Bombardier Sentinel R1

Knowing the location of your enemy has always been one of the basic principals of war led directly to the rapid development of aviation in WW1. However, by the time the Cold War had settled into an uneasy standoff along the borders of the Warsaw Pact (WP), the technology necessary for battlefield surveillance had progressed considerably from a man in a balloon with a pair of binoculars. During the Cold War all along the Central Region in West Germany, NATO forces were heavily outnumbered by the massed armoured formations of the WP. Had the WP armoured divisions ever managed to actually invade West Germany without their preparations being detected well in advance, the result would have been catastrophic. Only the most blinkered commentator would argue that, had such a conflict occured, the in place NATO forces could only have provided a sacrificial force to briefly delay the progress of a tide of main battle tanks and supporting forces, as they poured through the plains and passes of Germany en-route to the Channel and Atlantic seaboard.

Grumman OV-1D Mohawk

To provide a means of being able to monitor WP forces and provide advance warning of any manoeuvres that might precede an invasion, the US Army operated the Grumman OV-1 Mohawk, equipped with a large SLAR (Sideways-Looking Airborne Radar), that could look across the border into East Germany and detect large formations of vehicles. However, the capability of the OV-1 Mohawk was limited by its range, endurance and sensor capability. The US Army recognised that what was needed was a much larger aircraft, with a higher operational ceiling, range and endurance and this requirement eventually resulted in the entry into service of the Grumman E-8 Joint STARS – the most advanced battlefield surveillance aircraft to enter operational service.

The UK armed forces lacked any kind of battlefield surveillance aircraft whatsoever and since the mid-1970’s had a long standing, but fairly unsophisticated, requirement for an aircraft that could provide UK forces stationed in West Germany with target identification, which was known as CASTOR – Corps Airborne Stand-Off Radar. However, this was before the days of ‘Purple’ forces and all three services often appeared more interested in scoring points off each other, as they each competed for the largest slice of a limited defence budget, than inter-service co-operation. The Royal Air Force (RAF) was not in the least bit interested in seeing a significant part of the limited defence cake utilised to purchase and operate an aircraft purely to meet an Army requirement – particularly, when the Army wanted to operate the aircraft themselves. However, the RAF had a separate requirement for a more sophisticated high-altitude surveillance and intelligence gathering aircraft which could survey a wide expanse of the battlefield and identify potential armoured formations and troop concentrations for RAF ground attack aircraft, but was unwilling to compromise its specification to suit the needs of the Army – a fairly typical situation at that time.

G-DLRA 'CASTOR' Islander

But eventually the RAF’s intransigence and general unwillingness to co-operate was bypassed and in 1983 the MOD authorised the purchase of a BN-2T Islander (G-DLRA) for conversion into a specialised aircraft to research the CASTOR concept. On 17 May 1984 the Islander, after an usual conversion by Pilatus Britten-Norman (PBN), emerged from the hanger and with its long, flattened, circular nose, the aircraft was soon christened the ‘Flying Platypus’. But the unusual nose had a purpose, inside it carried a Ferranti Defence Systems multi-mode all weather radar, linked to two workstations inside the cabin. Throughout the next two years this rather crude and unusual aircraft was extensively test flown and proved that a radar equipped aircraft could detect, track and isolate ground targets using it’s built in Moving Target Indicator (MTI).

Canberra B(I)8 WT327

The Army was keen for the MOD to authorise the purchase of a squadron of CASTOR aircraft, but by the late 1980’s the MOD was also involved in funding a highly classified research programme into a Synthetic Aperture Radar (SAR), developed by Racal at the Royal Signals and Radar Establishment (RSRE) at Malvern – SAR soon proved capable of delivering radar pictures of almost photographic quality. Aware of similar developments in the USA, the RSRE decided to combine SAR with MTI and commissioned a Technology Demonstration Programme (TDP) using the new Racal radar and installed it in the bomb bay of Canberra B(I)8 WT327 based at Bedford. The Racal SAR/MTI radar worked well, however, by the early 1990’s, following the collapse of the Warsaw Pact and the subsequent defence cuts, the Army and RAF realising they had to cooperate and compromise on their two differing requirements to have any chance of succeeding in the new, much leaner, financial era and eventually, a new ‘purple’ arrangement was finally born.

Role of ASTOR

The MOD slowed moved forward with unusual speed and in 1992 began a Project Definition Phase (PDP) to outline a new requirement for a complete intelligence gathering and target acquisition system - known by the revised acronym of ASTOR for Airborne STand-Off Radar. Various industry teams were briefed on the requirement and invited to tender for the contract, but before long the competition was narrowed down to two teams, Lockheed-Martin's TeamASTOR and Raytheon Systems Limited ASTOR Team. Surprisingly, Northrop Grumman, which had only recently taken over Westinghouse Electronic Systems, the creators of the J-STARS radar, were excluded at this stage and appeared to be completely out of the competition. After 18 months the PDP ended and in Sep 1996 the competition moved forward to the Best And Final Offer (BAFO) stage. It was at this stage that the new Labour Secretary of State for Defence, George Robertson, intervened to allow Northrop Grumman to re-enter the competition, mainly as a result of an industrial agreement Northrop had negotiated with British Aerospace (BAe), with their WIZARD Team. By the end of Feb 1998 the three teams had all submitted their BAFO bids within a budget of £750M.

ASTOR role

The ASTOR specification called for three separate elements, firstly between four and five aircraft carrying a SAR/MTI radar and associated components, secondly six mobile Tactical Ground Stations (TGS) and lastly, two Operational Level Ground Stations (OLGS). The TGS and OLGS components would support both tactical and command level organisations. The ASTOR aircraft was required to be able to operate at around 50,000ft, giving a wide area of coverage for the radar, with less terrain shadow, as well as enabling the aircraft to operate well behind the front line and well clear of all but the most sophisticated air defences. This requirement ruled out the E-8C J-STAR, as it could only operate at around 42,000ft and anyway the MOD were not keen to acquire such an old and essentially obsolete airframe, particularly at the price Grumman were quoting. Having ruled out the conversion of a large passenger carrying aircraft on cost grounds, the MOD determined that the best airframe to meet the ASTOR requirement would be a long-range, reasonably large corporate jet, such as the Gulfstream V and the Bombardier Aerospace Global Express.

WIZARD Gulfstream V

Both Lockheed Martin’s TeamASTOR and Northrop Grumman’s / BAe WIZARD Team selected the Gulfstream V as their airborne component. With a range of over 5,000 miles the Gulfstream V ASTOR would have been capable of missions over 11 hours and had already been adapted into a SIGINT aircraft for the Swedish Air Force. The Bombardier Aerospace Global Express, selected by Raytheon Systems Ltd, had impressive performance being capable of 14 hour missions at up to 51,000ft in the ASTOR role. The ASTOR was planned to operate with a flight crew of two pilots and a rear crew of three radar operators – a mission controller and two mission analysts, each manning individual workstations. The aircraft would be permanently linked to the TGS and OLGS by secure data links, enabling interpretation of the data to occur independent of the aircraft. To keep costs down, both the aircraft and ground station workstations would have to be based on open architecture Commercial-Off-The-Shelf (COTS) hardware and software. The dual mode SAR/MTI radar would be able to continually scan a large area in Wide Area SAR or Spot SAR modes, with the MTI image overlaid on the SAR images.

Lockheed Martin’s Team ASTOR consisted of Racal Radar Defence Systems, Logica, GEC-Marconi Defence Systems, Marshall Aerospace and Gulfstream, together with various other British companies, and they planned to use the dual mode airborne surveillance radar developed by Racal. However, Raytheon’s ASTOR Team planned to use an improved version of their Advanced Synthetic Aperture Radar System (ASARS-2) installed in the U-2R. The Northrop Grumman WIZARD Team planned to use an improved version of the J-STARS radar. Finally, at the Paris Air Show on 16 Jun 1999, the MOD announced that Raytheon’s ASTOR Team had won the competition and would build the ASTOR. One of the reasons for the success of the Raytheon bid was the greater cabin volume and electrical power capability offered by the larger Bombardier Global Express aircraft.

ASTOR prototype at Greenville

By the time the award of the contract was announced, the cost of the ASTOR had risen to some £800M, but given the problems the MOD has continually encountered with the rising costs of defence projects, this was almost to be expected. The planned In Service Date (ISD) was announced as 2005 and the RAF also announced that it intended to base the aircraft at RAF Waddington, alongside the RAF’s E-3D AWACS and Nimrod R1s creating an ISTAR hub. The aircraft would all be built at the Bombardier factory in Canada and the first aircraft would be converted to the ASTOR role at Raytheon’s facility at Major’s Field, Greenville, Texas. The subsequent four aircraft would be converted at Raytheon’s facility at Broughton near Chester. As well as Bombardier, who would provide the basic Global Express airframe, the ASTOR team also consisted of Motorola, who would provide the TGS and OLGS, Lucas Aerospace, who would provide the electrical systems, Messier Dowty, who would provide the landing gear, Augusta/Westland, who would provide the doors and Rolls Royce / BMW, who would provide the two BR710 two-shaft turbofan engines, identical to those powering the new Nimrod MRA4 that will probably also be based at Waddington at some time in the future.

 Four Global Express aircraft at Broughton for conversion

Once the contract had been signed, Bombardier moved fairly quickly and converted its original prototype Global Express C-FBGX into the prototype ASTOR and on 3 Aug 02 the converted aircraft flew for the first time in its new configuration. The aerodynamic changes included the addition of a large 4.6 meter canoe-shaped fairing under the fuselage to house the dual mode radar antenna, delta fins under the rear fuselage to counteract the effect of the fairing, a small bullet fairing extension on the fin and a large Satcom antenna radome on the upper fuselage. A number of flight trials validated the new aerodynamic configuration and these continued throughout the remainder of 2002. On 31 Jan 2002 the first RAF Global Express, ZJ690, was flown to Raytheon’s facility at Greenville, Texas. On 26 May 04, after over two years of work, the initial stages of the conversion were complete and this aircraft flew for the first time in the Sentinel R1 configuration on a 4hr 24min test flight, displaying handling qualities little different from a standard Global Express. The second RAF aircraft, ZJ691, was delivered to Raytheon’s facility at Broughton, nr Chester on 29 Jan 03. The third and fourth (ZJ 692 and ZJ693) followed together on 16 Jan 04 and the fifth and final aircraft (ZJ694) arrived on 23 Jan 04 and work on all four aircraft is ongoing.

The ASTOR TGS support vehicle

Every bit as important as the aircraft are the ground stations, where the radar data will be received from the ASTOR and then interpreted by various intelligence experts, or forwarded onto other organisations. The six ASTOR TGS, each equipped with two workstations, will be housed in variants of the Steyr Pinzgauer 6x6 truck, allowing the TGS to easily deploy with field headquarters. The two OLGS will be based on ISO container sized shelters, housing the workstations and communication links and each ISO will contain three workstations. The ISO containers will be formed into an H configuration, giving a total of 9 workstations, although space also exists for an additional workstation in each ISO cabin. When not deployed on operations, the TGS and OLGS units will be based at a purpose built facility at Waddington. Raytheon already have a contract to provide Contractor Logistic Support at Waddington for the first 10 years of the anticipated 30 year life of the aircraft. The first Sentinel R1 ASTOR was due for delivery to the UK in early 2005 for flight trials at Boscombe Down.

The Sentinel R1

The key to the ASTOR is the Raytheon advanced dual mode SAR/MTI radar, based on the Hughes ASARS-2 radar system, initially developed for the U-2R. The ASTOR radar will be electrically steered in azimuth and mechanically in elevation, resulting in small blind spots to the front and rear of the aircraft. The radar can operate in either low-resolution wide-area swath mode, where it will provide a large number of strips of pictures that then join together to form a detailed image of the battlefield, or spot mode where it can apparently obtain a resolution of less than 3 meters from its normal operating altitude. The radar has also been reported as being capable of providing images of the battlefield at ranges of over 180nms when the aircraft is operating above 47,000ft, but I suspect its performance will eventually exceed this by some margin.

SAR Image

Data from the radar will either be transmitted continually, in near real-time, via Ku and X-band data links to the TGS and OLGS, using the Wideband DataLink Subsystem (WDLS) developed by Ultra Electronics and Cubic Defence Systems, or can be stored onboard during ‘off tether’ operations and then downloaded on landing. Some observers have been critical of the communication specification, citing the lack of SATCOM UHF voice facility, a limited radio fit and a poorly designed intercom/radio system, whether these factors will cause problems on actual operations remains to be seen. A Defensive Aids Group (DAG) integrated electronic warfare suite, based on the system developed by BAe Systems for the Nimrod MRA4, will also be installed. The DAG will include a missile warning system, a radar warning receiver, a towed radar decoy and chaff and flare dispensers.


Once the aircraft are delivered the operational trials will be split into two phases, the first phase will be co-ordinated by Raytheon and will mainly assess the systems compliance with the technical specification outlined in the customer requirements documents. The second phase, co-ordinated by the Defence Procurement Agency (DPA), will be more concerned with establishing how the system will actually be employed. Out of the trials should emerge the development of operational doctrine and procedures and, with the system having strategic, operational and tactical applications, these procedures will need to be unambiguous and robust to ensure the aircraft are allocated where they can be of most benefit. Clear priorities will need to be established for control of the system, determining who gets allocation of the asset and the product of its sensors. To enable this to be achieved without a continual squabble between competing organisations, commanders will have to have a detailed understanding of the technical capabilities of the aircraft and its sensors. The Sentinel R1 will eventually be capable of exchanging data with the E-8 J-STARS and with the planned NATO Alliance Ground Surveillance (AGS) system, which will be carried on a highly modified Airbus A321 and Global Hawk.

Sentinel R1 cabin layout

On 1 Apr 04, the reformed 5(AC) Sqn, manned by RAF and Army personnel, officially stood up at RAF Waddington and planned to commence its first training course in Autumn 04, in preparation to receiving the first aircraft scheduled for Sep 2005. However, the Army personnel responsible for manning the TGS and OLGS, who will also be collocated with 5(AC) personnel, will actually come under the command of HQ Land, rather than OC 5(AC) Sqn, and this could be a source of difficulties in the future. 5(AC) squadron will eventually comprise around 300 airmen and soldiers, all based at Waddington and eventually some RN personnel will probably be posted in to join them. Although the actual crew composition has yet to be formally announced, it seems fairly apparent that the two pilots will both be RAF officers and that they will only have limited access to mission information, via a small JTIDS display in the cockpit. In the rear cabin, the Mission Controller (MC) will initially be a commissioned WSO and he/she will optimise the sensor and command the mission crew. Eventually some Ops Spt (Int) or even Army Int Corps Image Analysts (IA) might be allowed to upgrade to MC. The two other IAs will be a mix of SNCO WSOp, TG14 and Army Int Crops personnel and they will be responsible for SAR imagery interpretation and MTI exploitation, particularly during ‘off tether’ operations. It seems highly unlikely that a proposal for an Ops Spt (Int) officer, acting as an Airborne Collection Manager (ACM), will ever see the light of day.

Sentinel R1 with AAR probe fitted

However, on 3 Oct 04, The Sunday Times reported that industry sources had indicated that the entire ASTOR programme would be put back over 6 months because of ‘engineering difficulties’ with the high-tech dual mode radar. Consequently, the planned ISD of Sep 05 would slip and the ISD would not now take place until sometime in 06 or even 07. However, plans are already underway to see if it might be possible to try and achieve an initial operating capability in 2007, by dovetailing the UK integration trials, operational testing and operational evaluation activities, with the complete system still eventually attaining full operational capability by 2008. However, there have also been persistent reports that, with a crew of 5 and a considerable amount of electronic equipment, the aircraft is already right up against its maximum permitted weight, making any additional enhancements, such as the three additional open architecture workstations originally planned, highly unlikely. The weight difficulties were probably a major factor in the decision to remove the requirement for an AAR capability, although other reports suggested that the AAR probe had to be removed as it destabilised the aircraft, and the money saved allowed additional equipment to be purchased for the TGSs. Any delay, for whatever reason, will be cited by some observers as yet another example of poor project management by the DPA, despite the governments much publicised introduction of 'so-called' SMART procurement.

Sentinel R1 airborne

The Sentinel R1s will eventually become a fundamental and absolutely vital element of the MOD’s planned Network Enabled Capability. Given the current emphasis on ‘purple’ operations, it seems rather odd that at present there are no plans for an TGS to be installed on any RN vessel, but I imagine this will eventually happen when the aircraft is already in service. Cruising at 51,000ft, with a range of 6,500 miles and an endurance of up to 14 hours, the vast amount of data collected by the Sentinel R1 will eventually be fused with data from the Nimrod R1, E-3D AWACS, Watchkeeper UAVs and even satellite imagery to provide a comprehensive overview of the battlespace. At long last British operational commanders, both in the field and in HQ’s at home and abroad, will be provided with the full spectrum awareness essential for conducting successful combined operations in the 21st century.