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Subject:
Jed article
french stratege
8/6/2005 11:39:38 AM
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Journal of Electronic defense article
Fighter EW: The Next Generation
by Bill Sweetman
Jul. 1, 2000
The fourth Lockheed Martin F-22 Raptor, aircraft 4004, is due to make its first flight from Marietta, GA, in late July. As the first F-22 to carry offensive avionics, its task is to demonstrate that a stealthy aircraft can be a fighter. Under a deal struck with Congress last year, the F-22 has to prove this key technology by the end of this year if the next ten aircraft are to be authorized.
The F-22 represents a radical departure from the traditional approach to EW. Passive systems, once considered to be defensive in nature, are now critical to detecting, tracking and even attacking the target. The active radar, while still a primary sensor, is used sparingly for specific tasks. Active jamming in the traditional sense has disappeared. The F-22 approach is echoed to some extent in most of today's advanced fighter programs, including the Dassault Rafale, Eurofighter Typhoon and Saab JAS Gripen. It is also fundamental to the future of the Joint Strike Fighter (JSF).
The F-22's EW philosophy is rooted in some of the earliest work on stealth. As the US Air Force (USAF) defined requirements and operational doctrine for the F-117 stealth strike aircraft and B-2 bomber, in 1980-81, a "Red Team" headed by Dr. Paul Kaminski was charged with looking for weaknesses and vulnerabilities in stealth technology. One of the Red Team's most important conclusions was that a stealth aircraft could not survive by low radar cross-section (RCS) alone, but by stealth and tactics.
In the case of the F-117, the Red Team's recommendation resulted in the development of one of the first automated mission-planning systems, but this left the aircraft dependent on a pre-programmed flight plan. The B-2 was designed to feature a sophisticated defensive management system (DMS) which would allow the crew to respond to threat radars not anticipated by the mission plan. The initial DMS was abandoned in the late 1980s. Its successor is the APR-50, developed by IBM Federal Systems (later acquired by Loral and now part of Lockheed Martin).
The USAF's Advanced Tactical Fighter project, which led to the F-22, presented greater challenges. In the air-to-air regime, the primary threats are airborne and move rapidly, making identification, location and tracking more complex. The F-22's sustained speed also shortens engagement timelines by as much as 40 percent.
At the same time, the fighter's classic tool for situational awareness ? a powerful search radar ? can render its stealth characteristics moot. Low-probability-of-intercept (LPI) techniques are not very compatible with continuous searches over a large volume. The fighter's stealth is also of little use if it has to close to visual range in order to identify its targets. Passive search and track and non-cooperative target recognition (NCTR) are not luxuries for a stealthy air-superiority fighter.
The solution to this problem on the F-22 is sensor fusion. The principal sensors are the Northrop Grumman APG-77 radar and the Sanders ALR-94 passive receiver system. The fighter also has two datalink systems: one using the standard VHF/UHF radio frequencies and the other, the intra-flight datalink (IFDL), a low-power LPI link which connects two or more F-22s at close range. The sensors are apertures connected to the fighter's Common Integrated Processor (CIP) banks in the forward fuselage.
The data from the APG-77, ALR-94 and the datalinks are correlated according to their azimuth, elevation and range. Data is combined into a track file, and the final target picture is obtained by choosing the read-out from the most accurate sensor. For example, the passive system may provide the best azimuth data, while the radar produces the most accurate range.
CIP software controls the APG-77 according to emission-control principles. The radar's signals are managed in intensity, duration and space to maintain the pilot's situational awareness while minimizing the chance that its signals will be intercepted. More distant targets get less radar attention; as they get closer to the F-22, they will be identified and prioritized; and when they are close enough to be engaged or avoided, they are continuously tracked.
Sensor fusion and emission control are closely linked. The more the datalinks and ALR-94 can be used to build and update the tactical picture, the less the system needs to use the radar. The IFDL provides another layer of protection against tracking, because any one F-22 in a flight can provide radar data to the others.
The APG-77 and ALR-94 are unique, high-performance sensors. The APG-77 has an active, electronically scanned array (AESA) comprising some 1,200 transmitter and receiver modules. One vital difference between an AESA and any other radar that has a single transmitter (including a passive electronically steered array) is that the AESA is capable of operating as several separate radars simultaneously. An AESA can change its beamform very readily, and its receiver segments can operate in a passive or receive-only mode. Unlike a mechanical antenna, too, its revisit rates are not constrained by the antenna drive, and it can concurrently revisit different points within its field of regard at different rates. The F-22 has space, weight and cooling provision for auxiliary side arrays on either side of the nose. If installed, these would provide radar coverage over almost 270°. The ALR-94, meanwhile, is the most effective passive system ever installed on a fighter. Tom Burbage, former head of the F-22 program at Lockheed Martin, has described it as "the most technically complex piece of equipment on the aircraft."
The F-22 has been described as an antenna farm. Indeed, it would resemble a signals-intelligence (SIGINT) platform were it not for the fact that the 30-plus antennas are all smoothly blended into the wings and fuselage. The ALR-94 provides 360° coverage in all bands, with both azimuth and elevation coverage in the forward sector.
A target which is using radar to search for the F-22 or other friendly aircraft can be detected, tracked and identified by the ALR-94 long before its radar can see anything, at ranges of 250 nm or more. As the range closes, but still above 100 nm, the APG-77 can be cued by the ALR-94 to search for other aircraft in the hostile flight. The system uses techniques such as cued tracking: since the track file, updated by the ALR-94, can tell the radar where to look, it can detect and track the target with a very narrow beam, measuring as little as 2° by 2° in azimuth and elevation. One engineer calls it "a laser beam, not a searchlight. We want to use our resources on the high-value targets. We don't track targets that are too far away to be a threat."
The system also automatically increases revisit rates according to the threat posed by the targets. Another technique is "closed-loop tracking," in which the radar constantly adjusts the power and number of pulses to retain a lock on its target while using the smallest possible amount of energy.
High-priority emitters ? such as fighter aircraft at close range ? can be tracked in real time by the ALR-94. In this mode, called narrowband interleaved search and track (NBILST), the radar is used only to provide precise range and velocity data to set up a missile attack. If a hostile aircraft is injudicious in its use of radar, the ALR-94 may provide nearly all the information necessary to launch an AIM-120 AMRAAM air-to-air missile (AAM) and guide it to impact, making it virtually an anti-radiation AAM.
Of course, there are some targets that do not emit signals. "We prefer it that way, because he's dumb," remarked one Boeing engineer. In this case, the F-22 can use its LPI features to track the target ? which is not a threat unless another radar is tracking the F-22 and datalinking information to the "quiet" aircraft ? and can, if necessary, identify it.
NCTR is a highly classified area. One of the few known techniques is jet-engine modulation, which involves analyzing the raw radar return for the characteristic beat produced by a combination of the radar-pulse frequency and the rotating blades of the engine. This technique is already used on operational radars (including the APG-70 in the F-15) but is vulnerable to countermeasures and dependent on target aspect.
Other NCTR techniques involve very precise range measurements. If the target's orientation is known, the distribution of the signature over very small range bins can yield a range profile which is characteristic of a certain aircraft type. It is possible that the F-22, which has a great deal of onboard processing power ? as well as a flexible, frequency-agile radar ? is designed to use an NCTR technique of this kind.
Unlike the Eurofighter Typhoon (discussed below), the F-22 does not have an electro-optical (EO) system for target identification. F-22 program managers have said consistently that they believe that the F-22 pilot will be able to identify any target ? emitting or not ? beyond visual range (BVR). "We are confident that we can demonstrate to our leadership that we know what's out there, and that we will operate with rules of engagement that reflect that fact," USAF program manager Gen Mike Mushala remarked at a conference in 1997.
The ALR-94 drives the F-22's defensive displays. The system determines the bearing, range and type of the threat, and then computes the distance at which the enemy radar can detect the F-22. The pilot is the decision-maker and is provided with timely, graphic information to guide defensive maneuvers. On the main defense display, usually shown on the left-hand screen in the pit, threat surface-to-air missile (SAM) and airborne early warning (AEW) radars are surrounded by circles that show their computed effective range. On the right-hand attack display, fighter radars are shown as blue beams extending towards the F-22's position.
The F-22 has no dedicated jamming systems. However, the APG-77 array can be used to generate powerful jamming beams over a certain frequency range.
Developing such a system has been a tremendous challenge. The F-22 avionics-development program is methodical and has learned from the experiences of other projects. From the outset, all of the software was designed on the same hardware with the same compilers and operating systems. "It was a tremendous advance," comments Boeing F-22 avionics deputy manager Gherry Bender. "We got beyond the hardware integration problems."
The complete system is being tested in three stages, starting with the ground-based avionics integration laboratory (AIL), then moving to the Boeing 757 flying test bed (FTB) and completing its tests on the F-22 prototypes. The AIL, located at Boeing Field in Seattle, WA, includes a tower-mounted sensor suite. The FTB is fitted with a sensor wing above and behind the pit, which accommodates the F-22's full-size wing-mounted antennas in their proper orientation. Internally, it features a complete CIP bank, an F-22 pit ? both the AIL and FTB support pilot-in-the-loop tests ? and multiple engineering workstations. The FTB has worked with Navy aircraft out of NAS Whidbey Island, WA, and with Air National Guard F-16s based at Albuquerque, NM.
The goal is to make the testing as realistic and repeatable as possible at each stage and, thereby, to minimize surprises at each succeeding stage. "The problem with integration is fault isolation," says Bender. "To do that, we need repeatability, combined with data gathering and reduction to get answers rapidly. If we can isolate faults on the FTB, it's a lot cheaper than doing the same on the F-22."
The first elements of the engineering-and-manufacturing-development (EMD) sensor suite for the F-22 were installed on the 757 in 1998, and powered up for the first time in December of that year. These first tests used Block-1 software, which comprised the basic operating system, navigation and some radar modes. Its primary goal was to unearth any basic problems "so that we wouldn't have to rewrite a lot of software later," says Bender. The Block-2 software, which integrates some EW and communication, navigation and identification (CNI) functions, has been operating on the FTB since October 1999, and will be loaded on to the fourth F-22 for its first flight.
Block 3.0 is the most crucial step forward, because it introduces sensor fusion among the radar, EW and CNI subsystems. A development version of Block 3.0, called Block 3S, has been flying on the FTB since April. Block 3S was added to the development program in early 1999, and includes sensor functions but not sensor fusion. "It is a risk-reduction tool," says Bender. "With the software controlling the sensors and fusion in the feedback loop, it's sometimes hard to unravel what happened. Did the sensor fail, or did it do what it did because we commanded it to do it?"
The real Block 3.0 is due to fly on the FTB in August before being loaded on Raptor 4004 in October or November. "It will be a challenge," says David Anderson of the F-22 Plans and Programs Division at Wright-Patterson AFB. "There is some risk there, depending on the availability of the aircraft and the software. The degree of risk depends on who you talk to." One area which is receiving some special attention, though, is throughput in the main computer. "We can't afford too much delay between the collection of the signal and the point where it is displayed to the pilot. We're overcoming that," says Anderson. But, he says, the team is confident that they will pass the milestone on schedule.
The schedule appears to be tight, with two to three months between the first flight of Block 3.0 on the FTB and its first flight aboard the F-22. "The current avionics schedule," notes a disapproving General Accounting Office (GAO) in its latest F-22 report, "shows Block 3...being completed five months before the completion dates the Air Force considered realistic in 1997." The first flight of 4004 slipped from February to May 2000 in the course of 1999, further delaying the flight testing of Block 2 aboard the fighter, and that date has since slipped to July. So far, however, the program has avoided disasters, and key changes (such as the implementation of Block 3S) have been implemented in time to avert problems.
UP NEXT: THE JSF
Both Lockheed Martin and Boeing are closely involved with the integration of the F-22 avionics, so it is not surprising that the proposed offensive avionics system for both JSF candidates takes the F-22 as a baseline. Sensor fusion, including the ability to detect, identify and locate pop-up threats quickly and accurately enough to attack them, is basic to the JSF. Both teams plan to fuse data on large-format displays and to use AESA radars in an LPI mode.
In many ways, JSF's goals are more advanced than those of the F-22. They include the fusion of synthetic aperture radar (SAR) and electro-optical systems in both the offensive and defensive modes. The JSF system is also intended to cost and weigh less than the F-22 hardware and to make extensive use of commercial, off-the-shelf (COTS) technology.
The JSF is planned to have five basic sets of sensors which, as on the F-22, will be entirely integrated into the central processor. Two of these form the Multi-Function Integrated Radio-Frequency System (MIRFS). The MIRFS/Multifunction Forward Looking Array (MFA) is the functional equivalent of the APG-77 radar and is being developed, under a separate competition, by Raytheon and Northrop Grumman; neither company is specifically teamed with either of the prime contractors on this part of the JSF program.
The MIRFS/Electronic Warfare System (EWS) is the all-around passive element of the RF system. The MIRFS/EWS will use its own dedicated antennas and the MFA. Sanders is the MIRFS/EWS supplier to both teams, basing its work on its experience with low-observable (LO) apertures for the F-22.
Two sensor packages make up the EO system. The forward-looking EO targeting system (EOTS) is an infrared (IR) system to locate and help identify targets. The objective is to fuse IR and SAR imagery to detect and identify targets automatically with the minimum emission level. The EOTS will also function as a long-range IR search-and-track (IRST) system to detect airborne targets and as an EO system for airborne target identification.
The Distributed Aperture Infrared System (DAIRS) comprises a set of staring focal-plane-array (FPA) sensors covering a complete sphere around the aircraft and will combine three functions: it will feed a video signal to the pilot's binocular, day-night helmet-mounted display (HMD); will act as a missile-warning system, and will serve as an IRST to detect airborne threats.
The DAIRS and EOTS are the subject of a parallel competition, like the MIRFS/MFA. Northrop Grumman and Lockheed Martin Missiles and Fire Control form one team, with Northrop Grumman being responsible for the DAIRS and Lockheed Martin taking the lead on the EOTS. Boeing is presumably working with Raytheon.
Both teams are using FTBs in the current demonstration and validation stage of the JSF program. Boeing began testing the JSF's integrated avionics on its 737-based Avionics Flying Laboratory (AFL) in December 1999 and plans a total of 50 missions. Lockheed Martin is using the BAC One-Eleven, which has served as an FTB for many Westinghouse and Northrop Grumman radars.
Although the teams have common suppliers in some areas (e.g., Sanders is the contractor for the MIRFS/EWS in both cases), there are detail differences. For example, Lockheed Martin has chosen Litton Advanced Systems to team with Sanders on the EWS, providing its unique expertise in electronic-support-measures (ESM) technology. In particular, Litton is applying its long-baseline interferometry processing to the Lockheed Martin JSF, providing the aircraft with twice the receiver capability of the ICAP-III Prowler to the Lockheed Martin JSF at half the size, weight and cost. BAE Systems is also a member of the team.
The EW and sensor systems proposed for the JSF would not be affordable using today's technology. One of the most costly aspects of the F-22 system is the need to provide separate antennas for all wavebands and aspects and to make those apertures compatible with stealth. F-22 antennas are installed in cavities lined with radar-absorbent material and covered with specially formulated materials which allow the signals of interest to pass through, while absorbing hostile signals. On the JSF, the goal is to reduce the cost and complexity of the antenna systems by making the antennas simpler and using a single antenna or aperture for many tasks.
Although Boeing and Lockheed Martin have demonstrated some key JSF functions on their test-bed aircraft, it is worth remembering that the F-22's avionics functions were demonstrated on the same level during the demonstration and validation phase of the ATF program in 1989-90. The GAO has said that several unspecified aspects of the JSF program are still not ready for EMD, and it is more than likely that the very sophisticated, yet low-cost technology proposed for the aircraft is among them.
EUROPE'S NEXT GENERATION
Three new-generation European fighters ? the Eurofighter Typhoon, Saab JAS Gripen and Dassault Rafale ? all embody definite advances over earlier-generation aircraft. None of the three is as stealthy (or as expensive) as the F-22 is, or as the JSF is intended to be, nor do they display the same level of electronic integration. They do, however, reflect an appreciation of the importance of electronics to the air battle.
The Typhoon's configuration reflects the need for high speed and acceleration, maneuverability ? particularly at supersonic speeds ? and a large weapons load for air-to-ground missions. It uses stealth technology in a limited fashion and, therefore, needs an active jamming system but does not need quite as sophisticated an ESM suite as the F-22. However, the Typhoon does incorporate sensor fusion to refine the information presented to the pilot and to reduce emissions from the radar. One very important emphasis is on positive target identification. "The biggest problem is when the poor pilot has CNN looking over one shoulder and the Minister of Defense looking over the other," comments Eurofighter marketing executive David Hamilton.
The Typhoon's principal long-range sensor is the ECR 90 radar. The radar has been the subject of some controversy because it is the only mechanically scanned radar among a group of AESA contemporaries. Hamilton calls it "the fastest mechanically scanned radar in the world," with a new antenna drive system and a low-inertia antenna. This agility makes it possible for the ECR 90 to perform some of the same tricks as an AESA, including interleaving air-to-ground and air-to-surface modes. Mechanically scanned radars have some advantages, too: e.g., no degradation of performance at the extremes of the scan angle, which can be important in a BVR engagement when the fighter launches a missile and then makes a supersonic turn away to evade the enemy's shot.
The ECR 90 also has excellent range, according to Eurofighter. It has demonstrated the ability to track an airliner target at 200 nm and to track a fighter at 100 nm. Raw range has not been a top priority for fighter radars in recent years, but opens up some interesting options when combined with long-range missiles (such as the Meteor selected by the UK in May) and high-bandwidth datalinks. A hostile pilot may realize that he is being tracked by a Typhoon that is too far away to be a threat, but fail to detect another aircraft that is closing silently to obtain positive identification (ID) and attack.
The ECR 90 signal ? whether from the fighter's own radar or from the wingman's ? is fused with data from the datalink, the ESM and the Typhoon's Fiar/Pilkington (the latter company is now part of Thomson-CSF) IRST system. The latter is a very sophisticated sensor, operating in both midwave (3 mm) and long-wave (11 mm) bands, which functions as a classic IRST ? detecting and tracking targets at extreme ranges and adding information to the track file ? and as a forward-looking IR (FLIR) sensor, acquiring a magnified, stabilized image of the target and allowing the pilot to obtain positive visual ID beyond the range of the human eye, by day or night. As in the case of the F-22, information from the different sensors is combined into track files and displayed to the pilot as a single target. A difference between the two aircraft, though, is that the target symbols on the main tactical display appear in different colors, according to which sensor has detected the target.
Since the radar has NCTR modes, there are four sensors on the Typhoon that can ID the target: the ESM, the identification-friend-or-foe (IFF) system, the radar and the IRST. According to Hamilton, Eurofighter is looking at future rules of engagement which could permit the pilot to attack if any two or more sensors can confirm it as hostile.
The Typhoon's defensive aids subsystem (DASS) is designed to support low-altitude, air-to-surface operations. Most of the equipment is housed in two wingtip pods, providing all-aspect coverage with two sets of sensors and simple installation. The DASS computer receives signals from three groups of sensors: the ESM, a pulse-Doppler missile warner (with one installation in the tail and two in the wing roots) and, on the UK aircraft, a laser warning receiver. Countermeasures are selected automatically, and the aircraft carries an onboard jammer, chaff and flare dispensers (permanently installed in the wings) and two expendable towed decoys. The latter are probably intended for use against monopulse tracking radars, and are carried in the right-hand wingtip pod. (Given the cable burn-through problems encountered by the Super Hornet, this is probably a smart place to put them.) The DASS and IRST will be installed on the full-operational-capability Typhoon, deliveries of which are due to start in 2003, after 40 radar-only initial-operational-capability (IOC) aircraft have been produced.
Dassault's Rafale takes a somewhat different approach to the related issues of stealth and EW. Dassault describes the fighter as "discreet" rather than "stealthy." It combines physical stealth ? with rather more blended contours and visible treatment of leading and trailing edges ? with tactical stealth, using an automated terrain-referenced navigation to take full advantage of terrain masking against ground threats.
The Thomson-CSF RBE2 radar is of the passive electronically scanned type, like the B-1's APQ-164, with a single power source, transmitter and receiver and a physically fixed array of phase-shifter modules to steer the beam. The radar has a single beam, but it can be pointed instantly in any direction so that it can use a wide variety of interleaved modes. ? Dassault describes a capability as the difference between "track while scan" and "track here while scan there." For example, the RBE2 can readily track airborne targets while searching for a target on the ground. Thomson-CSF is part of the European AMSAR consortium which is developing active-array technologies for future upgrades of the Rafale and Typhoon.
The Rafale's EW suite, known as Spectra, has been under development since the program started in 1985. It was developed by a team comprising Matra (now part of Aerospatiale), Dassault Electronique and Thomson-CSF, the latter two of which have merged to form Thomson-CSF Detexis.
Spectra has been a challenge because of the very tough requirements imposed by the customer. To preserve the fighter's stealth qualities, Spectra has been designed with low-observable antennas. For the same reason, it has been designed to locate RF threats very accurately ? within 1° ? and jam them with an equally narrow, high-power beam. It is also designed to meet demanding requirements for false-alarm rates. Spectra uses interferometric techniques to locate targets in range and bearing, with two prominent receiver antennas on the inlet ducts. It has electronically steered transmission antennas built into the foreplane root fairings and the tailcone and integrates IR and laser warning. Its internal processors make extensive use of application-specific integrated circuits.
Dassault has also indicated that Spectra uses "stealthy jamming modes that not only have a saturating effect, but make the aircraft invisible." This sounds a great deal like active cancellation, a LO technique in which the aircraft, when painted by a radar, transmits a signal which mimics the echo that the radar will receive ? but one half-wavelength out of phase, so that the radar sees no return at all. The advantage of this technique is that it uses very low power and provides no clues to the aircraft's presence; the challenge is that it requires very fast processing. Dassault, however, has never confirmed that active cancellation is used on Rafale.
Spectra, a high-rate secure datalink and the Rafale's front-sector optronics, including imaging IR (IIR), IRST, low-light video and a laser rangefinder/designator ? provide the fighter with the ability to launch a silent air-to-air attack. The fighter is also armed with a unique weapon: the IR variant of the Matra BAE Dynamics Mica, a long-range passive weapon that uses inertial navigation and updates from the launch aircraft for mid-course guidance, and an IIR seeker in the terminal phase.
The smallest and lightest of current fighters, the Saab JAS 39 Gripen nevertheless carries a full suite of passive and active countermeasures and the very effective Ericsson PS-05 radar. One of its most significant features in the long term, however, is its datalink. The Royal Swedish Air Force (RSAF) has been using datalinks longer than any other air force (going back to the J35 Draken in the early 1960s) and the datalink is a key to the Gripen's effectiveness and a major influence on tactics and doctrine. The JAS 39 has "given us a whole new air force, based to a large degree on information technology," comments an RSAF officer. The pilot's central tactical display is largely fed through the Tactical Information Data Link System. Using the datalink, a single Gripen can provide detailed, accurate and timely tactical information from its sensors to all other Gripens in the same combat area ? but with the sensors on these other fighters turned off. It also receives information from the S 100B airborne early warning and control aircraft and the S 102B SIGINT platform, based on the Gulfstream IVB.
The JAS 39 datalink and the AMRAAM missile are leading to a change in tactics in the RSAF, with a move to more widely spread formations which cover a wider area. Pilots no longer need to be in direct visual contact to be aware of the other members of their formation.
The JAS 39C Batch 3 variant of the Gripen ? which is also the basis for the export version of the fighter ? incorporates the new EWS 39 electronic-warfare system. Ericsson Saab Avionics was appointed prime contractor for this system in early 1998, and CelsiusTech ? also now a division of Saab ? was awarded a contract late in 1999 to develop the new RWR for this system. (CelsiusTech has delivered radar warners to the RSAF since 1957 ? seven years before the USAF started fitting such systems to fighters.) It will include functions such as automatic emitter identification and ranging.
Of the 64 Batch 3 Gripens to be delivered to the RSAF, 14 will be in a unique configuration. The two-seat JAS 39D, unlike the JAS 39B trainer, will be a dedicated information-warfare platform with a redesigned rear pit. The flight controls are removed and replaced by large-format displays. The RSAF concept is that the JAS 39D will host three levels of backseaters: a weapons-system officer, for high-workload missions involving precision-strike, long-range cruise missiles or suppression of enemy air defenses; a strike-package leader, in overall command of a multi-aircraft mission; or a scenario commander, who has authority over the entire force in a specific area. The overarching mission is to achieve information dominance ? that is, to ensure that friendly assets have the best information possible while destroying or jamming the enemy's information resources.
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