Yep, to paraphrase Bunker Hunt's "a billion dollars ain't what it used to be"... a nanosecond (or picosecond) ain't what it used to be. Things that used to be insignificant n'th order theoretical nuisances are now very real significant problems. ---------------- > But it's not one-to-one as you suggest. A 1 meter change in elevation corresponds to a frequency offset of about 1e-16. So for 1e-18 levels of performance you "only" need to know g, or your elevation to 1 cm accuracy.