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Comment by msravi | Hacker News Reader

Curious. Do missile systems also lose track of their position midway through a flight like our phones? Are the receivers radically better than the ones in our phones? What happens if a missile loses its coordinates because of a bad signal? Does it just continue and keep track of its position based on the last known position or does it abort? If it cannot lock on to a GPS signal at the start of its flight, does it fail to take off?

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Missile have their own very complex on-board guidance systems that don't require GPS at all. Newer missiles might use GPS as an extra layer of calibration, but I doubt they rely on it and most likely discard it if it's way off (otherwise GPS interference could potentially jam a missile).

There's a wiki on the various ways to guide a missile: https://en.wikipedia.org/wiki/Missile_guidance

Ballistic missiles use stars mid-flight and inertial guidance system during the terminal phase. Cruise missiles follow terrain.

Many of those systems were designed before GPS became operational, but even now relying on GPS is not particularly prudent.

The main things that cause loss of GPS 'lock' are the satellite signals being blocked (buildings, terrain, people, etc in the way), and multipath (signals being reflected off buildings, terrain, etc). A munition a few thousand feet above the ground has none of those things.

(There's also 'GPS jamming', which is totally a thing in war, but that's not really 'like our phones', generally.)

As other posts have mentioned, though, GPS is simply one input into weapons systems, not the input into weapons systems.

> Do missile systems also lose track of their position midway through a flight like our phones?

Civilian grade GPS receivers are intentionally crippled to stop working above some altitude/speed limits.

EDIT: It seems that the COCOM limits apply only after "1,900 km/h at an altitude higher than 18,000 m".

However, looking at the ublox receivers as an example, they have to be explicitly configured to operate at aircraft-level speeds, which smartphones likely aren't.

My iphone has picked up gps signals on short flights on several offasions, I have an almost flawless track on a flight from LGA to DCA, and on over the Mediterranean from Frankfurt to Lagos.

The LGA to DCA one had a max speed of 461mph (average 322) and altitude of 23,400 foot, with maximum negative gradient of 67.7%.

Maximum speed I've recorded overall (I've had the app open for 24,190 miles) is 673.3mph, and max altitude of 35,081 ft

Yeah, mine too! I got a perfect signal while flying from Las Vegas to San Francisco.

They not only have better receivers than in our phones, they have an entirely separate signal they can use- P-code (https://en.wikipedia.org/wiki/GPS_signals#Precision_code).

It's encrypted, so it's not for civilian use, and the ability to look at two signals of different frequency at the same time means that they have been able to have much more robust GPS than civilians for awhile.

Airplanes are big metal tubes that can block GPS signals, that's probably the problem you're having.

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