High precision ephemerides for solar-system bodies are available online using JPL’s Horizons system. Most analysis (including optical and radar observations) will be best served by using the Horizons system to generate custom observer-table ephemerides. The Horizons system can also be used to output ephemerides in the form of osculating orbital elements and/or Cartesian vectors (position, velocity, and acceleration). However, there are some ephemeris data files available for export (see below) for those rare cases when Horizons will not suffice.
High-precision ephemerides readable from their own software may find ephemeris data files (in SPK format) and a software toolkit needed to read those files from JPL’s NAIF website. SPK files for planetary and planetary satellite ephemerides are available from NAIF. Comet and asteroid SPK files may be generated using Horizons.
High precision ephemerides files for selected planetary satellites are available for download from the JPL planetary satellite ephemerides page. Additional information related to planetary satellites orbits and ephemerides is available.