Sentinel R1
Airborne Stand-Off Radar (ASTOR)

The Sentinel R1, also known as the Airborne Stand-Off Radar, is a reconnaissance aircraft based on the Bombardier Global Express airframe. The aircraft was originally intended for conventional war-fighting tasks such as strategic reconnaissance and tracking of armored formations, but was also used in tactical intelligence roles by the United Kingdom's Royal Air Force in Afghanistan as part of Operation Herrick.

The Sentinel R.Mk 1 provides long-range, wide-area battlefield surveillance, delivering critical intelligence and target tracking information to British and coalition forces. The aircraft has been operationally deployed in support of operations in Afghanistan, Libya and Mali, and is currently deployed in support of British and Coalition operations in Iraq and Syria.

Using its powerful multi-mode radar, the Sentinel’s mission crew identifies, tracks and images numerous targets over great ranges, passing the information in near real time to friendly forces. A team of intelligence imagery analysts from 1 Intelligence, Surveillance, Reconnaissance (ISR) Wing backs up the Sentinel crews, conducting in-depth forensic analysis of collected data, and using it to generate intelligence products for time-critical dissemination to commanders and decision makers, enabling them to execute current operations and plan future strategies.

During the early 1980s, the UK government identified a need for a battlefield reconnaissance system to provide awareness over a broad area. Expressed in the Corps Airborne Stand-Off Radar (CASTOR) requirement, the programme elicited responses from Thorn-EMI and Ferranti. The former’s radar was tested in an English Electric Canberra from 1982, while a Britten-Norman Islander modified to take the latter, first flew in 1984.

Under the mid-1980s Staff Requirement (Land/Air) 925, an existing commercial business jet aircraft was to be modified to carry the radar and air-to-ground data links. The aircraft was to be based on a modern, class-leading, large business jet. It would be able to operate above 40,000 feet and have an endurance in excess of 9 hours. The aircraft identified as suitable candidates for what became known as the Airborne Stand-Off Radar were the Bombardier Global Express and the Gulfstream V.

ASTOR was designed a ground surveillance system designed to provide information about the deployment and movement of enemy forces. It would use state-of-the-art radar technology to obtain high resolution imagery of static features and, operating in an alternate mode, it was to be capable of identifying and tracking moving vehicles. Imagery gathered would be transmitted in near-real-time to a network of distributed Ground Stations deployed with the front-line forces. Facilities within the Ground Stations would permit the display and analysis of imagery, thus ensuring that the tactical commanders were aware of the latest developments on the ground.

Experiences during operations in the Gulf in 1990-1991 further promoted the development of such a system. Widespread use of Synthetic Aperture Radar (SAR) and Ground Moving Target Indicator (GMTI) during the response to Iraq's invasion of Kuwait had clearly shown their value. In 1993, the UK Ministry of Defense approved the requirement for the ASTOR system.

In June 1999, the UK Ministry of Defence selected Raytheon Systems Limited, the UK subsidiary of the Raytheon Corporation, to build the ASTOR. The deal, worth £800 million ($1.3 billion), was one of the most intensely competed and thoroughly analyzed multi-mission programs in business aviation history. Two UK-based teams, led by Lockheed Martin Tactical Systems UK and Raytheon Systems Limited, were in competion following receipt of the 1995 Ministry of Defense project definition contracts. They were joined by a bid from Northrop Grumman for a deiravtive of the E-8 JSTARS mission systems package on a Gulfstream V.

The ASTOR system was expected to comprise 5 aircraft and 8 ground stations, together with comprehensive training and maintenance facilities at the main operating base. The equipment procurement costs were expected to be approximately £800 million. The in-service date was expected to be 2004/5, assuming contract award late 1999. Bids for the ASTOR contract were been received from Lockheed Martin, Northrop Grumman and Raytheon. The bids were under review by early 1999 and an announcement was expected in April 1999. All the proposed system solutions made extensive use of commercially available equipment to minimise risk and cost. Irrespective of the choice of eventual contractor, there will be a substantial UK industrial participation in the execution of the contract.

The joint RAF/Army battlefield surveillance system selected would be installed on 5 Bombardier Global Express business-jets, produced by Bombardier/Shorts, and would be based at RAF Waddington in Lincolnshire when it entered service, expected at that time to occur in 2005. This new capability would be supported by a team of some 350 RAF and Army personnel and some 50 civilian contractors. ASTOR consisted of a dual-mode SAR, utilizing the ASARS-2 radar technology, similar to those used by American U-2R spy planes and GMTI radar as used on the American JSTARS surveillance system. This allowed near-realtime monitoring of static and moving ground targets in all weather conditions. The Global Express aircraft was capable of operating at an altitude in excess of 45,000 feet (13,724 meters), almost twice that of J-STARS and had a range of up to 6,000 miles (9,660 kilometers). The aircraft had a mission endurance of up to 13 hours. The aircraft component was subsequently named the Sentinel R1.

The Global Express airframe offered numerous advantages over competing aircraft in its category and had set new standards in business aviation. These included the longest non-stop range in corporate aviation and a top cruise speed that reached nearly the speed of sound, as well as the largest cabin of any corporate aircraft at the time. It also featured the highest United Kingdom content by far, with 36 percent of the aircraft to manufactured in the United Kingdom. Bombardier Aerospace Shorts unit designed and manufactured 25 percent of the Global Express airframe in Belfast. Motorola, the world's leader in advanced tactical ground stations, would provide ground systems. Motorola planned to build 2 types of ground stations for the ASTOR project: Operational Level Ground Stations (OLGSs) for large, centralized headquarters facilities and Tactical Ground Stations (TGSs) for highly mobile, tactical platforms. The 8 planned ground stations were divided between 2 OLGS and 6 TGS.

The dual mode SAR/GMTI was the main sensor in the ASTOR system, enabling radar data to be available in near real time for processing and exploitation both on the platform and the ground. This provided monitoring of land-based targets to assess military capability and behaviour patterns. Targets could be classified at long range for interdiction, with wide area GMTI surveillance used to determine the position of a variety of vehicles travelling over a wide range of radial velocities.

ASTOR was not an airborne battle management or command and control system, such as JSTARs, although operators could analyze the imagery on board the aircraft, in the ground stations, and at other military sites as ASTOR passed the information in near real-time. The system would provide a 24-hour, all weather, battlefield surveillance capability. The radar range was such that the aircraft would be able to operate successfully at a safe 'Stand-off' distance behind the forward-edge-of-battle, greatly reducing the risk of loss to enemy action. ASTOR would be able to interface with the proposed military communications architecture and to be interoperable with other NATO forces. It would be a new capability for the UK Armed Forces, and the most advanced system of its kind anywhere in the world when it entered service. It would be a vital force multiplier in the modern conflict, where speed of battle was such that up-to-date information was crucial if troops were to be deployed effectively.

As part of the UK's 2010 Strategic Defence and Security Review, a successor to the Sentinel R1 and its associated ground component were planned. The Sentinel R1 would be retained at least as long as UK forces remained deployed with the International Security Assistance Force (ISAF) in Afghanistan. After that time, a new platform for the ASTOR system would be developed.

Sentinel was introduced in 2008 in the knowledge that a significant equipment upgrade would be required in the mid 2010s. The Defence Review in 2010 cancelled this expected upgrade bringing forward the likely out of service date.

The Sentinel R1 has been operationally deployed in support of a number of operations. Some operations are considered to be both conventional and counter-insurgency; for example operations in Afghanistan (Op HERRICK) and Iraq (Op SHADER). It has also been deployed on operations in Libya (Op ELLAMY), Nigeria (Op TURUS) and Mali (Op NEWCOMBE), all considered conventional operations.

In May 2011 British Army personnel took to the skies over Libya in RAF Sentinel aircraft, helping gather essential intelligence in support of NATO-led operations to protect civilians on the ground below. Among the Royal Air Force ISTAR (Intelligence, Surveillance, Target Acquisition and Reconnaissance) squadrons enforcing the UN mandate in the north African country is 5 (Army Cooperation) Squadron - a tri-Service set-up. Within 24 hours of combat operations beginning, the Sentinel R1 aircraft, flown by 5 (AC) Squadron, was using its sophisticated array of onboard sensors to surveil the unfolding situation and acquire ground targets.

Unlike Op HERRICK, Op ELLAMY is a more air-centric deployment. The team finds and tracks pro-regime armour using moving target indicators. They are also looking at radar pictures to determine whether an airfield is in use or being obstructed. The aircraft uses a sophisticated array of sensors to surveil and acquire ground targets. The resulting information is then brought back to the UK for analysis.

The Sentinel R1 aircraft of 5 (Army Cooperation) Squadron continued to keep a watchful eye on ground activity in Afghanistan during Janaury 2012 and quickly reported any new activity to ground commanders. Primarily, their tasking was in support of operations being undertaken across Helmand province. The majority of the activity was to provide pattern of life data in support of a number of disparate operations led by the US Marine Corps of Task Force Leatherneck and UK ground forces operating as part of Task Force Helmand. The Sentinel’s Ground Moving Target Indicator function continues to be highly sought after and saw 5 (Army Cooperation) Squadron fly for approximately 30 hours.

By August 2012 Surveillance aircraft based at RAF Waddington had notched up a collective total of 20,000 flying hours protecting British and other ISAF forces in Afghanistan. The high-tech, state-of-the-art Sentinel R1 aircraft of No 5 (Army Cooperation) Squadron and Shadow R1 aircraft of 14 Squadron have each clocked up 10,000 hours on Operation HERRICK. While based in Britain aircraft from both units have been permanently patrolling the skies above Afghanistan since 2009 gathering vital intelligence on insurgent activities.

The threat from terrorist groups in Mali demands a response based on strong international and regional partnerships. International assistance was given to support the Malian Government and Armed Forces and to support the African-led International Support Mission to Mali (AFISMA). The SENTINEL R1, a surveillance aircraft with a support team of about 60 men and women has been based in Dakar since January 2013. The operation has been aimed at supporting the efforts of the international community, led by France and African countries, to assist the Malian authorities in restoring peace and stability in Mali. The UK strongly supported French military intervention, which was made at the direct request of the Malian Government when terrorist groups advanced on Bamako in January and provided immediate logistical and surveillance capabilities to assist the military operations to halt the immediate threat from terrorism in Mali.

In 2017 the original service date for the Sentinel fleet was extended from 2018 to 2021. This will be achieved by careful fleet management practices, including the removal of one Sentinel from front line service with effect from 1 April 2017.

The Strategic Defence and Security Review 2015 determined that Sentinel should have an out of service date (OSD) of March 2021. Sentinel’s radar and mission system are now increasingly obsolescent and will face increasing reliability issues as time progresses. Retaining the capability would have required significant upgrade expenditure and the March 2021 out of service date has been retained. It is necessary to retire certain capabilities at planned OSDs to allow re-investment in future capability.

While no identical capability is operated by the UK (though similar capabilities exist in the NATO inventory), the UK does have a number of other intelligence, surveillance and reconnaissance capabilities that collect different types of intelligence information, including long-range strategic assets (Sentry, Rivet Joint and Poseidon) and shorter-range more tactically-focused assets (including Shadow, Reaper and Watchkeeper).