[Sois-tcoa] RE: SOIS Time Distribution Service

[Keith Scott]

Jane,

Point taken; I don't know how many missions will require high-fidelity time
synchronization between different things (possibly exceedingly
small/hardware only) on the S/C.  I do think that if we are to accomplish a
general time distribution mechanism that can function between different
processors, it probably won't have the one-cycle accuracy that you'd get
with a square wave.  Depending on how the high-fidelity time is distributed,
and how widely on board the spacecraft, there might be ways to use the
high-fidelity system itself to synchronize time.

I'd be perfectly happy for you (and anyone else with an interest) to express
a 'user' perspective.  Once we get to something implementable, if you can do
some prototyping, that'd be great!

		--keith 

>-----Original Message-----
>From: Jane K. Marquart [mailto:Jane.K.Marquart at nasa.gov] 
>Sent: Tuesday, November 30, 2004 1:48 PM
>To: Chris Plummer; 'Keith Scott'; 'Scott Burleigh'; 
>Stuart.Fowell at scisys.co.uk; arturo.B.Ferrer at nasa.gov
>Cc: 'Amalaye Oyake'; philippe.david at esa.int; 
>patrick.plancke at esa.int; Massimiliano.Ciccone at esa.int; 'Stuart 
>Fowell'; tyamada at pub.isas.ac.jp; Rick.Schnurr at nasa.gov; 
>'Abhijit Sengupta'; SCOTT.C.BURLEIGH at jpl.nasa.gov; 
>Ashton.G.Vaughs at jpl.nasa.gov; sois-tcoa at mailman.ccsds.org
>Subject: RE: SOIS Time Distribution Service
>
>Fellas,
>
>I shan't quibble about this, even though the missions at GSFC 
>that require 
>higher fidelilty are not a minority.  Let's aim to please the masses.
>As for abstractions, I'm still a youngster there (as was 
>obvious during our 
>TCONs meetings), so I'll leave that to the "grown-ups":<)   In Art's 
>absence, and until or if we replace him in TCOA,  would it be 
>helpful if I 
>took on the role of the user who would be reading the 
>recommendation and 
>trying to implement something from it?
>
>Jane
>
>At 12:27 AM 11/30/2004 +0100, Chris Plummer wrote:
>>Hi Keith,
>>
>>It gets my vote with a couple of provisos:
>>
>>Firstly, let's keep well away from the dreaded "API" 
>terminology. We are all
>>grown ups here and can handle abstraction! (-Oh boy....am I 
>going to regret
>>saying that). Instead let's stick with an abstract service interface
>>definition in true ISO 7498 style.
>>
>>Secondly, I'm sure that an NTP-like protocol is the right way 
>to go and that
>>we should concentrate our initial efforts in this direction, 
>but we must
>>also bear in mind that this kind of service is often provided 
>in hardware.
>>Therefore, we must be careful that we do not preclude the use 
>of different
>>mechanisms to provide the service.
>>
>>I fully agree that a high accuracy time service is not part 
>of the charter.
>>There will always be cases where one payload or system onboard the
>>spacecraft needs a high accuracy time service, but these are 
>the minority.
>>Most applications will want a modest accuracy, wall-clock 
>type time service,
>>and these are the applications we, SOIS, should cater for.
>>
>>Finally, sincere apologies on the part of Patrick and myself 
>that no-one
>>showed up for the time distribution session. We had had to dynamically
>>re-shuffle the agenda to cater for some of the joint meetings 
>and could not
>>get word of this to you. We will endeavour to do better next time!
>>
>>Cheers,
>>        Chris.
>>
>>-----Original Message-----
>>From: Keith Scott [mailto:kscott at mitre.org]
>>Sent: 29 November 2004 21:17
>>To: 'Scott Burleigh'; Stuart.Fowell at scisys.co.uk
>>Cc: 'Amalaye Oyake'; philippe.david at esa.int; patrick.plancke at esa.int;
>>Massimiliano.Ciccone at esa.int; 'Stuart Fowell'; c.plummer at skynet.be;
>>tyamada at pub.isas.ac.jp; Jane.K.Marquart at nasa.gov; 
>Rick.Schnurr at nasa.gov;
>>'Abhijit Sengupta'; SCOTT.C.BURLEIGH at jpl.nasa.gov;
>>Ashton.G.Vaughs at jpl.nasa.gov; sois-tcoa at mailman.ccsds.org
>>Subject: RE: SOIS Time Distribution Service
>>
>>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
>> >
>> >
>
>
>