Optimized sampling for CCD instruments: the Supermode scheme

High resolution remote sensing optical systems which are based upon the push-broom acquisition principle, have usually a significant Modulation Transfer Function (MTF) value at Nyquist frequency which leads to aliasing and sub optimization of the system: the sampling grid is not dense enough to catch all the spatial frequencies transmitted through the optics and the CCD. This papers analyses the image acquisition process from a signal processing point of view by reducing it to a three component set: MTF, signal to noise ratio and sampling grid. The classical push broom principle then appears to be ill-adapted since the cut-off frequency and the sampling rate are bound together. To overcome this limitation, the authors concentrate on the Supermode sampling scheme (adopted for SPOTS very high resolution panchromatic mode) which consists in processing two images acquired simultaneously shifted by 0.5 pixel.