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Subject: Re: Differences between GPS and WWV time
Smith_Peter@XXX.XXX
Date: Fri Jun 22 2001 - 15:25:58 EDT
Robert Owens [mailto:tugly@XXX.XXX] asked:
> Has anyone noticed a difference between GPS and WWV time. I hear some
units
> display a different time depending on software.
The GPS system broadcasts time as a week number (where Week 0 began
at 00:00:00 6 Jan 1980 UTC) and seconds-of-week. Somewhere else in the
broadcast message is the current difference between this strict
sequential count and UTC, since UTC has had a dozen or so leap seconds
inserted since the beginning of the GPS epoch. All but the most
primitive receivers correctly apply the leap second correction.
However, as Dan Allen noted, receivers' internal clocks often drift,
and shouldn't be trusted until the unit is tracking.
An interesting variation on the "Y2K Bug" of recent memory was GPS week
roll-over. The week field is 10 bits long, so in August 1999 it rolled
over from week 1023 to week 0. Some early GPS receivers needed software
updates or some other intervention in order to cope.
-- Peter
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