VLBA Astrometric Observations of the Cassini Spacecraft at Saturn
D. L. Jones (1), E. Fomalont (2), V. Dhawan (2), J. Romney (2), W. M., Folkner (1), G. Lanyi (1), J. Border (1), R. Jacobson (1) ((1) Jet Propulsion, Laboratory, (2) National Radio Astronomy Observatory)

TL;DR
This paper reports high-precision VLBA astrometric observations of the Cassini spacecraft around Saturn, improving the planetary ephemeris accuracy and linking inner and outer solar system reference frames.
Contribution
It introduces new VLBA-based measurements of Saturn's position that enhance the accuracy of planetary ephemerides and the inertial reference frame connection.
Findings
Saturn's position accuracy improved to ~0.3 mas
Combined data constrains Saturn's ephemeris (DE 422)
Expected threefold improvement in ephemeris accuracy
Abstract
The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemeredes), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric VLBI observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5 nrad), or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et…
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