On 08/15/2013 11:02 PM, unruh wrote: > 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.
As I just answered to David Taylor, it's all described this document: http://www.gps.gov/technical/icwg/IS-GPS-200G.pdf You might enjoy reading the earlier revisions as things have been modified over time, and to understand olrder receivers you need to look at the older spec, available here: http://www.gps.gov/technical/icwg/ The 1024 weeks period I have been speaking of comes from interpreting this document. Cheers, Magnus _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions