[fitswcs] FITS WCS Paper V: Time - Draft for Review
Arnold Rots
arots at head.cfa.harvard.edu
Fri Dec 23 14:49:42 EST 2011
Steve,
Thank you for your comments.
Here are some responses.
- Arnold
Steve Allen wrote:
> On Fri 2011-10-14T10:25:54 -0400, Arnold Rots hath writ:
> > Version 0.90 of the FITS WCS Paper V on Time is now available for review:
>
> Section 3.1
> We need to be very clear, so
> "At the present time"
> should become something like
> "Using existing FITS conventions"
>
> ISO 8601:2004(E) does define the meaning of calendar dates prior to
> 1582-10-15 in sections 3.2.1 and 4.1.2.1.
> They are defined as proleptic Gregorian calendar, with the proviso
> that parties must agree to use such early dates.
> Therefore, since we are asserting that we are agreeing that such
> dates are proleptic Gregorian those dates are defined by ISO
> and consistent with ISO.
>
> Section 4
>
> "If the HDU containd a table, ...
> may be replaced by columns ...
> with specific values for each row ("Greenbank convention")."
>
> I'm not sure that this is clear. Under what circumstances can any of
> the keywords summarized in Table 4 not be used as columns?
As it says: all keywords in the first two section of the table.
The other sections contain the CTYPE, etc., keywords that already
override the listed keywords and describe their us in the table
columns.
>
> Table 1
I moved most of this to Appendix A, although I did make some changes
to Table 1, in part also in response to Dick Manchester's comments.
But you should review teh result.
>
> I think there may be too many notes regarding the various time scales
> to fit well in the table.
>
> TAI should be avoided prior to 1972 because
> 1) TAI had not been authorized until the 14th CGPM in late 1971
> http://www.bipm.org/en/CGPM/db/14/1/
> http://www.bipm.org/en/CGPM/db/14/2/
> 2) TAI had not been available for any contemporary time stamping
> mechanisms prior to 1972-01-01
>
> TAI should be used with caution prior to 1977 because of the 1e-12
> change in rate, and for precision work TAI should always be corrected
> using TT(BIPMxxxx).
>
> TAI should be avoided now because of document CCTF/09-27
> http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf
> where the CCTF "stresses that TAI is the uniform time scale underlying
> UTC, and that it should not be considered as an alternative time
> reference."
This in particular I find a bit disturbing as it is likely to cause
more confusion than clarity.
>
> UTC should be used with caution prior to 1974 because the meaning of
> the name was unknown outside the metrology community.
> UTC should be used with extreme caution prior to 1972-01-01 because
> different contemporary sources of timestamps were providing different
> time scales.
> UTC should not be used prior to 1960-01-01 because coordination of
> broadcast time did not begin until then, and prior to 1961 only
> time sources in the US and UK were providing it.
>
> GPS system time should not be used before its inception date 1980-01-06.
>
> GMT has to be allowed after 1972 because
> 1) broadcasts of time continued to say "GMT"
> 2) the recommendation saying not to use GMT came in 1974
> 3) the IAU Comm 4 & 31 joint resolution saying not to use GMT came in 1976
> 4) the CCIR Rec. 535 saying not to use GMT came in 1978
> and most folks could not know of those recommendations/resolutions.
> So I'm not sure what "should/must/may" wording to use, nor what date.
>
> Section 4.2.1
>
> After TIMESYS is defined the clarity will be improved if
> "In relevant context it may be overridden by"
> is replaced by something like
> "In the context of axes in image arrays or table columns TIMESYS
> may be overridden by"
>
> Section 4.2.2
>
> "(as long as one does not venture too far from the earth's orbit),"
> I am not understanding what this is trying to communicate.
> The use of TOPOCENTER anywhere other than the surface of the earth
> begs questions of what meaning is intended.
I rephrased this.
>
> For a trendy example of this, note the recent OPERA superluminal neutrinos.
>
> Table 2
>
> We now have an object giving us extremely precise measurements of the
> center of mass of 4 Vesta, and next year that same will apply to 1
> Ceres, so we may find ourselves wishing we had defined Reference
> Positions for many of the minor planets.
> I wish we could get away with recommending "MPnnnnnn", but the advent
> of Pan-Starrs and LSST operations may quickly overflow 6 digits.
> Perhaps we should simply allow that a purely numeric value
> TREFPOS = "nnnnnnnn"
> shall be interpreted as the minor planet number?
I did not really sove this, but mentioned the problem.
>
> Table 3
>
> If Table 3 is to have precise meanings then ET cannot be in a column
> heading. ET could appear in the caption saying something like
> "To its commonly available precision ET may be used with a Reference
> Position anywhere within the solar system."
Removed ET
>
> If Table 3 is to have precise meanings then it seems to me we either
> have to give reference to a prescription for the 4-d transform to the
> EMBARYCENTER or else we have to omit that row.
Removed EMBARYCENTER
>
> Or, we could overload the table meaning further (at the risk of making
> it even less clear) by adding entries like "0.002 s" in the column TDB
> for TOPOCENTER, GEOCENTER, EMBARYCENTER as an indication that those
> are acceptable uses if the precision of the numbers is no better than 2 ms.
> But I really don't like that idea much. I'd rather delete EMBARYCENTER.
>
> Section 4.2.2, terrestrial coordinates.
I reaarranged and rephrased some.
>
> The last sentence demands the IAU 1976 ellipsoid (a shape from before
> satellite geodesy had matured), but the cartesian coordinates are
> specified as being ITRS. ITRS specifies that the length of the meter
> is consistent with TCG, not TT, whereas all recent ITRF publications
> eschew that and use a length of meter consistent with TT.
>
> Furthermore, any allowance of geodetic latitude or altitude
> again implies an ellipsoid.
>
> I will point out that VLBI avoids the issue of ellipsoid by specifying
> terrestrial coordinates in a cylindrical system: Rspinax, Longitude, Z
>
> But again I raise the spectre of the OPERA superluminal neutrinos.
> If we want precision we had better be consistently explicit, and
> I don't think the terrestrial coordinates here are safely explicit.
>
> Section 4.2.3, at its end
>
> The end note is too complex to be clear.
> We need a new section with a worked example showing the math for
> computing the time values associated with a pixel location along an
> axis, and we need to refer to that example here.
The example is already there in Table 8 and I now mention it here.
>
> Section 4.2.4, at its end
>
> EventRA,EventDec should now be referring to what is in Table 8, not Table 6
Oops, I forgot to fix that in the version I just sent out.
It's now fixed.
>
> Also, those celestial coordinates must necessarily be accompanied by
> RADESYS
> EQUINOX (if above not ICRS)
> and MJD-OBS or DATE-OBS
> either in the header keywords or as other columns as is pointed out by
> WCS Paper 2 section 3. I think we should include words saying that.
>
> Section 4.4.1, at its end
>
> As with section 4.2.3, it's just too complexly dense in meaning, so we
> really need a new section with worked examples showing the math for
> using TIMEOFFS and MJDREF/JDREF/DATEREF, and we need to refer to that
> section here.
Hm, I think the last sentence of this paragraph says it about as
clearly as one can.
>
> Section 4.4.2
>
> "but may be overridden, in appropriate context, by"
> would be much clearer as something like
> "In the context of axes in image arrays or table columns TIMSYER may be
> overridden by"
>
> Section 4.3
>
> I am happy to see tropical and besselian years,
> but these numbers are not useable.
> McCarthy and Seidelmann took their tropical year from Laskar, Table 5 of
> 1986A+A...157...59L
> and that's ancient by comparison to Simon et al., section 5.9.3 of
> 1994A+A...282..663S
> And besides that the calculation for Besselian year is not right.
> Arnold Rots and I should go play with polynomials and units.
>
> Section 5.2.1
>
> "time scales TDB and TCB may"
> could become
> "time scales TDB and TCB (or ET, to its precision) may"
>
> Section 5.2.3
>
> CRVALia does not have italic "ia"
>
> Appendix A interacts heavily with the things I wrote above about Table 1.
> We'll have to figure out how to harmonize which goes where.
> Also, the URLs for IAU documents have proved notoriously unreliable,
> so we will have to cite the IAU proceedings themselves as primary with
> the URLs being secondary references.
Still needs to be done.
>
> --
> Steve Allen <sla at ucolick.org> WGS-84 (GPS)
> UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855
> 1156 High Street Voice: +1 831 459 3046 Lng -122.06015
> Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
>
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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 head.cfa.harvard.edu
USA http://hea-www.harvard.edu/~arots/
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