Radar Ranging to Ganymede and Callisto

Arecibo observations from 1992 February to March have yielded the first successful radar range measurements to the Galilean satellites. Round-up time delays were measured for Ganymede and Callisto with accuracies of 20 to 50 micrometer (3 to 7 km) and 90 micrometer (14 km), respectively. Both satellites showed round-trip delay residuals (relative to the E-3 ephemeris) of about a millisecond, most of which can be attributed to errors in the predicted along-track positions (orbital phases). Using a simple model that assumed that all of the ephemeris error was due to constant orbital phase and Jupiter range errors we estimate that Ganymede was leading its ephemeris by 122 +/- 4 km, Callisto was lagging its ephemeris by 307 +/- 14 km, and Jupiter was 11 +/- 4 km more distant than predicted by the PEP740 planetary ephemeris.