Accurate insertion of airborne forces was always difficult during the Second World War, but in 1944 the Allies developed the Rebecca-Eureka guidance system. This required a pathfinder to set up a Eureka transmitter beacon on the ground prior to the main lift and send out and receive signals on one of five frequencies. A Rebecca was placed in each aircraft to receive the signal and respond to the Eureka and from that point the two would transmit to each other giving the pilots in the aircraft their bearing to, and distance from, the DZ or LZ from approximately 20 km away. A posed photograph of four Pathfinder officers synchronising their watches before take-off at RAF Harwell - all survived the war. Left to right - Lieutenants Bobby de la Tour, Don Wells, John Vischer and Bob Midwood. This is one of the most famous and atmospheric images associated with the British Army and D-Day.