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// Tales from software development

The RAF Sector Clock

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With the 70th anniversary of the start of the Battle of Britain I thought it was a good time to write about something related that’s provides an insight into how the RAF’s command and control system worked: the RAF Sector Clock.

RAF Sector Clock

The Sector Clock is a standard RAF clock of the 1930s and 1940s made by FW Elliot but it has a modified face that shows the space between each hour as a different coloured triangle. Only three colours, red, blue, and yellow, are used and they alternate around the circumference of the face. The clock was used in the sector control rooms where each day’s aircraft movements were plotted on a map table.

As each piece of information about Luftwaffe movements arrived it was added to the plot table using a wooden block that showed the approximate number of enemy aircraft, their height, and a colour that corresponded to the colour currently indicated by the minute hand of the Sector Clock.

This allowed the controllers to immediately identify how old the information on the table was but the most interesting aspect of this method was that as the minute hand of the clock entered a each coloured triangle, any plots on the table with the corresponding colour were removed. Any plots on the board with the same colour must have been placed at least 10 minutes earlier.

The principle was a simple enough: out of date information is worse than no information at all. It’s interesting that this principle was established before the computer age and what we think of as the era of information theory.

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Written by Sea Monkey

June 11, 2010 at 8:00 pm

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