Cassini ISS Mutual Event Astrometry of the Mid-sized Saturnian Satellites 2005-2012

TL;DR

This paper presents high-precision astrometric measurements of Saturn's mid-sized satellites using Cassini ISS images, significantly improving positional accuracy over previous observations and providing valuable data for orbital modeling.

Contribution

It introduces a method for precise satellite position estimation from mutual event observations, reducing errors and systematic biases, and offers a comprehensive dataset for future orbital studies.

Findings

Achieved an order-of-magnitude improvement in positional accuracy.

Demonstrated the effectiveness of mutual event astrometry in reducing systematic errors.

Provided a dataset with 2303 observations, including satellite and star positions.

Abstract

We present astrometric observations of the Saturnian satellites Mimas, Enceladus, Tethys, Dione and Rhea from Cassini Imaging Science Subsystem (ISS) narrow-angle camera (NAC) images. Image sequences were designed to observe mutual occultations between these satellites. The positions of satellite centres were estimated by fitting ellipsoidal shape models to the measured limbs of the imaged satellites. Spacecraft pointing corrections were computed using the UCAC2 star catalogue. We provide a total of 2303 astrometric observations, resulting in 976 pairs, the remainder consisting of observations of a single satellite. Mean residuals for the individual satellite positions relative to the SAT360 ephemeris were 4.3 km in the line direction and -2.4 km in the sample direction, with standard deviations of 5.6 and 7.0 km respectively, an order of magnitude improvement in precision compared to published HST observations. By considering inter-satellite separations, uncertainties in camera pointing and spacecraft positioning along with possible biases in the individual positions of the satellites can be largely eliminated, resulting in an order-of-magnitude increase in accuracy compared to that achievable using the individual satellite positions. We show how factors relating to the viewing geometry cause small biases in the individual positions of order 0.28 pixel to become systematic across the dataset as a whole and discuss options for reducing their effects . The reduced astrometric data are provided in the form of individual positions for each satellite, together with the measured positions of reference stars, in order to allow more flexibility in the processing of the observations, taking into account possible future advances in limb-fitting techniques as well as the future availability of more accurate star catalogues, such as those from the GAIA mission.