STEWART PENNEY / EDINBURGH
With "some fairly clever technology insertion" into existing radars, BAE Systems is predicting hugh improvements in resolution
AE Systems Avionics has developed a technology that marries microwave processing techniques with commercially available processors to give existing airborne radars improved resolution.
This innovation could give every radar a reconnaissance-like imaging capability in synthetic aperture and inverse synthetic aperture (SAR/ISAR) modes. The system will allow resolution improvements at the same rate as advances in commercial processors and digital signals processing.
Despite the advent of active, electronically scanned arrays (AESAs), traditional mechanically scanned antennas will remain in service for some years. There is potential for "some fairly clever technology insertion into current generation radars", says Professor John Roulston, BAE Systems Avionics director of technology. The system, he says, gives "higher resolution capability while retaining conventional equipment architecture. It provides an unconventional performance. This is a huge upgrade capability."
BAE Avionics has developed a system that combines radar's traditional gallium arsenide (GaAs) MIMC (microwave monolithic integrated circuit) processors with the commercial-off-the-shelf silicon-based integrated circuits that are already widely available. Roulston says this is the first time that radar resolution will improve at the same rate as the growth in computing power, "which does not necessarily happen with microwave circuits".
Improved resolution allows for greater discrimination, important for air-to-air and air-to-surface modes. Against an air target, this means aircraft raid numbers can be established at greater distances, formations more easily determined, and identification made possible. In air-to-ground modes, it provides the same discrimination but also improves SAR and ISAR modes. These modes produce radar imagery and the greater resolution provides the high quality pictures required for reconnaissance.
Roulston says that, for typical X-band radars, it is possible to achieve sub-1m (3ft) resolution with the upgrade: "It is low-cost technology insertion, but offers a significant [performance] impact."
There is a market for improved resolution, he says, because of the traditional design trade-off between detection range and target resolution. Every coherent-pulse radar could benefit, says Roulston.
BAE has demonstrated its system on a Blue Vixen radar (used in the BAE Sea Harrier FA2), but it could be fitted to the Blue Kestrel that equips the EH Industries Merlin HM1 maritime helicopter or any other Western-built coherent radar. The system, says Roulston, is applicable to coherent radars in "almost every military and paramilitary context". This includes civil marine radars, which have resolution but not necessarily detection capabilities as the design trade-off was reversed.
Coherent radars use Doppler processing and employ travelling wave tubes (TWT) for signal amplification. Older radars using a magnetron are not coherent, typically transmitting across a broad frequency range. They can be adapted to the new technique through the addition of a coherent-on-receive modification which has also been developed by the company.
Simple changes
Modifications are relatively simple, says Roulston. Although some changes to the analogue side of a radar would be necessary, few changes are required by the technique. "It is a simple modification to the analogue elements," he says.
The nature of radar makes the processing more complicated - modifying the transmitted signal means the incoming signal will also be altered. "The customised circuits are tightly connected - one on the transmit side, the other on the receive," Roulston says. "They're in combination and the signals are a matched pair."
On the transmitter side, the GaAs MIMC is complex and application-specific. On the receive circuit, the silicon element is "relatively pedestrian", says Roulston. But, he adds, "the performance of this system grows with the performance growth of silicon". Clock rates for silicon-based processors are now around 2GHz, which means the radar's bandwidth is also improved. Such chips are also available as commercial-off-the-shelf (COTS) items. Similarly, "all processing is based around techniques in the consumer market. This is not the first time we've exploited COTS in our radar systems," Roulston says.
The system, he adds, could improve resolution to the theoretical maximum. For a typical X-band radar transmitting at 3cm wavelength, the limit imposed by physics is in the order of five wavelengths. "A sensible limit is 20-50cm resolution because of the atmospheric limits, particularly in damp environments," he says.
Radar propagation is affected by water droplets, but 30cm resolution at a 50km (27nm)range in a partially moist atmosphere should be achievable. The aircraft's altitude will have an effect because height alters the radar grazing angle, which changes propagation effects. Physical properties, not processing capabilities, would form the boundary condition. Nevertheless, such resolution would be available to every aircraft in the fleet and eliminate the need for SAR pods, which are used or under development in a number of countries. SAR pods are costly and would not be available to every fighter. Such extension of reconnaissance capability would dramatically improve available intelligence and effectively integrate the sensor-to-shooter path in every aircraft, particularly important when attacking mobile targets.
A potential problem is that "hardly any" COTS chips are fabricated to military standards. For example, to reduce costs in the commercial market, COTS processors do not meet the military metal-thickness requirements, "which are there for a good reason", says Roulston. Metal thickness requirements are predominately life-cycle driven, and it is not clear whether commercial processors will meet the life-cycle requirements as they are.
Nevertheless, says Roulston, the limitations of COTS are generally accepted and the high resolution technology is mature, with the risk reduced to that of modifying existing equipment. A system could be in service a year after contract, he adds.
Map overlay
As well as the high-resolution SAR, BAE has developed a map overlay that improves the system's tactical utility. If it is to be useful, there has to be "ground truth" between the map and the radar image, says Roulston, but "this is difficult because of SAR distortion". However, because "we can correct to better than 1m resolution, targeting can be accomplished very accurately", he adds. The correction algorithms can be applied to any SAR image, giving BAE another upgrade offering, he says.
AESAs will allow resolution to be improved to theoretical limits by software, shaping the radar aperture to mimic the adaptive optics used by astronomers to improve Earth-based telescope resolution.
Such capabilities, however, are not the first priority of AESA radar development. A driver for such arrays - made up of hundreds of digitally controlled transmit/receive modules - is anti-jamming performance, says Roulston. Electronic scanning (e-scan) allows the radar to transmit a floodlight beam, but block the jamming on-receive by creating a virtual hole in the array pattern so the jamming energy is reduced. As the radar is still transmitting it will also detect the jamming platform by being aimed at a small angle off the jamming signal.
As radar develops, the same techniques will be available to the electronic warfare designer, "so we can expect more sophisticated jamming", Roulston says.
Multiple arrays allow near-instantaneous switching between modes so that the radar appears to be providing several functions at once - for instance, looking up to provide air-to-air target tracking, while looking down and producing SAR images. This is made possible by the zero time delay between mode switching. In addition, as there are many small arrays, rather than the one mechanically-scanned array, it is possible to provide safety-critical modes such as terrain-following radar - there is no single point failure, simply graceful degradation of performance.
BAE, EADS Germany and Thales have been working on the AMSAR e-scan technology demonstrator as a base for future programmes. BAE has also been working on an implementation plan for the technology. The Eurofighter's Captor radar should receive an e-scan array as an incremental upgrade during its service life.
Roulston says one advantage that e-scan will bring is the ability to produce multiple beams, allowing use of multi-static systems. Bistatic systems have been under consideration for some time, but the procedure only works where the two beams intersect; the area under surveillance can be increased by using multiple intersecting beams.
Equipping unmanned air vehicles with receivers makes it possible to create a multi-static system, says Roulston. BAE has funded technology acquisition work and demonstrated a multi-static SAR using a radar at its factory in Crew Toll, Edinburgh, working in tandem with the company's BAC One-Eleven radar testbed aircraft.
With bistatic operation, a UAV would not need a transmitter, so the sensor package would be smaller than the fighter's radar. The UAV would be small and light, improving its ability to operate covertly. The transmitter platform would stand back, providing the radar illumination. Data received at the UAV would be networked to a command system using the secure datalinks that UAVs will carry to allow integration with the C4ISTAR (control, command, communications, computers, intelligence, surveillance, target acquisition and reconnaissance) network.
C4ISTAR is becoming the driver for defence forces, says Roulston, and developing such systems has become an important activity for BAE.
Synchronisation
Roulston says technology development is under way, and the synchronisation problem is "pretty well solved". This latter point is crucial if the returns seen by a number of receivers are to be reconciled. A synchronisation signal could be taken from the GPS satellite system, but only a common carrier wave is required - all the participants in a system can synchronise to it and, because the receivers will generally be in line-of-sight with the transmitter platform, the latter can provide the necessary information.
Roulston says such a system would be "potent and the UAVs hard to detect". The illuminator, perhaps a two-seat Eurofighter, would be armed with long-range weapons, able to defend itself and prosecute targets while standing off.
Source: Flight International
