where in the P code you are from the state of the C/A code.

If the P code is a 48 bit PRBS then you only need to read 48 bits from the sequence to identify the current state, then extend that by using the same PRBS generator and lock to the correlation.

Where bit streams are involved, correlation can be tested for a time offset of many bits by using a shift register with one correlator per bit.

 

[senmeis]

Why C/A code? The P code receiver analyzes C/A code at the same time?

 

[Baluncore]

The P code receiver analyzes C/A code at the same time?

The receiver needs to know where it is before it can lock to the P code. The C/A and P codes have different chip rates, so they can be separated in the correlator. First lock to the C/A, then lock to the P.