1. Apparatus for a spaceborne or aerial platform, comprising: analogue to digital converter means for converting received SAR echoes into digital samples; a frequency demultiplexer for frequency demultiplexing a signal corresponding to a range line or an azimuth line of Synthetic Aperture Radar (SAR) data, and containing information about a plurality of target points, into a plurality of frequency channels, each frequency channel signal containing information about each of the plurality of target points; compression means for performing compression on the information contained in each frequency channel; a processor having range processing means for carrying out range compression of arrays of samples corresponding to range lines and azimuth processing means for carrying out azimuth compression on arrays of samples corresponding to azimuth lines, wherein at least one output of the range compression means and the azimuth compression means includes the apparatus, the processor being configured to generate a plurality of sub-images corresponding to the compressed information in the frequency channels into which a signal corresponding to a range line or an azimuth line is demultiplexed; image generator means for combining the sub-images; and a transmitter configured to send the combined sub-images to a ground station.