Arecibo Radar Astrometry of the Galilean Satellites from 1999 to 2016

Abstract

Harmon et al. Arecibo radar observations from 1992 provided some of the most precise line-of-sight distance (ranging) measurements of Ganymede and Callisto to date. We report 18 new ranges obtained at Arecibo from 1999 to 2016, among which are the first measurements of Io and Europa. We also report accompanying line-of-sight velocity (Doppler frequency) measurements. In 2015, we detected Europa, Ganymede, and Callisto with time-delay (range) resolutions as fine as 10 mu s (1.5 km) while Io was detected with 70 mu s (10.5 km) resolution. We estimated residuals for the radar measurements with respect to the latest JPL satellite ephemeris JUP310 and planetary ephemeris DE438. We found that the rms of the time-delay residuals are 29 mu s for Io, 21 mu s for Europa, 58 mu s for Ganymede, and 275 mu s for Callisto. When normalized by the measurement uncertainties, these correspond to the rms of 0.82, 1.25, 2.17, and 3.17 respectively. As such, the orbit of Callisto has the largest residuals and may benefit from an orbital update that will use radar astrometry. All Doppler residuals were small and consistent with their 1 sigma uncertainties.