Australian Aviation, 1994
by Carlo Kopp
© 1996, 2005 Carlo Kopp
The technology of Airborne Early Warning (AEW&C) systems is at a generational junction point at this time. We are now witnessing the next step in the lengthy evolution of this key technology, that being the transition from mechanically scanned antenna technology to electronically scanned phased array technology.
Phased array antenna technology has been in use for some decades, but most applications have been confined to ground based systems, due significant weight and size penalties associated with older families of electronic devices. The ongoing march of miniaturisation combined with significant improvements in microwave power transistor device technology has now allowed its wider use in airborne applications.
Conventional Antenna Systems
The purpose of a radar antenna is to focus a beam of electromagnetic energy into a desired shape and direction, and due some of the nicer properties of electromagnetism, the shape of the transmitted beam is identical to the shape of the antenna's sensitivity pattern when receiving. In an AEW&C application, and typically most airborne surveillance applications, the shape of the beam is designed to maximise the chances of detecting a distant and small target.
But this is not the only requirement which exists, as an AEW&C platform must also have the ability to track targets, find their altitude and resist the effects of jamming by inbound hostiles. These are often contradictory requirements in terms of what the antenna must do, as the search and detect function typically requires a broader beam to cover as large a possible volume of airspace in a single sweep, whereas the tracking function requires as tight a beam as is possible to provide the ability to resolve multiple closely spaced targets, and determine their position as accurately as possible.
A key issue in the design of such antennas is sidelobe performance, sidelobes being spurious and unwanted beams produced by the antenna in directions other than that of the principal beam, the mainlobe. Sidelobes have typically unhealthy effects on a radar system, which by its nature cannot tell whether the energy which it is receiving originated in the mainlobe or the sidelobe. In airborne lookdown radars, which are typically MTIs (Moving Target Indicators) in lower band systems, or pulse Doppler in the microwave bands, sidelobes can inject energy reflected off the airframe and underlying terrain with Doppler shifts which are very different from the Doppler being used as a reference by the electronics which sift through the mainlobe return searching for targets. This can degrade performance if appropriate measures are not taken.
There is another insidious side effect of sidelobes, and that is that they render the radar vulnerable to hostile jamming and anti radiation missiles (ARM). False target generator jammers will typically exploit sidelobing, injecting false target returns into the system via a sidelobe, thus creating the illusion of targets in the mainlobe, where none exist. ARMs will home in on the stray sidelobe emissions, which enable them to track the radar even if it is pointing elsewhere.
As is clearly evident, designing an antenna system for an AEW&C platform is not a trivial task, even for the experts, and many differing solutions have been devised over the last five decades.
The oldest and least effective approach was to produce one of a variety of rotating concave (dish shape) sections, typically based on paraboloid shapes, producing a narrow beam by using a horizontally wider section (ie horizontal orange peel shape), height finding being implemented by switching the beam via several mechanically offset antenna feeds. This meant that one of several beams was active at any time, and each of these beams covered a different range of altitudes. While this is a cumbersome arrangement, it is easy to build, and many systems in the fifties and early sixties used this arrangement. Its principal weakness is poor sidelobe performance, and slow response when height finding.
The venerable Lockheed E/RC-121 and WV-2 Warning Star (Connie) and Avro Shackleton AEW&C.2 used this class of antenna, albeit without height finding capability.
The next step in the evolution of AEW&C antennas was the use of fixed arrays. The idea of an antenna array is very simple and elegant. Instead of designing a single complex antenna shape, the array uses a group of much simpler antenna shapes, and combines their individual signals together. In the fashion, all the mainlobes are added together, and if this is done correctly, a much tighter single mainlobe is produced.
One of the key constraints was that of beamwidth, and the basic rule which applies is that for a given radar wavelength (frequency), the wider the antenna or antenna array, the tighter (narrower) the beam. Typically, the tighter the beam mainlobe, the weaker the sidelobes. This of course introduces a problem in airborne applications, as the bigger the antenna, the bigger the airframe required to carry it, and hence the cost will increase dramatically.
The use of arrays allowed the design of much more compact antennas, and the sixties E-2C AEW&C and seventies E-3 AWACS both capitalised upon this technology to maximise the performance of their antenna designs.
The E-2C uses a family of radars, the APS-125/138/139/145, all of which employ derivatives of the APA-171 antenna assembly in a dorsal radome (saucer shaped). The antenna arrangement hidden under the plastic is a horizontal array of UHF band Yagi antennas.
The bigger E-3A/B/C/D/F uses the larger and more sophisticated APY-1 or 2 pulse Doppler radar, which uses an E/F band microwave slotted planar array. The slotted planar array is a microwave antenna, which uses hundreds or thousands of tiny slots, each slot acting as a very simple antenna element. A complex network of waveguides and delay elements hidden behind the antenna array times the arrival of the microwave signals in such a fashion, that the antenna produces a very tight mainlobe beam, and very small sidelobes. As with a conventional radar, the transmitter uses a large Travelling Wave Tube (TWT) microwave amplifier (usually dual redundant in the E-3) which pumps the very powerful microwave signal into the antenna. In the opposite direction, the slots/waveguides/delay elements feed into a redundant receiver which then in turn feeds a conventional pulse Doppler signal processing chain. The antenna scans in azimuth by the whole antenna assembly being rotated upon its pedestal at 6 RPM, through 360 degrees.
Both the E-2C and the E-3 integrate a primary and secondary radar capability, the secondary/IFF antennas are mounted back to back with the primary radar antenna.
This family of microwave antennas was the first to see a limited application of of the phased array principles to be discussed, and this is typically used for the height finding function.
Phased Arrays - An Introduction
The phased array is extension of the idea of the planar array. In the planar array, the beam is fixed in direction and shape, because the timing of the microwaves fed into the array is fixed. However, if the timing can be varied, then both the shape of the beam and its direction can be changed. If this is done electronically, the shape and direction of the beam can be changed in a very small fraction of a second.
Needless to say, this can be as daunting a task as it appears to be, because several hundred or thousand array elements must be retimed simultaneously. The key elements in building such an array are the programmable phase shifter (or more colloquially, "shifter"), a device which can change the phase (ie time delay or timing) of the microwaves passing through it under electronic control, and the ubiquitous digital computer. Using the computer to control the shifters, the whole array becomes in effect an antenna with software programmable beam shape and direction.
Until the late eighties, building such a system involved a substantial volume of hardware, which meant that fully electronically steerable phased arrays were mainly used in surface based applications, such as the massive BMEWS ballistic missile warning radars and the somewhat smaller US Navy SPY-1 Aegis air defence radar, carried on the Ticonderoga class cruisers and more recently, the Arleigh Burke destroyer. The only known airborne applications were the large Flash Dance radar fitted to the gargantuan Soviet Foxhound air defence interceptor, and the attack radar in the Rockwell B-1B Lancer.
Airborne applications suffered mostly from the penalty of weight, as the first generation of phased array technology used a substantially conventional radar architecture. While the antenna changed, all else remained as was, but additional computer hardware was added to control the antenna shifters. This translated into a heavier antenna, an extra computer, and extra power loading on the electrical system resulting in bigger accessory generators.
The performance benefits of the phased array however justified the extra cost. The phased array could in a single antenna do the jobs of several purpose built antennas, almost simultaneously. Wide beams could be used for searching, narrow beams for tracking, flat fan shaped beams for height finding and narrow pencil beams for terrain following (B-1B). In a hostile jamming environment the benefits were even greater, as phased arrays allow the system to place a "null", an area of zero receiver sensitivity, over a jammer and thus in effect block it from entering the receiver chain. Another benefit, although minor in non-surveillance applications, is that there is no longer the need to mechanically point the antenna in the direction of the target. Typically a multiple sided antenna arrangement could provide 360 coverage, with fixed antennas covering all directions at once.
Less obvious benefits also flowed from this technology. One was the ability to rapidly scan a small sector of sky to increase the likelihood of detecting a small and fleeting target, unlike a slowly rotating antenna which can only scan a particular sector once per rotation, typically seconds apart. A small target like a low flying cruise missile may be almost impossible to track under such conditions. The phased array's ability to almost instantaneously change beam direction and shape in fact adds a whole new dimension to tracking, as multiple targets may be tracked by multiple beams, all of which are interleaved in time with a periodically scanning search beam. As an instance, a search beam may sweep 360 degrees periodically, while tracking beams can follow individual targets regardless of where the search beam is looking at, at the time.
Significant as these gains may have been, the first generation of this technology was simply too physically cumbersome to penetrate into the AEW&C environment. The E-3 uses a limited phased array capability, in that the APY-1/2 can height find through vertical beam steering, this was implementable at modest cost as the antenna slots could be controlled in horizontal "stripes" to achieve this functionality. A shifter is thus only required for each stripe, thus cutting their number down to something modest, rather than thousands.
Phased arrays do have their limitations, as all designs have. The principal limitation is the range of angles through which the beam can be steered. In practice, the limit is about 45 to 60 degrees off the vertical to the plane of the antenna, steering the beam to shallower angles degrades antenna performance significantly. Two effects are at play here. The first of these is that the effective length (width) of the antenna is reduced with increasing beam deflection angle (for technical readers the effective length L' becomes L'=L.cos(A), where L is the physical array length and A the angle off the antenna axis, at the array boresight L'=L, falling to 0.5L at 60 degrees and zero at 90 degrees), reducing array length in turn diminishes its ability to resolve targets at a distance, while also reducing antenna gain, a measure of its efficiency. The second effect is less apparent, but derives from the radiation pattern of the constituent elements, the slots, which radiate less with increasing angle off the vertical, thus reducing the power transmitted and the sensitivity. In effect, at extreme angles the mainlobe is both substantially weakened and defocussed (technical readers are directed to Eli Brookner's item in Scientific American Feb 1985, pp 76 for a more detailed discussion). So substantial is this reduction, that a typical situation would see antenna gain, and hence power radiated and sensitivity, cut down to 25% at 60 degrees off the vertical.
The application of phased arrays to AEW&C technology had to wait for another technological development, that being the active phased array. In an active phased array, each array element or group of elements has its own miniature microwave transmitter, dispensing with the single large transmitter tube of the older passive array technology. In an active phased array, each element is comprised of a module which contains the antenna slot, phase shifter, transmitter, and often also a receiver. In a conventional passive array, a single transmitter of several hundred kiloWatts of power feeds several thousand elements, each of which emits only tens of Watts of power each. A modern microwave transistor amplifier can, however, also produce tens of Watts, and in an active phased array design, several thousand modules each producing tens of Watts of power add up to an equally powerful mainlobe of hundreds of kiloWatts.
While the final effect is identical, the active array is far more reliable, as the failure of any array element merely degrades antenna performance by a fraction of a percent. This is graceful degradation, as the catastrophic transmitter tube failures which plague conventional radar simply cannot occur. A side benefit is the weight saving incurred by dispensing with the bulky high power tube, its associated cooling system and its large high voltage power supply.
Another powerful feature which may be exploited only in active arrays, is the ability to control the gain of the individual transmit/receive/shifter modules. If this can be done, the range of angles through which the beam can be swept is increased substantially, and thus many of the array geometry constraints which plague the conventional phased array may be circumvented. Such arrays are termed supergain arrays. From published literature it is unclear whether any existing or development designs use this technique, and the coverage limits indicated for some existing designs suggest that this is not the case as yet.
In summary it is fair to say that the active phased array outclasses conventional radar designs in almost all respects, providing superior performance, tracking capability and reliability, albeit at some penalty in complexity and possibly cost.
The venerable Hawkeye is the mainstay of US Navy AEW squadrons, as well as being used by Israel and Singapore. The APS-125/138/145 systems carried by subtypes of this aircraft are based on sixties UHF antenna technology, using the APA-171 radome which contains an array of Yagi antennas. Its strength is maturity and simplicity, and it delivers very good performance in its maritime environment (AEWA).
The AWACS is the flagship of US AEW technology, based on evolved versions of the 1970s APY-1 radar. The APY-1 and 2 radars are microwave E/F band systems, with mechanical rotodome scan in azimuth and phased array techniques in heightfinding. The system delivers superlative long range detection and tracking performance, but is penalised by size and weight, which impose the need for a large airframe (C-137/B-707 or widebody) and this reflects in high acquisition and support cost (AEWA).
The Hawkeye's UHF radar has been integrated with both the Lockheed P-3 and C-130 airframes, providing a mid range system with substantially better range and endurance performance than the E-2C. These derivative systems exploit the additional airframe volume available and use larger and newer computer and display technology, in comparison with the cramped E-2C airframe.
The best known phased array radar in use today is the US Navy's SPY-1 Aegis, a large passive array system fitted to Ticonderoga class cruisers. The large SPY-1 has four 3.65 x 3.65 m arrays, each with 4100 elements, and can concurrently track several hundred targets at a range of altitudes. Designed to counter saturation attacks by several hundred anti-ship cruise missiles, the radar relies heavily on its ability to flexibly allocate its system computing power to best advantage, and vividly illustrates the potential of modern phased array technology (LM Photo).
Fuente: http://www.ausairpower.net/aew-aesa.html
Ver blogger original: http://nubia-anc.blogspot.com/
Materia: CRF
