  This paper describes the definition of a typical next-generation space-based
weak gravitational lensing experiment. We first adopt a set of top-level
science requirements from the literature, based on the scale and depth of the
galaxy sample, and the avoidance of systematic effects in the measurements
which would bias the derived shear values. We then identify and categorise the
contributing factors to the systematic effects, combining them with the correct
weighting, in such a way as to fit within the top-level requirements. We
present techniques which permit the performance to be evaluated and explore the
limits at which the contributing factors can be managed. Besides the modelling
biases resulting from the use of weighted moments, the main contributing
factors are the reconstruction of the instrument point spread function (PSF),
which is derived from the stellar images on the image, and the correction of
the charge transfer inefficiency (CTI) in the CCD detectors caused by radiation
damage.
