Air Traffic Controller Eric Long with the 'daylight plotter' in the second Control Tower at Melbourne/Essendon in 1951. Aircraft movements within 20 miles radius of the airport could be tracked by the operator of an experimental Aerodrome Control Radar located in a hut on the other side of the aerodrome.

The aircraft's position was relayed electro-mechanically to the 'daylight plotter', located in the Control Tower, which drove a stylus over sensitised paper and plotted the track of the radar return (aircraft).

The blotting paper display was continuously soaked in a solution of potassium iodide, bi-carbonate of soda and water. Energising of the stylus with an electric current by the radar operator caused the iodine to come out of solution at the point touched by the stylus. The bi-carbonate of soda would cause the paper to be bleached after some time, allowing continual use.

This display could not be used for separation and was provided to enhance what we would today call situational awareness.

(Photo: Jack Gallagher/CAHS collection)

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