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Comment by mherdeg | Hacker News Reader

Hmm. I don't see any mention of whether this design complies with the COCOM Limits, an agreement that GPS devices should disable themselves when at an altitude above 60,000 feet and a speed faster than 1,000 knots.

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Probably not

Which is funny, because to know you're higher/faster than allowed you need to calculate that from GPS data

In practice, not sure how this is achieved, but for speed this may even be a limitation of the circuit (for example, if you can't compensate for the doppler effect at that speed or has to add some other processing - still, the doppler effect should be small)

1000 knots is still +/- 1.5 Mach (at sea level) so Concorde and fighter jets should use a special GPS (but this is very fast)

> Which is funny, because to know you're higher/faster than allowed you need to calculate that from GPS data

Actually, no. All military aircraft have inertial navigation packages that, among other values, provide altitude above the elipsoid, position, and velocity vector. When available, these systems are fixed periodically to GPS-based measurements.

Yes, but GPS modules can't expect to have this information input into them. See http://en.wikipedia.org/wiki/CoCom

You're right from the point of view of the aircraft/missile, where several pieces of information are put together (GPS, barometric, inertial, etc) for navigation purposes.

Right, minor brain fart. I was thinking at the systems level, but the requirement is for the module to cut itself out, not have an external system do it.

I don't think there's a requirement to completely shut down, only a requirement to cut off your data output. You can keep calculating your speed and position, you just stop telling the outside world.

The problem is, COCOM says:

GPS devices should disable themselves when at an altitude above 60,000 feet AND a speed faster than 1,000 knots

Whereas some vendors read it:

GPS devices should disable themselves when at an altitude above 60,000 feet OR a speed faster than 1,000 knots

Which effectively disables your cheap GPS dongle in your home-made stratospheric balloon, which is quite capable of reaching 30 km, above the COCOM altitude spec.

Sometimes it's quite tricky to figure out if a given receiver is an AND or an OR.

You could maybe beg/steal/borrow a GPS simulator[1] and test your candidate receivers with it. Best do it in a Faraday cage though, unless you want to upset some Very Serious Important Things[2], and (likely government) owners thereof.

Probably way out of budget range for hobby stuff though

[1] One is the WelNav GS720: http://www.t-e.com.tw/GS-720.pdf

[2] A Simple Demonstration that the Global Positioning System (GPS) is Vulnerable to Spoofing: http://www.ne.anl.gov/capabilities/vat/pdfs/GPS-Spoofing-%28...

Doppler shift due to observer speed will almost always (i.e. for anything other than an actual ballistic missile, and even then only a missile traveling west) be less than the doppler shift due to satellite speed for a stationary observer. So, artificially restricting the doppler spread isn't an effective method of implementing the COCOM restrictions. It would also make PLL lock-up more difficult.

Concorde is old enough that its operators probably never went to the trouble of doing the paperwork to have a GPS integrated with the rest of the NAV system. For its (relatively) short flights, an internal navigation system plus VHF or LF radionav works adequately (and there were very few, if any, GPS/WAAS approaches certified by the time of its retirement).

Those limits are to prevent its use in intercontinental missiles. If intends to build one, I guess he'll have bigger problems than the speed at which his GPS receiver works. If you were thinking about the case where he sells it to someone making a missile, I guess that if someone is able of getting a missile capable of intercontinental flight, a GPS receiver won't be the missing bit.

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