STANDARDS CHANGE REQUEST
                           ========================

     Purpose: Remove ambiguity about label time standards and GMT vs UTC in
              the PDS standards and reference documents
        Date: 2002-10-24
      Edited: 2002-11-12
Submitted by: Anne Raugh



Background
==========

Questions surrounding the inconsistent use of the "Z" time zone indicator
and the possible differences between the civil time at Greenwich (the "Z"
time zone), "Greenwich Mean Time" (defined differently on land and sea),
and Coordinated Universal Time (the appropriate time system for astronomy)
have arisen periodically throughout the history of the Planetary Data
System.  The issue has come to the fore as part of serious efforts to
support correlative searches based on recorded START_TIME.

Anne Raugh undertook the task of researching the differences between the
above-mentioned time standards, the results of which were presented at the
July 2002 Technical Session.  The consensus of the technical group at that
time was that the PDS documents should be modified to clearly state that
UTC is the preferred time standard for fields such as START_TIME, and
that a 'Z' suffix on a time string indicates the civil time zone at
Greenwich, England and is not synonymous with "UTC".

This SCR presents the list of specific changes identified in the PDS
standard documentation set, and notes one issue of PDS time zone notation
that remains outstanding.


Change Requested
================

The following emendations are recommended. 

"Standards Reference, Version 3.4"
----------------------------------
  Chapter 7
     pages 7-2 to 7-3, Section 7.3.1 (Conventional Times):
       o Replace the first two paragraphs with:

         Conventional times are represented as hours, minutes and seconds
         according to the ISO/DIS 8601 time format standard: HH:MM:SS[.sss]
         Note that the hours, minutes, and integral seconds fields must
         contain two digits. The colon (':') is used as a field separator.
         Fractional seconds consisting of a decimal point (the European-style
         comma may not be used) and up to three digits (thousandths of a
         second) may be included if appropriate.

         Coordinated Universal Time (UTC) is the PDS time standard and must
         be formatted in the previously described ISO/DIS 8601 standard format.
         The letter "Z", indicating the civil time zone at Greenwich (i.e.,
         GMT), may be appended to the time if desired and appropriate. Note
         that the relationship between UTC and GMT has varied historically and
         with observer context. UTC is the PDS time standard; a time with an
         appended 'Z' will be interpreted within the PDS as UTC, regardless
         of any changes or local variations in the GMT/UTC relationship.

       o Page 7-4, item #6. Alternate Time Zones, add this at the end of the
         paragraph:

         Note that only the Greenwich time zone may be indicated by its letter
         designator ("Z"); all other zones must be indicated by the hour offset
         from UTC.

     *** PROBLEM: This paragraph implies that time zones vary from UTC only by
     *** positive whole hours. This is not true. It makes sense to continue to
     *** follow the ISO/DIS 8601 standard for this as well:
     ***
     ***      HH:MM:SS.sss+/-hhmm   where +/- idicates either '+' or '-'
     ***                                  hh is 2-digit integer hours, and 
     ***                                  mm is 2-digit integer minutes
     ***
     *** However, this may have software implications.

  Appendix A
     page A-90, "LEAPSECONDS" line of KERNEL_TYPE table:
       o Change "(UTC or GMT)" to "(UTC)"


"Data Preparation Workbook"
---------------------------

Appendix D. Acronyms
  o Page D-3, entry for UTC:

    Remove the parenthetical comment: "(often called GMT)".


PDSDD.FULL File
---------------

  o SPICE_KERNEL Generic object definition description:

    Replace "(UTC or GMT)" with "(UTC)".