The STEREO mission uses a particular kind of ephemeris file known as SPICE for
the orbit and attitude information, and there are routines in the SolarSoft
"gen" tree for STEREO for handling the files.  The upcoming Solar Orbiter and
Solar Probe Plus missions will also use the SPICE ephemeris system, so the
STEREO software for handling SPICE files has been migrated to a separate
package called "SunSPICE".  This is explained in this document.

http://sohowww.nascom.nasa.gov/solarsoft/packages/sunspice/doc/sunspice.pdf

People using Linux or Mac OS X, and who have either the SECCHI or SSC trees
installed, should pick up this new package automatically when they update
SolarSoft, but Windows users need to take an additional step.  Edit the file
$SSW/site/setup/ssw_install.pkg and include something like the following:

package=_solarsoft_packages_sunspice
   comment=ssw_upgrade__solarsoft_packages_sunspice
   site=sohoftp.nascom.nasa.gov
   remote_user=anonymous
   remote_password=<YOUR-EMAIL-ADDRESS>
   remote_dir=/solarsoft/packages/sunspice/
   local_dir=C:\ssw\packages\sunspice
   mode_copy=true
   passive_ftp=true
   use_timelocal=false
   max_delete_files=99%
   max_delete_dirs=99%
   update_log=C:\ssw\site\logs\ssw_upgrade__packages_sunspice.log
   compress_patt=
   force_times=yes
   exclude_patt=[:\?]
#

You may need to edit the "local_dir" line to reflect the correct location on
your machine.  I'm told that if this the last package in the file, it needs to
be entered twice to make sure it's properly run.

The older STEREO SPICE routines are still in SolarSoft, but have been updated
to call the equivalent SunSPICE routines to do the actual work.