13 ms·

Well, funny story. When North Korea put up their first satellite it used GPS. Which, when you think about it, is pretty funny because denying entities like the

Well, funny story. When North Korea put up their first satellite it used GPS. Which, when you think about it, is pretty funny because denying entities like the North Korean regime use of GPS in that use case is a major reason the export controls exist.

toomuchtodo 12y ago
Which is very funny, as they could've used celestial navigation systems/star trackers (which are impossible to deny them from using) similar to that used with ICBMS. http://en.wikipedia.org/wiki/Attitude_control#Star_tracker http://en.wikipedia.org/wiki/Attitude_control#Star_tracker http://www.ballaerospace.com/page.jsp?page=104 http://www.ballaerospace.com/page.jsp?page=104 I've always thought star trackers would make wonderful backups to GPS for high-altitude aircraft (they work even during daylight hours depending on atmospheric conditions).
- vonmoltke 12y ago
  The P-3C Orion is fitted with a retractable sextant. Pre-GPS it was a primary navigation aid, but has been maintained post-GPS as an emergency system in case GPS fails. I don't know offhand of its replacement, the P-8 Poseidon, has any kind of celestial navigation system as a backup, but I would not be surprised if it did.
- matthewmcg 12y ago
  "depending on atmospheric conditions" is a big IF in the polar regions. A USAF U-2 on a high altitude polar air sampling mission accidentally wandered into Soviet airspace at the height of the Cuban missile crisis because auroras blocked the pilot's stellar fixes. See: http://www.vanityfair.com/politics/features/2008/06/missile_crisis_excerpt200806 http://www.vanityfair.com/politics/features/2008/06/missile_...
  - toomuchtodo 12y ago
    Have we come far enough yet with mapping the earths magnetic field that you could use it to cross check your location with the star tracker's data?
    - TTPrograms 12y ago
      It's extremely variable, and I assume unpredictable.
      - nitrogen 12y ago
        I seem to recall reading about magnetic field charts for aviation purposes, but their purpose was to translate from location to magnetic field orientation, not the other direction. https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world.jpg https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world... from https://en.wikipedia.org/wiki/Magnetic_declination https://en.wikipedia.org/wiki/Magnetic_declination
    - ufmace 12y ago
      Sort of. I know a little about this from working in the oilfield. If you think navigating in the sky is tricky, try navigating underground. We had the BGGM[1], a model of the Earth's magnetic field, that gives us magnetic field strength and direction, given lat/long, altitude, and date. Accuracy was around 0.5deg in direction and I think 0.5% in strength. I'm not sure about the validity of it at more exotic locations and high altitudes, though. We used it as an accuracy check when taking field measurements to determine direction. I can elaborate if anybody's interested. As for using the data to check aircraft location, if the model is reasonable valid in those conditions, it might work as a sanity check. I doubt it would be practical to reverse it and get location from field information, though. It's already tough to get really good field data in an environment filled with metal objects and electric currents, though. [1] http://www.geomag.bgs.ac.uk/data_service/directionaldrilling/bggm.html http://www.geomag.bgs.ac.uk/data_service/directionaldrilling...
- InclinedPlane 12y ago
  > they could've used celestial navigation systems/star trackers Well, that implies they have the expertise to do so. Using a GPS is orders of magnitude easier.
  - toomuchtodo 12y ago
    Star trackers are available off the shelf, and aren't export restricted. They're essentially precision CCDs with on-board logic. Disclaimer: I am a geo/navi/celestial positioning nerd.
trhway 12y ago
the North Korea supposedly doesn't have keys to decode the military GPS signal content - thus they are limited to few meters precision which they can get from consumer GPS and from the un-decrypted military signal (and just by using the GPS satellites as close "stars" for old fashioned celestial navigation). If they managed to get military keys then of course they could get a few feet precision. Of course this difference in precision doesn't matter that much even when your warhead is only modest 5-10kt nuke :)
- InclinedPlane 12y ago
  The point being that they had access to an illegal GPS receiver (which didn't have the limits of altitude and speed that all civilian receivers should have). But, of course, the civilian GPS signal is just a signal, and anyone can build a receiver capable of decoding it, so there's not a lot anyone can do to prevent that capability from getting into the hands of folks you don't want to have it.
  - trhway 12y ago
    >"illegal GPS receiver" illegal where? for sale in US? I'd not be surprised that GPS gadgets produced in China for US market are just loaded with different version of firmware containing those limits while the same gadgets going somewhere else - with no limits firmware, or may be with even stronger limits (until pretty recently many "US legal" consumer GPS gadgets were illegal in Russia for being "too precise" :)

toomuchtodo 12y ago

Which is very funny, as they could've used celestial navigation systems/star trackers (which are impossible to deny them from using) similar to that used with ICBMS. http://en.wikipedia.org/wiki/Attitude_control#Star_tracker http://en.wikipedia.org/wiki/Attitude_control#Star_tracker http://www.ballaerospace.com/page.jsp?page=104 http://www.ballaerospace.com/page.jsp?page=104 I've always thought star trackers would make wonderful backups to GPS for high-altitude aircraft (they work even during daylight hours depending on atmospheric conditions).

vonmoltke 12y ago
The P-3C Orion is fitted with a retractable sextant. Pre-GPS it was a primary navigation aid, but has been maintained post-GPS as an emergency system in case GPS fails. I don't know offhand of its replacement, the P-8 Poseidon, has any kind of celestial navigation system as a backup, but I would not be surprised if it did.
matthewmcg 12y ago
"depending on atmospheric conditions" is a big IF in the polar regions. A USAF U-2 on a high altitude polar air sampling mission accidentally wandered into Soviet airspace at the height of the Cuban missile crisis because auroras blocked the pilot's stellar fixes. See: http://www.vanityfair.com/politics/features/2008/06/missile_crisis_excerpt200806 http://www.vanityfair.com/politics/features/2008/06/missile_...
- toomuchtodo 12y ago
  Have we come far enough yet with mapping the earths magnetic field that you could use it to cross check your location with the star tracker's data?
  - TTPrograms 12y ago
    It's extremely variable, and I assume unpredictable.
    - nitrogen 12y ago
      I seem to recall reading about magnetic field charts for aviation purposes, but their purpose was to translate from location to magnetic field orientation, not the other direction. https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world.jpg https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world... from https://en.wikipedia.org/wiki/Magnetic_declination https://en.wikipedia.org/wiki/Magnetic_declination
  - ufmace 12y ago
    Sort of. I know a little about this from working in the oilfield. If you think navigating in the sky is tricky, try navigating underground. We had the BGGM[1], a model of the Earth's magnetic field, that gives us magnetic field strength and direction, given lat/long, altitude, and date. Accuracy was around 0.5deg in direction and I think 0.5% in strength. I'm not sure about the validity of it at more exotic locations and high altitudes, though. We used it as an accuracy check when taking field measurements to determine direction. I can elaborate if anybody's interested. As for using the data to check aircraft location, if the model is reasonable valid in those conditions, it might work as a sanity check. I doubt it would be practical to reverse it and get location from field information, though. It's already tough to get really good field data in an environment filled with metal objects and electric currents, though. [1] http://www.geomag.bgs.ac.uk/data_service/directionaldrilling/bggm.html http://www.geomag.bgs.ac.uk/data_service/directionaldrilling...
InclinedPlane 12y ago
> they could've used celestial navigation systems/star trackers Well, that implies they have the expertise to do so. Using a GPS is orders of magnitude easier.
- toomuchtodo 12y ago
  Star trackers are available off the shelf, and aren't export restricted. They're essentially precision CCDs with on-board logic. Disclaimer: I am a geo/navi/celestial positioning nerd.

trhway 12y ago

the North Korea supposedly doesn't have keys to decode the military GPS signal content - thus they are limited to few meters precision which they can get from consumer GPS and from the un-decrypted military signal (and just by using the GPS satellites as close "stars" for old fashioned celestial navigation). If they managed to get military keys then of course they could get a few feet precision. Of course this difference in precision doesn't matter that much even when your warhead is only modest 5-10kt nuke :)

InclinedPlane 12y ago
The point being that they had access to an illegal GPS receiver (which didn't have the limits of altitude and speed that all civilian receivers should have). But, of course, the civilian GPS signal is just a signal, and anyone can build a receiver capable of decoding it, so there's not a lot anyone can do to prevent that capability from getting into the hands of folks you don't want to have it.
- trhway 12y ago
  >"illegal GPS receiver" illegal where? for sale in US? I'd not be surprised that GPS gadgets produced in China for US market are just loaded with different version of firmware containing those limits while the same gadgets going somewhere else - with no limits firmware, or may be with even stronger limits (until pretty recently many "US legal" consumer GPS gadgets were illegal in Russia for being "too precise" :)