The First Two Years of juno Spacecraft Astrometry with the Very Long Baseline Array

The Very Long Baseline Array (VLBA) is a ten-antenna radio interferometer with baseline lengths up to 8000 km. It can provide astrometric measurements of spacecraft orbiting planets and other objects in our solar system with sub-nrad precision (5 nrad = 1 milli-arcsec). These measurements can be used to create a time series of positions for solar system objects in the inertial International Celestial Reference Frame, which in turn can be combined with other data to refine the planetary ephemeris. An accurate solar system ephemeris is critical for interplanetary spacecraft navigation, dynamical studies and tests of gravitational theories, the analysis of pulsar timing observations, predictions of transits, eclipses, and occultations, and other applications. We are using VLBA observations of the Juno spacecraft in orbit about Jupiter to provide accurate positions for the Jupiter system barycenter for the ephemeris development group at the Jet Propulsion Laboratory, using observing and data reduction techniques developed for similar observations of the Cassini spacecraft while it orbited Saturn from 2004 until 2017. The VLBA observations of Cassini helped to improve the accuracy of Saturn's orbit by nearly an order of magnitude, and we expect that our observations of Juno will produce a similar improvement in our knowledge of Jupiter's orbit. Astrometric positions are particularly useful in constraining the orientation (inclination and longitude of ascending node) of an orbit, while range measurements are most useful in constraining the semi-major axis and ellipticity. Juno's orbit around Jupiter has a longer period than initially planned due to a concern about the spacecraft main engine. The resulting extended mission duration will improve our constraints on Jupiter's orbit inclination beyond that originally expected. Our VLBA observations of Juno are scheduled during approximately every third or fourth perijove pass. During these times the Juno spacecraft is continuously tracked by the Deep Space Network and the most precise solutions for the orbit of Juno about Jupiter are available. A good spacecraft orbit solution is needed to transfer our spacecraft sky positions to planet system barycenter positions. VLBA astrometry of planetary spacecraft has previously been applied to Mars orbiting space-craft, and will also be used during the OSIRIS-REx mission to improve the accuracy of the orbit of the potentially hazardous asteroid Bennu.