both carry the same mission equipment. For many years, the RC-135V/W could be identified by the four large disc-capped MUCELS antennas forward, four somewhat smaller blade antennae aft and myriad of smaller underside antennas. Baseline 8 Rivet Joints (in the 2000s) introduced the first major change to the external RC-135V/W configuration replacing the MUCELS antennas with plain blade antennas. The configuration of smaller underside antennas was also changed significantly. RC-135W Rivet Joint (Project Airseeker) The United Kingdom bought three KC-135R aircraft for conversion to RC-135W Rivet Joint standard under the Airseeker project. Acquisition of the three aircraft was budgeted at £634m, with entry into service in October 2014. The aircraft formed No. 51 Squadron RAF, based at RAF Waddington along with the RAF's other ISTAR assets. They are expected to remain in service until 2045. Previously, the Royal Air Force had gathered signals intelligence with three Nimrod R1 aircraft,. When the time came to upgrade the maritime Nimrods to MRA4 standard, Project Helix was launched in August 2003 to study options for extending the life of the R1 out to 2025. The option of switching to Rivet Joint was added to Helix in 2008, and the retirement of the R1 became inevitable when the MRA4 was cancelled under the UK's 2010 defence review. The R1's involvement over Libya in Operation Ellamy delayed its retirement until June 2011. Helix became Project Airseeker, under which three KC-135R airframes were converted to RC-135W standard by L-3 Communications. L-3 also provides ongoing maintenance and upgrades under a long-term agreement. The three airframes are former United States Air Force KC-135Rs, all of which first flew in 1964 but were modified to the latest RC-135W standard before delivery. The three airframes on offer to the UK are the youngest KC-135s in the USAF fleet. As of September 2010 the aircraft had approximately 23,200 flying hours, 22,200 hours and 23,200 hours. 51 Sqn personnel began training at Offutt in January 2011 for conversion to the RC-135. The first RC-135W (ZZ664) was delivered ahead of schedule to the Royal Air Force on 12 November 2013, for final approval and testing by the Defence Equipment and Support team prior to its release to service from the UK MAA. The second (ZZ665) was delivered on 4 September 2015 and the third (ZZ666) in June 2017; the latter entered operational service in December 2017. RC-135X Cobra Eye The sole RC-135X Cobra