[AlexanderTheGr8]

"Interestingly, the only thing stopping civilians from using the P code is the knowledge of the value of its chipping sequence. If the formula to generate the P code was publicly known, there’d be nothing stopping civilian GPS receivers from locking on to it, with the exact same techniques as are used for the C/A code."

I didn't finish reading the whole thing but was curious. Is there any way of brute forcing it or some other trick to get the chipping sequence to get the P code for more precise GPS?

[minetest2048]

Kind of. The P code itself is publicly available in publicly available specification IS-GPS-200. They are xor-ed by W code, which like the sibling comment says, is cryptographically generated, which runs at a lower rate. This results in P(Y) code signal, which is then transmitted over the satellites.

If you know whats the P code roughly is, which you can know from L1CA code and handover words, you can try using both possible W code value (its 0 or 1) and that W code practically dissappear, until the W code move to its next iteration and you need to guess again. This strategy is called semi-codeless tracking.

People did this so they can access the signals at L2 frequency (1227.6 MHz), which because its at a different frequency than L1 (1575.42 MHz), they got slowed down by different amount through the ionosphere. So by comparing the signals together, you can remove the ionospheric effects and get more accurate position.

These days you don't need all of this, GPS now have public signals at L2 and now smartphones have these fancy dual-frequency receivers

[tgsovlerkhgsel]

I would assume that the chipping sequence is derived from a frequently-rotated cryptographic key.

[codyd51]

I believe you are correct. My understanding is that the root P key is rotated daily, and needs to be manually uploaded to any military hardware that needs to use it.