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Since year 86, the spot satellites provide images of the earth for commercial applications. Whereas the Spot IV satellite, to be launched in 97, will include a medium IR imaging capability, a more significant improvement of the system performances will be achieved with the future Spot IV observation modes a 5 m panchromatic resolution, as well as an along track stereoscopic capability. Trade off activities on concurrent concepts of instrument have been performed during feasibility phase, they have confirmed that the reuse of the Spot IV instrument concept provides the best compromise in terms of performance with respect to programmatic. Even so the demanding resolution implies significant improvement of every subsystem of the instrument. A new focal plane including 5 observation channels has to be designed. To cope with the 5 m resolution and the 60 km image width a monolithic 12000 pixels of 6.5 microns squared CCD array is to be developed. Data rate up to 4 Megapixels per second will also need to develop new components for the video electronics. The optics constituting the Schmidt telescope will require outstanding performances in terms of wave front error. A figure ranging (lambda) /18 for the complete telescope is needed, this will be achieved thanks to computer assisted polishing technique. For the same reason the minimisation of the in orbit mirror surface distortions will need to develop specific isostatic mounts. High sensitivity of the optical combination with regards to defocus will also lead to design improvement in the field of thermo- and hygroelastic behaviors of the structure and in the fixation concept of the telescope mirrors. Lastly new techniques for the optical alignment and the performance measurements will have to be developed to assess ultimate in orbit performances.
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