4 ms·

>there's no real reason why a spoofer can't set up a receiver at one location and rebroadcast the encrypted signal as received there I can't really see this wo

>there's no real reason why a spoofer can't set up a receiver at one location and rebroadcast the encrypted signal as received there I can't really see this working if you have an inertial navigation backup system. It would be easy to detect the large error in GPS position and just ignore the GPS.

jhayward 9y ago
You'll need two alternatives to GPS to be able to tell whether it's a malfunction in GPS, or the inertial nav. e-loran is such a third alternative.
- jjoonathan 9y ago
  Only if you assume they're equally likely to fail. Since INS is much less susceptible to spoofing and ought to have a fairly low nominal failure rate, in wartime it might be reasonable to assume disagreements are due to spoofing.
  - cpncrunch 9y ago
    Also, I believe most airliners have 2 or 3 INS systems, so it would be easy to see (or detect) if one of them has failed. It seems unlikely that all the gyros would silently fail...that would only happen if the gyros tumbled, or power failure to the gyros, but there would presumably be an error indication in either of those situations. (Also, these days they use ring laser gyros, which don't tumble or stop spinning).
    - jjoonathan 9y ago
      Speaking of which, are RLGs actually difficult to manufacture or is the military intentionally stopping them from getting into civilian hands? Even the best cell phone sensor fusion seems to result in an obnoxiously shitty map experience.
      - jhayward 9y ago
        The power requirements for even the most miniature RLGs is still pretty large relative to cell phone peripherals, and they are still physically pretty large. DARPA has had focus efforts in the MEMs arena for more than a decade now, including atomic clocks and optical systems but its not stuff you can put in a square mm of silicon and feed with a couple of microamps.
        jjoonathan 9y ago
        Huh, I'd have guessed the opposite: that a laser, optics, and a piezo vibrator would be easy to integrate, while a MEMS gyroscope would be much harder. I ask anyone who gives the slightest hint of familiarity with the field because I suspect that my surface-level understanding is just missing the juicy photonics challenge that's roadblocking integration. The alternative hypothesis of military heel-dragging (similar to the case of bolometers) is much less exciting, but verifying it would at least allow me to stop poking around :)
      - cpncrunch 9y ago
        Phones use MEMS sensors, which are much cheaper than laser gyros. I'm not sure if google maps uses the accelerometer, or just the GPS and compass. In my experience, phone maps work well if the device has a good GPS signal. It's just the compass that gives a shitty experience. I wish google would ignore the compass and just use GPS direction when you're moving. They used to do this, but now they seem to rely in the inherently crappy compass for direction.
        jjoonathan 9y ago
        > Phones use MEMS sensors, which are much cheaper than laser gyros Yes, but why? To my surface-level understanding, RLGs sound like they would be easier to integrate. > It's just the compass that gives a shitty experience. Yep, that's my experience too, and that's why I'd be ecstatic to hear that the compass was being replaced by a RLG (which could be re-locked to true North every time GPS got a decent movement vector). Then again, I half suspect that something is FUBAR'din the current compass stack. I have one of those fluxgate-magnetometer current probes at work and can report that everywhere I've used it I have seen magnetic field vary as a high-SNR continuous function of direction, yet my phone's maps app has a ton of lag, hysteresis, and jankiness when rotating in the same damn field. Maybe we just need to wait for the cheap compasses to get better.
jhayward 9y ago
This is kind of fun, so to play some more: suppose the spoofing platform is mobile; say, a UAV that starts out very very near the target of the spoofing, and gradually moves away at a rate designed to confuse such a check. You can see why military stuff gets expensive. The world gets complicated when folks are actively trying to screw with you.
Nokinside 9y ago
Large commercial ships already have INS backups and it can be spoofed just fine using carry-off attack. Spoofed signal gradually diverges from the correct signal and inertial navigation can't spot the difference.

jhayward 9y ago

You'll need two alternatives to GPS to be able to tell whether it's a malfunction in GPS, or the inertial nav. e-loran is such a third alternative.

jjoonathan 9y ago
Only if you assume they're equally likely to fail. Since INS is much less susceptible to spoofing and ought to have a fairly low nominal failure rate, in wartime it might be reasonable to assume disagreements are due to spoofing.
- cpncrunch 9y ago
  Also, I believe most airliners have 2 or 3 INS systems, so it would be easy to see (or detect) if one of them has failed. It seems unlikely that all the gyros would silently fail...that would only happen if the gyros tumbled, or power failure to the gyros, but there would presumably be an error indication in either of those situations. (Also, these days they use ring laser gyros, which don't tumble or stop spinning).
  - jjoonathan 9y ago
    Speaking of which, are RLGs actually difficult to manufacture or is the military intentionally stopping them from getting into civilian hands? Even the best cell phone sensor fusion seems to result in an obnoxiously shitty map experience.
    - jhayward 9y ago
      The power requirements for even the most miniature RLGs is still pretty large relative to cell phone peripherals, and they are still physically pretty large. DARPA has had focus efforts in the MEMs arena for more than a decade now, including atomic clocks and optical systems but its not stuff you can put in a square mm of silicon and feed with a couple of microamps.
      - jjoonathan 9y ago
        Huh, I'd have guessed the opposite: that a laser, optics, and a piezo vibrator would be easy to integrate, while a MEMS gyroscope would be much harder. I ask anyone who gives the slightest hint of familiarity with the field because I suspect that my surface-level understanding is just missing the juicy photonics challenge that's roadblocking integration. The alternative hypothesis of military heel-dragging (similar to the case of bolometers) is much less exciting, but verifying it would at least allow me to stop poking around :)
    - cpncrunch 9y ago
      Phones use MEMS sensors, which are much cheaper than laser gyros. I'm not sure if google maps uses the accelerometer, or just the GPS and compass. In my experience, phone maps work well if the device has a good GPS signal. It's just the compass that gives a shitty experience. I wish google would ignore the compass and just use GPS direction when you're moving. They used to do this, but now they seem to rely in the inherently crappy compass for direction.
      - jjoonathan 9y ago
        > Phones use MEMS sensors, which are much cheaper than laser gyros Yes, but why? To my surface-level understanding, RLGs sound like they would be easier to integrate. > It's just the compass that gives a shitty experience. Yep, that's my experience too, and that's why I'd be ecstatic to hear that the compass was being replaced by a RLG (which could be re-locked to true North every time GPS got a decent movement vector). Then again, I half suspect that something is FUBAR'din the current compass stack. I have one of those fluxgate-magnetometer current probes at work and can report that everywhere I've used it I have seen magnetic field vary as a high-SNR continuous function of direction, yet my phone's maps app has a ton of lag, hysteresis, and jankiness when rotating in the same damn field. Maybe we just need to wait for the cheap compasses to get better.

jhayward 9y ago

This is kind of fun, so to play some more: suppose the spoofing platform is mobile; say, a UAV that starts out very very near the target of the spoofing, and gradually moves away at a rate designed to confuse such a check. You can see why military stuff gets expensive. The world gets complicated when folks are actively trying to screw with you.

Nokinside 9y ago

Large commercial ships already have INS backups and it can be spoofed just fine using carry-off attack. Spoofed signal gradually diverges from the correct signal and inertial navigation can't spot the difference.