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    Re: Atomic clock and local time
    From: David Pike
    Date: 2014 Nov 26, 15:29 -0800


    I just went outside and switched on a Garmin etrex20, GPS 72H, and GPS Map 60CSx.  All read the same time to the second.  Then I brought them inside and compared them to the USNO time on this site, and that appeared to be about 0.8 seconds fast on them.  Could this be due to an offset inserted to allow for transmission time within the internet and the laptop?  Interestingly, the Oregon Scientific radio controlled clock on my desk is six seconds ahead of the GPS sets, but I don’t know how often it updates itself or the last time the battery was changed.  Thinking about it, consumer GPS time has to have the 16 leap seconds added at some stage, because so much else depends on it these days other than positioning including power distribution, communications timing, and million dollar financial dealing.  The only other way all these systems could work is if they too worked on raw GPS time.  As far as I know, the Nav Message includes the difference between the individual satellite time based on three £1000s dollars onboard atomic clocks and GPS master time unless Selective Availability is set to other than zero, in which case it’s falsified on purpose.  The GPS receiver clock (costing a few cents) is slewed into line with GPS time until the tetrahedron generated by the intersecting range spheres of the four selected satellites is shrunk to zero.  At which stage the 16 leap seconds are slipped in I know not, but I wish I did.  Dave


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