An orbit estimation for the spacecraft at closest approach phase to the planet

This paper describes the results of an investigation of the expected navigation (orbit determination) accuracy during the approach phase of a planet orbiter mission. The target planet is "Venus". This expected accuracy was determined for Earth-based Doppler observation data with and without the addition of ranging or on-board optical observation data. The sensitivity of this expected accuracy to errors effecting the radiometric and optical data also was examined. It was found that unless planetary ephemerisis errors are reduced below their anticipated levels, the addition of ranging data does not significantly improve the estimate of the spacecraft\´s state near encounter. However, the addition of optical data could make a significant improvement in the encounter accuracy when the spacecraft is outside the sphere of influence of Venus. This improvement diminished as the spacecraft deeply penetrated the sphere of influence.