4 ms·

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 lie

by 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)
- jcims 5y ago
  Speaking of Starlink, there was a news story posted here not too long ago about using Starlink satellite signals to determine location (within 8 meters IIRC) https://news.ycombinator.com/item?id=28656566 https://news.ycombinator.com/item?id=28656566

by gh02t 5y ago

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)
jcims 5y ago
Speaking of Starlink, there was a news story posted here not too long ago about using Starlink satellite signals to determine location (within 8 meters IIRC) https://news.ycombinator.com/item?id=28656566 https://news.ycombinator.com/item?id=28656566