[Spice_discussion] Delta_t from Earth PCK?
ernest.t.wright at nasa.gov
Thu Jun 18 19:06:05 PDT 2015
Thanks for replying! I should've explained what I'm trying to do. I've
calculated the circumstances of the 21 Aug 2017 solar eclipse using
earth_070425_370426_predict.bpc and I'd like to get some idea of the
delta_t (ET - UT1) that's implicit in that kernel for that date, for a
couple of reasons.
Your comments in the kernel say that beyond the end of the EOP's
coverage (July 2007), some things including UT1-TAI are "held constant."
Based on the really good agreement I'm getting with eclipse
calculations others have done, I'm guessing that doesn't literally mean
that the implicit delta_t is frozen at the 2007 value. Or I got lucky
and made two mistakes that cancel each other out. Anyway, I wanted to
The other reason is that I'd like to be able to tell people what delta_t
I'm willing to do the work necessary to back out the polar motion if
that's necessary, but I don't need high accuracy.
For ET <-> UTC, I've been making good use of ET2UTC and TIMOUT. My
understanding is that those are independent of the PCK, depending only
on the leap seconds kernel.
On 06/18/2015 07:43 PM, Nat Bachman wrote:
> 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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Ernest T. Wright ernest.t.wright at nasa.gov
Scientific Visualization Studio http://svs.gsfc.nasa.gov
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