  The design of an Airwatch type space mission can greatly benefit from a
flexible simulation code for establishing the values of the main parameters of
the experiment. We present here a code written for this purpose. The cosmic ray
primary spectrum at very high energies, the atmosphere modelling, the
fluorescence yield, the photon propagation and the detector response are taken
into account in order to optimize the fundamental design parameters of the
experiment, namely orbit height, field of view, mirror radius, number of pixels
of the focal plane, threshold of photo-detection. The optimization criterion
will be to maximize counting rates versus mission cost, which imposes limits
both on weight and power consumption. Preliminary results on signals with
changing energy and zenith angle of incident particles are shown.
