In designing the Typhoon the consortium partners had to keep in mind the hostile environment in which the aircraft may be operating. A fighter aircraft will not only be subjected to attack from enemy aircraft but also from increasingly capable ground based defences. In both cases modern anti-aircraft weaponry is becoming ever more advanced and not as easily fooled as they may have once been. Therefore the Eurofighter pilot has to not only engage and eliminate enemy aircraft but also deal with threats against their aircraft. Since the Typhoon is a single seat fighter this task is made even more critical.

History of DASS

The original plan for Eurofighter's defensive systems was, like the other major systems, intended to be a collaborative effort between all the member nations. However by 1991 a fully integrated detection and protection system was looking to be extremely costly. This led an already cost sensitive German Government to withdrew from the joint project. Following this decision Spain also declined to continue with the project. This left Britain and Italy to form a joint, 60% GEC-Marconi (now BAE Systems) and 40% Elettronica consortium called EuroDASS, European Defensive Aids Sub-System which was subsequently awarded the £200M ($340M) defence system contract in 1992. This left Germany to initiate its own defence system projects including a new DASA developed Towed Radar Decoy (TRD). However it soon became clear that to reach a similar level of effectiveness to the joint DASS project would be prohibitive.

By 1995 Spain had decided to opt back into EuroDASS and a £29M ($39M) contract was signed with Elettronica to allow INDRA to participate. During 1997 tentative discussions also occurred between DASA and EuroDASS on re-entering the project. Problems occurred however on which systems would be utilised. For example DASA were keen for their TRD (which was still under development) to be used, however GEC already had a proven TRD in operational service with the RAF. In May 1998 a £180M ($276M) production investment contract was signed between Eurofighter and EuroDASS. In October 2001 the German Government announced its intention to place an order for DASS with EADS. This finally brings all four partners back into the EuroDASS consortium.

The modular nature of the DASS has also resulted in each consortium nation refining their individual DASS fits. At this time it is becoming increasingly clear that only the Royal Air Force Typhoon's will employ the full set of detection and counter measure systems originally envisaged. However this can be seen as being of potential benefit for overseas purchases who will be able to specify individual fits tailored to their requirements. An additional benefit of using such a modular system is that upgrade paths can be somewhat simplified. At the present time (mid-2000) discussions are already underway as to future improvements in the DASS architecture for each subsequent Tranche. This will lead to later Eurofighter's benefiting from complete 360° spherical detection and protection capabilities. However even in its current state DASS represents one of the most advanced self-protection systems available.

Threat Detection

As is common with all the other sensor systems the Typhoon DASS is tightly integrated within the AIS. Therefore the system is able to access data not only from its own dedicated sensors but from all those available to the aircraft including off-board platforms via the MIDS datalink. All parts of DASS are controlled by the single Defensive Aids Computer or DAC. During development this unit, constructed by Teldix GmbH utilised Motorola 68020 CPU's. However each of the DAC units in production aircraft will utilise five specially packaged, ruggedised and hardened PowerPC processors supplied by Radstone Technology under a £2.35M contract to BAE Systems. This change will significantly enhance the processing capacity of the system and allow for simple future upgrades.

The DAC provides a fully automated capability to analyse and respond to any threat the Eurofighter may encounter. For a single seat fighter this should prove extremely useful. Through the DAC and AIS the pilot can obtain a 360° threat picture around the aircraft including target identity and even their zones of lethality. The data can be output to any of the MFDs or be viewed along with all the available sensor data via the Moving Map. Additionally the CAMU system is utilised to provide both verbal and simple audio warnings of current threats.

Radar Warning Receiver (RWR)

One of the fundamental sensors available to a modern fighter is its radar. However the use of such a system also puts an aircraft at risk since, as an active system it emits electromagnetic radiation. The DASS is equipped with Radar Warning Receivers (RWRs) designed to detect such emissions. The particular units used are Super Heterodyne (SuperHet) based wide-band receivers and are located in the port side pod (both front and rear) and within the aircraft's fuselage giving full 360° coverage in azimuth (elevation coverage is currently unknown).

Port Side DASS pod © WAPJ

These units are combined with on-board processing systems enabling not only a bearing to be determined but also the likely type of radar (and thus the platform it is deployed on). This is achieved using a stored database of radar signatures forming part of the ESM, Electronic Support Measures suite. Through the use of high speed digital signal processing the ESM will attempt to map the detected emission to its database. One of the weaknesses of current (and more so older) RWR systems is a difficulty in countering Low Probability of Intercept (LPI) enabled emissions. These use various techniques to try and hide the emitted signal. Although it is unclear whether the DASS RWRs possess a capability to detect and classify such emissions it is known the Marconi (now BAE Systems) have been working hard in this area for some time.

Missile Approach Warner (MAW)

Although the goal of any fighter pilot is to remove an enemy before they have fired there will of course be occasions when this is not the case. To enable tracking of such missile launches the DASS incorporates three Missile Approach Warners (MAW), one each in the port and starboard wing roots (near the cockpit) and one in the rear fuselage (near the tail). The units are derived from the Plessey PVS2000 MAW which utilises an active, pulse-doppler radar for detection. Since the units are active they are able to detect not only radar guided ordnance but also passive weapons such as infra red guided short range missiles. To increase the effectiveness of the system the MAW is also directly linked to the flare launchers allowing an instantaneous response to a local launch.

Two possible upgrades to the MAW system have been discussed as part of on-going DASS improvements for inclusion on either Tranche-2 or more likely Tranche-3. The first of these is a UV based system. These were discounted for the initial install of DASS because GEC-Marconi thought the technology immature at the time. A UV system detects the characteristic emmisions from the missile plume. The advantage is it can be highly discriminating against counter measures. However since it is reliant on detecting the plume of the missiles engine its primarily useful only during the burn phase.

PIMAWS © BGT

The second and more likely potential replacement, announced by Germany's Bodenseewerk Geratechnik (BGT) in June 2001, is termed PIMAWS, or Passive Infra-red Missile Approach Warning System. PIMAWS began development in 1997 as an advanced technology demonstrator jointly funded by the German Ministry of Defence and BGT, the project is due to complete in 2003. The system is intended for installation as Line-Replaceable Units in both of the wing-tip pods achieving full spherical coverage. The detector portion of the system is a step-stare infra-red sensor operating in the 3-5µm frequency range allowing for both air to air and surface to air missile detection up to the post-burn-out phase. All the resulting data is fed into a custom designed image processing system utilising a Systolic Array Processor and DSP. Over 64 targets can be tracked simultaneously and through a built-in threat library they can also be determined. All the resulting data can be fed via a STANAG-1553B databus to the rest of the avionics, here one unit acts as the master the other the slave. This should allow the information to be fused with the other AIS sources. Although ground tests have been carried out and flight tests are scheduled for October 2001 no decisions have been reached by EADS or the partner nations as to whether PIMAWS will be integrated into Eurofighter.

Laser Warning Receiver (LWR) : RAF only

It is not uncommon to find aircraft fitted with laser range finding equipment to accurately determine distances. There are also laser guided weapons available which ride the beam to their target. To counter such threats the RAF Typhoon's will be equipped with a Laser Warning Receiver, LWR. Mounted below the nose the units will be capable of detecting any incoming laser radiation and determine its bearing.

Protection

Starboard side TRD pod © BAE Systems

As well as providing a set of dedicated sensor systems to detect any threats to the Typhoon the DASS also incorporates a full range of countermeasures. Through direct links with the DAC and AIS these countermeasures can be automatically engaged as required although the pilot can also utilise them at will utilising the VTAS system. Many parts of the DASS are located within wing tip mounted pods, including the ECM/ESM systems, TRDs and two RWRs. To enable quick maintenance and turn around times these pods are fitted as Line Replaceable Units (LRUs).

Chaff and Flares

Countermeasure Basics

Since the development of radar by Britain and Germany before World War II there have been numerous developments in the technology used to defeat it. The first such system, developed by Britain (and also by Germany) code-named Window is the now familiar and most basic defence, Chaff. This comprises thousands of small pieces of aluminium cut to a length appropriate for air interception radar frequencies. These shards when dropped by an aircraft can lead to false returns appearing on the enemy fighters radar. Although this technique is now quite old it is often still very effective. However, modern radar's featuring modern digital signal processing will be fooled far less easily than they once were. It is therefore now typical to find active radar countermeasures being deployed, referred to as Electronic Counter Measures. These emit radio frequencies in such a way as to either fill the enemy radar with noise (so called noise jamming) or by causing false echo's to appear (known as false target or repeater jamming). Although increasingly sophisticated ECM techniques are now available the newest radar's have also advanced and are less likely to be spoofed. To this end several companies now produce Towed Radar Decoys which can be externally deployed by an aircraft. These can then either jam an opposing radar or simulate a potential target, luring any missiles away from the aircraft.

Radar is not the only concern of countermeasures in modern air combat, there is also a threat from passive Infra Red guided weaponry. As with Chaff a very simple defence in the form of pyrotechnic flares tuned to emit radiation at similar frequencies to the jet exhaust of a fighter have been deployed for many years. However as with chaff modern IR missiles and detectors are likely to be less easily fooled by such techniques. To help overcome these problems alternative systems have been developed. These include expendable decoys such as Texas Instruments GEN-X or active measures such as DIRCM, or Directional Infra-Red Counter Measures which uses a laser to overpower the infra red seeker of a missile or other detector.

Although almost as old as radar itself chaff is still one of the fundamental defences available to a fighter aircraft. The Eurofighter's DASS includes internally carried Chaff dispensers within the outer (short range missile) wing pylons thus freeing the pylons for weapon carriage. The system can be controlled automatically by the DAC and AIS or manually by the pilot. In addition the on-board ECM generators can be used to illuminate the chaff cloud with a radio beam becoming what is termed Jaff. This increases the performance of the otherwise passive chaff substantially but even here the usefulness of chaff is being eroded by modern active radar missiles and digital signal processing techniques.

The development of Infra Red guided weaponry also led to the deployment of a simple yet effective counter in the form of pyrotechnic Flares. As with the chaff, the DASS specification includes flare launchers carried internally within the inboard wing fairings. The system can be operated in three ways; manually by the pilot, automatically by the DAC and, in response to an immediate threat, by the Missile Approach Warner's. The release pattern is automatically controlled to minimise the risk of the incoming missile recognising the flares for what they are. However, against missiles featuring an imaging capability such as the Russian Vympel AA-11 and more so ASRAAM, Python-4, IRIS-T, etc. the effectiveness of flares may be substantially eroded.

Both the chaff and flare dispensers will be supplied by SAAB Technologies of Sweden in the form of their BOL dispenser. This is basically a cylinder containing chaff or flare packages. These individual packets are moved along the container electro-mechanically before being dumped out the rear and into the aircrafts slipstream where they disperse. The system is fully self-contained and has a proven record being in use with several European air forces as well as the U.S. Navy. Additionally the BOL system is not limited to simple chaff/flare packets but can carry a range of different expendable decoy systems.

Electronic Counter Measures (ECM)

Although chaff is a simple and often effective counter measure against enemy radar there is a further, active, method. This comes in the form of Electronic Counter Measures or ECM which essentially emit radio frequency energy directed at the threat. The aim of such a system is to either fool the opposing radar into thinking the aircraft is somewhere else or overpowering it completely thus rendering it useless.

The basic DASS specification includes a radio frequency jammer located in the port side pod allied with the DAC and ESM systems, including a techniques generator. When combined with both the DASS and other sensors the on-board techniques generator can deploy an appropriate type of jamming for the current threat. Although the exact specifications of the system are classified the system should be capable of decoying and jamming all types of modern radar; Continuous Wave (CW), Pulse and Pulse-Doppler. The form of ECM generated can be altered on the fly by the DAC as and when required.

At this time it is still unclear whether the Cross Eye (X-Eye) system developed by Italy's Elettronica will be deployed as part of the base ECM specification. This system uses two widely spaced radio frequency emitters (one in each wing tip pod) to generate two identical repeater type jamming beams. These can be directed at an incoming missile or aircraft and under tight control they can cause a significant aiming error within the opposing systems radar. This should result in the opposing missile or aircraft believing the Eurofighter is in an entirely different location.

Towed Radar Decoy (TRD) : RAF only

One of the most recent additions to the arsenal of active radar counter measures is the Towed Radar Decoy or TRD. These systems deploy a unit externally from the aircraft on some kind of cable. As part of the basic Royal Air Force specification the DASS incorporates either one or two TRDs (the number is dependent on other systems present in the starboard pod, i.e. Cross-Eye) as standard. The units will be carried in the Starboard side pod and will be capable of being recovered or jettisoned as the situation may dictate.

The RAF's DASS TRD is to be a development of BAE Systems Ariel system which has already been successfully deployed on RAF Tornado's and Nimrod's. The major physical difference is a decrease in the overall size enabling carriage within the Typhoon's pods. The unit is deployed from the pod on a 100m Kevlar^® cable containing a Fibre Optic (FO) link and a separate power distribution line. Through the FO cable the DAC's techniques generator can send commands to the decoy based radio frequency emitter. The TRD can produce a range of jamming techniques to fool or lure the missile away from the aircraft.

So far the DASS TRD has been successfully deployed at both subsonic and supersonic velocities aboard the DA2. The final system will be cleared using BAE Systems IPA1 during the later stages of trials beginning in 2002. It is also possible that other Towed Decoys will be deployed on non-RAF Eurofighter's. In particular DASA's Sky Buzzer system may be made available at some future point.

Sources :

[1] : BAE Systems, UK
[2] : World Air Power Journal, 35, Winter 1998
[3] : Eurofighter 2000, Hugh Harkins, Key Publishing, 1997
[4] : DASA Electronic Warfare, Germany
[5] : Texas Instruments
[6] : Tactical Technologies Inc.
[7] : Teldix GmbH, Heidelberg, Germany
[8] : Flight International, June 2000
[9] : Surveillance and Warning Systems, BGT, Germany

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