On 2013-08-15, Magnus Danielson <mag...@rubidium.dyndns.org> wrote: > On 08/15/2013 10:22 AM, David Taylor wrote: >> On 15/08/2013 08:34, Rob wrote: >>> David Taylor <david-tay...@blueyonder.co.uk.invalid> wrote: >>>> On 14/08/2013 17:44, Rob wrote: >>>> [] >>>>> How does a "good" receiver know the correct time? Does it rely on >>>>> local (backed-up) storage, or is there some way of receiving it via >>>>> the almanac? Or are "good" receivers hardwired as well, only with >>>>> a different valid span? >>>>> >>>>> I would not be surprised when "good" receivers turn out to have just >>>>> a different moment or mode of failure. >>>> [] >>>> >>>> Some receivers have battery backup, in fact all but one of the receiver >>>> types I use have this. >>> >>> Ok but what happens when the battery is replaced? >> [] >> >> Hope and pray? Wish for a large capacitor or flash-rom? >> >> I had thought that either ephemeris or almanac data might contain the >> real UTC time, but apparently it does not. Obviously a system >> designed too far in advance of the Year2000 fuss and bother! > They completely avoid it by not numbering it that way. They have their > own numbering scheme that fit's the system, and the conversion over to > UTC is an added feature. It's all in ICD-GPS-200 for the current set of > details, and in the ION red book series for the early stages. > > GPS and GPS problems is best understood if you realize that everything > is counted in the GPS clock machinery with it's own set of gears. > Conversion isn't that hard and it is done every second in the GPS receiver.
That is fine, but I think that the question is "what are those internal geers" and "do those internal geers have a rollover time"? Ie, for how long a time period is there a unique mapping from the internals of GPS and the time (UTC or whatever). Obviously the oscillations of the H atoms in the H laser clocks have a rollover of picoseconds. Somewhere in those sattelites is some counter with a lot longer period before it rolls over. > > Cheers, > Magnus _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
