Raytheon Sentinel

Sentinel R1
ZJ692 landing at Waddington Air Show 2006
Role	Airborne battlefield and ground surveillance aircraft
Manufacturer	Raytheon (modifications) Bombardier (airframe)
First flight	26 May 2004
Introduction	2008
Status	Active service
Primary user	Royal Air Force
Produced	Raytheon UK
Number built	5
Unit cost	£954m (for a fleet of 5 aircraft plus 8 vehicle-based ground stations)
Developed from	Bombardier Global Express

The Raytheon Sentinel is an airborne battlefield and ground surveillance aircraft operated by the Royal Air Force. Based on the Bombardier Global Express ultra long range business jet, it was adapted by Raytheon to meet the RAF's requirements. Originally known as the ASTOR (Airborne STand-Off Radar) programme the aircraft is operated by a RAF squadron manned by both air force and army personnel. The Sentinel is interoperable with other allied systems such as JSTARS and the NATO Alliance Ground Surveillance (AGS) system.

In 2010 the UK Conservative-Liberal Democrat coalition government's Strategic Defence and Security Review announced its intention to "withdraw the Sentinel airborne ground surveillance aircraft once it is no longer required to support operations in Afghanistan." Sentinel has supported the British Army in Afghanistan. One Sentinel aircraft was deployed to assist French forces in Mali on 25 January 2013. The 2010 decision was reversed in 2014 by Prime Minister David Cameron and in the 2015 SDSR, the British government announced that the type's retirement would be delayed and that it would remain in service "into the next decade".

ASTOR has its roots in the British Army's Corps Airborne Stand-Off Radar (CASTOR) programme which in 1984 modified a Britten-Norman Islander (G-DLRA/ZG989) with a large nose radome for battlefield surveillance. The 1991 Gulf War confirmed the requirement for such an aircraft, but the end of the Cold War made funding difficult. The production contract was signed in December 1999 with a projected in-service date of 2005.

The first flight of the modified prototype was in August 2001, which validated the modifications required for the ASTOR system. The first production Sentinel R1 made its 4.4-hour maiden flight on 26 May 2004. The aircraft entered operational service with V (Army Co-operation) Squadron of the Royal Air Force based at RAF Waddington and flew its first operational sortie in Afghanistan in February 2009.