[Spice_discussion] Delta_t from Earth PCK?

[Ernie Wright]

Hi Nat.

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 
check.

The other reason is that I'd like to be able to tell people what delta_t 
I used.

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.

- Ernie


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
>     UNITIM
>
> 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
NASA/Goddard Space Flight Center, Code 606.4      voice: +1 301 286-1569
Greenbelt, MD 20771                                 fax: +1 301 286-1634