[Sea-time] FW: SEA-Time telecon minutes
Shames, Peter M (312B)
Peter.M.Shames at jpl.nasa.gov
Tue Feb 5 15:30:47 UTC 2019
From: "Christian.Stangl at dlr.de" <Christian.Stangl at dlr.de>
Date: Tuesday, February 5, 2019 at 3:06 AM
To: Peter Shames <Peter.M.Shames at jpl.nasa.gov>, "Erika.Sanchez at jhuapl.edu" <Erika.Sanchez at jhuapl.edu>
Cc: "osvaldo.peinado at dlr.de" <osvaldo.peinado at dlr.de>
Subject: AW: SEA-Time telecon minutes
Dear Erika and Peter,
I have some input for the definition sections of “CCSDS_Time_definitions - 22Jan19.docx”:
- §2.7: The term “clock synchronization” is still open or misleading compared to “time synchronization” of §2.8
in my opinion a clock synchronization – when it is not understood with time sync according §2.8 – could be more a “clock adjustment” e.g. where we adjust the drift of a S/C based oscillator due to aging process or radiation in space. Also the adjustment of the offset when it’s a clock based on an epoch
- §2.3: I miss the term “epoch” as the reference date for time measurement. We should also list the examples like GPS (1980-01-06T00:00:00) and the UNIX Time Scale (1970-01-01T00:00:00)
- When speaking from Time scales, TAI, GPS, UTC and time itself the leap seconds become important; there I always find the input of http://leapsecond.com very helpful ;-)
On http://leapsecond.com/java/gpsclock.htm they show a very nice and short definition of different time scales to avoid confusion of terms like “time”, the “epoch” of a “time scale” etc:
§ Local time is the date/time reported by your PC (as seen by your web browser). If your PC clock is accurate to a second then the other time scales displayed above will also be accurate to within one second.
§ UTC, Coordinated Universal Time, popularly known as GMT (Greenwich Mean Time), or Zulu time. Local time differs from UTC by the number of hours of your timezone.
§ GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not perturbed by leap seconds GPS is now ahead of UTC by 18 seconds.
§ Loran-C, Long Range Navigation time, is an atomic time scale implemented by the atomic clocks in Loran-C chain transmitter sites. Loran time was zero at 0h 1-Jan-1958 and since it is not perturbed by leap seconds it is now ahead of UTC by 27 seconds.
§ TAI, Temps Atomique International, is the international atomic time scale based on a continuous counting of the SI second. TAI is currently ahead of UTC by 37 seconds. TAI is always ahead of GPS by 19 seconds.
- The selection of their links (of leapsecond.com) may be also very helpful, but some are deprecated like the link to our (German) PTB which operates German Time Reference
- Regarding the term “Sun barycenter” I find following definition helpful (source: https://tycho.usno.navy.mil/systime.html):
• Dynamical Time replaced ephemeris time as the independent argument in dynamical theories and ephemerides. Its unit of duration is based on the orbital motions of the Earth, Moon, and planets.
· Terrestrial Time (TT), (or Terrestrial Dynamical Time, TDT), with unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides. TDT = TAI + 32.184 seconds.
· Barycentric Dynamical Time (TDB), is the independent argument of ephemerides and dynamical theories that are referred to the solar system barycenter. TDB varies from TT only by periodic variations.
- Regarding Time Transfer: this might be very dependent on the location of time data source and –data sinks (Ground-Ground or S/C<->Ground, etc.); European spacecraft follow very often the PUS standard (https://ecss.nl/standard/ecss-e-st-70-41c-space-engineering-telemetry-and-telecommand-packet-utilization-15-april-2016/) “Time management service”.
Best regards, Christian
Von: SEA-TIME [mailto:sea-time-bounces at mailman.ccsds.org] Im Auftrag von Shames, Peter M (312B) via SEA-TIME
Gesendet: Dienstag, 22. Januar 2019 21:05
An: Shames, Peter M (312B) via SEA-TIME
Betreff: [Sea-time] SEA-Time telecon minutes
Dear SEA Time BoF members,
I have uploaded the minutes of the meeting (thanks Erika) and the other submissions (revised definitions from the telecon and draft Charter) to the CCSDS CWE site. They are all in the Meeting Materials for 2019 folder:
If you do not have read/write access to this please contact the CCSDS Secretariat and request it (CCSDS Secretariat secretariat at mailman.ccsds.org<mailto:secretariat at mailman.ccsds.org>). As a working member of the BoF you should be granted access to everything, but it may take approval of your agency representative (nominally the CMC member) for it to be granted.
You should all now have a calendar invite for the next Telecon, on 5 Feb 2019 @ 0700 Pacfific (with the standard offsets for those distant from the Left Coast Barycenter).
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