Precise Position Measurements of Jupiter, Saturn and Dranus Systems with the Very Large Array

Abstract

The discovery of Quasi-Stellar Objects (QSO’s) and the development of Very Long Baseline Interferometry (VLBI) techniques since the early 1960’s have afforded an extremely precise coordinate grid system or reference frame on the sky. Since the QSO’s are both stellar-like objects and strong emitters of radio flux, their positions can be measured with both optical and radio techniques. However, the radio VLBI measurements on simple, compact sources are by far the most precise, the useful unit being micro-radians. The origin of the VLBI reference frame is tied to the stellar reference frame by the measurements of Hazard et al. (1971) who precisely measured the optical position of the QSO 3C273 with lunar occultations, which, when combined with VLBI position measurements of the same source, ‘‘tied’’ the VLBI and stellar position frames together. Their reported accuracy is ±0.15 arc seconds, but there is good reason to believe that the error in that frame tie is significantly smaller. Radio tracking measurements of spacecraft associated with the inner planets, such as the Viking Orbiters around Mars, combined with VLBI measurements indicate that the frames of the inner planets (i.e. their ephemerides), stellar system, and the VLBI reference sources are consistent to better than ±0:’03 (Newhall et al., 1984).