As in any modern combat aircraft the Eurofighter Typhoon pilot requires as much information about the outside world as can possibly be obtained. That information needs to be delivered quickly and accurately to render it useful. To this end the Typhoon offers a suite of on-board sensors as well as being capable of utilising data from external platforms.
Traditionally each sensor in an aircraft has been treated as a discrete source of information. However this can result in conflicting data and hence an increased pilot workload. Additionally it limits the ability to effectively automate systems. To overcome this the Typhoon employs what is now known as sensor fusion techniques (in a similar fashion to the U.S. F-22 Raptor). Considering that the Typhoon's avionics systems are still based on a typical distributed, or federated system (i.e. individual components linked via a standard databus) rather than a centralised architecture (which the F-22 has gone some way towards implementing) this seems like quite an achievement.
In the Eurofighter this fusion of all data sources is achieved through the Attack and Identification System, or AIS. The AIS combines all data from the major on-board sensors along with any information obtained from off-board platforms such as AWACS, ASTOR, JSTARS or even other Eurofighter's via the Multi-function Information Distribution System, or MIDS link. Additionally the AIS integrates all the other major offensive and defensive systems such as the; DASS, Navigation, ACS and Communications.
The physical design of the AIS comprises of essentially two separate units; the Avionic Computer (AC) and Navigation Computer (NC). These are linked via the STANAG-3910 databus to the other major systems such as the ACS, ECR-90/CAPTOR, PIRATE, etc. Both the NC and AC are identical in design and comprise a modular system based on Motorola 68020 CPU's with 68882 Maths co-processors. In addition several custom RISC based processors are utilised to accelerate floating point and matrix operations.
By having such a single source of information pilot workload should be reduced by removing the possibility of conflicting data and the need for cross-checking, improving situational awareness and increasing systems automation. In practice the AIS should allow the Eurofighter to identify targets at distances in excess of 150 nm and acquire and auto-prioritise them at over 100nm. In addition the AIS offers the ability to automatically control emissions from the aircraft, so called EMCON (from EMissions CONtrol). This should aid in limiting the detectability of the Typhoon by opposing aircraft further reducing pilot workload.
The contract for Eurofighter's prime sensor system was awarded to EuroRADAR with the ECR-90 system. EuroRADAR is a consortium of partner nation companies comprising of; DASA, INISEL and FIAR, the group is led by GEC-Marconi (now BAE Systems). The ECR-90 was selected over competing units such as an upgraded APG-65 offered by Hughes. The first trials of the system (ECR-90-A) were carried out in the UK on-board a modified BAC-111 in January 1993. Problems were encountered during these tests which were traced to interactions between the radar and the Eurofighter's radome. The problems were subsequently fixed by an antenna redesign carried out by DASA. The first Eurofighter compatible model, the ECR-90-C was fitted to the German DA5 and tested in February 1997, two aircraft were successfully tracked. Both the DA5 and the British DA4 are acting as the ECR-90 development aircraft. Full development of the ECR-90 was frozen in February 1999 with the system now entering the production phase. This completion of production coincided with a name change for the system, ECR-90 will now be known as CAPTOR.
CAPTOR is a third generation coherent X-band (8 to 12 GHz) multi-mode Pulse Doppler system developed from GEC's Harrier FA.2 Blue Vixen system. It offers twice the power output of the APG-65 combined with long range search and track and continuous illumination (for semi active missiles). The real time control software, written in ADA to MIL-STD 2167A comprises some 500,000 lines of code. The unit itself is highly modular comprising some 61 Shop Replaceable Items (or SRIs) and around 6 Line Replaceable Units (or LRUs) weighing in at 193kg. This should enable quick maintenance turn-around as well as simple upgrade paths.
The operating modes available to CAPTOR fall into three basic classifications; long range air to air, close range visual and air to surface. For long (beyond visual) range combat CAPTOR will automatically select an appropriate mode depending on the current situation. For example, long range look-up detection will typically find the system selecting a Low Pulse Repetition Frequency (LPRF). However for look-down situations a high pulse repetition (HPRF) will generally be used. For situations where both look-up and look-down need to be covered simultaneously or where range and velocity data is required a medium rate would be used. In addition CAPTOR will automatically initiate Track While Scan (TWS) for a list of targets (the exact number of possible tracked targets remains classified). The system employs Data Adaptive Scanning (DAS) to improve tracking of its selected targets while minimising unnecessary movement of the antenna. For close-in combat situations CAPTOR will automatically adjust its mode for a high precision single target track. A further useful function in this mode is the ability to slave the radar directly to the Helmet Mounted Sight. The resulting data can then be used to queue short ranged air to air weaponry such as ASRAAM. Finally CAPTOR has a range of air to surface modes including; beam mapping, sea and surface search, Ground Moving Target Identification (GMTI) useful for picking out moving surface targets such as armour, spot mapping and surface ranging. The Synthetic Aperture Radar (SAR) mode offered by Tranche-1 Eurofighter's gives a 1m resolution, test flights have occured reducing this to 0.3m (this improvement should be introduced for Tranche-2). As with the air to air and close range modes, air to surface modes can be automatically selected by the system as required. Just like most Typhoon systems VTAS provides the pilot with an easy way of communicating with the radar to manually change modes, alter target selection and the like.
In 1997 Marconi indicated CAPTOR had detected fighter sized aircraft at ranges of well over 160km and larger aircraft at double that. More recent information indicates the systems range accuracy is within 10 metres while it can obtain a target angle to within 1 miliradian. The system is capable of tracking 20 air targets simultaneously, automatically identifying and prioritising them. When in the track list the appropriate weapon can be automatically selected (a function in part of the ACS) and using auto-attack the aircraft can be flown under autopilot to a selected air target. All of these capabilities are designed to significantly reduce the workload of the pilot during combat operations.
Even though CAPTOR features a mechanically steered array, BAE Systems have indicated that the low inertia non-counterbalanced antenna coupled with four high torque, high precision samarium-cobalt drive motors allows extremely high scanning speeds. As a consequence of this the radar can interleave different operations such as air and ground mapping. This is quite an achievement for a non-phased array system.
CAPTOR is designed to be highly resistant to ECM and passive countermeasures. To this end it includes a unique separate (third) data channel exclusively for screening ECM sources. In extremely bad ECM environments the AIS should limit the effects of reduced CAPTOR information through its other data sources.
IFF - Identify Friend or Foe
The CAPTOR specification incorporates an Identify Friend or Foe capability. The system, developed by the UK division of Raytheon, Germany's DASA, Italy's MID and Spain's Enosa incorporates both the interrogator and a Mode-S Transponder. It was designed from the outset for NATO interoperability and is fully compatible with the IFF MK XII standard but is still upgradeable to future standards. As with the other avionics systems the IFF units utilise ADA for most of their code and incorporate built-in cryptographic functions. In addition the units are fully integrated with the other avionics sub-systems via a MIL STD 1553B databus and of course the radar itself.
CAPTOR also includes the ability to carry out non-coperative IFF, or Non-Cooperative Target Recognition (NCTR). Although no information has been released on how CAPTOR achieves this there are some possible techniques. One uses highly accurate range data to define a characteristic return pattern for an aircraft in a known orientation. An alternative method relies on a different but still characteristic return generated by exposed, rotating engine fan blades. The resulting data can then be compared to stored information and a match determined.
Future of CAPTOR ... AMSAR
In the next few years there will be a number of upgrades to the systems software till Final Operational Capability is attained. Following this a number of hardware upgrades are planned. These involve changing a number of both the shop replaceable items and line replaceable units. These upgrades will focus on improving resolution and ECCM capabilities. The next upgrades will see a switchover to off-the-shelf components. Even with these improvements there are a number of fundamental weaknesses in the CAPTOR's design such as; relatively slow scanning speeds compared to newer technology arrays, relative ease of detection, effects on RCS, etc.
To overcome these problems a project was launched in 1993 with 50/50 funding from the UK and France and technical input from Germany. This project termed, AMSAR or Airborne Multi-mode Solid-state Active-array Radar aims to provide the Typhoon and Rafale (and other future European air systems) with an entirely solid-state advanced active array (although the Rafale is equipped with a phased array radar, the RBE.2, it is a passive system rather than a solid-state active array). A consortium company was formed soon after called GTDAR (or GEC-Thomson-DASA Airborne Radar).
The program has an intended length of 11 years and is split into three phases. The first two of these examined the feasibility and requirements for a new generation of active arrays as well as new methods for fabricating the expensive Microwave Monolithic Integrated Circuit (MMIC) modules. The target price for the modules is around £400 to £500 compared to several thousand at present. In addition a bench scale unit was to be constructed demonstrating the overall feasibility of the project. Both of these phases were completed by mid-1998 with the testing of a 144 module array utilising an advanced MMIC featuring a custom ASIC and a multi-layer ceramic substrate housed in a metal matrix composite unit.
Following a successful demonstration of the 144 module array the British, French and German Defence Ministries have authorised the third stage of the project to proceed. This will see the construction of a 1000+ module full-scale unit which will subsequently undergo flight testing aboard BAE Systems's Canadair avionics test aircraft. If the project proceeds to schedule the unit will be competed in 2001 and flight trials will occur in 2002.
Although tranche-3 definition will not occur for some years it is highly likely that, assuming all goes well and the costs are kept down AMSAR will be incorporated. Such a system would yield very noticeable improvements in capability and further reduce the chances of detection of the Typhoon by opposing platforms. In addition the program combined with certain other European ventures (such as the UK's FOAS project) is going further by examining the integration of multiple arrays integrated into the aircraft structure, a so called Conformal Smart Skin Array. Using high speed wide band optical links and a centralised processing system the entire aircraft would become one giant integrated sensor. Although this will not be of any use to the Eurofighter it may well be deployable on its follow-on as well as FOAS.
The CAPTOR is an active system, it operates by transmitting radio waves. Whenever the radar is operational the power it outputs can be detected by an enemy using a Radar Warning Receiver (RWR). Even Low Probability of Intercept (LPI) radar's such as the American APG-77 or European AMSAR risk detection. There are really only two ways to solve this problem. One method (which Eurofighter also makes extensive use of) is to utilise data gathered from other platforms such as AWACS, Nimrod, JSTARS, ASTOR or even other fighters. However this requires such platforms to be available and for the datalinks from those platforms to be jam resistant.
An alternative method is to use an on-board passive system for detection. The PIRATE, or Passive Infra Red Airborne Tracking Equipment is a 2nd generation Imaging Infra Red (IIR) system and performs this duty of passive detection. PIRATE is constructed by the EuroFirst consortium led by Pilkington-Thorn Optronics (now Thales Optronics).
PIRATE incorporates both a Forward Looking Infra Red (or FLIR) and Infra Red Search and Track (or IRST) capability. The system itself utilises a highly sensitive Infra Red sensor mounted to the port side of the canopy. This equipment scans across wavelengths from 3 to 11 µm in two bands. This allows the detection of both the hot exhaust plumes of jet engines as well as surface heating caused by friction. By supercooling the sensor even small variations in temperature can be detected at long range. Although no definitive ranges have been released an upper limit of 80nm has been hinted at, a more typical figure would be 30 to 50nm. The use of processing techniques further enhances the output, giving a near high resolution image of targets. The actual output from the system can be directed to any of the Multi-function Head Down Displays mounted within the cockpit. Additionally the image can be overlaid on both the Helmet Mounted Sight and Head Up Display.
The IIR sensor is stabilised within its mount so that it can maintain a target within its field of view. Up to 200 targets can be simultaneously tracked by the system using one of several different modes; Multiple Target Track (MTT), Single Target Track (STT), Single Target Track Ident (STTI), Sector Acquisition and Slaved Acquisition. In MTT mode the system will scan a designated volume space looking for potential targets. In STT mode PIRATE will provide high precision tracking of a single designated target. An addition to this mode, STT Ident allows for visual identification of the target, the resolution being superior to that provided by CAPTOR. Both Sector and Slave Acquisition demonstrate the level of sensor fusion present in the Typhoon. When in Sector Acquisition mode PIRATE will scan a volume of space under direction of another Typhoon sensor such as CAPTOR. In Slave Acquisition the use of off-board sensors is made with PIRATE being commanded by data obtained from an AWACS for example. When a target is found in either of these modes PIRATE will automatically designate it and switch to STT.
Once a target has been tracked and identified PIRATE can be used to cue an appropriately equipped short range missile, i.e. a missile with a high off-boresight tracking capability such as ASRAAM. Additionally the data can be used to augment that of CAPTOR or off-board sensor information via the AIS. This should enable the Typhoon to overcome severe ECM environments and still engage its targets.
Although the radar and FLIR are the primary sensor systems there are other data sources available within the Typhoon. Of these the most notable are the Radar Warning Receiver (or RWR), Laser Warning Receiver (or LWR) and Missile Approach Warner (or MAW). These three devices provide the AIS with additional information about the external situation. For example the RWR is capable of detecting and classifying, via an on-board stored signature database, radar signals from other aircraft or ground systems. In an aircraft with a reduced radar cross section (as is the case with the Typhoon) this should enable the pilot to detect opposing systems before he/she is detected themselves. In a similar manner an opposing aircraft (or ground system) using a laser range finder (or guidance system) is at risk of detection by the LWR. The active MAW incorporated within the Typhoon also gives information over a full 360° of approaching missiles thus giving that extra time required to take evasive manoeuvres.
The webmasters would like to thank Marconi Avionics (now BAE Systems), and particularly Jane High for providing information on the CAPTOR, AMSAR and the future of airborne radar.
 : BAE Systems, Warton, UK