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.

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.

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.

Don't you mean... M-code? (meaning "military" code)

https://en.wikipedia.org/wiki/GPS_signals#Military_.28M-code...

Though I'm honestly surprised I've "known" of GPS for this long and never realized there were a variety of codes. Legacy--some still operating, modern, military, and more. I'm going to enjoy binge reading this wiki/math stuff tomorrow.

No, I'm talking about the P code or precise code. It's modulated with the encrypted W code to create the encrypted Y code.

m-code is the new hotness that is stuck in development hell currently.

Yeah. As I understand it, previously it wasn't possible to do dual-band GPS without using the encrypted military P(Y)-code because the unencrypted C/A code was only on the L1 frequency, and for best accuracy you need dual-band to measure and compensate for ionospheric delay.

is there any reason to be even more accurate than a 5 meter radius? I mean, if you are targeting a person in a building, i can't see why it has to be more accurate than that. Navigation of a drone that is less than 5 meters in size may require higher accuracy, i suppose...

The chip doesn't just improve maximum accuracy, it also vastly reduces the deviation when in poor conditions such as inside a building. The technique to do so just happens to also make peak operating conditions more accurate.

Self-driving vehicles. You can buy GPS-based automatic steering systems for tractors right now [1] (there's still an operator on board, so it's not truly autonomous)

Obviously the self-driving cars of the future will also need other sensors as GPS doesn't work under tunnels and bridges.

[1] https://agriculture.trimble.com/precision-ag/products/steeri...

That's precisely the problem TomTom and Here HD Maps aim to solve.

Airplanes are already using the higher-accuracy GPS+GNSS to land safely on the runway in low-visibility conditions, guided in by the Flight Management Computer. It makes a big difference whether the plane is 5 meters above the runway or zero meters above the runway.

Commercial airliners use a radar altimeter, not GPS/GNSS to assess altitude.

Most if not all airliners equipped with autoland systems use ILS rather than GPS for position as well.

Agreed, but only the big iron. I spent 8 years working for Universal Avionics, who develops precision WAAS-SBAS GPS flight management systems for GA / BizJets: http://uasc.com/home/nextgentechnologies/precisionaviationwi...

A problem that I usually have with Google Maps is that, when I am driving in (or next to) an elevated highway, the GPS usually does not know if I am in the highway or in the road beneath it.

Knowing the lane of the highway you are may be useful as well.

Determining lanes for map navigation is a big problem. I had a buddy that was responsible for mass export of Verizon telemetry data from Verizon to a third company. Their "holy grail" was to be able to determine lanes through triangulation in a timely manner (2008-2009ish). Don't know if the problem ever got solved and it would have only been relevant in dense metropolitan areas.

Imagine pedestrian navigation. 5 meters (a rare, perfect accuracy) would place you in the center of a street instead of the sidewalk.

5 meters is the difference between one room and the next room. Huge potential for indoor navigation and contextual AR applications.

Mapping the boundaries of a property is usually desired to much higher precision than 5 meters!

Autonomous vehicles and drones (used for delivery) can benefit a lot from such accuracy.

Wow really? You would post that on a technology board?

Yesterday I read an idea from a guy who wanted to create a mariokart-style "ghost" via AR to pace you while you are on a run. That's one example.

Guess I'm being downvoted for snark. But the definition of technology is to advance the state of the art. None of us would be here if we thought everything was already good enough.

> There are some really fancy civilian receivers out there that can leverage Y code to a limited extent without knowing the encryption key

Would you have links to any examples?