Hı, Does gps l2 frequency signal, carrying p code, can be used without permission of US. If not how dual frequency gps receiver decrypt p code. I mean what is authorization process software or hardware? Thanks.
You can attempt to use the P code without permission.
Unfortunately it is encrypted, with what is known as the W code, to produce the actual transmitted Y code. This prevents full use of its increased precision.
Fortunately, the W code is applied at a different rate to the P code, which allows some intelligence to be gleaned from the signal without knowledge of the W code, and this method is actually supported officially. See the wikipedia article for links out into the documentation.
At a quick scan (correct me if I've got this wrong) it appears that although this semi-coded approach will not allow you to use the 10.23Mb/s chip rate precision, it will allow some L1 to L2 correlation to estimate better the variable delay due to ionospheric propagation.