Airborne units (2) acquire images as electronic signals either directly
or indirectly from first ground based units (2a) and transmit them
by broad band radio to second ground based units (3, 4), either directly
or indirectly via second airborne units (1), which units (3, 4) record
the received image signals for a period of time sufficient to permit a section
of the received image to be selected by narrow bandwidth means (5, 6).
