[Spice_discussion] how are the long-term predicts made for ITRF93?
Goodson, Troy D (US 392C)
troy.d.goodson at jpl.nasa.gov
Wed Jul 13 15:51:34 PDT 2022
Nat,
Thank you. That's the information I was looking for. I should've thought to get the comment section with "commnt -r earth_200101_990628_predict.bpc" :/
Troy.
https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/ug/commnt.html
> On Jul 13, 2022, at 1:49 PM, Bachman, Nathaniel J (US 392N) <nathaniel.j.bachman at jpl.nasa.gov> wrote:
>
>
> Hi Troy,
>
> Here's the complete comment section of the file
>
> earth_200101_990628_predict.bpc
>
>>>>
>
> Low accuracy, long term predict earth PCK
> =========================================
>
> Original file name: earth_200101_990628_predict.bpc
> Creation date: 2020 April 11 18:17:00 PDT
> Created by: Nat Bachman (JPL/NAIF)
>
> Data Sources
>
> Input file: extended EOP created from
>
> JPL EOP file latest.long
> (Copied from WWW URL
> http://epic.jpl.nasa.gov/nav/eop/latest.long)
>
> Source EOP metadata:
>
> $ JPL Earth Orientation Parameter File
> $ Last Data Point 6-APR-2020
> $ Predicts to 28-JUN-2020
> $
> EOPLBL='EOP. LAST DATUM 6-APR-2020. PREDICTS->28-JUN-2020, UT1TYP=UT1. '
> EOPFNG='Enter MAKE_EOP 6-Apr-2020 11:04:43 linked 12-Jan-2017 00:15:37 '
> EOPUT1='UT1'
> EOPTYP='EOP'
> EOPTIM=' 6-Apr-2020 11:04:43 '
> EOPTRF='ITRF93'
> EOPCRF='ICRF93'
>
> Input file: leapseconds kernel naif0012.tls
>
>
> Coverage
>
> Start time: 2020 JAN 01 00:01:09.183 TDB
> Stop time: 2099 JUN 28 00:01:09.182 TDB
>
> UTC Epoch of last datum: 6-APR-2020
>
> Particulars
>
> This is low-accuracy predict PCK file giving the orientation of the Earth as
> a function of time for the interval shown above. This PCK file is made from
> an extended EOP file which consists of the JPL EOP file named above, with
> four extra records appended. These records are designed to add 79 years to
> the coverage of the EOP file without affecting interpolated data values for
> times within the coverage interval of the original EOP file. Data values
> of records in the extended interval are constant, with the exception of
> the TAI-UT1 offsets. Those are adjusted so that the corresponding values
> of TAI-UT1R are constant.
>
> The terrestrial frame whose orientation is given by this file is ITRF93; the
> inertial base frame is Ecliptic of J2000. Rotational effects included are:
>
> -Precession (1976 IAU model)
> -Nutation (1980 IAU model)
> -Nutation corrections
> -Rotation through true sidereal time
> -Polar motion
> -Nutation corrections
>
> Euler angles have been extracted from the Ecliptic-to-ITRF93 rotation matrix,
> and Chebyshev polynomials were fit to the Euler angles. The Chebyshev
> expansion degree was 49; the degree of the retained polynomials is 20.
> The coverage interval for each set of polynomials is 1 day or less.
>
> This file represents the same earth orientation as does the input EOP file
> for the time range:
>
> 01-JAN-2020 UTC
> 28-JUN-2020 UTC
>
> For epochs after the end of EOP coverage, polar motion and nutation
> corrections from the EOP file are held constant at values obtained by
> extrapolating 1 day from the last source EOP record. TAI-UT1 values are
> set so that the corresponding TAI-UT1R values at each epoch of the
> extrapolated records are equal. Details of the EOP extrapolation process
> are given below.
>
> Let n be the number of data records in the original EOP file. Let
> MJD(i) be the epoch of the ith record expressed as a modified
> Julian date. The added records have the following
> characteristics:
>
> Record n+1: Epoch is MJD(n) + 1. Nutation correction and polar
> motion values are obtained by extrapolating to the
> epoch a cubic Lagrange interpolating polynomial
> fitted to the corresponding data in records n-3 : n.
> TAI-UT1 is obtained by converting to TAI-UT1R the
> TAI-UT1 data in records n-3 : n, extrapolating an
> interpolating polynomial as for the other data, then
> converting the extrapolated TAI-UTR1 value to TAI-UT1.
> UTC string is consistent with the MJD epoch.
>
> Record n+2: Epoch is MJD(n) + 2. Nutation correction and polar
> motion values match those of record n+1. TAI-UT1 is
> set so that the corresponding value of TAI-UT1R at
> this epoch is that corresponding to TAI-UT1 at the
> epoch of record n+1. UTC string is consistent with the
> MJD epoch.
>
> Record n+3: Epoch is MJD(n) + 3. Data values are as described for
> record n+2, adjusted for this epoch.
>
> Record n+4: Epoch is MJD(n) + 28854. Data values are as described
> for record n+2, adjusted for this epoch.
>
> In all of the four records described above, the values of TAI-UT1
> have been selected so the corresponding values of TAI-UT1R are
> constant and equal to the value resulting from extrapolating the cubic
> interpolating polynomial for TAI-UT1R, fitted to the final four values
> of TAI-UT1R derived from the original EOP file, to the epoch of
> record n+1.
>
> The expected behavior of the interpolation algorithm is as follows:
>
> Time range Interpolation characteristics
> ============ =============================
>
> Epoch of record 1 to
> epoch of record n-1: Interpolation is identical to standard EOP
> file.
>
> Orientation error relative to that provided
> by a binary PCK derived from the source EOP
> file is non-zero due to differences in the
> Chebyshev fitting process. Error is at the
> 1.1 nanoradian level.
>
> Epoch of record n-1 to
> epoch of record n: Nearly identical to standard EOP file;
> small round-off error will occur.
>
> Orientation error is at the 1.1 nanoradian
> level.
>
> Epoch of record n to
> epoch of record n+1: Some ringing of interpolated data
> will occur. This interval has duration 1 day.
>
> Orientation error relative to that provided
> by a binary PCK derived from a JPL EOP
> released 4 days after the source JPL EOP file
> (thereby providing accurate predicted data
> 4 days further into the future--up to 2020
> July 2) is at the 225 nanoradian level.
>
> Epoch of record n+1 to
> epoch of record n+2: Some ringing of interpolated data will occur.
> This interval has duration 1 day.
>
> Orientation error measured as for the
> interval described above is at the
> 245 nanoradian level.
>
> Epoch of record n+2 to
> epoch of record n+3: Ideally nutation corrections and polar
> motion will be constant, as will TAI-UT1R
> derived from TAI-UT1 values in the file.
> Some round-off error should be expected.
> This interval has duration 1 day.
>
> Orientation error measured as for the
> interval described above is at the
> 265 nanoradian level.
>
> Epoch of record n+3 to
> epoch of record n+4: Data are as described for records n+2 to
> n+3. This interval has duration 79 years.
>
> Orientation difference relative to the
> IAU_EARTH reference frame's rotation model
> ranges from the 1.25 milliradian level at
> the interval start to the 4.75 milliradian
> level at the end.
>
>>>>
>
> So, as you see, the only data source is the cited JPL EOP file.
>
> Since TAI-UT1 is frozen at the end of the EOP coverage interval, the orientation
> from the PCK for times after that epoch will be off by, on average, about 1 arcsecond for
> each future leapsecond. Nutation and polar motion will be off too, but the magnitudes
> of those errors are dominated by that of the spin error.
>
> This file enables operation of applications that require Earth binary PCKs
> having very long coverage, but that require only very low accuracy. For example,
> using this file, view periods can be computed with an accuracy of a few minutes.
>
> Please let me know if you have further questions.
>
> Best regards,
>
> -Nat Bachman (JPL/NAIF)
>
> Nathaniel.Bachman at jpl.nasa.gov
>
>
>
>
> ________________________________________
> From: Spice_discussion <spice_discussion-bounces at naif.jpl.nasa.gov> on behalf of Goodson, Troy D (US 392C) <troy.d.goodson at jpl.nasa.gov>
> Sent: Wednesday, July 13, 2022 12:23 PM
> To: spice_discussion at naif.jpl.nasa.gov
> Subject: [Spice_discussion] how are the long-term predicts made for ITRF93?
>
> The tutorial https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/23_lunar-earth_pck-fk.pdf
> as well as https://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck/aareadme.txt
> describe long-term predicts for ITRF93:
>
> Low accuracy, long term predict kernel. The extended predict region of this
> kernel---the time interval following the end of the predict region of the
> input EOP file---does not estimate changes in UT1-TAI. The dates in the
> file name are the file's coverage start and stop times:
>
> earth_200101_990628_predict.bpc
>
>
> But, I believe IERS only provides predicts for 1 year. See https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html
> and EOP2 files from https://eop2-external.jpl.nasa.gov are only predicted for about 78 days.
>
> What is the source for the long-term prediction in a BPC kernel like earth_200101_990628_predict.bpc?
>
> Thanks,
>
> Troy.
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