The UK's lengthy search for an airborne stand-off radar has resulted in a unique surveillance system

Stewart Penney/LONDON


Simultaneous announcements in London and Paris of the winner of the UK Airborne Stand-Off Radar (ASTOR) competition ended a battle which has raged for the best part of this decade and concludes studies that date to the late 1970s.

A decision was once expected in early 1997, but a change of government and the resulting 1998 Strategic Defence Review conspired to push the date back. Delay in selecting a winner has had a knock-on effect on the ASTOR in-service date, which has slipped from 2003 to 2005. The registered bidder and the government expect a final contract to be signed this year.

Raytheon, offering a system developed from the Lockheed Martin U-2's ASARS-2 synthetic aperture radar (SAR), beat bids from a Lockheed Martin-led team offering a Racal radar, and a Northrop Grumman/British Aerospace team proposing a system based on the US company's Radar Technology Insertion Programme (RTIP), in development for the US Air Force's Joint Surveillance and Target Attack System (Joint-STARS).

Shortlist selection followed a competition in 1993/5 for project definition studies - won by Raytheon and Lockheed Martin - which were completed in September 1996. Following an invitation to these teams for best and final offers in early 1997, Northrop Grumman was controversially allowed back into the contest after high-level talks between the US and UK governments.

The UK Ministry of Defence and Raytheon are now locked in final contract negotiations for the £800 million ($1.3 billion) programme. The contract will be for five Bombardier Global Express airborne platforms and eight Motorola-built ground stations, six of which will be highly mobile tactical units able to keep pace with Army brigade level headquarters as they move forward. Once in service, ASTOR aircraft will be based at RAF Waddington alongside the Royal Air Force's Boeing E-3 Sentry AEW1 airborne warning, control and command squadrons.

ASTOR will provide 24h, all-weather surveillance of moving, static and fixed targets in near real time, information which will be handed-off to friendly ground forces or other air assets such as airborne command and control, attack aircraft or other stand-off surveillance assets.

UK Undersecretary of State for Defence John Speller says ASTOR will "give battlefield information quicker and better" and will be available for short-notice deployment for "national, NATO and UN operations". He says key discriminators in naming Raytheon as the preferred bidder included industrial participation, affordability and the "best balance".

As with many recent competitions, the air platform has been a relatively minor element - as long as it is capable of carrying the system to its operating position. The key ASTOR components are the radar and ground stations.

Pooled technologies

Although based on the ASARS-2, ASTOR's sensor will work as an SAR and a moving target indicator (MTI) radar. It will incorporate technologies developed for Raytheon's HISAR radar, the SAR integrated with the Teledyne Ryan Global Hawk unmanned air vehicle (UAV) and in the ASARS-2 improvement programme which recently entered flight tests.

Raytheon Systems director of surveillance and reconnaissance systems Peter Robbie says the ASTOR radar antenna will be, at 4.6m (15ft)-long, three times the length of the ASARS-2. As with all recent UK competitions, UK workshare was a key aspect of each bid. Robbie says 75% of the radar will be manufactured in the UK. Development and production of the antenna is entrusted to Marconi Avionics Radar and Countermeasures division in Edinburgh. Raytheon Systems at Glenrothes, also in Scotland, will produce the power supply. The remaining work will be at Raytheon's El Segundo site in California. Robbie says all the intellectual property rights will be in the UK.

Unlike its competitors, Raytheon based its ASTOR bid on a passive antenna. It is electronically scanned in azimuth with a mechanical elevation scan. Passive does not indicate non-transmitting. Such an antenna, explains Raytheon ASTOR director Richard Anderson, has a single high power transmitter which feeds the arrays, each of which has an electronic scanning device - a phase shifter - to steer the radar beam. An active antenna has a lower power transmitter at each array. Anderson says Raytheon could have offered an active array as it uses the technology on other programmes, but ASTOR "looks" close to the horizon, negating the need for rapid switching of the radar beam. A passive antenna is also lighter, cheaper and needs less cooling, says the company.

Industry observers note that a passive-antenna approach is also considered less risky than an active sensor. At an operating altitude of 51,000ft (15,600m) and a 2° grazing angle, ASTOR will have a stand-off range of more than 300km (160nm).

Speller says ASTOR will be procured under Smart Procurement rules. A central tenet of the purchasing initiative is that programmes should enter service as quickly as possible and upgraded incrementally throughout their operational life. Capability improvements will often be driven by software improvements, possible because of the exponential growth of hardware capabilities, particularly processing power.

Smart procurement means the radar is "fully compliant-but includes a specific radar growth path", says Anderson. A potential upgrade is the US-led RTIP in which the UK continues to have an interest, says a Ministry of Defence source. A UK/US RTIP memorandum of understanding could be continued, although discussions are informal, he adds. Raytheon is teamed with Northrop Grumman on RTIP.

A mid-life upgrade including RTIP technology would provide obvious interoperability benefits if the USAF and NATO use the system. Robbie says that in 10-15 years' time, technology will have make antennas lighter, shorter and more efficient in terms of power use. Antenna length is a critical factor for MTI performance. An ASTOR-length antenna will have the equivalent capability of today's 7.3m-long Joint-STARS antenna.

The radar operates in three coverage modes: swath SAR, spot SAR and MTI. The first provides a large area "map-like" image while spot SAR produces a greater resolution image of a smaller area that looks like a high quality black-and-white photograph but which is unaffected by cloud cover or battlefield effects such as smoke. MTI provides a view of moving targets, both on the ground and slow-speed airborne targets such as helicopters.


Although the radar performs only a single function at any one time, interleaving between the functions allows continual updating of each radar mode. ASTOR crews numbers are dramatically fewer than for Joint-STARS - five people on ASTOR compared to 21 on Joint-STARS - but the UK requirement does not include battle management from the air. Mission system operators will sit at Sun Microsystems workstations which will be identical to those in the ground stations.

The mission crew will control the radar's data collection, adjusting search patterns to meet emerging needs. They will also perform some data processing to aid this process. Racal will provide the mission planning system. Robbie says most of the software is commercial-off-the-shelf, reducing cost and risk.


Typically, an area would be monitored using the MTI that would detect an SAR image and track patterns, probably overlaid on a map to aid tactical awareness. An SAR image superimposed on an area of interest provides greater detail and allows positive identification of targets.

Speller says ASTOR would have been "extremely useful" during recent operations over Kosovo, where the tactical picture of the disposition of Serb military forces was confused by the movement of refugees and internally displaced people. "ASTOR would have identified and tracked refugees," he says. As one source notes, NATO attacks on refugee convoys could have been avoided as its historical plot would reveal it had originated in a farmyard or village rather than a barracks, while the SAR image would have helped positive identification of the vehicles.


Radar data will be broadcast to ground stations via two datalinks. Ultra Electronics and Cubic will provide the two-way broadcast link and L-3 Communications will supply the wide-band equivalent. As well as "tethering" the aircraft to ground stations, the datalink will also allow data transfer to and from naval assets, attack helicopters, UAVs, offensive aircraft, ground systems such as long-range artillery and reconnaissance aircraft.

This gives the force commander a vast view of the battlefield and allows the rapid hand-off of key information that will negate some advantages enjoyed by highly mobile targets such as tanks or tactical missile systems. A satellite communications (satcom) link will allow "off-tether", non-line-of-sight data transmission.

High bandwidth datalinks enable secure transmission of radar data, which removes the need for large mission crews and allows the army commander to manipulate the picture to best meet the ground manoeuvre element's needs. A smaller mission crew allows the ASTOR to be mounted in a business jet which in turn allows the radar sensor to be carried to higher altitudes than would be possible with a converted airliner. The higher the antenna, the further the stand-off range.

Raytheon Systems and Motorola will integrate the ground stations, Motorola also supplies the Joint-STARS equivalent. This reduces risk, says Raytheon's Robbie, as Motorola has already solved the problem of maintaining computer serviceability as the tactical ground stations are moved forward with army headquarters, often over rough terrain.

Reduced logistics costs

The tactical ground stations will be mounted on Steyr Pinzgauer 6x6 trucks customised by Marshalls Special Vehicles. Robbie says the truck is in service with the British Army, a factor that will reduce introduction costs as a logistic support train is in place. A tactical ground station consists of three trucks - workstations, communications and support - two towed generator trailers and a datalink trailer.

The larger operational level ground stations will consist of two transportable 7m containers, one for the operator workstations and the other dedicated to communications.

Another Raytheon team member is the UK Defence Research and Evaluation Agency's Malvern facility. Malvern started developing ASTOR-type technologies in the 1970s and worked closely with competition loser Racal on technology development programmes. Malvern-developed SAR algorithms and image analysis functionality will be integrated into the airborne and ground workstations.

Raytheon selected the Global Express as its air platform, although both its competitors settled on the long-range business jet's arch rival, the Gulfstream V. Both aircraft meet the general requirement to operate at 51,000ft and fly 12,000km (6,500nm) or for 14h without refuelling. In terms of industrial participation the Global Express has a political advantage as the forward fuselage was designed and is made by Shorts in Belfast. Anderson believes Shorts' design responsibility for this component - which will receive the majority of the modifications - will aid the conversion process. Shorts also makes engine nacelles for the BMW Rolls-Royce BR710 that powers both business jets.

Robbie says major factors in selection of the Global Express were its take-off and landing performance, cruise fuel consumption and its bigger airframe. The nosewheel to wingbox length is superior and it has 20% more internal volume. Another factor in favour of the Global Express, Robbie adds, is the use of two Lucas Aerospace electrical generators on each engine supplying ample power for the radar and avionics and providing growth potential.

Despite the Global Express' long-range capabilities, the aircraft will also be equipped with an in-flight refuelling probe. The Global Express will have aircraft availability levels not normally associated with military aircraft, says Robbie. Business jets are designed for high dispatch reliability and low maintenance requirements.

British Aerospace Systems and Equipment will provide the fairings over the satcom antenna on the upper forward fuselage and the under fuselage canoe which covers the radar scanner. Anderson says windtunnel tests were performed with "worst case" fairings and "[the] aircraft performed well, with almost negligible impact".

Before delivery of the first ASTOR aircraft to Raytheon's Greenville, Texas, plant for modification, the company will flight test the fairings on the prototype Global Express. First conversion will be performed at Greenville by a team including Shorts and specialists from Raytheon's modification and finishing centre at Broughton in North Wales. Anderson says Greenville will integrate ASTOR systems on the first aircraft as it has long experience of modifying aircraft for special purposes, such as the USAF's Boeing RC-135 reconnaissance assets. It is also a designated modification centre and can put the resulting aircraft through the certification process. The four remaining aircraft will be converted in Wales, where export modifications of ASTOR will also be carried out.

One element of ASTOR which remains to be firmed up is its defence aids system (DAS). Raytheon will review the system before final design is completed. Vinten will supply a chaff and flare countermeasures system while a towed decoy will also be integrated. Raytheon and Marconi are developing towed decoys for other UK MoD programmes. The aircraft will almost certainly be fitted with the Northrop Grumman Nemesis directional infrared countermeasures system which Marconi is integrating with 14 UK large fixed wing aircraft and helicopters.

Teaming surprise

Although the DAS is open for change, the MoD has opted for the Raytheon bid without seeking any last-minute teaming adjustments. This came as a surprise to many as rumours were rife before the decision was announced that Raytheon would either give Racal a bigger, radar-related role on the team or would buy the company's radar activities.

Raytheon will also supply training and logistics. Most training will be performed at Waddington, where a rear-crew trainer, ground station crew trainer and classrooms will be installed. The costly aircrew flight simulator will be procured using a private finance initiative approach and will be located away from ASTOR's home base. Raytheon will offer training time to other Global Express operators. Although the army and RAF will perform first line maintenance, deeper levels and logistics are Raytheon's responsibility.

Robbie says a potential export market exists for around 40 systems, and the market will be considered "more closely". NATO's air to ground surveillance (AGS) need provides the largest potential sale - six to 12 aircraft - but the requirement is poorly defined and the alliance has already split in two. Canada, Denmark, Norway and the USA are studying an RTIP-based solution mounted on an Airbus airliner. France, Germany, Italy and the Netherlands are pushing the European SOSTAR. The UK Government will promote ASTOR as an AGS solution.

In the Asia-Pacific region, Japan, South Korea and Australia could all become operators of ASTOR-like systems. In the Middle East both Saudi Arabia and the United Arab Emirates have speculatively expressed an interest in such aircraft.

Although the USAF is keen to buy more Joint-STARS aircraft, there is a potential market for up to 20 ASTOR-like aircraft that are cheaper to operate and of a size that more closely matches the needs of future conflicts. Integration on a business jet also sidesteps problems with the availability of Boeing 707 airframes suitable for overhaul and conversion and avoids the costs of operating elderly aircraft.

Source: Flight International