Active phased array antenna technology promises significant improvements in AEW&C platform performance. While the technology base has yet to mature, the first designs using this technology are beginning to appear.
AEW&C using Active Phased Arrays
While the application of active arrays to AEW&C systems has been under discussion for many years, only two designs have been built to date and the technology has yet to reach full scale operational deployment. Applying the active array to an AEW&C platform introduces some interesting problems with antenna placement.
Conventional AEW&C systems have traditionally employed a rotodome, the characteristic saucer shaped radome which covers a conventional antenna, the rotodome rotating in order to scan 360 degrees about the platform. The placement of the rotodome on pylons, elevated well above the aircraft's fuselage, ensured that the antenna had an unobstructed field of view about the aircraft. This was achieved at considerable cost in weight, as the fuselage required strengthening to carry the structure, which itself wasn't featherweight. In typical designs, the rotodome is designed with an aerodynamic profile which produces, under cruise conditions, lift equal to the weight of the rotodome assembly, thus alleviating structural loading in cruise, but not necessarily in other configurations, unless the rotodome is built to tilt and thus change the AoA of its aerodynamic section.
The rotodome arrangement is readily applicable to active array systems, as a four or three sided array may be fixed in the same position as the rotodome, providing 360 degree coverage and good clearance from the aircraft's structure. Lockheed have proposed a three sided array in this configuration, fitted to either a new airframe or the existing S-3 Viking airframe, to meet USN E-2C replacement requirements. The three or four sided array arrangement may be applied to any airframe able to accommodate a rotodome, and may well become the standard in years to come. Its only significant failing is that it retains much of the cost and weight penalties of the rotating rotodome.
Another configuration derived from this idea is that of the Swedish Ericsson Erieye, which uses a two sided array in a beam shaped structure, carried above the fuselage of a twin engined commuter airframe. The two sided array used in this arrangement is almost as long as the APY-2 antenna of the AWACS, potentially providing similar angular resolution performance at range, on a very small airframe.
This arrangement however suffers from an obvious and significant operational limitation, as it cannot provide 360 degree coverage, using conventional active phased array technology. With each array scanning a 120 degree sector, the two sided array has a 60 degree blind sector over the nose and the tail of the aircraft, and degraded antenna performance beyond 45 degrees off the beam of the aircraft. With Sweden's compact geography this would probably not be an issue, as multiple platforms would cover a single area, and operating in pairs, the aircraft could patrol in two racetrack orbits set 90 degrees apart to provide overlapping coverage. The success of this scheme then devolves down to the capability of the computer datalink networking which links the platforms to each other or the ground air defence centre, to ensure that a comprehensive picture of the air situation exists at whatever is the central command post.
In a heavy ECM environment, where platform to platform or platform to ground datalink function is interfered with, the two sided array has thus a major limitation. Producing a three or four sided array with similar array length results in a structure with a size comparable to an E-3 AWACS radome, which in turn requires at least a 737 sized aircraft to carry it, thereby largely defeating the apparent cost advantage of the linear array concept.
A possible resolution would be the use of a supergain array, where the ultimate size of the blind sector would be determined by the array's module parameters and array length.
Another alternative which exists is the use of a rhombic four sided array geometry, with a 60-120-60-120 degree arrangement of arrays. While the rhombic arrangement will provide full 360 degree coverage, its effective antenna length is halved in the nose and tail sectors. The result is a compromise between the bulky but excellent four or three sided array, and the compact but partially blind two sided array. No publicly discussed proposals to date have involved the rhombic arrangement.
An idea which has created some excitement in the engineering community is the concept of conformal active arrays or "smart skins", where active arrays are embedded in the skin of the aircraft, thus avoiding the structural, aerodynamic and weight penalties of an external radome. However, close examination of most existing airframe designs suggests they may not always accommodate this concept without some other penalties, such as coverage limitations like those suffered by the two sided array concept.
The Israeli Phalcon system, which uses a B-707 airframe, was reported initially to have been the first implementation of this scheme. The aircraft's public debut has however shown this not to be true, as the aircraft uses fuselage mounted boxes for its main sidelooking arrays, and a nose mounted radome for a smaller forward looking array.
The fuselage mounted linear arrays provide for excellent coverage over the beam aspect 120 degree sectors, but the nose and (reported optional) tail mounted arrays which "plug" the holes in beam array coverage are much shorter due fuselage diameter and thus would suffer a major loss in resolution performance fore and aft. Again the use of supergain array techniques could alleviate this problem.
As with two sided and rhombic array configurations, the Phalcon arrangement may or may not be suitable for a given operational environment. Where the threat axis is defined unambiguously and the aircraft's patrol racetrack can be aligned appropriately, the coverage limitations may not be of significant importance. Where the threat can approach from multiple axes concurrently, full 360 degree coverage is almost mandatory.
Westinghouse's MESA system which is currently in development, uses a podded arrangement of sidelooking linear arrays carried by a C-130, with some reports suggesting that supplementary nose and tail arrays could be fitted to plug nose and tail coverage.
Other alternatives do exist. An arrangement publicised by Boeing for the now defunct USN E-X program involved the use of an airframe with a trapezoidal (diamond shaped) wing, with arrays embedded in the wing surfaces to provide 360 degree coverage. The S-3 sized aircraft had been proposed to replace the E-2C. Recent budget cuts have put its future very much in doubt.
The dilemma faced by designers is simple: active arrays provide the potential for a small airframe to have E-3 AWACS class antenna performance, however antenna coverage requirements will force the use of a either a mast mounted radome or a new airframe geometry, both negating the potential cost advantage offered by the antenna technology.
Other alternatives may yet exist, but investigating their suitability will require some effort by airframe manufacturers. Most modern airliner airframes have a wing sweep of about 60 degrees, which suggests that the leading edges of the wings have the almost ideal geometry to accommodate conformal arrays. A six to eight metre conformal array embedded in the leading edge, inboard, would automatically provide two sides of the three sided array configuration. The problem is that this would preclude the use of leading edge lift devices over at least 30% of the span, and also preclude the use of wing mounted engines, which would obstruct the array's field of view. The unresolved issue is the third side of the array, which could only be implemented by placing an array on the tail of the aircraft. A six to eight metre array length will require, by default, a beam structure of similar size.
Closer examination of available airframes suggests that those with aft mounted engines (B-727, MD-80) would be geometrically most suited to leading edge array placement, but the positioning of the powerplants would cause difficulties in positioning the tail array. The optimal geometry would see engine pods mounted above the wings (cf VFW-614), and the tail array beam structure fitted to the end of the fuselage, or at the top of the vertical stabiliser. The scale of change would again force a new airframe design, or substantial rework of an existing design.
The application of supergain array techniques could of course alleviate many of these difficulties, and it remains to be seen how soon this technology becomes adapted to an AEW&C application. Clearly as the technology of active phased arrays matures, designers will settle upon the most suitable configurations, but as is often the case, the simple and orthodox solutions may ultimately prevail simply because they were a good idea in the first place. The pylon mounted radome may be with us for some time to come.
The Australian Perspective
Australia has some unique problems in acquiring its future AEW&C capability. These revolve primarily about the operational requirements associated with the range of missions to be performed, further complicated by the geography of our landmass. Projected RAAF AEW&C operations can be basically divided into the support of air defence (DCA or Defensive Counter-Air), and the support of maritime operations and OCA (Offensive Counter Air). Existing doctrine is focussed on DCA (AAP1000-Ch.5).
Local air defence of point targets can at a minimum, be performed by a smaller aircraft with lesser radar range, the requirement being centred on the ability to quickly get an aircraft aloft to a station within 200 NM of the runway in use, and to provide 2-4 hours of time on station. Given the concentration of potential targets in the North into specific areas and potential threat axes, 360 degree coverage may not be a mandatory requirement. Supported by long range threat warning from Jindalee, this mission can be performed readily by a smaller twin engined aircraft. The proximity to a land base means that the requirements imposed insofar as Command/Control/Communications go are modest, because land based facilities may be used to support the mission. Wider coverage of the large expanses of the North would however preclude this approach.
Providing AEW&C support for DCA in the air-sea gap, maritime operations and OCA becomes a more demanding affair, as the area of operations may be several hundred nautical miles from the operating base, over the ocean, out of the reach of land based UHF comms and exposed to enemy air attack from many axes. These conditions impose the requirement for considerable range and endurance, to ensure that the AEW&C aircraft can remain on station for a substantial time, and also demand comprehensive C3 capability and 360 degree radar, IFF and ESM coverage. Good radar range is also to an advantage, as is transit speed to station and inflight refuelling capability.
All AEW&C systems in use today reflect these operational requirements. The E-3 has superb radius, endurance, radar coverage and a comprehensive C3 suite, which allows for wholly autonomous operation as an airborne command post. The former Soviet Mainstay was designed to a similar requirement. The E-2C, optimised for local air defence in the maritime carrier environment, has limited range, endurance, C3 capability and radar performance, although it does provide the necessary 360 degree coverage. The SAAB-2000/Erieye has endurance and range in the class of the E-2C, limited C3 capability, and limited coverage, reflecting the local air defence requirements of the congested airspace of the Baltic, and predictable threat axes.
In the Australian context, the question is whether we can effectively capitalise upon the emerging technology of active arrays. The RAAF in defining its AEW&C requirement will ultimately have to decide whether to opt for a twin engined system limited to localised DCA operations, closely coupled to ground C3 facilities and hence depart from established doctrine, or whether to opt for an extended AEW&C umbrella satisfying existing DCA doctrine and encompassing RAAF OCA and RAN operations, and hence select a longer ranging four engined aircraft capable of performing as a self contained command post. Existing RAAF DCA doctrine (AAP1000-5.41,5.42) stresses the latter approach.
The central question in this matter is whether the RAAF will define its requirement in the context of recent doctrine, opting for a longer ranging platform, or whether it will yield to the inevitable financial pressure from the government to select a smaller and less capable aircraft. The selection of off-the-shelf candidates which meet stated doctrine and obvious requirements is both limited and split between older conventional technology and newer array technology. The alternative would then be a custom integration exercise, combining active array radar with a platform and array geometry not necessarily used at this time by the radar vendor.
What is significant in this context is that judicious choice of airframe and array geometry could minimise integration costs, while providing scope for domestic integration work which would in turn bolster our domestic defence industry, and ease longer term support costs for the design once in service. It is central in this debate that the government recognise that short term acquisition cost advantages may not translate into a cost or operational advantage over the whole life cycle of the AEW&C system, and hence that the government does not pressure the RAAF in the direction of short term expediency.
Active array technology promises major gains in the capability of both medium sized and smaller AEW&C systems, but to capitalise fully on these gains will require further integration effort. Australia has in many senses a unique operational environment and it would not be wise to bend requirements to fit established and very much development systems. We can hope the government will recognise this when it eventually proceeds with the AEW&C acquisition.
The Boeing E-3 AWACS was the first AEW&C platform to use a limited amount of phased array technology in its APY-1/2 surveillance radar. The APY-1/2 utilises a slotted planar array which scans azimuth mechanically and height electronically. With the closure of 707 airframe production, Boeing will integrate the system with a newer widebody airframe.
The Ericsson Erieye system uses an active phased array radar mounted in a two sided array geometry. The whole array is contained in a large beam shaped structure carried above the fuselage of a commuter twin airframe. The limitation of the two sided array is that it can only cover two 120 degree sectors abeam of the aircraft, leaving 60 degree blind sectors over the nose and tail of the aircraft, and reduced antenna performance from 45 degrees off the beam aspect. Another limitation stems from the use of an airframe too small to accommodate a comprehensive self contained command, control and communications system, and other sensors such as a capable ESM and track association system.
Pic.3 (Phalcon - not enclosed)
The Israeli Phalcon is the first full scale application of phased array technology, using arrays along the fuselage and under the nose and tail. While providing full 360 degree coverage, the smaller size of the nose and tail arrays will limit angular resolution in the nose and tail sectors, thus degrading system performance in these areas. While cheaper than external pylon mounted radomes in terms of structural modifications, conformal arrays require suitable airframe geometry if they are to be used to full advantage.
This AMSS Lockheed proposal for an E-2C replacement depicts a three sided array geometry. Three sided and four sided arrays offer 360 degree coverage without significant degradation in angular resolution against azimuth, but incur the cost, weight and drag penalties of the radome structures. At the time of writing no design using this geometry has been flown.
Ver blogger original: http://nubia-anc.blogspot.com/
Materia: CRF
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