[SEA-TIME] Time scales

[Lux, Jim (US 3370)]

I ran across this handy diagram from Wojciech Owczarekm, who's a PTP evangelist.
There is also an extensive list of time scales, with comparisons.
https://www.ucolick.org/~sla/leapsecs/timescales.html



As far as epochs and leap or not, for Annex B, I have the following list (so far).

I think a question is "what has been used or is likely to be used in flight" While the epoch and scale for various video and audio streams might not be directly used in flight, we might want to talk to the video compression working group - timestamps are a standard feature of video streams: and, for instance digitized RF, in VITA.49, which has a integer seconds (32 bits) and fractional seconds (64 bits) timestamp.  The integer seconds  provides for 4 (2 bits) types - none, UTC, GPS, Other. If UTC, it's seconds (including leapseconds) since 1 Jan 1970 00:00:00.  Fractional seconds is either None, Sample Count, Real Time Picoseconds, Free Running Count (at some TBD rate).
VITA 49 also calls out various time stamps for things like position fixes, attitude data, etc. (i.e. if you have a satellite collecting waveforms, you can put state vector information in the data stream, and there's a "at this time, the state was X".



TDB = Barycentrical Dynamic Time -
TCB - Barycentric Coordinate Time
TT - Terrestrial Time - same rate as TAI - defined to be different: TT-TAI =  32.184 seconds
TAI - equal to UT2 on 1 Jan 1958 00:00:00
JPL Ephemeris Time - differs from TT by small periodic terms <2ms - linear (mx+b) offset from TCB
Various OS time scales
01-Jan-1601 (JD 2305813.5) is WNT (Windows NT) epoch
17-Nov-1858 (JD 2400000.5) is VMS epoch  (MJD 0)
01-Jan-1900 (JD 2415020.5) is NTP epoch  (MJD 15020
01-Jan-1970 (JD 2440587.5) is UNIX epoch (MJD 40587) - no leap seconds counted
06-Jan-1980 (JD 2444244.5) is GPS epoch  (MJD 44244)
01-Jan-2000 (JD 2451544.5) is Y2K epoch  (MJD 51544)
SMPTE ST 12-2 epoch is 00:00:00 1 Jan 1958 but epoch is rarely used,   Counts in Hours:Minutes:Seconds:Frames - so "leap seconds" aren't generally considered
There are proposals to make a better Longitudinal Time Code for video/audio that incorporates both longer and shorter references (UTC for date, for instance): ST-309 RD 46:2019
ST309 is time zone and date.  Date as either YYMMDD or 6 digit MJD.
ST-304.3 Timestamping Compressed Motion Imagery - 8 byte precision time stamp as specified in MISB ST 0603. Microseconds since 1970 derived from UTC, but no leap seconds.

IRIG - Year and day of year, year is 00-99
IEEE 1588 Precision Time Protocol - allows user defined, and has three predefined: IEEE 1588 UTC; GPS UTC; NTP UTC
1588 UTC 1 Jan 1970 00:00:00 - no leapseconds, basically offset from TAI
GPS 6 Jan 1980 00:00:00 no leap seconds - offset from TAI, only 1024 weeks before rollover (encodes 10 bit week, then seconds of week)
NTP 1 Jan 1900 00:00:00 has leap seconds



[JPL logo]
James Lux
Data Standards Program Manager
3x   |   Flight Communications Systems Section (337)
JPL
4800 Oak Grove Drive
Pasadena, CA 91109
M: 818.395.2714
O: 818.354.2075
jimlux at jpl.nasa.gov<mailto:jimlux at jpl.nasa.gov>


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.ccsds.org/pipermail/sea-time/attachments/20251031/a22db23c/attachment-0001.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.png
Type: image/png
Size: 2081 bytes
Desc: image001.png
URL: <http://mailman.ccsds.org/pipermail/sea-time/attachments/20251031/a22db23c/attachment-0002.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: timescales.png
Type: image/png
Size: 194364 bytes
Desc: timescales.png
URL: <http://mailman.ccsds.org/pipermail/sea-time/attachments/20251031/a22db23c/attachment-0003.png>