The Time Ephemerides software project consists of software and associated data to create, manipulate, and interpolate time ephemerides of the Earth (and potentially other solar system bodies). These time ephemerides consist of convenient approximations to relativistic time-dilation integrals which have been determined using numerical quadrature of quantities supplied by JPL ephemerides of the major bodies of the solar system. Time ephemerides of the Earth help to determine general-relativistic corrections for Earth-based clocks due to the time varying velocity relative to the solar system barycenter and time-varying gravitational field experienced by such clocks. Such corrections are fundamentally important since they help determine the independent variable of the JPL ephemerides, corrections to pulse-arrival times of pulsars, determinations of precise radial velocities, interpretation of spacecraft-ranging data, etc. For important background information on time ephemeris calculations see this paper. For example, equation 3 of that paper defines the relativistic integral approximated by the various time ephemerides of the Earth.

Software Subprojects

The Time Ephemerides software is divided into the following subprojects:

  • ephcom2
  • timeephem_generate
  • timeephem_interpolate

The ephcom2 software project is a major revival of the ephcom1 software for manipulating and interpolating ephemeris data in standard JPL ascii and binary forms. The ephcom1 software was last released under the LGPL by Paul Hardy in May 2004 at the website. The ephcom2 software fixes several important bugs (and many minor bugs) in ephcom1 and adds some useful additional functionality (a convenient Fortran interface and a CMake-based build and test system). The ephcom2 software is licensed under the LGPL. For a more detailed look at ephcom2 please see here.

The timeephem_generate software project contains software to perform the necessary quadrature of the time-dilation integral discussed above based on data supplied by a particular JPL ephemeris. The result of these calculations is an ascii version of a time ephemeris which is similar in form to the ascii form of the corresponding JPL ephemeris.

The timeephem_interpolate software project contains software to manipulate and interpolate the ascii time ephemeris data supplied by the timeephem_generate project.

The timeephem_generate and timeephem_interpolate software have been used previously to calculate the TE200 and TE405 time ephemerides (corresponding to the JPL ephemerides DE200 and DE405) that were discussed in the above paper. Both software packages are being modernized (including a new CMake-based build and test system) and will depend strongly on ephcom2. Both software packages should be released in the forthcoming months under the LGPL.