by Tom Clancy
A horizontal view of the vertical coverage obtained by a typical airborne fighter radar, the APG-63/70. Jack Ryan Enterprises, Ltd., by Laura Alpher
As good as the APG-63 was, the follow-on radar system for the dual-role F-15E Strike Eagle had to be even better. Hughes engineers used the APG-63 as the basis for the new APG-70 radar. When it was tested in 1983 on a modified two-seat F-15B, it was obvious that the Eagle’s eyes had gotten even sharper. To keep costs and airframe modifications to a minimum, the APG- 70 used the same antenna, power supply, and transmitter as its predecessor. But the brains of the system were all new. A new radar data processor, PSP, and other modules replaced older APG-63 LRUs. The software package was completely new, with greater flexibility, making future modifications even easier. The APG-70 can simultaneously track and engage multiple airborne targets with the new AIM-120 AMRAAM air-to-air missile. To support the F-15E’s ground-attack mission, there is a high-resolution ground-mapping mode (crews tell us it can routinely pick up high-tension power lines), and an even finer synthetic-aperture-radar (SAR) mode, which produces in just seconds a black-and-white photographic-quality picture of the ground for use by the WSO. SARs use a processing technique that uses the aircraft’s horizontal motion to “fool” the radar system into “believing” the antenna is actually much larger than it really is. By overlapping multiple return echoes from several scans, and matching them up with the Doppler shift from the various objects in each individual scan, a very high resolution image can be created. Objects as small as 8.5 feet/2.6 meters can be clearly seen in the SAR mode at a range of around 15 nm./27.4 km. The ability to clearly pick out buildings or even vehicles from the radar image at long ranges and in almost any weather greatly simplifies the targeting problem for an aircrew.
Another remarkable feature of the APG-70 is called Non-Cooperative Target Recognition (NCTR). “Cooperative” target recognition depends on the transponders carried by friendly aircraft, which return the proper coded reply when they are “interrogated” by an IFF system. The relatively low reliability of this method has led to very restrictive rules of engagement (ROE) that require several independent means of verifying that a target is really, truly an enemy before a pilot is allowed to shoot it. All air commanders live in fear of “fratricide” or “blue-on-blue” accidents, and the tragic shootdown of two Army helicopters in Northern Iraq in 1994 by F-15Cs suggests that this fear is well founded. NCTR, which is quickly becoming standard on many U.S.-designed radars, is the ability to classify a target by type while it is still beyond visual range. How this is done is highly classified; and even mentioning NCTR around an Air Force or contractor site is likely to raise eyebrows and tighten lips. Nevertheless, NCTR was used in Desert Storm. One possible means discussed in open sources is to focus a high-resolution radar beam on a head-on target and count the number of blades in the opposing aircraft’s engine fan or compressor. Knowing the blade count tells you the type of engine and can give you a good idea as to whether the target is hostile.
The APG-70 also has a Low Probability of Intercept (LPI) mode, designed to defeat the Radar Warning Receivers (RWRs) and Electronic Support Measure (ESM) detectors on enemy aircraft, by using techniques like frequency-hopping and power regulation.
The key to the APG-70’s capabilities is raw computer power. Compared to earlier F-15s, the Strike Eagle has a five-fold increase in computer processing capability, a ten-fold increase in system memory and storage, and software which is easier to reprogram and use. Troubleshooting is simplified by Built-In Test (BIT) software that routinely checks on the health and well-being of major systems and can isolate a fault to a particular LRU. These capabilities make the F-15E Strike Eagle the most dangerous bird of prey in the air today. Yet even as the “Mud Hen” (as the early crews called the F-15E) was finishing up its testing in 1990, the U.S. Department of Defense was already looking into ways to shorten the time it took to get advanced computer technology into military systems.
In 1980, the Pave Pillar program was initiated by the USAF, with the goal of developing an advanced avionics architecture that could be built out of standard modules containing next-generation digital integrated circuits. With this approach, all of the sensors, communications, navigation, and weapon systems management subsystems will talk to each other over a local area network (LAN), and processed information will be presented to the crew as needed or requested. This significantly reduces pilot workload, allowing him or her to concentrate on flying the plane—a must if future aircraft are to have only one human on board. The new F-22 is the first aircraft to benefit from the Pave Pillar program, and the increase in computer power will make the avionics system of the F-15E Strike Eagle look like a pocket calculator by comparison.
The F-22 carries two Hughes Common Integrated Processors (CIPs). They give the new fighter a hundred-fold increase in computer-processing power over the Strike Eagle. When new sensors or other systems become available, there is room for a third CIP, if required. To accommodate this increase in processing capability, the F-22 data bus bandwidth has been increased to 50 Mb/sec. By comparison the F-15E’s data bus carries only 1 Mb/ sec. Since the F-22’s APG-77 radar is no longer a stand-alone system, the radar antenna will be just one of a number of sensor arrays, including the electronic-warfare and the threat-warning systems. Data from all of these sensors will be fused together, processed by the CIPs, and displayed to the pilot on one or more color flat-panel multi-function displays (MFDs). Now let’s take a look at what the F-22’s new APG-77 radar will do.
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 120° multiple-bar search pattern. However, instead of taking fourteen seconds to sweep a 120°, 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 t
o 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 360° 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 191° at an airspeed of 510 knots, that the aircraft is in a 10° 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 ba
ses 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.
