early 1990s, the U.K. Ministry of Defence launched an Airborne Stand-Off
Radar (ASTOR) program that was a ground surveillance system
designed to provide information about the deployment and movement of
enemy forces. The requirement stands for an existing modern,
class-leading, large commercial business jet aircraft was to be modified
to carry the radar and air-to-ground data links. In 1999, the Raytheon
Ltd proposal was selected to build ASTOR. The major components of ASTOR
program included the use of five Global Express business jets that
equipped with the ASARS-2 radar technology, used on U-2 reconnaissance
aircraft. Once the contract had been signed, Bombardier moved fairly
quickly and converted its original prototype Global Express into the
prototype ASTOR. The first flight of the modified prototype was made in
2001, which validated the modifications required for the ASTOR system.
The first production Sentinel R1 made its 4.4 hour maiden flight in
2004. The aircraft entered operational service with the RAF based at RAF
Waddington and operated by 5 Squadron.