USNO/AA Technical Note 2016-01 A Difference in the Evaluation of Ephemerides Arising from Using the SPICE Format Specification Rather Than Export Format File Specification
US Naval Observatory Washington United States
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The Jet Propulsion Laboratory JPL makes its planetary and lunar ephemerides available in two formats. The First is the export format devised by JPLs Solar System Dynamics group. The second is the SPICE Double Precision Array File format devised by JPLs Navigation and Ancillary Information Facility. JPLs requirement for the evaluation of these ephemerides is that they are precise to 0.5 mm Newhall, 1989. A comparison of the export format and SPICE format versions of the same ephemeris found differences as large as 20 mm. The largest differences occur for Mercury, the planet with the smallest semi-major axis. It is shown that the differences arise not from a difference in the values of the coefficients in the files but because the export format specifies the use of an extended precision time argument while the time argument in the SPICE specification is double precision. Evaluating the SPICE format version of the ephemeris using an extended precision time argument reduces the differences in the evaluated positions to meet the 0.5 mm requirement.
- Celestial Mechanics