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te_gen-2.0.0 results have been released

This file release (http://sourceforge.net/projects/timeephem/files/Time%20Ephemerides/) contains ascii and binary time-ephemeris results that have been generated for most JPL and INPOP planetary ephemerides by the recently released te_gen-2.0.0 software. These results include the Newtonian (O(1/c^2)) and post-Newtonian (O(1/c^4)) components of a time-ephemeris integral evaluated at geocentre and the corresponding components of the time-ephemeris vector that helps determine the location-dependent correction to that integral for observation locations anywhere in the vicinity of the Earth. These combined results provide a superb approximation to the general-relativistic clock correction that must be made to determine the independent time variable of planetary (and time) ephemerides from Earth-based times such as TT (terrestrial time). Such results are also required to accurately interpret pulsar pulse arrival times or any other astronomical or spacecraft observation that requires accurate timing. Irwin and Fukushima (1999, http://articles.adsabs.harvard.edu/full/1999A&A...348..642I\) give important additional information concerning time ephemerides and Irwin (2013) is preparing an update to that paper to be consistent with the latest IAU time definitions (which are also used in the te_gen-2.0.0 implementation). When that research note is completed it will be linked from http://timeephem.sourceforge.net.

The time ephemerides given here depend on masses and motions of the solar-system bodies that are provided by the corresponding planetary ephemerides so use (with some exceptions noted below) the same gravitational model that was used to calculate the corresponding planetary ephemeris. That gravitational model ultimately depends on a least-squares-fit to astrometric observations of solar-system objects (see, e.g., Standish and Williams, 2010, ftp://ssd.jpl.nasa.gov/pub/eph/planets/ioms/ExplSupplChap8.pdf). Modern planetary ephemerides and corresponding time ephemerides tend to be an improvement on earlier ones because of improved methods (e.g., the treatment of the asteroids) used for the fit and because there are obviously more high-quality astrometric data to be fitted as time progresses. From comparisons (Irwin, 2013) between the present time ephemeris results it appears the errors due to astrometric fitting uncertainties for modern time ephemerides are of order 10^{-10} seconds in the epoch range from 1900 through 2100.

The present results use ascii and binary formats that are well-documented as part of the recent ephcom-3.0.0 software release and which are suitable for storing planetary, time, and (in principle) asteroid ephemerides. These formats are necessarily different from the traditional JPL ephemeris formats which are only suitable for storing planetary ephemerides. Because of these necessary format differences, manipulating (e.g., converting from ascii to binary form) or interpolating (i.e., determining time-ephemeris results at a particular value of the independent time variable for the binary form of these time ephemerides) the present results can only be currently done using the ephcom-3.0.0 software.

The present results do not include the direct effect of asteroids and figure effects on the gravitational potential calculated by te_gen-2.0.0 (although that potential does include the indirect asteroid and figure effects on the motions of the major bodies in the solar system). The missing direct effect of the asteroids is (by far) the more important of the two effects and corresponds to an error in the present time ephemerides consisting of a sum of quasi-periodic terms whose combined amplitudes rarely exceed 1.5 x 10^{-11} seconds. These errors are smaller than the above estimate of errors in modern time ephemerides due to astrometric fitting uncertainties. Nevertheless, I call on all those groups publicly distributing planetary ephemerides to also publicly distribute the corresponding exact asteroid ephemerides used for these calculations to make it possible to remove this small source of error from te_gen calculations. Of course, public distribution of these asteroid ephemerides should be encouraged in any case in the interest of open science since, for example, the quality of the asteroid model is an important factor distinguishing the quality of one planetary ephemeris calculation from another.

Posted by Alan W. Irwin 2013-01-18