Sector clock
A sector clock or colour change clock was a round colour-coded clock used at military airfields and observation posts in the United Kingdom to help track the movements of enemy aircraft and coordinate and control air defences. == Overview == Developed in the United Kingdom during the First World War and originally known as "colour change clocks", the Royal Flying Corps (RFC) introduced sector clocks in 1917 to aid in the chronological monitoring and position plotting of German aircraft.