found during operations that the target might not appear on the radar; in these cases, the navigator would select a nearby feature that would be visible, a bend in a river or a radio tower for instance, and measure the angle and distance between it and the target. They would then attempt to guide the aircraft so that the selected aiming feature was in the proper location relative to the centre of the display, by no means a simple task. Offset bombing allowed the navigator to dial these offsets into the display, which caused the entire display to move by that amount. The navigator then guided the aircraft so that the selected feature passed through the centre of the display, which was much easier to arrange. Through the same period, the API was replaced by the more advanced Navigation and Bombing Computer (NBC), which, when combined with Mk. IX and Green Satin radar, formed the Navigation and Bombing System (NBS). Green Satin made highly accurate and completely automatic measurements of wind speed and direction, allowing the NBC to perform dead reckoning calculations with a very high degree of accuracy. This further automated the navigation process to the point where separate navigators and bomb aimers were no longer needed, and some aircraft were designed with a crew of only two. Development proceeded at a slower rate due to post-war austerities. Flight testing of the smaller Mk. IX began in 1950 on an Avro Lincoln, followed by the Mk. IXA in 1951 on Handley Page Hastings or Avro Ashton aircraft. As this was too late for the Canberra, which entered service in 1951, early models had to be modified with a conventional glass nose for optical bombing. The Mk. IVA remained in service until 1956 when the Mk. IX finally entered service on the V-force. The first use of NBS in combat was in 1956, when Vickers Valiants performed long-range strikes on the Egyptian Air Force at Cairo Airport. The system remained in service with the V bomber force (Valiant, Avro Vulcan and Handley Page Victor) throughout their lifetime. The last use in combat was made by the Vulcans of the Operation Black Buck flights in 1982 during the Falklands War, which used the system as the primary navigation and bombing aid throughout the round trips to and from Ascension Island. Mk. IX was also used on the Handley Page Victor, the