[Spice_discussion] Delta_t from Earth PCK?

Nat Bachman nathaniel.bachman at jpl.nasa.gov
Thu Jun 18 16:43:14 PDT 2015

Hi Ernie,

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
40 microseconds.

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.

Best regards,

   -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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