aircraft, either from earlier RC-135 variants or from tankers and transports. In 2005, the RC-135 fleet completed a series of significant airframe, navigation and powerplant upgrades which include re-engining from the TF33 to the CFM International CFM-56 (F108) engines used on the KC-135R and T Stratotanker and upgrade of the flight deck instrumentation and navigation systems to the AMP standard. The AMP standard includes conversion from analog readouts to a digital "glass cockpit" configuration. Operational history The current RC-135 fleet is the latest iteration of modifications to this pool of aircraft dating back to the early 1960s. Initially employed by Strategic Air Command for reconnaissance, the RC-135 fleet has participated in every armed conflict involving U.S. forces during its tenure. RC-135s supported operations in Vietnam War, the Mediterranean for Operation El Dorado Canyon, Grenada for Operation Urgent Fury, Panama for Operation Just Cause, the Balkans for Operations Deliberate Force and Allied Force, and Southwest Asia for Operations Desert Shield, Desert Storm, Enduring Freedom and Iraqi Freedom. RC-135s have maintained a constant presence in Southwest Asia since the early 1990s. They were stalwarts of Cold War operations, with missions flown around the periphery of the USSR and its client states in Europe and around the world. Originally, all RC-135s were operated by Strategic Air Command. Since 1992 they have been assigned to Air Combat Command. The RC-135 fleet is permanently based at Offutt Air Force Base, Nebraska and operated by the 55th Wing, using forward operating locations worldwide. The 55th Wing operates 22 platforms in three variants: three RC-135S Cobra Ball, two RC-135U Combat Sent, and 17 RC-135V/W Rivet Joint. On August 9, 2010, the Rivet Joint program recognized its 20th anniversary of continuous service in Central Command, dating back to the beginning of Desert Shield. This represents the longest unbroken presence of any aircraft in the Air Force inventory. During this time it has flown over 8,000 combat missions supporting air and ground forces of Operations Desert Storm, Desert Shield, Northern Watch, Southern Watch, Iraqi Freedom and Enduring Freedom. On 22 March 2010 the British Ministry of Defence announced that it had reached agreement with the US Government to purchase three RC-135W Rivet Joint aircraft to replace the Nimrod R1, which was subsequently retired in June 2011. The aircraft, to be styled as 'Airseeker', were scheduled to be delivered by 2017 at a total cost of around £650 million, including