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Technically, yes it is possible for an autopilot to use dead reckoning for navigation without GPS at all. (we didn't use GPS to navigate to the moon, right?) Ho

Technically, yes it is possible for an autopilot to use dead reckoning for navigation without GPS at all. (we didn't use GPS to navigate to the moon, right?) However, all commercial autopilots that I know of rely on GPS to some degree. The difference is the use of low-cost gyros and accelerometers. They are good enough when augmented with GPS, but not good enough all alone. Edit: the low-cost sensors can be augmented with sensors other than GPS too, like vision processing. This has been demonstrated but is not yet in widespread usage. I expect to see this become more common due to demonstrated vulnerabilities in GPS.

jandrewrogers 14y ago
State-of-the-art inertial navigation technology has approximately the same precision as state-of-the-art GPS. Even modern run-of-the-mill interferometer-based inertial systems are precise enough for most purposes. The unique value of inertial navigation is that it requires neither receiving nor sending a signal that can be jammed or spoofed, hence why the military uses it for everything.
- Cushman 14y ago
  Out of curiosity, how precise are we talking? I assume the top-level military tech is classified, but if I buy off-the-shelf parts, how far can I fly and still land on the runway using pure dead reckoning?
- asynchronous13 14y ago
  1960's inertial navigation systems have better resolution than today's consumer GPS. The issue is not the technology, it's the cost, size, and weight. The small, light, and low-cost sensors fueling the current wave of UAVs are not the most accurate technology available, but that doesn't mean they aren't useful.
  - einhverfr 14y ago
    The concern I would have with inertial navigation over long distances is the possibility of drift. You might have good resolution over short distances, but what about hours and hours? What if your vectors are not quite right, or you get drift in your gyros (and LR gyros are prone to drift in certain circumstances)?
    - runjake 14y ago
      Which is why aircraft normally have multiple INS units. INS has been used as primary nav for endurance missions (>24hrs) in the US military for decades. Citation: Myself, I worked on a variety of systems including INS and GPS in the military.
      - einhverfr 14y ago
        But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together. LR gyros are also limited by physics to a certain limit of precision, and this is the specific mechanism by which they can drift (from what I understand though building larger gyros might help). So flying for a long time.... I don't know to what extent we know how much drift might occur. It would certainly be possible to build in some checks here though since landing in Iran instead of Afghanistan would seem to show some discrepancy between the two systems. I am just not sure you can be sure that relative position will be as accurate as absolute position over an extended period of airtime.
        jessriedel 14y ago
        > But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together. I'm am pretty sure this is very wrong, although I'm not an expert enough to downvote you. Having multiple systems allows one to average out (statistically independent) drifts; it's not just a protection from failure. I don't think there's anything special about a "slow turn" which will cause military-quality INSs to have correlated drifts.
- rdtsc 14y ago
  Extra bonus points : if you get above the cloud cover, can also use constellations to correct accumulated errors. I heard some Russian ICBMs had a special window so that cameras can see constellations for that purpose.
  - wazoox 14y ago
    And the SR71 had it from the start, too. http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro-Inertial_Navigation_System http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro-...
  - sneak 14y ago
    Oh wow, this is why I keep coming back to HN again and again. Thank you for teaching me something amazing!

jandrewrogers 14y ago

State-of-the-art inertial navigation technology has approximately the same precision as state-of-the-art GPS. Even modern run-of-the-mill interferometer-based inertial systems are precise enough for most purposes. The unique value of inertial navigation is that it requires neither receiving nor sending a signal that can be jammed or spoofed, hence why the military uses it for everything.

Cushman 14y ago
Out of curiosity, how precise are we talking? I assume the top-level military tech is classified, but if I buy off-the-shelf parts, how far can I fly and still land on the runway using pure dead reckoning?
asynchronous13 14y ago
1960's inertial navigation systems have better resolution than today's consumer GPS. The issue is not the technology, it's the cost, size, and weight. The small, light, and low-cost sensors fueling the current wave of UAVs are not the most accurate technology available, but that doesn't mean they aren't useful.
- einhverfr 14y ago
  The concern I would have with inertial navigation over long distances is the possibility of drift. You might have good resolution over short distances, but what about hours and hours? What if your vectors are not quite right, or you get drift in your gyros (and LR gyros are prone to drift in certain circumstances)?
  - runjake 14y ago
    Which is why aircraft normally have multiple INS units. INS has been used as primary nav for endurance missions (>24hrs) in the US military for decades. Citation: Myself, I worked on a variety of systems including INS and GPS in the military.
    - einhverfr 14y ago
      But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together. LR gyros are also limited by physics to a certain limit of precision, and this is the specific mechanism by which they can drift (from what I understand though building larger gyros might help). So flying for a long time.... I don't know to what extent we know how much drift might occur. It would certainly be possible to build in some checks here though since landing in Iran instead of Afghanistan would seem to show some discrepancy between the two systems. I am just not sure you can be sure that relative position will be as accurate as absolute position over an extended period of airtime.
      - jessriedel 14y ago
        > But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together. I'm am pretty sure this is very wrong, although I'm not an expert enough to downvote you. Having multiple systems allows one to average out (statistically independent) drifts; it's not just a protection from failure. I don't think there's anything special about a "slow turn" which will cause military-quality INSs to have correlated drifts.
rdtsc 14y ago
Extra bonus points : if you get above the cloud cover, can also use constellations to correct accumulated errors. I heard some Russian ICBMs had a special window so that cameras can see constellations for that purpose.
- wazoox 14y ago
  And the SR71 had it from the start, too. http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro-Inertial_Navigation_System http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro-...
- sneak 14y ago
  Oh wow, this is why I keep coming back to HN again and again. Thank you for teaching me something amazing!