3 ms·

Apologies for the brevity, I'm on a tablet with an annoying keyboard. There's two things that make GPS trickier to jam: - it's spread spectrum with relatively

Apologies for the brevity, I'm on a tablet with an annoying keyboard. There's two things that make GPS trickier to jam: - it's spread spectrum with relatively hign coding gain. The signal itself is pretty much imperceptible above the noise floor until you phase lock onto the signal and despread. A really effective jammer would likely need to send out a similar but corrupted signal, not just noise. - we know where the signal will be coming from. First that it will be coming from the sky, and once we have a copy of the almanac, we've got a pretty good idea where in the sky each SV will be. Directional antennas could further mitigate the jamming. You're not wrong about it being jammable, but it's not trivially simple. p.s. (because kb sucks) While the spreading codes for the civilian signal are commonly available and could be replicated in a jammer, the military spreading codes are not available. A well-funded adversary might be able to get ahold of them anyway, but it'd take some effort/treason.

jessaustin 11y ago
I'm sure you're correct, but this account seems to contradict all the horror stories we heard about LightSquared a few years ago. Is the explanation simply that civilian GPS uses crap radios and military GPS does not? I always sort of felt sorry for LightSquared, because they were obviously victims of regulatory capture.
- tonyarkles 11y ago
  I just read through the LightSquared story and am pretty intrigued! One of the fascinating demonstrations of the fact that GPS is relatively interference robust is that Galileo (ESA GNSS system) runs on the same frequencies as GPS; the different spreading codes mean that receivers can choose which signal to listen to. Reading about the LightSquared system though, they do talk about a quite real concern: overloading the receiver's frontend entirely. That's one of the concerns that would be addressed by using directional antennas (if you're receiving interference, rotate your antenna so that the source is in an antenna null). As far as I know, most civilian receivers wouldn't have a directional antenna, they'd have an omni so that they work in any orientation; it'd suck if your iPhone could only receive GPS signals if it were sitting flat. On a military vessel, it'd make a lot more sense to mount an upward-cone-looking antenna high up on a mast to avoid sources of surface interference. I'm super curious now though! I've got a software GPS receiver set up, I'd like to try injecting noise into the (virtual) signal chain and see how much noise power is necessary to lose lock. If anyone's curious, GNSS-SDR works great with $20 RTLSDR radios: http://gnss-sdr.org/source-code http://gnss-sdr.org/source-code Edit: I did a bit more research, and it sounds like overloading the front-end is the most common approach to jamming. People are turning on multi-watt transmitters in their vehicles (e.g. to block fleet management GPS from detecting their location). Despite the amount of power they're putting out, they seem to be limited in range to around 50 feet. It's also pretty satisfying to see http://www.novatel.com/assets/gajt/pdf/gajt-white-paper.pdf http://www.novatel.com/assets/gajt/pdf/gajt-white-paper.pdf which indicates that the solution I came up with (directional antennas with nulls pointed at the interference sources) is a solid strategy for mitigating jamming.

jessaustin 11y ago

I'm sure you're correct, but this account seems to contradict all the horror stories we heard about LightSquared a few years ago. Is the explanation simply that civilian GPS uses crap radios and military GPS does not? I always sort of felt sorry for LightSquared, because they were obviously victims of regulatory capture.

tonyarkles 11y ago
I just read through the LightSquared story and am pretty intrigued! One of the fascinating demonstrations of the fact that GPS is relatively interference robust is that Galileo (ESA GNSS system) runs on the same frequencies as GPS; the different spreading codes mean that receivers can choose which signal to listen to. Reading about the LightSquared system though, they do talk about a quite real concern: overloading the receiver's frontend entirely. That's one of the concerns that would be addressed by using directional antennas (if you're receiving interference, rotate your antenna so that the source is in an antenna null). As far as I know, most civilian receivers wouldn't have a directional antenna, they'd have an omni so that they work in any orientation; it'd suck if your iPhone could only receive GPS signals if it were sitting flat. On a military vessel, it'd make a lot more sense to mount an upward-cone-looking antenna high up on a mast to avoid sources of surface interference. I'm super curious now though! I've got a software GPS receiver set up, I'd like to try injecting noise into the (virtual) signal chain and see how much noise power is necessary to lose lock. If anyone's curious, GNSS-SDR works great with $20 RTLSDR radios: http://gnss-sdr.org/source-code http://gnss-sdr.org/source-code Edit: I did a bit more research, and it sounds like overloading the front-end is the most common approach to jamming. People are turning on multi-watt transmitters in their vehicles (e.g. to block fleet management GPS from detecting their location). Despite the amount of power they're putting out, they seem to be limited in range to around 50 feet. It's also pretty satisfying to see http://www.novatel.com/assets/gajt/pdf/gajt-white-paper.pdf http://www.novatel.com/assets/gajt/pdf/gajt-white-paper.pdf which indicates that the solution I came up with (directional antennas with nulls pointed at the interference sources) is a solid strategy for mitigating jamming.