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Can someone more familiar with the situation explain why they jam GPS at all? You'd think the vast industry around the military could replace actual jamming wi

Can someone more familiar with the situation explain why they jam GPS at all? You'd think the vast industry around the military could replace actual jamming with training aids. Even if the bespoke training tools are expensive, I doubt they're as expensive as an airliner wandering in to an active missile range. Not to mention the economic disruption to air traffic throughput.

gh02t 5y ago
The signal GPS uses is actually very very weak and can be jammed pretty easily just by spewing a bunch of noise in the general area of the spectrum where it lies. Military grade receivers have some countermeasures that civilian equipment doesn't (e.g. the military signals have much wider bandwidth to work with, and you can also do some antenna magic), but jamming is still a problem for them too and it's a pretty active area for R&D. https://www.afcea.org/content/jam-proof-signals-guide-navigation https://www.afcea.org/content/jam-proof-signals-guide-naviga... is a good read on the subject.
- andrepew 5y ago
  The article makes it seem like they're testing the capability of field personnel rather than doing R&D. For the actual R&D stuff, I can see why they'd need to do something in the field to see how it behaves. But I would also imagine that sort of testing to be fairly infrequent with long experiment cycles.
- topspin 5y ago
  "and you can also do some antenna magic" Just thinking about what that would involve. Very high gain antennas could track individual GPS satellites and probably defeat jammers. A phased array system (similar to Starlink's 'dishy', but probably larger due to the much lower frequency) might also work. Either solution would make many if not most uses of GPS either physically impractical and/or too costly.
  - oceanplexian 5y ago
    You can defeat jamming in a fairly unsophisticated way by using an antenna that can only receive signals from a specific direction (i.e straight up), while everything else is filtered out. If the jammer is to the side or below you it wouldn’t do much good. The problem is for GPS to work you need to be able to see a few of the sats and some will be closer to the horizon.
    - topspin 5y ago
      "antenna that can only receive signals from a specific direction" That's what "high gain" means; the main lobe is strongly focused in some direction. Actively tracking multiple moving satellites makes this approach mostly unworkable.
      - Dylan16807 5y ago
        You were specifically talking about "very high gain" to track individual satellites. Having a 60-120 degree cone pointed straight up is a different category of solution entirely.
  - mike_d 5y ago
    GPS has a type of frequency hopping. Rather than jumping around from frequency to frequency, it slides up and down the spectrum like if you were turning an analog knob. The movements are determined by a pseudo random number generator unique to each satellite. Civilian GPS receivers have these PRNG keys so they can follow the movements up and down and stay locked on to the carrier while filtering any potential interference or jamming. For military use they have a second PRNG that lines up with the civilian PRNG once every 10 cycles. So between every ping your phone or car GPS gets, there are 9 more military only ones that move around the spectrum randomly and have some other security features. To jam GPS you either need the PRNG keys to know when to move your jamming, or just blanket the entire spectrum with random noise. If you take the blanket everything approach receivers may still be able to pick out the carrier (because it is moving and a detectable change). (This isn't actually how it works, but is a simplified example of the concept. GPS is an amazing protocol that I don't even understand most of. There is some really cool stuff like using two different frequencies to measure the delay introduced by the ionosphere. The Coast Guard manages a lot of the civilian side of things and has plenty of rabbit holes you can go down: https://www.navcen.uscg.gov/?pageName=GPSmain https://www.navcen.uscg.gov/?pageName=GPSmain)
pigbearpig 5y ago
Probably the same reason Netflix used/uses Chaos Monkey to actually cause problems, rather than just relying on training aids. You need to find the problems you didn't anticipate.
- topspin 5y ago
  We've got submarine crews that can't avoid smashing nuclear powered warships into sea mounts. Switching off the crutches seems like a very necessary training approach.
- SavantIdiot 5y ago
  I spent years working on CPU validation. One classic method is random template based injection. You do three setup things: 1) pick a bunch of registers and busses that you will toggle to 1/0 in the RTL, 2) then code a bunch of mutexes of things that absolutely cannot happen (e.g., and AND cannot act like an XOR), 3) and then write checkers that look for bad conditions that should not happen. Then you press go literally run "coverage" by generating trillions of random test patterns and see if they toggle checker failures. There are behavioral tricks to reduce the search space (hierarchy helps), but it is one classic method. It can only be done a few times, so you really have to nail down your revision before you each up weeks of compute time on thousands of servers running random noise!
takk309 5y ago
Remember, GPS is a military tool first. Civilians get to use it at the militaries discretion. They could turn the selective availability (which was turned off in 2000) back on and remove a lot of civilian accuracy. Maybe civilian pilots are the ones that need to training aids more than the military. It seems to me that they should be able to navigate without gps in a safe manner. Edit: fixed term for selective availability.
- Crosseye_Jack 5y ago
  Didn't they actually remove selective availability from the new sat's that were launched. https://web.archive.org/web/20080113123316/http://pnt.gov/public/sa/ https://web.archive.org/web/20080113123316/http://pnt.gov/pu... > In September 2007, the U.S. Government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that has been of concern to civil GPS users worldwide for some time.
  - pmorici 5y ago
    I think it is a moot point because there are 5 position constellations now days (US, EU, China, Russia, Japan) and many popular consumer chipsets can use multiple if not all of them.
    - KennyBlanken 5y ago
      It's not moot for aviation, where GPS units can be decades old, in part due to how expensive avionics are due to an absolutely mind-boggling amount of FAA regulations. The FAA is a giant lumbering dinosaur that spends a lot of time making sure private pilots generate huge reams of paperwork and restricting the kinds of equipment that can go into various categories of planes, when the overwhelming majority of GA crashes are 1)controlled flight into terrain and 2)running out of fuel. We'd have a lot less controlled flight into terrain if the FAA would unclench its avionics regulations asshole. Thankfully anyone can run an iPad with Foreflight, or any number of iOS and Android apps, that provide rudimentary terrain awareness and backup navigation.
GCU-Empiricist 5y ago
SO the US military is heavily reliant on GPS, and they know it, and so do professionals on the other side. There is a lot of good reason to think that if the US gets into a Large State action, which is relatively unlikely but. . . , then the opposition will employ GPS jamming to negate some of the more smart and collateral damage avoiding systems. I've been out for almost a decade, so I don't know if secondary powers have reliable and ubiquitous GPS jamming capabilities, but I'd bet dollars to donuts IRAN has spent significant funding to acquire GPS jamming. All of this gives a a context for why the military both uses training aids (marker blackout over GPS, manually disabled data links, and whatever more high tech the have devised) and real operate in jamming training, possibly as final training, certification training/exams, if my experience in sibling forces is any guide.

gh02t 5y ago

The signal GPS uses is actually very very weak and can be jammed pretty easily just by spewing a bunch of noise in the general area of the spectrum where it lies. Military grade receivers have some countermeasures that civilian equipment doesn't (e.g. the military signals have much wider bandwidth to work with, and you can also do some antenna magic), but jamming is still a problem for them too and it's a pretty active area for R&D. https://www.afcea.org/content/jam-proof-signals-guide-navigation https://www.afcea.org/content/jam-proof-signals-guide-naviga... is a good read on the subject.

andrepew 5y ago
The article makes it seem like they're testing the capability of field personnel rather than doing R&D. For the actual R&D stuff, I can see why they'd need to do something in the field to see how it behaves. But I would also imagine that sort of testing to be fairly infrequent with long experiment cycles.
topspin 5y ago
"and you can also do some antenna magic" Just thinking about what that would involve. Very high gain antennas could track individual GPS satellites and probably defeat jammers. A phased array system (similar to Starlink's 'dishy', but probably larger due to the much lower frequency) might also work. Either solution would make many if not most uses of GPS either physically impractical and/or too costly.
- oceanplexian 5y ago
  You can defeat jamming in a fairly unsophisticated way by using an antenna that can only receive signals from a specific direction (i.e straight up), while everything else is filtered out. If the jammer is to the side or below you it wouldn’t do much good. The problem is for GPS to work you need to be able to see a few of the sats and some will be closer to the horizon.
  - topspin 5y ago
    "antenna that can only receive signals from a specific direction" That's what "high gain" means; the main lobe is strongly focused in some direction. Actively tracking multiple moving satellites makes this approach mostly unworkable.
    - Dylan16807 5y ago
      You were specifically talking about "very high gain" to track individual satellites. Having a 60-120 degree cone pointed straight up is a different category of solution entirely.
- mike_d 5y ago
  GPS has a type of frequency hopping. Rather than jumping around from frequency to frequency, it slides up and down the spectrum like if you were turning an analog knob. The movements are determined by a pseudo random number generator unique to each satellite. Civilian GPS receivers have these PRNG keys so they can follow the movements up and down and stay locked on to the carrier while filtering any potential interference or jamming. For military use they have a second PRNG that lines up with the civilian PRNG once every 10 cycles. So between every ping your phone or car GPS gets, there are 9 more military only ones that move around the spectrum randomly and have some other security features. To jam GPS you either need the PRNG keys to know when to move your jamming, or just blanket the entire spectrum with random noise. If you take the blanket everything approach receivers may still be able to pick out the carrier (because it is moving and a detectable change). (This isn't actually how it works, but is a simplified example of the concept. GPS is an amazing protocol that I don't even understand most of. There is some really cool stuff like using two different frequencies to measure the delay introduced by the ionosphere. The Coast Guard manages a lot of the civilian side of things and has plenty of rabbit holes you can go down: https://www.navcen.uscg.gov/?pageName=GPSmain https://www.navcen.uscg.gov/?pageName=GPSmain)

pigbearpig 5y ago

Probably the same reason Netflix used/uses Chaos Monkey to actually cause problems, rather than just relying on training aids. You need to find the problems you didn't anticipate.

topspin 5y ago
We've got submarine crews that can't avoid smashing nuclear powered warships into sea mounts. Switching off the crutches seems like a very necessary training approach.
SavantIdiot 5y ago
I spent years working on CPU validation. One classic method is random template based injection. You do three setup things: 1) pick a bunch of registers and busses that you will toggle to 1/0 in the RTL, 2) then code a bunch of mutexes of things that absolutely cannot happen (e.g., and AND cannot act like an XOR), 3) and then write checkers that look for bad conditions that should not happen. Then you press go literally run "coverage" by generating trillions of random test patterns and see if they toggle checker failures. There are behavioral tricks to reduce the search space (hierarchy helps), but it is one classic method. It can only be done a few times, so you really have to nail down your revision before you each up weeks of compute time on thousands of servers running random noise!

takk309 5y ago

Remember, GPS is a military tool first. Civilians get to use it at the militaries discretion. They could turn the selective availability (which was turned off in 2000) back on and remove a lot of civilian accuracy. Maybe civilian pilots are the ones that need to training aids more than the military. It seems to me that they should be able to navigate without gps in a safe manner. Edit: fixed term for selective availability.

Crosseye_Jack 5y ago
Didn't they actually remove selective availability from the new sat's that were launched. https://web.archive.org/web/20080113123316/http://pnt.gov/public/sa/ https://web.archive.org/web/20080113123316/http://pnt.gov/pu... > In September 2007, the U.S. Government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that has been of concern to civil GPS users worldwide for some time.
- pmorici 5y ago
  I think it is a moot point because there are 5 position constellations now days (US, EU, China, Russia, Japan) and many popular consumer chipsets can use multiple if not all of them.
  - KennyBlanken 5y ago
    It's not moot for aviation, where GPS units can be decades old, in part due to how expensive avionics are due to an absolutely mind-boggling amount of FAA regulations. The FAA is a giant lumbering dinosaur that spends a lot of time making sure private pilots generate huge reams of paperwork and restricting the kinds of equipment that can go into various categories of planes, when the overwhelming majority of GA crashes are 1)controlled flight into terrain and 2)running out of fuel. We'd have a lot less controlled flight into terrain if the FAA would unclench its avionics regulations asshole. Thankfully anyone can run an iPad with Foreflight, or any number of iOS and Android apps, that provide rudimentary terrain awareness and backup navigation.

GCU-Empiricist 5y ago

SO the US military is heavily reliant on GPS, and they know it, and so do professionals on the other side. There is a lot of good reason to think that if the US gets into a Large State action, which is relatively unlikely but. . . , then the opposition will employ GPS jamming to negate some of the more smart and collateral damage avoiding systems. I've been out for almost a decade, so I don't know if secondary powers have reliable and ubiquitous GPS jamming capabilities, but I'd bet dollars to donuts IRAN has spent significant funding to acquire GPS jamming. All of this gives a a context for why the military both uses training aids (marker blackout over GPS, manually disabled data links, and whatever more high tech the have devised) and real operate in jamming training, possibly as final training, certification training/exams, if my experience in sibling forces is any guide.