[Sea-time] A request for coordination from the SEA Time BoF

Berry, David S (3920) David.S.Berry at jpl.nasa.gov
Tue Feb 19 19:18:23 UTC 2019


The "Navigation Data - Definitions and Conventions" is well understood to be non-normative. It doesn't "define" any time systems, just provides descriptions of time systems that have been defined by other authoritative entities. This document is cited by virtually all Navigation WG standards as an INFORMATIVE reference.

The Navigation WG itself does not (or has not) dealt with issues of time correlation and/or time synchronization as a CCSDS entity. It may be the case that this function resides in flight dynamics in some of the CCSDS Agencies, but I don't know. Of course we do depend upon timetag accuracy, as do many other space mission functions, but to my knowledge the timetag functions are handled by either tracking networks or spacecraft flight teams or both.

The most current information related to Time Systems provided by the Navigation WG (and targeted for use as values for keywords in Navigation WG standards) is now ON the SANA at https://sanaregistry.org/r/time_systems?sort=name  . At present this is a "candidate" registry, though we hope to have it "approved" in the not too distant future. If the names and descriptions are technically incorrect, we would of course like to correct that, but the material has been reviewed by knowledgeable WG members and I believe it to be substantially correct. The only time related TERMS we are defining are the NAMES of the time systems, and a description of the time system. Based on my understanding of the CCSDS SANA Registry Management Policy, I believe that the SANA is meant to be a normative source of "assignments" in instantiations of CCSDS Standards. So for example, there are many "time related terms" in the CCSDS Terms Registry, but very few of them are in the Nav WG Time Systems registry. The term in the CCSDS Terms Registry would in general NOT be acceptable in an instantiation of one of the Nav WG standards.

Regarding your heartburn with respect to the Nav WG intent for the CCSDS "Navigation Data - Definitions and Conventions", Green Book, we have indeed been discussing major revisions to the document, with the POSSIBLE future step of eliminating it. The first step has been to ensure that the terms in that document's glossary are present and correct in the SANA https://sanaregistry.org/r/terms registry, after which time we plan to remove the glossary from the document and refer to the SANA registry (this avoids duplication, inconsistency, etc.). The terms to which I refer are not "invented on the fly", but rather are terms used in common spacecraft flight dynamics activities. Also, please note that the CCSDS "Navigation Data - Definitions and Conventions" was first published in September 2004, and the "realized SANA" and the Magenta Book paradigm came along substantially later; if the Green Book needs to be changed, that can be discussed, but the existence and contents of this Green Book have not been an issue through 3 editions. And again, our https://sanaregistry.org/r/time_systems?sort=name registry doesn't define TERMS, it specifies a set of VALUES that are acceptable for various keywords in instantiations of our standards that relate to the time system used.


From: "Shames, Peter M (312B)" <Peter.M.Shames at jpl.nasa.gov>
Date: Tuesday, February 19, 2019 at 10:13 AM
To: "Berry, David S (3920)" <David.S.Berry at jpl.nasa.gov>, "Wilmot, Jonathan J. (GSFC-5820)" <jonathan.j.wilmot at nasa.gov>
Cc: Ramon Krosley <r.krosley at andropogon.org>, "Gramling, Cheryl J. (GSFC-5950)" <cheryl.j.gramling at nasa.gov>, Jon Hamkins via SEA-TIME <sea-time at mailman.ccsds.org>
Subject: A request for coordination from the SEA Time BoF

Dear David and Jonathan,

During today's SEA Time BoF telecon we were working through the set of definitions that we need for time, clock, correlation, synchronization, etc, all in the aim of providing time management services.  In the process of defining these terms we have been reviewing the existing definitions in the SANA as well as definitions from authoritative normative external sources such as the BIPM and NIST.

We have, in the course of exploring what has been defined in the SANA, stumbled across definitions that came from various MOIMS Navigation WG (Navigation Data—Definitions and Conventions. Green Book. Issue 3. May 2010, CCSDS 500.0-G-3) and SOIS App WGs (Spacecraft Onboard Interface Services—Time Access Service. Silver Book. Issue 1. January 2011, CCSDS 872.0-M-1-S), as well as those in the SEA Time Code document, (Time Code Formats. Blue Book. Issue 4. November 2010, CCSDS 301.0-B-4).  Here is where things start to get interesting and why I am reaching out to you.

As best I can tell these three documents are the primary sources of definitions for terms related to time.  They are what comes up in the CCSDS Terms registry.  But here's the issue:

  1.  The Time Code format document does not itself specify normative definitions.  All of the terms it uses come from other sources like BIPM and NIST.  So far so good.
  2.  The SOIS Time Access service is now a Silver Book, but we understand (from Ray Krosley) that it is under consideration to be brought back to life, possibly in some new form.  I would guess that it would be well served by the Time Code Format definitions, plus any new ones for time correlation and synchronization that the SEA Time WG standardizes.  TBD.
  3.  The Nav Definitions and Conventions is a Green Book, which, in normal CCSDS parlance means that it is by definition non-normative.  That said, this document contains a broad range of terms related to navigation, including some related to time, clocks, etc.  We all know that navigation references time as well as position reference frames.  And we know that this is used as a reference by other Nav WG docs.

We need to make sure that these different uses of the set of terms related to time, correlation, synchronization are aligned, and to the extent possible, identical.  David, I heard from Cheryl Gramling today that the Nav WG has been reviewing what is in the SANA and proposes to make changes.  The fact that you are doing that is excellent and aligned with CCSDS policies, but I need to ask that we find a way to coordinate with the Time BoF.  I know that the BoF is not yet a formal entity, it will be shortly (I hope).  The regular crew of 8-11 people have an excellent background in time and time keeping, so I want to suggest that when terms relating to time are adopted that this group be asked to concur, if not to take the lead.

To that end would you please send your new list of terms to me for distribution to the Time BoF?  I want to make sure that we concur, or if not, to provide you with feedback about any issues we see.

On a related topic, Cheryl said that you have in mind defining or updating terms in the SANA and then eliminating the existing Green Book.  She may have mis-stated your intention, but I have to say that with my SEA AD CESG "hat" on that gives me major heartburn for a few reasons:

  1.  The CCSDS process and publication procedures (CCSDS A02.1-Y-4) say that formal terms are to be included in a normative section of a normative document, whether that is a Blue Book or a Magenta Book (both are normative) and that common terms shall be adopted (Sec A11).
  2.  The CCSDS process document says that "Magenta Books may … define means for applying or using standards or for documenting reference architectures and other formal specifications, or … document reference models or reference architectures to assist in the design, use, description, or selection of one or more standards."  Since you intend this document to be used across other Nav WG documents I assert that it must be a Magenta Book, and not a Green Book.  This is in line with a recent CESG discussion on a similar SLE Reference Architecture Blue Book that is now to be re-issued as a Magenta Book for consistency with CCSDS guidelines.
  3.  The Registry Management Policy (CCSDS 313.1-Y-1) in Sec 3.4.4 says that Glossary items must be derived from published documents and that terminology from authoritative sources is to be preferred.  There is no part of any CCSDS procedures that permits SANA Glossary terms to be invented on the fly without being sourced in a published document.

Bottom line, let's figure out how to arrive at a set of terms that meets the needs of these three WG, and thereby the needs of other WG that need to reference, but not uniquely define, these terms.  Ok?

Thanks, Peter

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