SUMMARY An ephemeris for precise gravity tide predictions must match the nominal sensitivity of the superconducting gravimeter, 1 ngal. It is shown that this requires over one thousand separate spectral lines in an ecliptic ephemeris, which implies several thousand spectral lines in an equatorial ephemeris, and a similar number in a tidal catalogue. Important tidal effects also arise from the direct tidal attraction of the planets, and indirectly from the planetary perturbations to the luni-solar ephemeris. Indirect planetary effects amount to nearly 100nga1, and are larger than direct planetary effects, which are only about 10 ngal. Venus, Mars, Jupiter, and Saturn all influence the tide at or above the ngal level. The precision of the ephemeris has been tested by comparison with published values of the right ascension, declination, and sine horizontal parallax of the Sun and Moon over a 10 year period. This indicates that the tide raising potential generated from the ephemeris is good to better than 3 x lo-’ m2 s-’, or 1 ngal in gravity. INTRO D UCTI ON Superconducting gravimeters are now installed in 12 sites around the world, with more installations planned for the future. These instruments have a nominal sensitivity of lngal, which is so much of an improvement over mechanical spring instruments that it necessitates a revision in the processing of gravity station data, as well as a whole range of corrections that need not be performed on the data from mechanical spring instruments. This paper will deal with one aspect of this problem, that is, the prediction of the forcing function for gravity tides at a level which matches the sensitivity of the instrument. The forcing function is here defined as the direct geocentric attraction of the tide raising bodies at the equatorial radius of the Earth. The prediction of the actual tide, which includes an Earth response, and is measured along the local vertical, is a much more difficult problem and its solution to the ngal level awaits, among other things, an accurate prediction of the forcing function. The ephemeris described here will produce gravity tide predictions which are spectrally correct to the ngal level. That is, every partial tide whose amplitude exceeds a ngal, as well as all smaller waves whose sum exceeds a ngal, is represented.
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