by Tom Clancy
The APG-77 is nothing like older radar systems. The antenna is a fixed, elliptical, active array which contains about 1,500 radar Transmit/Receive (T/ R) modules. Each T/R module is about the size of an adult's finger and is a complete radar system in its own right. The AN/APG-77 T/R module is the result of a massive technology development program by Texas Instruments and the DoD. As planned, each module will cost about $500 per unit (depending on the quantity ordered), a price that was set when the program was first begun almost a decade ago. The APG-77 has no motors or mechanical linkages to aim the antenna. Even though the antenna doesn't move, the APG-77 is still able to sweep a 120deg multiple-bar search pattern. However, instead of taking fourteen seconds to sweep a 120deg, six-bar search pattern like the APG-70, the APG-77 will search the equivalent volume almost instantaneously. This is because the active array can form multiple radar beams to rapidly scan an area.
The most impressive capability of the APG-77 radar is LPI (low probability of intercept) search. LPI radar pulses are very difficult to detect with conventional RWR and ESM systems. This means the F-22 can conduct an active search with its APG-77 radar, and RWR/ESM-equipped aircraft will probably be none the wiser. Conventional radars emit high-energy pulses in a narrow frequency band, then listen for relatively high-energy returns. A good warning set, however, can pick up these high-energy pulses at over two times the radar's effective range. LPI radars, on the other hand, transmit low-energy pulses over a wide band of frequencies (this is called "spread spectrum" transmission). When the multiple echoes are received from the target, the radar's signal processor integrates all the individual pulses back together, and the amount of reflected EM energy is about the same as a normal radar's high-energy pulse. But because each individual LPI pulse has significantly less energy, and since they do not necessarily fit the normal frequency pattern used by air-search radars, an enemy's warning system will be hard-pressed to detect the pulses long before the LPI radar has detected the target. This will give the F-22 a tremendous advantage in any long-range engagement, as the pilot doesn't have to establish a lock-on when firing AMRAAM missiles. Thus, the first indication that a hostile aircraft will have of an attack by an F-22 will be the screams from his radar-warning receiver telling him that the AMRAAM's radar has lit off, locked on, and is in the final stages of intercept. By that time it's probably too late for him to do anything except eject.
Finally, the APG-77 has an improved capability to conduct NCTR. Since it can form incredibly fine beams, the signal processor can generate a high-resolution radar image of an aircraft through Inverse Synthetic Aperture Radar (ISAR) mode processing. An ISAR-capable radar uses the Doppler shifts caused by rotational changes in the target's position with respect to the radar antenna to create a 3-D map of its target. Thus, where ISAR processing is used, it is the target that provides the Doppler shift, and not the aircraft that the radar is mounted on, which is the case in SAR processing. With a good 3-D radar image, an integrated aircraft-combat system could conceivably identify the target by comparing the image to a stored database. The computer would then pass its best guess to the pilot, who could, if desired, check for himself by calling up the radar image on one of the multi-function displays. If this sounds like a scene from a Star Trek movie, remember that it's all done by software in the F-22s CIPs, and additional capabilities are only a software upgrade away.
Although radar will continue to be the main sensor of combat aircraft for decades to come, infrared sensors are increasingly important for both air superiority and ground-attack missions. In Desert Storm, FLIR-equipped aircraft (such as F-117A, F-111F, F-15E, and F-16C) made precision bombing attacks around the clock. For the air-superiority mission, an aircraft needs an IRST system, while a specialized ground-attack aircraft needs a FLIR system. The differences between these two IR sensors stem from different mission requirements.
IRSTs are wide field-of-view sensors that look for targets in both the middle and long IR bands. IRSTs use automated detection and track routines, designed to find targets in highly cluttered backgrounds. Modern IRSTs are stabilized, gimbaled staring arrays that can scan large areas and detect aircraft at ranges out to 10 to 15 nm./18.2 to 27.4 km. — although 5 to 8 nm./9.1 to 14.6 km. is a more reasonable range against a non-afterburning, non-IR stealthy aircraft. Stabilized means that the sensor automatically compensates for the motion of the aircraft. Gimbals are the supporting bearings that make this possible by allowing the sensor head to rotate on multiple axes. A staring array is like an insect's eye — it consists of many independent detector elements arranged more or less hemispherically rather than a single element that must be mechanically driven to sweep the whole field of view.
FLIRs can be either wide or narrow field-of-view sensors. However, image quality is not particularly good with a wide field-of-view FLIR, and such systems are usually for navigation purposes only. Because FLIRs are designed to provide a higher-resolution picture than an IRST, they have a higher data rate and do not undergo as much signal processing. Essentially, FLIRs are IR television cameras, which must provide a clear image so that an operator can identify the picture with the world's smartest sensor, a Mark 1 human eyeball. Most ground-attack FLIR systems are mounted in external pods or turrets. The Low-Altitude Navigation and Targeting Infrared Night (LANTIRN) system used on the F-15E and F-16C consists of two such pods. The AAQ-13 navigation pod is equipped with a wide field-of-view FLIR for navigation and a terrain-following radar for all-weather navigation. The AAQ-14 targeting pod has a narrow field-of-view FLIR for precise target recognition, along with a bore-sighted laser designator. The FLIR systems used by F-15Es and F-111s in Desert Storm were the cameras that brought you some of the amazing nighttime footage of laser-guided bombs going down Iraqi command post ventilation shafts.
Only a few years ago, radar-warning receivers were widely regarded as noisy and unreliable nuisances in the cockpit. Today, however, no sane combat pilot wants to fly in harm's way without a good RWR/ESM suite. Most combat aircraft have RWRs which are tuned to provide a warning only when an enemy fire control radar has established a lock-on. That means they work about as effectively as smoke alarms do when you are in the same room with the fire. With the greatly increased computer power available to the F-22A, a fully integrated ESM and electronic-warfare (EW) system is now finally possible. ESM is basically a wide frequency band passive radar receiver. It is designed to find radar signals, analyze them, and classify the type of radar that is producing the emissions. This has already been done on specialized EW aircraft such as the EF- 111A Raven, which are packed with so many electronic black boxes and festooned with so many antennas that they have little direct combat capability.
In addition to the standard ESM package, dedicated missile-warning systems are being investigated for installation on the F-22. Historically, 80 % of all aircraft shot down never saw the opponent that killed them. With a missile-warning receiver providing 360deg spherical coverage, a pilot will know when an enemy missile has been fired at him. Based on data from the missile-warning receiver, other aircraft systems could automatically deploy expendable countermeasures (chaff and flares) and sound an aural warning to the pilot. This will improve the pilot's reaction time to an incoming missile, reducing aircraft losses in high-threat environments.
DISPLAYS
Human senses set a limit to how much data pilots can handle before they become overloaded. The key to managing this flood of data is to give the pilot only processed information relevant to the current situation. In other words, we need "pilot friendly" cockpits: If you don't get the message, it doesn't matter if the computer had the right answer or not. Earlier, we noted the sheer number of gauges, switches, and screens that an early F-15 pilot had to be aware of in order to fly the plane. However, once he went into combat, all he needed to do was put the wide-angle HUD onto the enemy aircraft, which allowed him to keep his eyes out of the cockpit.
The HUD displays all relevant tactical and aircraft-systems information in
a clear and concise manner — once you understand what all the numbers and symbols mean. The HUD is tied to and controlled by a series of switches mounted on the engine throttle and control stick. Called Hands On Throttle and Stick (HOTAS), this system allows a pilot to avoid having to go "head down" into the cockpit while in a combat situation. On the Vietnam-era F-4E Phantom, the pilot had to reach below his seat to find the selector switch for the 20mm cannon! Today, the pilot of an F-15 or F-16 has only to flip a selector switch to control everything from radar modes to weapons selection.
A drawing of a notional Heads-Up Display (HUD), showing the symbology that a pilot would typically see.
Jack Ryan Enterprises, Ltd., by Laura Alpher
A lot of important data is crammed onto the HUD. For example, a pilot can tell that he is on a course of 191deg at an airspeed of 510 knots, that the aircraft is in a 10deg climb, and that the target is up and to the left of the plane's present course. A short range IR-homing missile can be selected to engage the target, once the pilot is in a proper position to shoot. Unfortunately, when pilots take their eyes off the HUD to look around (and a good pilot will do that often to check his "six" — the sky behind him), all that data is lost to them until they look forward again. The HUD is just an image projected onto a glass screen mounted above the instrument panel. Since it is a fixed display, it can't follow the pilot's eyes when they look around.
Or can it? Right now, helmet-mounted HUDs are under development in the U.S. and Great Britain (and Israel and Russia both have operational systems). The helmet-mounted HUD supplements the standard HUD, providing enhanced situational awareness. If the aircraft carries air-to-air missiles with slewable seekers (called high off-boresight seekers), like the Russian AA-11 Archer or the Israeli Python-4, the pilot can attack targets that are offset from the aircraft's nose. You can attack a crossing target without wasting time or energy maneuvering for position, which gives you a tremendous advantage in a high-speed, multi-aircraft dogfight or "furball."
Future possibilities include virtual-reality (VR) displays, voice-command recognition (remember the book and movie Firefox?), VR control gloves, VR bodysuits, or eye motion command controls. In skies filled with stealthy, silent attacks, there is no time to waste.
THE "EDGE": COMING USAF AIRCRAFT
So what about the "edge"? What's the next step in combat aircraft design?
Two new combat aircraft will be arriving at USAF bases in the next decade or so; both incorporate elements of the technologies we have talked about. Each is a state-of-the-art solution to some problem that USAF planners identified over the last decade or two, and thus represents the thinking of the late stages of the Cold War. This fact alone has made some folks question their utility and affordability, given the changes in the world scene in the last five years. Nevertheless, given the lessons of the 1991 Persian Gulf War, as well as the general acceptance that the U.S. military in the 21st century will be a "home-based" force, these systems will be vital to maintaining the credibility of the USAF.
Northrop Grumman B-2A Spirit
Two B-2s, without escorts or tankers, could have performed the same mission as a package of thirty-two strike aircraft, sixteen fighters, twelve air-defense suppression aircraft, and fifteen tankers.
— GENERAL CHUCK HORNER, USAF (RET.)
The most expensive airplane ever built is a hard sell to taxpayers and legislators who are increasingly cynical about defense contractors and increasingly skeptical about military procurement. But to understand the B-2, you have to understand the threat that it was designed to overcome and the almost unimaginable mission it was created to perform. One of the things that helped to bankrupt the Soviet Union was an obsessive, forty-year attempt to build an impenetrable air-defense system. The National Air Defense Force (known by its Russian initials, PVO) was a separate service, co-equal with the Soviet Army, Navy, Air Force, and Strategic Rocket Forces. It was designed to keep the U.S. Air Force and the few strategic bombers of the other Western allies from penetrating the Russian heartland and decapitating the highly centralized Soviet command and control system, as well as their top military and political leadership. Ultimately, the only Western plan for defeating the system was the Doomsday scenario, using nuclear missiles to "roll back" the successive layers of air defense so the bombers could get through to their targets.
In the 1970s, the Russians began to develop mobile ICBM systems that could shuttle around the vast spaces of the Soviet Union on special railroad trains or giant wheeled vehicles. The Soviets knew that every fixed missile silo could be pinpointed by satellite imagery and targeted for destruction; every Soviet ballistic missile submarine could be tracked by sonar arrays and trailed by a U.S./NATO attack boat; but what could you do to kill a mobile missile complex? The proposed U.S. solution was to hunt down the mobile missiles with an aircraft so revolutionary that nothing in the Soviet arsenal could touch it.
The first pre-production B-2A Spirit stealth bomber in front of its hangar at the Northrop Grumman factory at Palmdale, California.
Craig E. Kaston
An invisible airplane that traveled at the speed of light, armed with precision "death ray" weapons, would have been ideal. But a subsonic airplane which was almost invisible to radar and IR sensors, carrying a few nuclear-tipped missiles, was sufficient if (and it was a big if) its development could be kept so secret that the other side would have no time, and no data, to develop effective countermeasures. Thus was born the B-2A Spirit. The origins of the B-2 design date back to experimental aircraft of the 1920s, when the visionary Horten brothers of Germany designed their first "flying wing" aircraft, without conventional tail surfaces and with a cockpit smoothly blended into the thickened wing section. Their goal was low drag (they were unaware as yet of the advantages of a low radar cross section). The problem with all-wing aircraft is that they are inherently less stable than the more normal kind with fuselages and tail sections; and crashes of various prototypes led to the shelving of the Hortens' project (although a very ambitious twin-jet-powered version was under development at the end of the Second World War). In the 1940s, the brilliant and eccentric American engineer Jack Northrop designed the XB-35 heavy bomber, a propeller-driven flying wing, and later the YB-49, a promising eight-engined turbojet bomber (which compromised the purity of the design by adding four small vertical fins). Unfortunately, the manual flight controls of the time were inadequate to solve the inherent stability problems of pure flying wing designs, and the Air Force canceled the project. Despite the problems inherent in the flying wing design, it does have one undeniable characteristic: It is tough to see on radar. Thus, the stage was set for the development of the B-2.
Originally called the Advanced Technology Bomber (ATB), the B-2 began development in 1978 as a black program, which means that it was not published in the Air Force budget and its existence was revealed only to a limited circle of legislators. In 1981 the Northrop/Boeing team's proposal was selected, and full-scale development of the new bomber followed. It took seven years, including a major redesign in the mid-1980s, when the USAF changed the original B-2 specification to include a low-level penetration capability. (Shortly before his death, under a special security dispensation, Jack Northrop was allowed to see a model of the B-2—the vindication of the idea he had championed four decades earlier.)
The first B-2 pre-production aircraft (known as Air Vehicle #1) was rolled out at Palmdale, California, on November 22nd, 1988, and the first flight was on July 17th, 1989. The first B-2A squadron (of eight aircraft) of the 509th Bombardment Wing at Whiteman AFB, Missouri, are scheduled to reach IOC (initial operation capability) in 1996. Given the official Air Force designation of Spirit, each aircraft will be named for a state; the first five are "Spirit of California," "Spirit of Missouri," "Spirit of Texas," "Spirit of Washington," and "Spirit of South Carolina." General Mike Loh, the ACC commander, likes the designation because, like a ghost, the B-2 will be able to come and go without being seen.
A c
ombination of several advanced technologies made the B-2 possible. Foremost among these was computer-aided design/computer-aided manufacturing, known as CAD/CAM in the aircraft industry. The F-117A had to employ awkwardly faceted flat surfaces, because this was the only solution available in the mid-1970s to the earlier-generation computer hardware and software on which it was designed (millions of radar cross section calculations were necessary to validate the design). The B-2, designed on vastly more powerful computer systems, could have smoothly contoured aerodynamic surfaces because, by that time, the billions of necessary calculations could be performed relatively quickly.
Moreover, the B-2 was the first modern aircraft to go into production without requiring a prototype, or even a development fixture. Designed with advanced three-dimensional CAD/CAM systems, which are used to fix parts, the B-2's virtual development fixture allowed every component to be fit-checked before it was manufactured. As a result, when the first B-2s were assembled, something happened that was unprecedented in aviation history, possibly in the entire history of engineering development and manufacturing. Every part fit perfectly the first time, and the finished aircraft precisely matched its designed dimensions within a few millimeters over a span of 172 feet/52.4 meters.
