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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Many platforms today use UTC (it is
probably the most widely used) and I believe Constellation will be using a </span></font>1/6/1980
(GPS Epoch). <font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 color=black face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;color:windowtext;font-weight:bold'>From:</span></font></b><font
size=2 color=black face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma;
color:windowtext'> sis-ams-bounces@mailman.ccsds.org
[mailto:sis-ams-bounces@mailman.ccsds.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>Scott Burleigh<br>
<b><span style='font-weight:bold'>Sent:</span></b> Thursday, September 04, 2008
4:16 PM<br>
<b><span style='font-weight:bold'>To:</span></b> sis-ams@mailman.ccsds.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [Sis-ams] consistent
"time-zone"?</span></font><font color=black><span style='color:windowtext'><o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt'>Ray, Timothy J. (GSFC-583.0) wrote: <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'>Dear WG members,</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'> </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'>When using time-tags in other protocols, we’ve
run into problems when multiple locations each use their own “local
time” – e.g when it’s 8am at JPL it’s 11am at Goddard,
and maybe 5pm in <st1:country-region w:st="on"><st1:place w:st="on">England</st1:place></st1:country-region>.
So, typically Greenwich Mean Time is specified. </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'> </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'>May I suggest that we insert a note in the vicinity
of table 5-1 (the MPDU header format) that says something like
this: “We recommend that the value of the time-tag be
consistent with Greenwich Mean Time if practical”
or “In the absence of any overriding agreements,
all time-tags shall use Greenwich Mean Time”.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'> </span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial'>Of course, there may be scenarios where it is
impractical/impossible to know the time on Earth. So, let’s not be
too authoritative, but it seems worthwhile to make the spec a little tigher.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt'>I think we're actually okay on this already,
Tim. The spec says we're using standard CCSDS unsegmented time codes with
preambles, and the only standardized time code defined in the Time Code Formats
spec is 1958 January 1 epoch (TAI). Two AMS implementation could have a
private side agreement to standardize on some other epoch, but the only way for
an AMS implementation to be certain of being interoperable with every other AMS
implementation is for it to use that 1958 epoch time. And that time is
defined to be TAI, which is just a few leap seconds away from UTC, which is
essentially GMT -- so I think this is already as tightly defined as we can make
it without deviating from the CCSDS time standard.<br>
<br>
Scott<o:p></o:p></span></font></p>
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