Report of the IAU Commission 4 Working Group on Standardizing Access to Ephemerides and File Format Specification

TL;DR

This report recommends standard formats (SPK and PCK) for accessing planetary ephemerides, details their specifications, and introduces new data types for improved precision and compatibility in astronomical computations.

Contribution

It standardizes ephemeris data formats (SPK and PCK), describes their specifications, and introduces new data types, enhancing interoperability and accuracy for planetary position data.

Findings

Adoption of SPK and PCK as standard formats for ephemerides.

Introduction of new data types, including Chebyshev velocity-only and TCB-based types.

Compatibility with major ephemerides like JPL DE, INPOP, and EPM.

Abstract

The IAU Commission 4 Working Group on Standardizing Access to Ephemerides recommends the use of the Spacecraft and Planet Kernel (SPK) format as a standard format for the position ephemerides of planets and other natural solar system bodies, and the use of the Planetary Constants Kernel (PCK) format for the orientation of these bodies. It further recommends that other supporting data be stored in a text PCK. These formats were developed for use by the SPICE Toolkit by the Navigation and Ancillary Information Facility of NASA's Jet Propulsion Laboratory (JPL). The CALCEPH library developed by the Institut de mecanique celeste de calcul des ephemerides (IMCCE) is also able to make use of these files. High accuracy ephemerides available in files conforming to the SPK and PCK formats include: the Development Ephemerides (DE) from JPL, Integrateur Numerique Planetaire de l'Observatoire de Paris (INPOP) from IMCCE, and the Ephemerides Planets and the Moon (EPM), developed by the Institute for Applied Astronomy (IAA). The bulk of this report is a description of the portion of PCK and SPK formats required for these ephemerides. New SPK and PCK data types, both called Type 20: Chebyshev (Velocity Only), have been added. Other changes to the specification are (i) a new object identification number for coordinate time ephemerides and (ii) a set of three new data types that use the TCB rather than the TDB time scale for the ephemerides, but are otherwise identical to their TDB versions.