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Comment by galangalalgol | Hacker News Reader

Don't listen to the rule followers! There are no SDRs out there with enough power to be much of a nuisance to anyone. Do look up what the band is for. Do watch it for a few hours or days to confirm it is clear, but seriously if you don't interfere with anyone the FCC isnt paying you a visit. Even if you do insist on following the rules, just stick to ism bands. 315 ( key fobs) (2.5ghz wifi bluetooth) and one around 900. You can go full blast on any sdr and not get close to the half watt limit.

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Whatever you do, PLEASE PLEASE PLEASE DO NOT TRANSMIT BETWEEN 1.100 Ghz and 1.600 Ghz. GPS receivers operate on very low power signals and it's entirely possible to accidentally jam them even without high power signals. If you are near the approach path for an airport you can cause serious problems.

In other words, uhh, don't use these projects:

https://github.com/Mictronics/pluto-gps-sim

https://github.com/osqzss/gps-sdr-sim

Or if you use them, do so in a completely RF shielded environment. You wouldn't want to wreck havok with drones, Lime scooters, Bird Scooters, cell phones, etc. Right? :)

Not to mention automated aircraft landing systems, ambulance GPS, fire brigades and various cellphone and/or power grid synchronization systems.

Yeah that ia definitely one no to stomp on. Vls an tacan are good ones too but they are immediately apparent if you stare at the band. Gps is so quiet you would only know by looking it up. Which you should absolutely do before TXing.

GPS is so quiet it is actually below the noise floor and needs all kinds of black magic to be recovered.

Is the system different for Civilians? It seems like a system that is so fragile would be a pretty big vulnerability for something that is in such high use by the Military.

The military has a higher bitrate (and encrypted) signal, which leads to higher positional accuracy if you have the code. It's no less vulnerable to brute force jamming though, which is why military platforms typically solve this by having GPS receivers with high receive selectivity upwards and very low selectivity in the horizontal plane (where hostile platforms carrying jammers are likely to be).

Source: studied weapons engineering in Naval College.

The military does not have a different system, though - at least in the past, not sure how things are now - they had access to different modes of precision, and it is possible to augment GPS by using ground based reference beacons if you want.

https://en.wikipedia.org/wiki/Differential_GPS

For additional precision of reliability.

As for the signal levels at the antenna, that is just a function of distance of the transmitters and the crappy antenna on the receivers. The fragility is to some extent overcome by using multiple satellites (more than you need a fix for), but of course these can still be overcome by a jammer.

If you really want a stronger signal you have a couple of options, the first one would be a better antenna, the second to chill the receiver pre-amp.

The best analogy is to imagine someone who is 100 yards away from you shouting at the top of their lungs and then someone who is whispering up close straight in your ear. That's just the physics side of it and no amount of trickery is going to change that in a way that will make the system more robust against jammers. Any radio based system can be jammed like that.

It is very spoofable:

https://www.maritime-executive.com/editorials/patterns-of-gp...

It's also illegal to transmit unlicensed in the L band. Fortunately though, GPS uses spread spectrum signals which are incredibly resistant to interference, in addition to using several frequencies simultaneously and several reference recievers on the ground, it is extremely unlikely you will jam GPS.

Also don't stomp on ham bands even if they appear empty. They are almost universally awesome people. There are emergency channels, that should fall under "look up what the band is for" but, idiots...

Less than a watt could get you around the world.

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