Navigational utility of high-precision radio interferometer for Galileo's approach to Jupiter

摘要

The effect of very long baseline interferometry (VLBI) measurements of 2-nanoradian (nrad) accuracy has been studied for use in Galileo's approach to Jupiter's moon Io. Of particular interest is reducing the error in the minimum altitude above Io's surface. The nominal tracking strategy includes Doppler, range, and onboard optical data, in addition to VLBI data with 25-nrad accuracy. For nominal data, the altitude error is approximately 250 km with a data cutoff of 19 days before closest approach to Io. A limited number (two to four) of 2-nrad VLBI measurements, simulating a demonstration of improved VLBI data, were found to reduce the altitude error by 10 to 40 percent. Improving the accuracy of the VLBI measurements of the nominal tracking strategy to 2 nrads, to simulate the results from an operational few-nrad VLBI capability, was found to reduce the altitude error by an approximate factor of four. This reduction in altitude error is attributed to the ability that VLBI data give to help determine the along-track component of Jupiter's ephemeris. This capability complements the ability of the onboard optical data to determine the radial and cross-track components of Jupiter's ephemeris.