In an image data transmission system carried by a craft, an image pickup apparatus (24) picks up optical images from an object zone to produce a succession of first image signals representative of the optical images and a succession of second image signals having a predetermined delay time relative to the first image signals. The first image signals are previously processed by a reference signal generating circuit (44) to monitor frequencies of occurrences for a plurality of levels predetermined from a minimum level to a maximum one and to determine a reference signal necessary for dis. tinguishing the second image signals. Amplitudes of the second image signals are classified by a signal classifying unit (45) with reference to the reference signals into classified image signal successions which falls within at least two amplitude ranges. The classified image signal successions are individually encoded by at least two predictive encoding units (52 and 541 into encoded image signal successions, respectively. The encoded image signal successions are combined by a combining unit (43) into a processed signal succession which is sent to a terrestrial station.
IMAGE DATA TRANSMISSION SYSTEM