5 ms·

By the way, has anybody tried to crack P(Y) code using a kind of "rainbow table"? (I'm not advocating that ofc, just curious...) I mean, it gets it's security b

By the way, has anybody tried to crack P(Y) code using a kind of "rainbow table"? (I'm not advocating that ofc, just curious...) I mean, it gets it's security because the parameters to its generating function are not known. But the function repeats after about a week, and presumely can't be changed due to the large numbers of legacy devices in the field. So you could just record the whole signal for a week (I did a back of the envelope calculation some time ago and it would be about a TB IIRC). Then just build some kind of index, and after recording a few seconds, you can find your position in the code, and syncronize to it. Also, I've been wondering, how effective the old scheme with a secret military and a public code was - certainly an old GPS reciever shows up sometimes at a military surplus store. I wouldn't be surprized if enemy nations bought one of those, and extracted the secret parameters from the now quite old chips using modern technology.

MertsA 9y ago
What's really interesting is that there are some commercial receivers out there that will use P(Y) Code even though they can't decrypt it. IIRC the weakness was something along the lines of the lower rate W code made it so that while you couldn't decrypt it you could still use it as a reference to increase the accuracy of the C/A code. E.G. P code 1001011010010110101010100010101 W code 00001111000000001111111100001111 And since it's modulated with the W code this would leak information. Not really sure how off base that part is, that's just a rough explanation I was told many years back.
nullc 9y ago
P code itself has a period of a week but the W code that scrambles it doesn't have a known period AFAIK. Semi-codeless receivers can use it anyways. The reason is that the W code has a much lower data-rate (500KHz I think). Take the incoming signal, correlate with the aligned P code. The resulting signal will be a MHZ wide signal, bandpass to it, and square the result. The squaring wipes off the residual W code. Because you bandpass filtered before the squaring you don't get so much squaring loss. Lock the resulting carrier, and use RTK signals and long observations to resolve integer ambiguities. For a dual frequency rx you can exploit that P(Y) is the same on both frequencies and correlate them against each other to find the ionospheric delays.

MertsA 9y ago

What's really interesting is that there are some commercial receivers out there that will use P(Y) Code even though they can't decrypt it. IIRC the weakness was something along the lines of the lower rate W code made it so that while you couldn't decrypt it you could still use it as a reference to increase the accuracy of the C/A code. E.G. P code 1001011010010110101010100010101 W code 00001111000000001111111100001111 And since it's modulated with the W code this would leak information. Not really sure how off base that part is, that's just a rough explanation I was told many years back.

nullc 9y ago

P code itself has a period of a week but the W code that scrambles it doesn't have a known period AFAIK. Semi-codeless receivers can use it anyways. The reason is that the W code has a much lower data-rate (500KHz I think). Take the incoming signal, correlate with the aligned P code. The resulting signal will be a MHZ wide signal, bandpass to it, and square the result. The squaring wipes off the residual W code. Because you bandpass filtered before the squaring you don't get so much squaring loss. Lock the resulting carrier, and use RTK signals and long observations to resolve integer ambiguities. For a dual frequency rx you can exploit that P(Y) is the same on both frequencies and correlate them against each other to find the ionospheric delays.