[LEAPSECS] it's WP7A week in Geneva
M. Warner Losh
imp at bsdimp.com
Sat Oct 3 12:23:07 EDT 2009
In message: <4AC732C8.3060409 at rubidium.dyndns.org>
Magnus Danielson <magnus at rubidium.dyndns.org> writes:
: Poul-Henning Kamp wrote:
: > In message <1254164464.4ac107f0211cd at webmail.unb.ca>, "Richard B. Langley" writ
: > es:
: >> <http://gauss.gge.unb.ca/ITU_future_UTC_(CGSIC_49th).pdf>
: > Thanks.
: > Although I do appreciate what the context conveys, I did smile when I
: > noticed that the last slide mentions "GPS time" but ends with
: > "Only UTC can be disseminated".
: > I don't know if GPS or POSIX time is the most diseeminated timescale,
: > but I suspect they both beat UTC by an order of magnitude when it comes
: > to number of receivers...
: Almost every GPS receiver also applies the UTC corrections to produce a
: UTC time. Infact, many of them can't externally produce any other time
: than UTC. Some has the ability to deliver GPS time or UTC time. Only a
: fraction of the receivers made only use GPS time, but then for some
: specific use.
: So it is UTC and not GPS you should compare.
Ummm... Every GPS receiver calculates GPS time. Most are able to
produce UTC time. But only after they "warm start" or after they get
the UTC offset from the almanac. What they do before that is somewhat
ill defined on some, over defined on others.
Until they have this data, they can't start producing real UTC time.
12.5 minutes is the figure that's normally given for this. If you
have a failure and have to swap in a new GPS receiver, you'll not have
UTC time for at least 12.5 minutes (usually more like 15-20 minutes
when all is said and done). A 12.5 minute down time means your annual
reliability can only be 4 9's, not 5 9's... This is why many
receivers remember the last UTC offset values and warm start with them
if they have only been off a short period of time...
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