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"Team Ada: Ada Advocacy Issues (83 & 95)" <[log in to unmask]>
Date:
Tue, 9 May 2000 07:42:08 -0700
Reply-To:
Joe Vlietstra <[log in to unmask]>
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Joe Vlietstra <[log in to unmask]>
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At 03:03 PM 5/9/00 +1000, Geoff Bull wrote:
>> Dale Stanbrough wrote:
>> > i would disagree. In the same way that we use GMT as
>> >  standard for time,
>Isn't GMT defunct?

The problem is that GMT is ambiguous.  In civil time keeping,
GMT is interpreted to be the same as UTC; for navigation, GMT
is interpreted as UT1.  These two interpretations may differ
by as much as 0.9 seconds.

begin ramble

UTC is kept within 0.9s of UT1 by occasionally (about every
18 months) entering a leap second adjustment.  Although it is
possible for a computer to predict when a leap second SHOULD
be entered, the actual authority/responsibility for making the
adjustment rests with BIPM.  BIPM announces a leap second
adjustment 6 months in advance and the parameters of the leap
second adjustment is broadcasted by GPS.

UTC can be used as a basis for a calendar package as long as
leap seconds are accomodated.  The easiest way to do this for
a large computer installation is to attach a GPS-based network
time protocol (NTP) server onto the network.  Even then, we
white knuckle the leap second adjustment and this solution
isn't economical for smaller systems.

end ramble

Note that the Consultative Committee for Space Data Systems
(CCSDS) has an interesting solution to time keeping problem:
use a preamble byte to identify the time code format which
follows. (See CCSDS Recommendation 301.0-B-2, www.ccsds.org).
Joseph P Vlietstra               [log in to unmask]
Aerojet Electronic Systems       [log in to unmask]
1100 West Hollyvale Street       (626) 812-2865
Azusa, CA 91702

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