Context :

Aims : 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 .

Methods : 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 compare observed-minus-computed ( O - C ) residuals based on inter-satellite separations with those based on individual satellite positions , relative to the SAT360 and NOE-6-2012-MAIN ephemerides .

Results : We provide a total of 2303 astrometric observations , resulting in 976 pairs , the remainder consisting of observations of a single satellite .
We obtain mean residuals for the individual satellite positions relative to the SAT360 ephemeris of 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 .
We show that , 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 themselves .
We demonstrate 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 .

Conclusions :