We have used the Very Long Baseline Array (VLBA) to make phase-referenced astrometric measurements of the Juno spacecraft in orbit about Jupiter since August 2016. These measurements are combined with solutions for Juno’s orbit to provide accurate positions for the Jupiter system barycenter in the International Celestial Reference Frame (ICRF). As of January 2021 a total of 12 observing epochs have been obtained. Our primary error sources are ionosphere/troposphere calibration, spacecraft orbit determination, and phase reference source position uncertainties. We use global ionosphere maps derived from navigation satellite measurements to calibrate ionosphere delays, and multiple short scans on sources covering a wide range of elevations at each antenna to calibrate troposphere delays. We schedule VLBA epochs during Juno’s perijove passes when the spacecraft Doppler signature is largest, as this provides the strongest spacecraft orbit constraints. Reference source positions are obtained from ICRF3 catalog, and the continual improvements to these positions from regular VLBI absolute astronomy observations. We estimate that the rms uncertainties in our Jupiter system barycentric positions are 0.2-0.3 milli-arcsec in right ascension and 0.3-0.4 milli-arcsec in declination. The ultimate result of these observations will be an improved orbit for Jupiter in the JPL planetary ephemeris.
This work have been partially supported by a grant from the NASA Planetary Astronomy Program to the Space Science Institute. Portions of this work were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The VLBA is a facility of the National Science Foundation operated by the National Radio Astronomy Observatory. This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under license, and the Astronomical Image Processing System developed by NRAO.