Radar dish on board HMS Belfast. Radar is an acronym for radio direction and ranging. Ship radar systems emit a radio signal across the air in all directions. When the waves hit enemy ships or aircraft the signal is reflected back (an echo) and picked up by a reciever dish. The system knows the exact location of the ship from the time it takes for the signal to return to the reciever dish. The speed of moving objects can be measured by the change in the reflected signal's frequency.