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The GPS P(Y) and Military codes exist to (hopefully) prevent spoofing.
by jof 9y ago
The GPS P(Y) and Military codes exist to (hopefully) prevent spoofing.
- anovikov 9y agoThey aren't. A spoofer doesn't need to know what the signal means/be able to decrypt it. Just retransmit signal received from a different place at higher power. Only way to distinguish it from a real one is timing, but that requires an atomic clock, which is $15,000 and too expensive for most applications. But, military grade GPS receivers use virtual beam forming to achieve a very high attenuation of spoofing signal so they are extremely hard to spoof, they always get the real signal as stronger.
- lisper 9y agoYeah, I'm not terribly worried about military drones. But as a civilian pilot I worry a lot about whether the non-military-grade GPS in my airplane is telling me the truth.
- jschwartzi 9y agoHopefully GPS isn't your sole source of information. When I'm out scrambling I make sure that I carry multiple navigational aids so that I can cross-check. I also pay attention to terrain features before trips as well as during so that I can locate myself, or in the worst case make my way to the handrails that I've identified on my map.
- lisper 9y ago> Hopefully GPS isn't your sole source of information. Nowadays, in modern airplanes, it often is your only source of positional information when you're in instrument conditions (which is, of course, when it matters most). > I carry multiple navigational aids Yeah? Like what?
- jschwartzi 9y agoCompass, Altimeter, topographic maps, notes about the route that I've made. Even a watch can aid navigation if you can calculate your speed and see any terrain features. If anything doesn't jive with what I expect or what I'm seeing I try to understand why.
- anovikov 9y agoWhy don't planes just carry a good INS? In today's world, it shouldn't be that bad.
- lisper 9y agoMoney. INS is still very expensive relative to GPS, and GPS is pretty frickin' reliable 99.99% of the time.
- jcims 9y agoWould commodity six axis sensors have enough precision to do a reasonable job at INS?. Buy a pack of iPhones and use some kind of quorum protocol to reduce the risks associated with one going bad?
- lisper 9y ago> Would commodity six axis sensors have enough precision to do a reasonable job at INS? I don't know, but my guess would be no. If it were that easy someone would have done it already. But who knows? Maybe there's an untapped opportunity here. Why don't you buy one of these: https://vetco.net/products/9-axis-inertial-navigation-module-for-arduino https://vetco.net/products/9-axis-inertial-navigation-module... and take it with you the next time you fly (or drive) and see how accurate it is?
- heartbreak 9y agoYeah I doubt the engineers forgot about replay attacks when designing their military GPS.
- throwaway2048 9y agoThey arn't very easy to avoid, say you capture the signal from a satellite that is not visible to the receiver, or being jammed out, unless you have an extremely high precision clock, you can just delay the signal rebroadcast and spoof away.
- derefr 9y agoEverything involved in GPS requires all the nodes (both the senders and the receivers) to have "extremely high precision clocks." That's the whole idea, really: you, the receiver, have a clock, and a map of where the various GPS satellites will be around the earth at given times. You "hear" the current time announced from three satellites (along with their station IDs), and compare those times to your clock to figure out the flight time of the data, and thus the distance to the satellites. Then you take the satellites' known positions on the map at the current time, plus the flight times, and triangulate your own position. If one of the three times you've received is a "lie", then its relative time will correspond to an impossible distance for that GPS satellite to be relative to where the map says it should be (e.g. over the horizon relative to you), and relative to where the other two satellites that you heard from are. (Theoretically you could receive such signals—using reflectors, like HAMs do—but GPS discounts this possibility and just considers it invalid data.)
- ianburrell 9y agoGPS receivers do not have clocks. Atomic clocks are expensive and large; there is no way you get one every device. Only the satellites have atomic clocks. The receiver get the time from the satellites. It basically compares the time delay between the satellites to determine position and time.
- 9y ago
- daniel-cussen 9y agoAtomic clocks are $2000 now. SA.45, specifically. And they're smaller.
- anovikov 9y agoGreat! That is a big difference. Cheap enough for every jet aircraft, most turboprops, and some of the most expensive guided munitions (cruise missiles, or nukes like B.61 Mod 12 for sure).