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Comment by _heimdall | Hacker News Reader

When Israel was waiting for a retaliatory strike from Iran they jammed GPS in the region. I never found a clear explanation of how it was done technically, this would make total sense if their system also was targeting atomic clock signals rather than GPS itself to confuse incoming missiles or aircraft.

That does raise an interesting question though - do missles actually depend on the standard atomic clock signals? Maybe that isn't how they did it, that seems like a dependency you wouldn't want in a weapon.

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GPS signals are atomic clock signals. The receiver triangulates its position by comparing the time delays between the signals originating from different satellites. The receiver itself doesn't require a good clock since it only compares signals with each other.

And you can even update your clock info from the GPS signal. So the only dependency is GPS or similar.

But would Iranian missiles even use GPS? Isn't accuracy limited for civilian use for precisely this reason?

No. The US stopped degrading civilian GPS accuracy in 2001[1]. Although the US retains the ability to degrade civilian GPS in specific target areas.

Regardless, if you’re building a long range missile, you need some ability for it to navigate. If you’re not using GPS, then what would you use instead? Additionally there’s nothing preventing you from using multiple navigation systems in tandem and fusing the results together, which is almost certainly what these missile do.

Sensor fusion reduces the impact of stuff like GPS jamming, but certainly doesn’t eliminate it. The over all system will be less accurate with fewer inputs, and if you’re the one faced with a high speed missile flying at you, I suspect you’ll take every edge you can get, regardless of how small the impact might be.

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

>Regardless, if you’re building a long range missile, you need some ability for it to navigate. If you’re not using GPS, then what would you use instead?

US ICBMs and submarine-launched ballistic missiles use a combination of inertial and celestial navigation: in space of course there are no clouds to obscure the stars:

https://en.wikipedia.org/wiki/Celestial_navigation#:~:text=I...

Many cruise missiles use terrain contour mapping. In principle at least it seems like it should work for airplanes too.

Doesn't really work if you're trying to navigate over an ocean. Cruise missile also operate over relatively short distances (1,000km - 5,000km, most being 1,000km - 3,000km) compared to long haul flights, plus they also fly at very low altitudes (which helps them avoid enemy radar) which makes terrain contour mapping easier.

Not to mention, if you're designing a cruise missile, you're not that bother about how your navigation system might interfere with other aircraft, or ground systems in the area. I doubt having thousands of planes flying around shooting radar straight down at the ground would work particularly well.

GPS Accuracy used to be limited, but that ended decades ago.

There are rules about GPS hardware that say that they should cease working above certain speeds and altitudes for guided missile purposes. But that is a firmware issue. I’m sure the Iranians have figured that out if the are even using off the shelf hardware.

A error correction technique I learned as a young land surveying assistant is to put a gps antenna on a known fixed point location. The delta between the fixed point and the point of measurement is cancelled out to get a more accurate read.

We did this to trial some new (at the time) surveying equipment when the primary equipment was optical. It would save time for really long measurements through the forest and mountainous terrain .

You can even subscribe to services which do this for you! There are a few companies with large-scale networks of fixed receivers, and you can get the observed offset from a node near you via the internet, usually via "NTRIP".

Getting correction data from a node a few dozen kilometers away isn't quite as good as having your own fixed base station a stone's throw away, but it's way more convenient and for a lot of applications plenty accurate.

GPS signals are relatively low power (American GPS broadcasts at 25 watts and the signal is a tiny fraction of a mW at sea level). In theory, it's easy to pump out noise over it, especially the civilian frequencies that Iran would in theory be using.

Depending on the receivers and what (combination?) of GPS/GLONASS/GALILEO/BAIDU Iran uses, you could easily overwhelm them.

There have been cases of delivery drivers using jammers to stop companies from tracking them, only to interfere with airport landing systems, which is a concern as a lot of warehouses are near airports.

EDIT: power at ground level is miniscule

> GPS signals are relatively low power (American GPS broadcasts at 25 watts and are ~10-15W at sea level)

Did you lose 16 orders of magnitude for the sea level values? GPS signal strength on the ground is usually below -135dBm per square meter. That gives you a couple of femtowatts with commonly used antenna, if you're lucky.

Easy to jam doesn't begin to describe it.

Shit, you're right. I blame the time change.

My understanding is you just flood the spectrum at the frequency that GPS is operating at

GPS signals are weak since they come from far away

GPS (as well as most satellite) signals are weak because it's strong enough for line of sight even from so far away. They only transmit at 25W. Comparatively, an FM/TV signal will often broadcast at tens of thousands of watts and up.

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