A study has been carried out for the European Space Agency (ESA) on strategies for radiometric and spectral calibration of data produced by optical sensors observing Earth surface from Earth orbit. The study relates specifically to data produced by high-resolution imaging spectrometers and imaging radiometers, such as the PRISM instrument which is currently under development by ESA. The typical instrument specification includes 50m spatial resolution on ground, with 10nm spectral resolution through the visible/near-IR and short-wave IR spectral wavebands, plus imaging in a few selected wavebands in the thermal IR. This paper is limited to discussion of the first step in the radiometric calibration process, in which raw digitized data from the space instrument are converted to accurate values for at- sensor radiances. Relative spectral response information, for each resolved spectral band, must accompany the calibrated radiance data. The calibration process, for at- sensor radiances, requires accurate characterization of the space-instrument response characteristics. Part of the specification for the PRISM instrument is given as an example of performance requirements, and a possible design form for PRISM is briefly described as an introduction to discussion of the relevant response characteristics. Critical pre-flight and in-flight characterization requirements are noted, and methods for in-flight characterization are discussed briefly, including use of on- board hardware and space-views, and vicarious calibration using ground target areas.