The definition and preliminary design of a thermal imager for earth observation applications has been performed, justified by a thorough analysis of user requirements. A survey of international programmes and other sources have been used to derive the radiometric requirements at ground level. Then instrument requirements at top of atmosphere have been obtained by means of the usual split-window techniques for land and sea. Preliminary instrument radiometric performances have been estimated on the basis of a review of possible instrument concepts (detectors and scan modes). A trade-off analysis between instrument requirements and performances led to the identification of two classes of instruments - the first based on high performance, cooled infrared detectors, and the second relying on microbolometer technology, with lower performance but not constrained by the need for a cryocooler. The applications feasible by means of each of them have been identified. The chosen instrument baseline was that using uncooled microbolometers, for which the best spatial and radiometric resolution achievable has been assessed, in order to cover as many applications as possible in view of the analysis of requirements. The selected baseline has been further detailed, up to a complete outline of the instrument, in order to confirm the achievable performance and assure its feasibility.