From mynameisvlad link given below[1] the restrictions are to prevent intercontinental missile navigation not regular missile navigation. The official limits are a speed (1,000 knots) and an altitude (18,000 m), but many manufactures do an "or" test instead.

"In GPS technology, the term "COCOM Limits" also refers to a limit placed on GPS tracking devices that disables tracking when the device calculates that it is moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).[2] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications.

Some manufacturers apply this limit only when both speed and altitude limits are reached, while other manufacturers disable tracking when either limit is reached. In the latter case, this causes some devices to refuse to operate in very high altitude balloons."

[1]https://en.m.wikipedia.org/wiki/CoCom

The non-mobile version: https://en.wikipedia.org/wiki/Coordinating_Committee_for_Mul...

Is it really so hard for a determined terrorist to build his own gps receiver? Or at least modify one to remove that limitation?

They don't need to. There is a great many of more cost-effective ways of causing havoc, and even if they really wanted to use GPS-guided missiles, it's probably a better bet to build slower missiles with consumer hardware, but build a lot of them - missile interceptors are very expensive. If a DIY $1k missile needs to be shot down by $1M Patriot, they'll be doing lots of damage with a couple of them, without even hitting anything.

Yes. Look at the ham hobby, hardly any of them can pick up GPS signals, despite the technology being literally everywhere

That's not true at all. You can use a $20 SDR card to receive GPS signals and and then can decode them on any commodity laptop, or a small computer like a raspberry pi.

e.g. https://www.rtl-sdr.com/receiving-gps-with-an-rtl-sdr-dongle...

Receiving the signals is only a part of the problem, and even getting a "fix" isn't all that hard. Maintaining an accurate fix, and at the same time refining away previous errors, on a fast moving vehicle, is a dark art.

Software architectures are the limiting factor here.

Every app on your phone cares about "Where am I now", but no app wants to know "Where was I 5 seconds ago, but with more accuracy than you knew when I last asked".

Neither android not iPhone have an API to allow GPS hardware to refine the accuracy of historic position locations.

Possible maybe but unusual. 99% of ham is narrowband, GPS is spread across a megahertz. Somebody goofed and made a TV tuner chip that could be used as an SDR at that sample rate, and I'm sure the powers that be arent happy.

You what?

Any sensitive Spectrum Analyser or SDR can see the bump in the spectrum caused by the GPS Signals. And quite a few amateurs have built homebrew GPS receivers.

Or they can buy a little GPS module and use the data stream for a huge range of projects.

Most modern digital Ham communication methods require a 10MHz feed from a GPS module to maintain sufficient time and frequency accuracy.

Too late to edit so replying to myself. I was mostly wrong. The GPS limits are much higher than I thought, and accuracy of INS is too low, although I still think that limit is artificial.

Yes the real answer is some impractical nonsense. Star navigation? Are you serious?

I beg to disagree. My phones have all gotten a lock on planes so far.

The limits are higher altitude and faster speed then you've ever been. If the Concorde was around you would see it firsthand. Weapons would be worthless against first world defences at airliner height and speed. Gps receivers shut down on weather balloons regularly, it's a known issue

The limits are real, encoded in public law. I don't understand how you could possibly disagree with me on this point

Ignore the comment above, obviously ignorance :) Thanks for explaining.

It's okay, I was wrong about what the limits are too. They've been increased a lot

The reason your GPS receiver does not work on a plane is likely because it's processing algorithms aren't tuned for those speeds. The popular Ublox NEO-6M for example needs to be explicitly switched to "airborne" mode.

The fact that you are enclosed in a faraday cage also isn't helping, but they make planes from plastic now, so this shouldn't always be the case.

Planes don’t usually reach the limits, but they exist. The limits seem to be 1200mph or 59000ft altitude.

https://en.m.wikipedia.org/wiki/CoCom

1200mph and 59000ft altitude

Unfortunately many cheap GPS chips use the (incorrect) or definition, not and. It was a pain to find one that works above 59000ft but very slow speed for a weather balloon project some years ago.

I like using a NSF funded mobile app "Flyover Country" [1] to identify features in the landscape when I fly. You can download the maps and data for you flight path a head of time and then use it in airplane mode on the plane. GPS works fine (by the window) and the app locates your position on the map.

https://flyovercountry.io/

Odd that you'd think that aircraft would be covered by these restrictions considering the fact that they use GPS to navigate.

There's not a ton of civilian craft cruising about at nearly Mach 2.