[Spice_discussion] Re: SPKs for HST?

Hal Weaver hal.weaver at jhuapl.edu
Sun Dec 31 09:37:39 PST 2006

Hi Chuck,

In response to one of Dick French's comments, HST's orbital parallax is a
huge effect when observing Earth-approaching objects, which happen to be
among my favorites (e.g., comets passing within 1 AU of Earth) ;-)

Over the years, I've always used HST's orbit files directly (both predicted
and post-facto), which can be read by the STScI's version of Percy, the s/w
package originally developed by the NAIF group to support Hubble
observations of moving targets. I run Percy on my local machine, after
setting up the various environment variables properly and putting the
relevant reference files in their expected locations.

The HST orbit files are available to users with access to the STScI's
computer system (which all Hubble users can get), and I typically download
the orbit files I want and read them into Percy to enable me to generate
HST-centric ephemerides.

As the HST orbit files are "official" reference files, and presumably have
considerably higher accuracy than orbits generated from TLEs, perhaps a
system can be set up so that users can access the HST orbit files they need
and manipulate them using spicelib.

Percy itself is also publicly available, but it currently runs only under
Sun Solaris, which is a major shortcoming in today's computing environment.
Although I currently have a SunBlade workstation, it's no longer my primary
computer (a Mac laptop now serves that purpose), and I don't know how much
longer I'll keep the Sun workstation around.

Getting back to the question at hand, I recommend the following course of

1) The STScI should make HST orbit files (both predicted and post-facto)
publicly available in a convenient place (e.g., from its MAST archive, or
from an http or ftp website).

2) The NAIF group can enhance spicelib with a utility that either reads HST
orbit files directly, or generates a BSP file derived from the HST orbit
file. The relevant s/w can be obtained by the STScI's software support
group, the people who maintain Percy.

Personally, I'd like to have access to BSP files for the Spitzer and Chandra
observatories too.

Happy New Year everyone!



P.S. I'm cc'ing Tony Roman at the STScI on this message, as Tony is
currently Hubble's moving targets guru and may be able to offer other useful

Harold (Hal) Weaver
The Johns Hopkins University Applied Physics Laboratory
Space Department
11100 Johns Hopkins Road
Laurel, MD 20723-6099

email: hal.weaver at jhuapl.edu

240-228-8078 (Washington Line)
443-778-8078 (Baltimore Line)
410-978-5172 (cell)

On 12/29/06 5:52 PM, "Charles H. Acton" <Charles.H.Acton at jpl.nasa.gov>

>        A question about production of SPICE SPK files for HST
> Occasionally over the last year or two NAIF has been asked to make an
> SPK file (trajectory file) for the Hubble Space Telescope, to support
> reduction of solar system observations taken by HST.  On these
> occasions NAIF has gotten so-called "two line elements" for HST from
> the Space Track website (space-track.org) and used our MKSPK utility
> to make an SPK from those elements.
> This process is available to anyone, after first registering with
> Space Track in order to be able to get 2-line elements.  (It could be
> used for other earth orbiting satellites as well, as long as 2-line
> elements sets are provided by Space Track.)
> That said, we must also note that there have been some times when the
> 2-line elements returned from Space Track are really not sufficient
> to produce a good orbit solution at all time within the bounding
> element sets... still a mystery to us.
> The Space Telescope Science Institute also makes (or gets from GSFC)
> orbit files for HST, apparently both "predict" for planning and
> "definitive" for data analysis. I think--but am not sure--that both
> predictive and definitive orbit data are provided in a single format:
> *.orb.  (Used to be *.orx?)
> A question for SPICE users: would it be useful if some entity such as
> StSciI or NAIF were to set up a regular HST SPK production process,
> making those SPK files available through whatever distribution
> channel is available (e.g. the archive website at STSciI,
> http://archive.stsci.edu/, or the NAIF server at JPL?  If you answer
> "yes," what caveats (requirements) would go along with your answer?
> 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?
> Of course there is no promise that anyone can and will take some
> positive action on this.
> Thanks in advance for your thoughts on this. You can answer to the
> entire distribution list or just to Chuck Acton, as you like.
> Chuck Acton
> NAIF Manager/JPL

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