Copenhagen Suborbitals has a blog-post about those limits:

  By default all GPS units have some built-in limits (so-called COCOM
  limits) that prevents them from providing any data if the velocity
  exceeds 515 m/s at altitudes above 18 km. These artificial limits are
  built into GPS receivers to prevent bad guys from using them in
  missiles and other nasty stuff.

... and proceed to figure out a way around ...

https://copenhagensuborbitals.com/gps-without-limits/

Please don't use code blocks for quoting, makes it incredibly hard to read on mobile.

> to prevent bad guys from using them in missiles and other nasty stuff

I was confused. So here's a translation: Bad guys = US enemies or non-allies. Not bad guys in any moral sense.

More like parties that are not technologically advanced nation states. Russia has its own satellites, China will find 10 ways around the problem, but a few random dudes won't be able to DIY a cruise missile with cheap parts that would be precise enough to be dangerous.

Or at least that's how I always understood it.

Actually civilian GPS would work fine in low-cost cruise missiles because they fly relatively low and slow. The restrictions are to protect against ballistic missiles.

Nothing at all: http://www.aholme.co.uk/GPS/Main.htm

That's really cool! I'd like to build something like that One Of These Days, when I have more free time.

Also gives you the point - if one guy can build a receiver on his own... I mean, I don't want to make it easy for rogue nations and terrorist groups to get ICBM-mounted nuclear weapons, but with the difficulty of building and testing the missiles and bombs, it seems a little silly to expect restricting GPS receivers to accomplish anything.

Technically speaking, such a receiver would be classified as a munition and prohibited from export out of the US: https://en.wikipedia.org/wiki/GPS#Restrictions_on_civilian_u...

But practically speaking, almost all GPSes are made outside of the US and then imported...

How do these folks do the math? Do they understand general relativity? Or do they just pull the equations from somewhere?

There is no general relativity involved in calculating your position. Solving for your position is basic geometry, if you want to have better resolution, have good model for the effects of the atmosphere on the radio waves.

http://www.aholme.co.uk/GPS/user_position_solution.pdf

In "Exploring Black Holes" by Taylor & Wheeler, project A is titled 'Global Positioning System', where they say locating your position accurately depends crucially on General Relativity.

Before the satellites are launched, their atomic clocks have to be tuned to offset for time dilation.

GPS satellites are all tuned this way, and it has the desired effect.

However, for people implementing GPS receivers, the signal arrives already corrected for time dilation. So the people making that part of the system don't have to know about general relativity at all :)

Huh, is the whole thing a myth then? I'd definitely read that GR is necessary, but now that I'm searching I see mentions of it being a myth.

I remember this question arising on HN about a year ago. GR is a necessary consideration in the design of the satellite system.

See page 5 of DTIC document: Relativistic Effects in the Global Positioning System

http://www.dtic.mil/dtic/tr/fulltext/u2/a158720.pdf

Without correction the drift due to combined coarse relativistic effects would be on the order of 100,000 nanoseconds per DAY.

In fact the first NTS-2 satellite was launched with the clock unadjusted for GR, due to doubters, but after three weeks the syntheziser had to be activated to compensate. The calculated uncorrected drift on that clock alone was 38,000 nanoseconds per day.

As the document notes, to further refine Navstar to greater levels of precision would require tackling yet another set of tougher relativistic effects, plus more mundane atmospheric influences.

GPS, Relativity, and pop-Science Mythology http://www.alternativephysics.org/book/GPSmythology.htm

Boy do I miss those types of sites. I need to find more of them.

Here's another, very detailed 4-part "build your own GPS" article from the early 90s:

http://lea.hamradio.si/~s53mv/navsats/theory.html

No FPGAs, RPis, or SDRs --- it's mostly done in discrete logic with a 68000 doing the back-end processing. As a bonus, he also implemented GLONASS. Furthermore, the software takes up less than 32KB of ROM.

Note that while well explained, this is a very 90s way of implementing a GPS receiver. As I understand it modern receivers use FFT-based correlators that have much better cold-start performance than the old-fashioned design because they can test very large numbers of possible correlations at once (generally hundreds of thousands of them).

Anecdotically, some chinese GPS chips for cell phones don't have those restrictions, but they maybe have others. The restrictions have become dubious anyway, as most nations who have that kind of rocket have the facilities to build GPS receivers (OK, I don't know about NK...).

Good luck. You're right but implementing a GPS receiver using a SDR is quite a feat. It's sort of been done but not in a way that would make it practical to use for e.g. a hypothetical terrorist cruise missile.