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Ray, Timothy J. (GSFC-583.0) wrote:
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<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Dear WG members,</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; 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 England. So, typically Greenwich Mean Time is specified. </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; 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></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; 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></p>
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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<br>
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