3 ms·

I think it is important to separate the use of GPS as a distributed clock from using GPS as a navigation system. These are different concerns and there are many

I think it is important to separate the use of GPS as a distributed clock from using GPS as a navigation system. These are different concerns and there are many commercial systems that use GPS for one or the other exclusively. The US military has never used GPS for navigation because it relies on state-of-the-art INS almost exclusively. GPS is for building accurate maps, not navigation. It wasn't expected to survive a serious war when it was designed. A challenge for the commercial market is that 1) INS is relatively expensive, whereas GPS is almost free these days, and 2) state-of-the-art INS is considered a cornerstone military tech and US R&D in this area is highly classified and controlled. This makes it a solved problem for the US military but not for anyone else. For many commercial navigation cases you can solve this with sufficiently advanced localization tech (basically, AI) but the tech isn't there yet. Using GPS as a shared clock source could be solved in most cases by choosing a different clock to use. Before GPS was cheap and ubiquitous, other clock sources were a thing and frankly should be quite cheap now if there was a big enough market for them.

lyu07282 3y ago
Is that really true? INS/inertial navigation systems accumulate errors over time, GPS is still required for them, its not a replacement?
- jandrewrogers 3y ago
  Advanced INS drifts very slowly, it only needs to meet a precision target for a finite period of time. Military INS only accepts GPS corrections within the (classified) drift model and larger military platforms crowdsource position from a network of INS units distributed throughout the hardware to lower the noise floor. For this reason, the amount of deflection you can induce by spoofing GPS undetected in military systems is measured in meters, which for high-precision work might be enough. The trope that you can make a military system fly off in a completely different direction by spoofing GPS is fiction. There is a fair amount of circumstantial evidence that state-of-the-art US military INS tech can exceed the precision of non-spoofed GPS for long durations. Some new military system specifications no longer accept GPS corrections. It is important to note that US military INS is a different animal than what is widely available in the commercial world. It is exotic tech and a closely held secret that has been under continuous R&D for half a century. The ubiquitous laser gyros we use now in high-precision applications were developed for US military applications in the 1960s and eventually de-classified.
  - lxgr 3y ago
    Very interesting. I hope some of that progress will eventually trickle down into e.g. civilian aviation – GPS jamming seems to be a growing concern there.
  - throw0101c 3y ago
    > Some new military system specifications no longer accept GPS corrections. Curious: are there specific MILSTDs / STANAGs / whatever that you can cite for this?
lxgr 3y ago
> The US military has never used GPS for navigation because it relies on state-of-the-art INS almost exclusively. GPS is for building accurate maps, not navigation. Huh, interesting. What's the purpose of the (encrypted) Y-code then? Sure, an INS is essential for situations in which GPS is denied in whatever way, and it would be a problem to not regularly do exercises with it disabled in order to make sure no dependencies creep in. But why not make use of it in peacetime or in a conflict with an adversary not capable of denying GPS? > other clock sources were a thing and frankly should be quite cheap now if there was a big enough market for them. Do you have any examples? The only thing that comes to mind for me are the various LF time beacons that some radio controlled clocks still use, like WWVB, DCF77 etc., but these are presumably orders of magnitude less accurate since you need to know your position and propagation conditions at high precision to determine propagation delay.

lyu07282 3y ago

Is that really true? INS/inertial navigation systems accumulate errors over time, GPS is still required for them, its not a replacement?

jandrewrogers 3y ago
Advanced INS drifts very slowly, it only needs to meet a precision target for a finite period of time. Military INS only accepts GPS corrections within the (classified) drift model and larger military platforms crowdsource position from a network of INS units distributed throughout the hardware to lower the noise floor. For this reason, the amount of deflection you can induce by spoofing GPS undetected in military systems is measured in meters, which for high-precision work might be enough. The trope that you can make a military system fly off in a completely different direction by spoofing GPS is fiction. There is a fair amount of circumstantial evidence that state-of-the-art US military INS tech can exceed the precision of non-spoofed GPS for long durations. Some new military system specifications no longer accept GPS corrections. It is important to note that US military INS is a different animal than what is widely available in the commercial world. It is exotic tech and a closely held secret that has been under continuous R&D for half a century. The ubiquitous laser gyros we use now in high-precision applications were developed for US military applications in the 1960s and eventually de-classified.
- lxgr 3y ago
  Very interesting. I hope some of that progress will eventually trickle down into e.g. civilian aviation – GPS jamming seems to be a growing concern there.
- throw0101c 3y ago
  > Some new military system specifications no longer accept GPS corrections. Curious: are there specific MILSTDs / STANAGs / whatever that you can cite for this?

lxgr 3y ago

> The US military has never used GPS for navigation because it relies on state-of-the-art INS almost exclusively. GPS is for building accurate maps, not navigation. Huh, interesting. What's the purpose of the (encrypted) Y-code then? Sure, an INS is essential for situations in which GPS is denied in whatever way, and it would be a problem to not regularly do exercises with it disabled in order to make sure no dependencies creep in. But why not make use of it in peacetime or in a conflict with an adversary not capable of denying GPS? > other clock sources were a thing and frankly should be quite cheap now if there was a big enough market for them. Do you have any examples? The only thing that comes to mind for me are the various LF time beacons that some radio controlled clocks still use, like WWVB, DCF77 etc., but these are presumably orders of magnitude less accurate since you need to know your position and propagation conditions at high precision to determine propagation delay.