[Sois-tcoa] RE: SOIS Time Distribution Service

[Keith Scott]

Stuart,

I was supposed to lead a time distribution group meeting on Tuesday when I
got into Toulouse, but nobody else showed up.  Attached are the slides I was
going to present to see if there was consensus to 're-synch' time
distribution around (my interpretation of) what's in the TCOA charter.
Namely, I think time distribution should provide a mechanism (API) for
_applications_ running on different _nodes_ of a spacecraft to get the
current time.  I think that distributing the 'current time' between nodes of
the spacecraft is in scope, and think that an NTP-like protocol could work
for this.  The important things here are that there are applications,
presumably running on some sort of operating systems and general-purpose
hardware (nodes).

I think previous discussions of time distribution got off-track (partly my
fault, since I hadn't gone through the TCOA charter carefully until
preparing for Toulouse).  In particular, I do NOT think that the time
distribution mechanisms we propose have to handle the extremely
high-fidelity/low overhead cases that Art was concerned about.  Instead I
think that if there are instruments that want to just count rising edges off
a common clock, great.  Those instruments can do whatever they'd normally do
-- put copies of the counter in their data stream.

If you want to 'synch' that square wave clock to 'the current spacecraft
time' as defined by the time distribution service, I'd simply have some node
with access to the square wave snap a copy of the counter and timestamp it
with the time distribution service (TDS) time.  This leaves some ambiguity
between the 'square wave time' and the 'TDS' time (since the TDS resolution
will likely not be one cycle of the square wave the counter is watching),
but it also decouples the two services.  It would be very difficult for a
reasonable time distribution service to cover all of Art's requirements.

So, how do people feel about backing off to this interpretation of what's in
the TCOA charter?  I'm hoping we can handle a lot of this type of discussion
over email, and it would be nice to get this activity moving in some
direction; now's the time to speak up!

		--keith

>-----Original Message-----
>From: Scott Burleigh [mailto:Scott.Burleigh at jpl.nasa.gov] 
>Sent: Monday, November 29, 2004 2:18 PM
>To: Stuart.Fowell at scisys.co.uk
>Cc: kscott at mitre.org
>Subject: Re: SOIS Time Distribution Service
>
>At 03:26 AM 11/26/2004, you wrote:
>>Hi Scott,
>>
>>Hope you had an easy journey back to your side of the pond.
>>
>>I wonder if you could help me. I'm putting together a proposal for the
>>on-board software for a European reference mission and in 
>particular the
>>communication services.
>>Of course I'm making extensive reference to the current 
>state-of-the-art
>>with regard to SOIS.
>>
>>I have a reasonable handle on TCONS, MTS, File Transfer, C&DA 
>etc but not
>>on the Time Distribution Service. I have the presentations 
>Art gave at the
>>Montreal meeting but I've no information on any progress 
>made/what happened
>>in Toulouse. If you have any info, presentations, reports or 
>commentary I
>>would really appreciate it. The proposal has to be submitted 
>3rd December
>>so if you have anything I really could do with it by 30th Nov.
>
>Hi, Stuart.  I don't know what happened in the Time 
>Distribution Service 
>area at Toulouse, but if anyone does know I think it would be 
>Keith Scott; 
>I'm copying him on this reply.  Keith, any information you can 
>pass on to 
>Stuart?
>
>Scott 
>
>
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