A search for extrasolar planets using the ESO VLTI PRIMA facility will become feasible in 2007. An astrometric accuracy of 10 micro-arcseconds will allow us to detect sub-Uranus mass planets (MUranus = 14.5×MEarth) around the most nearby stars, as well as to conduct a planet search around stars of different ages. Most of the PRIMA hardware subsystems are currently being developed by industry. At the same time a scientific Consortium has formed that will deliver the differential delay lines and astrometric software for PRIMA to ESO. In this paper we describe the planned efforts by the Consortium related to the “PRIMA astrometry operations and software”. These activities include an overall “PRIMA astrometry error budget”, a “PRIMA astrometry calibration and observation strategy”, the “PRIMA astrometry observation preparation tools” and the “PRIMA astrometry data reduction tools”. We describe how all these components fit together in an overall approach to the flow of knowledge within the project. First by quantifying the fundamental limits of the VLTI infrastructure and the astronomical sources under study. Followed by elimination or suppression of the errors through either a hardware change to the system, software control of the system, or a proper calibration and observation strategy. The ultimate goal is being able to calibrate all PRIMA astrometric data acquired over the full lifetime of PRIMA (5 to 10 years) to a uniform accuracy of 10 micro-arcseconds. This will allow identification of long-term trends in the astrometric parameters due to planetary companions around nearby stars and to determine the distances and proper motions for the selected sources.
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