Time Series 101

[François Bonnarel]

Hi Arnold,

Thnaks for that clear synthesis of what is needed to know.


On 19/05/2017 04:29, Arnold Rots wrote:
> Following up on yesterday's time series session, I thought it might be
> helpful to summarize the basics on time information.
> It really isn't that complicated.
> For reference and more information than you really want, you can turn
> to FITS WCS Paper IV: A&A 574, 36 (2015).
>
> 1. Required:
> 1.1 Record time stamps in JD, MJD, ISO-8601, or elapsed time
> (if in elapsed time, you need to give a zero point in a time stamp 
> that is not
> provided in elapsed time, of course)
> 1.2 Provide the time scale used (TT, TDB, TAI, GPS, ET, UTC, TCG, TCB)
> 1.3 Provide the reference position (place where the time is measured)
>
> 2. Note the following:
> JD and MJD do not imply a time scale; it needs to be provided separately.
> JD and MJD are dimensionless, though a unit of "day" is implied.
> It's a bad idea to mix UTC with JD or MJD, since not all UTC days are 
> the same length.
> Use the restricted form of ISO-8601: 
> [[+|-]c]ccyy-mm-dd[Thh[:mm[:ss[.ss...]]]]
> No time zone characters.
> TDB runs on average synchronously with TT, but corrects for the 
> relativistic
> effects caused by deviations in the earth's orbit from perfect 
> circularity and
> constant gravitational potential.
>
> 3. Recommendations:
> Also provide an estimate of the uncertainty in your timestamps.
> Avoid UTC. It is trivial to convert the times provided by, e.g., space 
> agencies
> to TT immediately when you get them and it will save headaches later on.
> We do it for Chandra (and RXTE).
> Use TT: it's the official IAU time scale, continuous with ET and the 
> one solar
> system ephemerides are based on.
> TAI and GPS are acceptable alternatives, with constant offsets from TT.
> Use the same reference position for time and space and make sure it is
> commensurate with your time scale. For instance, when you convert to
> the barycenter, also convert to TDB.
> Beware that the barycenter is not the heliocenter.
> Be specific in labeling the time axis; e.g.: JD(TT;GEOCENTER)
> or MJD(TDB;BARYCENTER).
> Use proleptic Gregorian dates for ISO-8601.
>
> 4. Do never use:
> TJD, HJD, BJD, etc.
> These are not officially recognized and suggest certain metadata 
> values, but
> leave considerable ambiguity as to what those metadata values actually 
> are.
> Instead, specify your metadata explicitly. It avoids confusion later 
> on and isn't
> much more work.
>
> 5. What if you deal with incomplete data?
> If you don't know the time scale and/or reference position, you can 
> provide them
> as UNKNOWN and set the systematic error/uncertainty to, say, 1000 s.
> 100 s will do if only the time scale is unknown.
>
> 6. What else is there to know?
> Quit a lot, especially the so-called coordinate time scales (TCG and TCB).
> Because TDB runs on average synchronously with TT, but in a very 
> different potential
> well, you may have realized that this requires different values for 
> fundamental
> physical constants in the barycenter. That's awkward and the 
> coordinate time scales
> fix that by running at different rates. Eventually these may come into 
> more common
> use, but at least for my lifetime I assume we will be sticking with TT 
> and TDB.
> More in the cited A&A paper.
>
> Hope this is helpful,
>
Definitely
Cheers
François
> Cheers,
>
>   - Arnold
>
> -------------------------------------------------------------------------------------------------------------
> Arnold H. Rots            Chandra X-ray Science Center
> Smithsonian Astrophysical Observatory                   tel:  +1 617 
> 496 7701
> 60 Garden Street, MS 67 fax:  +1 617 495 7356
> Cambridge, MA 02138 arots at cfa.harvard.edu <mailto:arots at cfa.harvard.edu>
> USA http://hea-www.harvard.edu/~arots/ 
> <http://hea-www.harvard.edu/%7Earots/>
> --------------------------------------------------------------------------------------------------------------
>

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