Abstract: A marine radar has a screen on which is displayed a radar image and several representations of the different control functions of the radar such as range, gain and sea clutter setting. The radar also has a key pad with three keys and a tracker ball that is connected to the radar control unit. The control unit generates a marker on the screen that is movable by the tracker ball. To change one of the control functions, the tracker ball is displaced until the marker moves into the region of the representation of the appropriate control function. The control unit then produces three boxes on the screen below the control function to indicate the different options available for that function. The user can select the desired option by pressing the appropriate one of the three keys.