Isn't the GPS signal fairly low-power, and thus, trivial for an enemy to jam?
Or would they have to get their jammer close enough to be in range of the ship's guns (and presumably its loud radio broadcasts would give away its position)?
Or would they have to get their jammer close enough to be in range of the ship's guns (and presumably its loud radio broadcasts would give away its position)?
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.
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
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 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.
We're coming to the end of a golden era where RF electronic reasons used to mean significant jamming was somewhat unrealistic at an analog electronics level. In the "next big war" jamming is going to be cheap and ubiquitous on the battlefield (or for that matter, in terror attacks or whatever).
There is an interesting economic warfare aspect where a jammer drone is inherently always going to be cheaper than the antiaircraft missile or antiaircraft drone that shoots it down due to certain precision flying and targeting requirements. A jammer has to keep the right side up and not hit anything, and anti-jammer has a much more complicated task of finding, reaching, and destroying a jammer. So it being cheaper to jam than to unjam means only the wealthiest aggressors with good logistics support will be able to decide when GPS will be usable on a battlefield. Also unjaming has certain risks... sure go ahead shoot down that jammer drone, the site that launches a missile or even a drone will get plastered about one minute later by what amounts to counterbattery artillery fire. Now you have GPS data but no anti-aircraft cover... hmm, probably not a good trade. This is before meta level games are played where you want to lure someone into an ambush by jamming the heck out of everything but one heavily armed and prepared "empty" mountain pass. In the future I think you can pretty much count on GPS not being available or at least not being reliable on the battlefield, and when the enemy does let you use GPS they're probably messing with you.
The concept of drones being cheaper than what it takes to shoot them down has a meta problem first discussed in anti-star-wars technology in the 80s where MIRV ICBMs had decoys... You can expect X drones of which only Y contain expensive jammer hardware, making the cost ratio of anti-jammer drones even worse compared to jammer and decoy drones.
Is this 100% true? I don't know much about RF jamming, but isn't it sufficient to fix on the strongest source of the RF signal you're interested in and guide a cheap penetrator at it?
DARPA's Exacto bullets are doing that with optical guidance. Would it be that much harder with radio guidance?
The jammer is a NASCAR, get to the track, turn left, make noise. Its remarkable how simple the psuedocode will be to write and debug.
The anti-drone missile/drone is a high speed police pursuit full of all kinds of adaptive tracking and fail safes and IFF (yeah well you hope it'll have IFF) and low latency judgment calls and 3-d interception math and classified anti-jam anti-flare and predictive algorithms in case the target has anti-interception code.
But I don't know if that's the only solution if being economical ends up being important.
Spike strips are really good at stopping cars, especially if they're running around a track in a circle.
That being said, there were also stories of anti-radar missiles being redirected by modified microwave ovens during the Bosnia-Serbia war. Can't find a solid link to that thougn