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 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
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
