6 ms·
> GPS doesn't need a constant feeding of data from ground stations This is not the case. USNO steers the clocks(4x) on the satellites. I think they do this vi
by dfc 13y ago
> GPS doesn't need a constant feeding of data from ground stations
This is not the case. USNO steers the clocks(4x) on the satellites. I think they do this via the AF but I have always been a little uncertain of the exact method:
GPS time is automatically steered to UTC(USNO) on a daily basis to keep
system time within one microsecond of UTC(USNO) modulo 1 second, but during
the last several years has been within a few hundred nanoseconds. The
rate of steer being applied is +/-1.0E-19 seconds per second squared.[^1]
One of the reasons for JPS's need for direct contact with ground stations is that the newer satellites do not contain the any rubidium frequency standards and use an experimental time synching system.
[^1]: http://tycho.usno.navy.mil/gpstt.html http://tycho.usno.navy.mil/gpstt.html
- icegreentea 13y agoGPS ground updates correct for clock skew and disturbances in orbits. The Japanese system also seems to upload the differential measurements of their entire ground reference points to rebroadcast to individual receivers. In some senses, the technique is just an augmentation/expansion of WAAS [0] to Japan (WAAS is strictly North American). [0] http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System EDIT: And to the OP, you would need some sort of GPS+ device.
- rasz_pl 13y agoW T F??? Why wouldnt they include rubidion clock on sats? Its at most $10K expense on something costing tens of millions to launch. Does this mean GPS will stop working when Zombies/Pandemic/whatever strikes and wipes out US military?
- cyanoacry 13y agoA really big problem with satellite technology are the harsh environments they go through. You're dealing with something like 10-20g's of vibration peak-to-peak on ascent, and then you subject it to the freezing colds and searing heat of space, all while doing this in vacuum. Since those clocks need to be super precise, it's easier to have an accurate reference on the ground where everything's not getting shaken and baked to bits.
- dfc 13y agoThe majority of GPS satellites do not have one rubidium clock on board, they have four. For a discussion of the experimental time sync system planned for QZSS see: http://web.archive.org/web/20130411122243/http://unsworks.unsw.edu.au/fapi/datastream/unsworks:4274/SOURCE02 http://web.archive.org/web/20130411122243/http://unsworks.un... I am curious before reading my comment did you think that GPS would continue to work without navy.mil and af.mil?
- rm445 13y agoI would have done. Having once convinced myself that four satellites shouting out the (accurate) time is sufficient to locate oneself in three dimensions, I assumed that was all there is to it.
- rasz_pl 13y agoHell yes I did. Whole point of Rubidium clocks is you dont have to correct them. I was hoping GPS would stay up for at least 5-10 years - I assumed all they needed was periodic orbit corrections. Well, there goes my GPS to the rescue Zombie survival plan :/. Time to learn how to read stars, or better yet - find android app that does it automagically from a picture :)
- sliverstorm 13y agoOr you could learn to use a map and compass. I know, I know, antique and all that, but reliable.
- dfc 13y agoThat is going to take the party out of Tactical Air Control Party.
- cnvogel 13y agoAccording to this "Fact Sheet", the latest GPS generation currently in orbit (Block IIF) has two Rb and one Cs. http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=18670 http://www.losangeles.af.mil/library/factsheets/factsheet.as... The next generation (III) will take away the Cs and only rely on (three) Rb: http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=18830 http://www.losangeles.af.mil/library/factsheets/factsheet.as... The current GPS constellation status can be seen here, Plane/Slot describes the position in orbit, SVN is the "Space Vehicle Number", "PRN" is typically the "Sattelite Number" as displayed on a GPS receiver, because it describes the "Pseudorandom number (sequence)" used to decode signals of this particular satellite. Clock is just for information what the satellite right now uses as a timebase. As of 2014-04-27 http://www.navcen.uscg.gov/?Do=constellationStatus http://www.navcen.uscg.gov/?Do=constellationStatus
- CamperBob2 13y agoRubidium clocks aren't magically perfect at all timescales. They're better than quartz at long timescales, but they still need periodic recalibration.
- akira2501 13y agoNot right away, it will just be slightly less accurate. Ignoring all other sources of errors, for extremely high precision work you still might want to correct for the relativistic difference in time between the satellite and the surface; but in a post-apocalypse setting, I don't think you'll mind the small drifts. There's still plenty of other maintenance, like delta-v maneuvers that need to get done or the system will eventually fail. Likely within a year.
- Someone 13y agoIn japan, in the typical post-apocalypse setting, small drifts are indeed not a problem. Who cares if your GPS has drifted by a few centimeter? That apocalypse may have moved the country by meters. This system will probably be useful for quickly determining what moved in an earthquake.