After seeing long-range surveillance's effectiveness during the first Gulf War, the UK acquired Airborne Stand-Off Radar (ASTOR) technology and integrated it into its all-seeing Sentinel surveillance system.
Originally developed to monitor the movements and formations of enemy armor on the battlefield, Sentinel consists of Air, Land and Support segments. The Air segment is made up of five converted Bombardier Global Express aircraft, known as the Sentinel R1, which are outfitted with Synthetic Aperture Radar (SAR) and Ground Moving Target Indicator (GMTI) technologies. The Land segment comprising two transportable Operational Level Ground Stations and six mobile Tactical Ground Stations. The Air and Land segments share data between themselves and command via high-speed Link 16 datalinks.
The Sentinel R1 aircraft is powered by dual Rolls-Royce BR710 turbofan engines, each producing 14,750 pound-foot thrust, providing a top speed of Mach .75 and enabling a 40,000-foot service ceiling. Missions in an R1 can routinely last nine hours and require a crew of five—pilot, co-pilot, an Airborne Mission Commander and two image analysts. While some of the analysis is performed onboard, the R1's primary objective is data collection, not interpretation.
Each R1 is outfitted with a Dual Mode Radar similar to that used by the U2 spy plane—Raytheon Systems' SAR/MTI radar, which can operate at high altitude (up to 47,000 feet), over long distances, and in inclement weather. SAR radar emits pulses in a modified grid pattern and employs a relatively long aperture to produce a high-resolution, 2D image of the battle field or focus on individual targets. MTI Radar is also employed to track vehicle movements, differentiating between moving and stationary objects by comparing sequential pulses—much in the same way astronomers search for supernovae. Together, SAR/MTI radar can spot the strength, location and speed of enemy troops from up to 160km away.
Altogether, the UK's Ministry of Defense has spent approximately £954 million for the entire Sentinel system—including the five aircraft and eight associated ground stations. [Wikipedia - RAF - Airforce Technology - Spyflight - IEEE]
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