Multi-spectral seekers, extended range and increased velocity are in vogue for the next generation of guided weapons.
Douglas Barrie/LONDON Andrzej Jeziorski/MUNICH Julian Moxon/PARIS
Over the next decade, Western air forces will revamp their inventories of both within and beyond-visual-range (WVR/BVR) missiles. The operational requirements which missile manufacturers are being asked to meet will push the next generation of weapons well beyond present engagement envelopes.
In meeting emerging operational requirements, missile manufacturers are beset with difficulties, not the least of which is the developing nature of the target.
While Russia is no longer the baseline perceived threat for the NATO alliance, its combat aircraft and associated weapons-systems remain the measure against, which Western systems are specified.
This is not least because such aircraft as the Sukhoi Su-27 Flanker, and its successor the Su-35, along with variants of the Mikoyan MiG-29 Fulcrum and possibly the MiG-31 Foxhound A, are likely to be increasingly visible on the export market.
Like its Western counterparts, the Russian air force (VVS) is moving towards integrated defensive aids, which tie in the aircraft's offensive on-board sensors in the shape of the air-intercept radar and the infrared search and track (IRST) system. Rearward-facing radar are also being introduced on the new generation of combat aircraft now in development.
What this effectively means is that the target aircraft crew stands a greater chance of identifying the threat, whether from a radar-guided or infra-red-guided missile and taking the appropriate countermeasures.
As Royal Air Force Sqn Ldr Paul Colley pointed out at a recent Shephard Air Power conference on air combat and air-delivered weapons, this means that the target aircraft is often capable of defeating the attack. This can be achieved by using electronic countermeasures (ECM) or by the aircraft manoeuvring to the extent that the missile engagement is a kinematic failure.
BVR WEAPONS
Colley considers that, with the bulk of the present generation of air-launched weaponry, "...the overall kill rate is very low". Colley's comment is revealing in the light of the RAF's requirement for a future medium range air-to-air missile (AAM) as specified by Staff Requirement (Air) 1239. This weapon will provide the primary BVR engagement capability for the RAF's Eurofighter EF2000 combat aircraft.
In its initial form, SR(A) 1239 looked like a requirement, built around the US Hughes AIM-120 advanced medium-range AAM. As the RAF reviewed the potential emerging threats, however, it became apparent that the requirement was becoming more demanding, both with regard to missile "fly-out" time and in its tail-chase engagement capability.
SR(A) 1239 has an importance above and beyond being merely the UK's BVR programme for Eurofighter. The winner of the requirement may become the de facto standard for all four Eurofighter nations: Germany, Italy, Spain and the UK. It will form the focus of a collaborative future European BVR missile programme, around which industry will coalesce.
This is already apparent from those companies which have responded to the UK Ministry of Defence's (MoD's) request for information. BAe Dynamics, Saab Missiles and GEC-Marconi have teamed to offer the proposed S225X, Daimler-Benz Aerospace (DASA) is offering the A3M and Matra is pushing a derivative of the Mica, while Hughes is offering an upgraded variant of the AIM-120. With the British Aerospace/Matra missile divisions merger imminent, the French company is also likely to find, itself involved in the S225X programme, in some shape or form.
All of the missiles being proposed have rocket/ramjet power plant configurations in at least one of their design iterations. Kentron of South Africa is also considering offering a ramjet missile.
DASA's AAM project manager, Franz-Josef Scholz, is a keen proponent of a collaborative BVR programme, unsurprisingly based round the A3M. "We at DASA had the feeling that it's time to have a European alternative to the Hughes AMRAAM," he says. He sees the A3M as a "nucleus for European co-operation".
Such a view is echoed by BAe and Matra, each of which would like to see its contender become the hub for a collaborative programme. Hughes, meanwhile, would like to see its AIM-120 pre-planned product-improvement programme form the basis of transatlantic collaboration. This would also take out the European threat.
As contenders for SR(A)1239 are quick to point out, the RAF is asking for the development of a weapon which, because of the missile-recess dimensions on the EF2000, will have to be the same size as the AMRAAM, but have a range well in excess of 100km (55nm). The AMRAAM has a maximum range of 50km. Scholz says that the A3M weighs around 165kg, only slightly more than the 157kg AMRAAM. Keeping the weight within the design parameters, however, will continue to prove a challenge to all competitors.
As now configured, all the SR(A)1239 contenders will have active-radar seekers. The RAF's BAe Skyflash, which is fitted to the Panavia Tornado F3 air-defence aircraft, is only semi-active. This requires that, the engaging aircraft continues to illuminate the target, with its own air-intercept radar for the duration of the engagement.
In the case of the active missile, the weapon is fitted with its own pulse-Doppler transmitter with which to illuminate the target once it is within acquisition range.
For long-range engagements, active missiles are provided with target updates via a datalink from the launch aircraft, to ensure that the parameters to allow for active acquisition are met.
For the front end of SR(A) 1239, GEC is likely to offer a development of the work it has carried out in conjunction with Dassault Electronique for the Matra Mica 4A J-band (10-20 GHz) active seeker. Hughes is likely to offer a derivative of the AIM-120s I-band (8-10Ghz) seeker, while DASA will offer a high-energy K-band (28-40GHz) seeker. A prototype of this seeker has already been built in support of Germany's next-generation surface-to-air missile programme.
K-BAND-SEEKER BENEFITS
Scholz argues that the benefits of pushing a K-band-seeker solution include the ability to "burn through" most ECM, along with the ability to provide target-position data for a directional warhead. Although a directional warhead is not included in DASA's current proposal, he sees such a development as a distinct possibility.
"This is a capability which is inherent to a K-band seeker. You cannot do this with an I-band seeker. You need the short wavelength of the K-band to be able to determine the target's position precisely," he says.
DASA is also the only one of the contenders to consider offering a dual-mode seeker from the outset. BAe, by comparison, is looking to offer at some time between 2005 and 2010 a multi-spectral seeker in time for a mid-life upgrade (MLU) of the missile.
As proposed, the A3M, says Scholz, would be fitted both an active (K-band) active seeker and possibly a semi-active receiver antenna. The latter would home in on radar-energy returns from the target being illuminated from the launch aircraft's radar.
BAe and, in all likelihood, Hughes and Matra, will offer dual-mode-seeker options as part of an upgrade programme from whatever emerges to meet SR(A)1239. As it stands, there are unconfirmed reports that the US Navy is working on a dual-mode variant of the Hughes AIM-54 Phoenix, with an IR seeker in the nose tip as well as an active-radar seeker. This may be a technological spin-off from the cancelled advanced air-to-air-missile programme, which was intended to replace the AIM-54.
BAe and UK Defence Research Agency (DRA) officials spell out the attractions of providing a dual-mode capability for medium- and long-range AAMs: it provides improved resistance to countermeasures and aim-point selection. The latter could also eventually lead to an autonomous target-recognition capability, using very-large-array imaging-IR (IIR)-sensor technology.
Marrying active-radar and IIR-seeker technology within one missile, however, raises various issues. BAe, GEC and the DRA are all examining issues such as whether there should be common or split apertures for the radar and IIR seekers.
Providing a common aperture for both radar and IIR seekers demands the use of advanced-materials technology. Split apertures, meanwhile, pose problems, with missile-airframe design.
A split-aperture approach may result in an asymmetric missile design. The traditional smooth lines of a radome could be succeeded by a missile front-end more resembling what one military source describes as a "shark's jaw". The "dog tooth" would house the IIR sensor, while the active-radar seeker would continue to radiate and receive throughout the front of the radar.
Such an approach would demand that, at least in the final phase of the engagement, the missile flies in such a way as to ensure that the target is always within the IIR aperture.
More arcane design solutions could get round this problem, by placing the IIR seeker at the front end of the missile, while using advanced materials shaped in such a way as to refocus the radar energy around the tip, both when it is being transmitted and received. Other options include a conventional radome, with up to four IR windows, mounted towards the front of the missile body to provide 360° coverage. Exactly which solution - or, more likely, solutions - will emerge remains to be seen.
Dual-mode AAMs have also attracted the attention of Israeli missile manufacturer Rafael, which is also examining IIR/active-radar-seeker combinations. The company says that it is funding a research programme into sensor-fusion technology for next-generation weapons.
The AAM is not the sole domain of the dual-mode-seeker solution. Dassault Electronique, along with DASA and Bodenseewerk Geratetechnik, is now looking at the development of a passive dual-mode seeker for a next-generation anti-radiation missile (ARM).
The seeker, part of the Franco-German anti-radar future (ARF) programme, would use both a passive radar and an IR seeker. Gerard Buoy, Dassault Electronique's seeker-division managing director, says: "We're hoping that full-scale development will go ahead in 1996/7." A prototype dual-mode seeker has already been test flown on a helicopter-mounted captive flight-test installation.
As described by Dassault Electronique, the passive radar seeker would be used to HOME in on the emitting air-defence radar to within around 3km of the target, when the IIR seeker would be used for terminal guidance.
The problem with using passive guidance for ARM seekers is that modern air-defence radar systems are becoming increasingly efficient, with a reduction in side-lobe emissions. These have traditionally provided the signal on which ARMs home.
Alternative homing sources include detecting and "flying down" the main beam. This, however, as RAF Sqn Ldr Paul Hull pointed out at a recent air-power conference, also entails limitations, the most problematic of which is that the missile itself will be detected. Consequently, the radar can be blanked in the appropriate elevation or azimuth, effectively leaving the ARM blind.
DUAL-MODE SEEKER ADVANTAGE
By opting for a dual-mode seeker, Dassault is, at least in part, overcoming this problem. If the passive seeker loses the radar, the IIR seeker may still locate the target. If the missile is detected beyond the IIR seeker's range, and the target radar is turned off, then the missile may still use a memory mode to fly into the area where the radar emissions were last detected.
Another approach, put forward by Hull, is to use a combined sensor approach, but with the passive-radar cueing system mounted on another platform, either an aircraft or possibly a drone. The ARM would be fitted with either an IIR or millimetre-wave (MMW) terminal seeker. As with the ARF, this would allow the user to attack elements of the air-defence unit other than just the radars.
Alongside working on a dual-mode front end for the ARF, Dassault Electronique is also developing a miniature synthetic-aperture radar, (SAR) for cruise-missile applications, such as variants of the Matra Apache. Buoy's preference for SAR, rather than an MMW radar stems from his belief that better accuracy can be achieved with the former. Unlike IIR, it is not affected by the weather, he says.
A development version of the SAR seeker is in flight test, says Buoy. The company is working on a version, which will be small enough to fit into a missile airframe, with a further two years of tests planned. "The challenge," says Buoy, "is to reduce both the cost and size to the level of current imaging infra-red systems."
The initial anti-runway version of the Apache is fitted with MMW radar. Later variants, however, including that being offered for the UK's Staff Requirement (Air) 1236 for a conventional stand-off missile, may have a dual-mode seeker. Matra is looking at using MMW, coupled with a pop out IIR seeker, for final target-acquisition and attack for the French MoD's APTGD long-range missile programme.
The majority of the other main contenders for SR(A)1236 are likely to offer single-mode IIR-based seeker solutions. As with SR(A)1239, however, it is possible that the winning contender will be given an MLU which will include the development of a dual-mode system. Options could include SAR/MMW along with IIR/MMW.
MMW and IR seeker solutions are also being bid to meet procurement programmes in Europe and the USA. The USN has opted for an IIR seeker for the unitary warhead variant of its joint stand off weapon after examining this technology alongside MMW, SAR and laser radar-seeker options.
The RAF's requirement for a stand-off anti-armour capability, SR(A) 1238, is also likely to be fulfilled with either an MMW or IR/IIR guided weapon.
MMW has been in development for the past ten to 15 years and is viewed as a relatively mature technology. In the air-to-ground role, MMW frequencies are appropriate because, for a given aperture, the azimuth beam-width is at a minimum. This provides good cross-range resolution.
For frequency range, there are two "atmospheric" windows at 35GHz and 94 GHz, with the lower frequency preferable for long range, although the choice is not clear-cut.
The seeker of the GEC-Marconi Brimstone missile being offered for SR(A)1238 operates in the 94GHz band. In target detection, while the Brimstone is an active-radar emitter, the company argues that, as the low-power narrow beam scans objects only 2-6s ahead of the missile, reaction time on the part of a target is minimal.
The solution to SR(A) 1238 being offered by BAe is a variant of its advanced short-range air-to-air missile (ASRAAM), which is fitted with a Hughes IIR seeker. The company argues that using IIR would potentially allow the user to lock on either before or after launch. While an MMW seeker could also be used in a lock-before-launch mode, BAe argues that the power requirements are prohibitive.
WVR MISSILES
The ASRAAM, in its air-to-air role, is also being proposed for the USN/US Air Force AIM-9X Sidewinder replacement programme. After years of apparent complacency, it became apparent that even late-model AIM-9 Sidewinders were substantially out-classed by the Russian Vympel R-73 (AA-11 Archer). The likely proliferation of this weapon on the export market, coupled with Vympel being well advanced on a follow-on programme, has belatedly spurred the USA into action.
As with their medium- and long-range counterparts, considerable development work is going into improving IR seekers for short-range missiles. The initial generation of "hot-spot" detectors has long since been superseded with imaging infrared arrays.
With the current generation of technology, fielded in ASRAAM, for instance, the seeker must still be gimbaled, to provide an adequate field of view. With the very large arrays in development, the need to gimbal the detector will become redundant. Next-generation IIR missiles are likely to be fitted with staring-sensor arrays. Besides an improved detection capability, such technology also offers the promise of both weight and cost savings.
Considerable work is also being carried out in improving IIR missiles' resistance to IR countermeasures (IRCM). Seekers with two distinct spectral bands have been developed, to provide two-colour discrimination. This is intended to defeat IRCM such as flares, which have tended to emit in one spectral band. As the countermeasures become more sophisticated, then so do the seeker heads.
Israeli missile manufacturer Rafael is rumoured to have fitted its still-classified Python 4 short-range missile with a seeker which can discriminate in three spectral bands. This makes the weapon extremely difficult to "spoof" with IRCM. Not surprisingly, the USA is also looking at Python 4 technology as part of its AIM-9X programme.
Improving the IIR seeker also offers advantages in target recognition and aim-point selection. With the level of technology already fielded in the Hughes ASRAAM seeker, target-type recognition is possible.
The UK's DRA has a research programme under way examining advanced processing techniques which allow the missile to determine which kind of aircraft it is attacking, be it a twin-engine fighter, or a four-engine turboprop transport.
With this level of processing capability the missile can then "choose" the most vulnerable point to target.
Such a capability, as previously suggested, may also offer a solution to one of the main problems of air-to-air combat - just exactly which side you are firing at. IIR technology has the potential to provide a level of target recognition, which would greatly improve identification-friend-or-foe concerns, particularly in BVR engagements.
Such a potential reward, no matter how arcane the technology might seem now, is reason enough for continuing the considerable expenditure into advanced-seeker technology.
Source: Flight International
