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Re^4: Converting GPS seconds to readable time&date

by n3toy (Hermit)
on Jan 11, 2009 at 16:31 UTC ( #735522=note: print w/replies, xml ) Need Help??

in reply to Re^3: Converting GPS seconds to readable time&date
in thread Converting GPS seconds to readable time&date

The time displayed on the GPS is not really accurate, depending on what you consider accurate. I think some could be as much as a second or maybe two off. But it will be close enough to compare against to see if it is outputting UTC or GPS time. Also, in the setup menu of your GPS there just may be a field to change from one to the other.
  • Comment on Re^4: Converting GPS seconds to readable time&date

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Re^5: Converting GPS seconds to readable time&date
by hobbs (Monk) on Jan 11, 2009 at 22:01 UTC
    Not true. If you have a good GPS device that's meant to do time-transfer it should be able to get you within 10-100 microseconds of the correct time. Keeping accurate time is the foundation of the whole GPS system :)

      Indeed. At my prior company, they use GPS clocks for controlling the signalling on their digital networks, which means they expect the time to be accurate so that they can send 1.5 million bits per second and not have to "slip" any bits, which means they expect accuracy on the order of 1 microsecond (and searches show 0.5..1 microsecond accuracy for GPS-based clocks). But that is more about frequency accuracy than about agreement with UTC.

      Some results agree with 10 microsecond accuracy vs UTC. Turning on my GPS (which I hadn't used recently) showed it quite at odds vs my WWV clock. Going to the window so it could sync up with satelites got it to agree with my WWV clock to within a small fraction of a second (just holding them next to each other so I could watch the seconds displayed click along in perfect sync as far as my eyes could tell).

      So, yes, GPS units display accurate "human" time (usually in your selected timezone, not just UTC), not something that is 20 seconds "off" nor something only accurate to within several seconds.

      - tye        

      If you have a good GPS device that's meant to do time-transfer

      I do, and their (I have two, and neither have a display) PPS accuracy is within 20 nanoseconds of UTC (assuming you have accounted for the cable length from the antenna to the device). But the time displayed on a typical handheld consumer GPS for LBS (location based servies) are not "accurate" to that level. (Although some quality handheld GPSs have a PPS (Pulse Per Second) that probably meet the specs you quote.) It is good enough to check against a known source to see if it is outputting UTC or GPS, as per the node I was commenting on.

      The time displayed on a handheld GPS is derived from the processing of C/A code and is "old" by the time the display is updated.

      But you are correct in that time-transfer is the whole foundation behind GPS, and LBS is only an application of the system.

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