Pointing and tracking system for UVISS on the International Space Station

The characteristics of the fine pointing and tracking system of UVISS, UltraViolet Italian Space Surveyor, which is proposed to fly as an external payload attached to the ISS (International Space Station) Express Pallet is presented. The system is derived from the pointing system of the UVSTAR telescope that has flown three times on the Shuttle. The electro-optical architecture of the tracking telescope, including the ICCD detector and the computational unit based on a DSP board with direct high speed interface to the camera and the on-board stellar catalog is described. The simulations performed to determine the instrument expected tracking precision and performances considering the dynamics of the ISS are reported. Finally the utilization of a high frequency tracking error correction based on the detection of the spectrographs zero-th order signal is considered as an independent tracking error measurement which provides information and corrections of possible residual tracking errors and could be used for alignment check as well.