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Comment by salmonellaeater | Hacker News Reader

Consumer GPS devices have a speed and altitude limit [1], which I initially thought should preclude using it in a fighter jet, but I was wrong (only something extreme like an SR-71 would hit this limit):

> ...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).[3] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications.

[1] https://en.wikipedia.org/wiki/Coordinating_Committee_for_Mul...

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An F-15 can exceed 59k feet and 1200 mph. It has a service ceiling of 65k feet, and a maximum speed of over 1600 mph.

That makes sense - the limits are designed to stop you guiding a missile with one, not guiding a jet plane.

Unfortunately the safeguards can also kick in with high altitude balloons which have no speed beyond ambient wind.

The GPS has to shut off when going faster than a certain speed or higher than a certain altitude. IIRC the GPS maker can decided which they want to follow, and most chose to stop working at high altitude. It’s possible but more difficult to find a GPS that will work with high altitude balloons.

I doubt they hit the speed limit - but balloons can go up to 100,000 feet which is above the altitude limit.

For everyone else that notices that COCOM limits require both speed and altitude, the problem is that some manufacturers have implemented these safeguards with an OR rather than an AND. So it’s possible to get tripped up by either limit in practice (unless you know for sure your specific device implements this as an and).

>> > ...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).[3] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications.

> That makes sense - the limits are designed to stop you guiding a missile with one, not guiding a jet plane.

Only some kinds of missiles, though. It seems like those limits would be quite acceptable for cruise missile guidance, and I'd think GPS would be more useful there than in an ICBM.

Probably because the threat surface of a cruise missile is similar to a low-flying manned aircraft. There's lots of civilian use cases for GPS under those same conditions, where there's relatively fewer uses for speeds and altitudes in excess of the COCOM limits.

I take it it’s much easier to get rid of cruise missile than ICBM?

I just had a very dark thought suppose a nefarious entity got hold of starship, added a multi ton nuclear payload and sent it towards some city center.

How does it get prevented?

Probably wouldn't even need a nuclear payload for that. If it was a sufficient size starship impacting at light speed I assume the impact would be tremendous.

The basic idea of missile defense is to intercept the incoming missile with another missile. What really matters is detection with enough time to react and fire those intercepting missiles. There are both cruise and ballistic missiles that can travel at hypersonic speeds which drastically reduce the viable interception window. Cruise missiles are actually scarier to modern militaries, in part because they are harder to detect separately from other aircraft.

>enough time to react and fire You mention the intercept window, but it's more complex than interceptable / not interceptable, and time is not the only factor at play.

It's much easier to intercept the missile earlier rather than later, even if the window allows for a later intercept.

It's easiest during ascent phase, because: 1) the rocket is still accelerating (slower target), 2) the rocket is still burning (bright target) 3) before the payload(s) has separated from the booster (big target) 4) before warheads and decoys deploy (single target)

There are some other aspects to consider as well, like a trajectory-changing near-miss during early ascent will have a much greater impact than the same in late ascent, (depending a lot on the target type).

Which is why US missile defense began as an entirely ship based program, to be deployed where ascent interception would be possible.

Although this has changed a great deal since then; technical abilities and limits revealed through testing, as well as political/ budget constraints have shifted the tactical role of the system more state-side, not without controversy.

There are no hypersonic cruise missiles. There are cruise missiles that are supersonic for at least part of their trajectory, though that is often achieved by using a final ballistic trajectory.

And no, cruise missiles are not scarier than ballistic missiles. There is no difficulty detecting them or distinguishing them from other aircraft. Ballistic missiles are far harder to deal with because the time to intercept is tiny and they have gravity on their side.

It also is kinda misguided.

If you have the capability to acquire/build a ballistic missile of that class, you most likely can (will) build your own GPS/GNSS receiver. Hell you can take a software defined radio and make your own software-based GPS receiver that does NOT disable itself at those described thresholds (some folks have done this for extremely high-altitude balloon experiments).

Selling it of course is another question.

People under-estimate how difficult geography/ geodesy/ navigation used to be, pre-NAVSTAR GPS. This difficulty was "built-in" to the military defensive posture/strategy/tactics of many countries, to the extent that simple possession of a GPS receiver was/is a crime. And not a misdemeanor, but a treason level offense.

Keep in mind, the programs that evolved into NAVSTAR started in the 50s\60s and the tech specs were pretty much set by 1980, along with the legal framework and military policy regarding it's operation.

Ironically, it was some of the systematic weaknesses if the original design that led to it being so much more useful (and accurate) than it was ever intended to be.

But it is still a military program, with military style regulations, so it shouldn't be too surprised that some of the 1960's era rules haven't evolved too much.

It's more surprising to me that so many of the restrictions have been relaxed. (Even if it's policy playing catch-up practice.)

Or even reverse engineer the firmware to remove the restriction. I'm also guessing that there are plenty of Chinese made gps devices that don't have that limitation. Simply because, why bother. Assuming you can find one accurate enough on AliExpress, I'd say there is a good chance it has no special restrictions.

This is also what I would expect the reality of the situation to be, regardless of policy.

Not to mention that one would assume if someone can make a GPS-guided missile, they can sure as hell alter/find a workable GPS for their purpose...

It's not Joe Average will try to make a missile with a GPS from his Garmin watch...

But any number of non State actors would. The IRA or ISIS or Hezzbollah for example. They probably don't have the tech capabilities to RE their own GPS firmware, but non-orbiting rocket science isn't that hard. Iron Dome does okay against simple rockets, but a big guided rocket (V2 size) would be a different problem entirely.

Obviously drone attacks make use of GPS, and it's a worry to many -- for example, there is frequently GPS jamming when Putin is visiting somewhere.

Realistically, this is the kind of restriction that's in place to impede the high-frequency & low-capability regime in the space of non-nation-state (or, historically e.g. the 70s, non-peer nation) threats.

That makes sense. However in today's ever expanding electronics-wonderland, I wonder if it matters anymore.

I think, post HK protests, it's going to be harder to order precursor parts from China. All sorts of jamming and drone tech is being used there by both sides. Hopefully we see restraint and mutual de-escalation and we don't see any use of energetics.

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