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Comment by tweedledee | Hacker News Reader

I think it’s for military protection. I had a flight canceled due to GPS jamming. I think it was to protect a U2 flight. Putin also flies around with GPS jammers.

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Wouldn't that mean that Putin's flights can't use GPS to navigate? But maybe he worries more about someone intercepting his plane than he does about crashing due to a navigation mistake. And his risk assessment may in fact be accurate...

You can jam 99.99% off the time except when your software takes a reading. So you get clear signal everyone else can’t.

I'm not sure that'd work very well as the primary strategy. A hot start can take 20+ seconds and the attacker has no particular advantage in time to fix when the interference stops.

I'd assume a russian military plane would primarily use GLONASS. GLONASS has the advantage of being more resistant to narrowband single tone jamming because it uses FDMA rather than CDMA like GPS. The signals are also in slightly different frequency ranges.

Obviously we navigated acceptably, including aircraft, long before the development of GPS in the 1970s. GPS is easy and precise and can make risky edge cases (like very low altitude flight) safer or allow higher density in air spaces and such, but it's not necessary particularly not at the government aircraft level. Inertial guidance, ground based Doppler navigation systems, and even automated celestial navigation in combo can produce great results just at a much, much higher expense. That isn't an issue for the leader of a major power though.

The SR-71 was a cool example of a system that used a hybrid of automated celestial navigation and inertial. Fully self-contained.

Presumably Putin's flights, on Russian-built aircraft (specifically the Il-96), use GLONASS?

Of course, the VC-25A used as Air Force One has a cockpit station for a navigator, specifically so that it can use old-style navigation methods if access to GPS and other navigational aids is lost. Putin's modified Il-96-300PU may well also have one.

Yeah, the original 747s flat out had a sextant port in the roof. Could navigate by the stars the old fashioned way. Given how crazy precise (and expensive) inertial guidance systems were developed for ICBMs in the Cold War I have to imagine though they have one on Air Force One, along with an automated celestial guidance system for recalibration. The AIRS (advanced inertial reference system) used in the LGM-118A Peacekeeper missiles have to be some of the most mindblowingly precisely made machines in human history. Drift of something like 1.5/10000° per hour! .003° per week (not that they'd ever run that long obviously, ICBM and all). At any rate, highly precise IGS is doable for enough money, even with older tech though these days I'd assume they're using fiber optic gyros or something like that. It's a neat area.

It should be possible for the jammer to subtract off its own signal rather more effectively than anything which doesn't have access to the pRNG it uses to generate the interference.

The thing is, GPS signals are really weak at the receiver. They're technically below the noise floor, and receivers have to use shenanigans to get a usable signal at all. If you're transmitting the jamming signal, you're really close to it, and therefore it is really strong near your receiver. I wouldn't like to calculate how many orders of magnitude stronger than the real GPS signal, but it would be a lot. Generally, subtraction works when the two signals are similar in strength, and you still lose definition in the received signal. GPS signals are so on the borderline of being able to be received that this would just not work at all.

> They're technically below the noise floor, and receivers have to use shenanigans to get a usable signal at all.

This is nonsense.

There are other accurate systems for navigation available for aircraft, like the network of Non-Directional Beacons (https://en.wikipedia.org/wiki/Non-directional_beacon). It's a bit more of a pain in the butt to fly, because you're going from waypoint-to-waypoint, rather than a course relatively towards your destination, but it was how we did aircraft navigation up until relatively recently.

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