[Spice_discussion] Re: SPKs for HST?

[Richard G. French]

"Charles H. Acton" <Charles.H.Acton at jpl.nasa.gov> writes:
>
>The alternatives are:
>
>   - NAIF hand make an SPK for HST on the rare occasion when such is
>needed;
>
>and/or
>
>   - You the customer can get the 2-line elements (or perhaps an *.ORB 
>file?) and use NAIF's MKSPK file to make an SPK on those occasions 
>when you need such.
>
>Are there earth orbiters other than HST for which SPKs would be useful?

To me, the most important things about the HST SPK file are:
1) that it be accurate at the 100-meter level, or at least that its
accuracy be clearly known
2) that it have a clear pedigree, so that others who want to duplicate or
test some published results will have an easy and unique way to identify
some property of the SPK file itself.
3) that the file have the NAIF or STSCI imprimatur of official approval
4) that the file be directly usable with the SPICE toolkit, which is the
gold standard for planetary science spacecraft trajectory manipulation

These makes me prefer that there be an end-product SPK file publicly
available, since otherwise one is left in the uncertain situation of
knowing if someone else's 2-line elements file conversion-to-SPK routine
had an error in a reference frame, epoch, ET-UTC correction, etc.

There are not that many HST observations that I can think of where knowing
the parallax of the HST orbit and/or its orbital velocity are important.
Stellar occultations by planetary rings and atmospheres certainly fall
into this category. Collectively, we could probably come up with the full
set of past HST observations for which SPK files would be desirable.

Dick French