[Spice_discussion] Delta_t from Earth PCK?
nathaniel.bachman at jpl.nasa.gov
Thu Jun 18 16:43:14 PDT 2015
Presuming by delta_t you mean
TT - UTC
you can get this quantity by calling the SPICE routines
DELTET will give you
TDB - UTC
and UNITIM can convert TDB to TT (called TDT in
SPICE documentation) and vice versa.
A restriction on the above approach is that DELTET will
not give you correct results for times during leapseconds.
The conversion performed by DELTET is accurate to about
SPICE doesn't provide a way to obtain UT1 from
a time specified in another time system. Trying to recover it
from the high-precision earth PCK would require you to model
polar motion and back that out from the orientation of the
ITRF93 frame relative to the ICRF.
-Nat Bachman (JPL/NAIF)
Nathaniel.Bachman at jpl.nasa.gov
On 06/18/15 14:22, Ernie Wright wrote:
> It seems like I should be able to infer a delta_t from any of the binary
> Earth PCKs, but I can't seem to wrap my head around how to do it.
> If I PXFORM from Earth-fixed (supplied by the PCK) to true equinox and
> epoch of date equatorial coordinates, I should be getting something like
> UT1, I think. (The TOD frame should cancel out the precession and
> nutation built into the Earth orientation, leaving the diurnal rotation.)
> I have the vague notion that the next step would be to subtract the "TAI
> rotation", but I really don't know what that means.
> - Ernie
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