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Sensors Take on Multitasking Activities

February 2001
By Robert K. Ackerman
E-mail About the Author

Researchers even consider Greek mythology to stay ahead of enemy technological advances.

Future U.S. Air Force sensors will serve multiple roles as detectives, guards, messengers and avengers. New active and passive systems will network, exchange information, formulate opinions and even lead the fight against adversaries on the ground and in the air.

Existing technologies will see improved capabilities amid reduced costs. Advances in signal processing are opening up new areas of sensor application. And, the process of designing and building state-of-the-art sensor systems is inspiring Air Force scientists to develop countermeasures to equivalent enemy technologies.

“Our goal is for the warfighter to know all the militarily relevant information that he needs to know when he needs to know it,” explains Dr. Donald W. Hanson, director of the Sensors Directorate at the Air Force Research Laboratory (AFRL).

This directorate is located at three sites. Most personnel are located at the Wright Research Site at Wright-Patterson Air Force Base, Ohio, but others can be found at the Rome Research Site in New York and the Hanscom Research Site in Massachusetts.

Hanson adds that the directorate plays an important role in information superiority, which is one of the Air Force’s six core competencies. While commercial off-the-shelf technologies can serve many roles in information superiority, some data can be collected only by military-specific sensors. 

The directorate conducts research into radar, radio frequency (RF), electro-optic and infrared sensors. Hanson notes, however, that the Sensors Directorate also is tasked with developing countermeasures for use against an adversary’s sensors.

Among work with RF sensors, Hanson describes how electronically steered phased array antennas are vital to a host of applications ranging from unmanned aerial vehicles (UAVs) to air superiority fighters. The technology is not new, but factors such as cost, complexity, maintainability and reliability have hindered its widespread use. The agility of these electronically steered antennas provides more functionality than a mechanically steered beam that follows the same path repeatedly in each cycle. An electronically steered beam can search in a random pattern or can be teamed with multiple beams to track and detect multiple targets.

The directorate is making noteworthy improvements in phased array technology, Hanson allows. He predicts that the antenna that will equip the Joint Strike Fighter will be a significant improvement over the one on the F-22, which was a leap ahead of its predecessor.

Other advances may equip the airborne warning and control system (AWACS) to replace its mechanical azimuth-scanning beam. An AWACS aircraft could search and track targets in different areas by tasking individual beams. While the current AWACS has a 10-second update rate, equipping it with electronically steered radar would give it a much better rate. Similarly, the joint surveillance target attack radar system (JointSTARS) could be upgraded considerably with a newer phased array antenna system.

Hanson notes that, if the functions of these two aircraft eventually are moved to space platforms, electronically steered antennas would be vital to effective deployment. The Sensors Directorate is working with the AFRL’s Space Vehicles Directorate on possible space-based sensor systems, he adds.

These phased array antennas also have an improved signal-to-clutter ratio, which especially is relevant for ground detection and tracking radars. The Kosovo Operation taught the Air Force that some targets can defy detection by contemporary sensors, Hanson notes. Natural concealment under forest cover or camouflage kept many potential targets from Air Force sensor systems.

The Sensors Directorate is heavily involved in the Air Vehicles Directorate’s sensorcraft program (see page 16). Hanson relates that this craft’s sensors would operate at several RF frequencies, such as very high frequency (VHF) for foliage penetration and X-band frequencies for improved image resolution. The craft also would include electro-optic and infrared sensors.

Among the sensor types that Hanson envisions for this craft are radars that span the full range of frequencies. Hyperspectral sensors also are on the wish list as are passive sensors to perform electronic surveillance. Bistatic operation that permits the craft to operate in a passive mode is a possibility. The craft’s role might even expand to include countermeasures.

Hanson continues that, even if the concept craft is not built, its enabling technologies will help guide the directorate’s technology development program. “Right now, the most important thing about sensorcraft is that it is a vision that gives us a guidepost for developing technology over the next several years,” he offers.

Increasingly sophisticated sensors mandate more intensive data processing. “Someone must make an intelligent assessment of what these signals mean,” Hanson notes. So, a major element of the directorate’s work involves automatic target recognition (ATR). The goal is to process this data “in a reasonably rapid way” to discover the most important elements that are embedded in the data.

The AFRL’s Sensors Directorate is working with the Defense Advanced Research Projects Agency (DARPA) on automatic target recognition. Hanson cites model-based ATR, which encompasses understanding the physical characteristics of the prospective target, as a prime area of AFRL expertise. Understanding these physical characteristics enables development of a model from which the target’s signature can be predicted under many different conditions.

“If you had a test program where you tried to look at every target that might be of interest in all the various orientations from which the target might be seen, then that could be a very significant problem,” Hanson explains. “But, if you can develop a model that adequately represents all the objects that you have an interest in, you then can use the model to predict what that object is going to look like in many different orientations.”

The Sensors Directorate also is working with the AFRL Information Directorate on sensor fusion, which is a key component of ATR. Hanson explains that ATR becomes more achievable with information from more than one sensor. “Obviously, the more information you can get on a particular object, the more likely it is that you will be able to determine what that object really is,” he emphasizes. Data fusion from various sensors remains a key part of the directorate’s ATR thrust.

This sensor fusion capability is migrating toward missiles and other munitions. The capability is especially important for the “endgame” in which a missile closes in on its target, Hanson relates. His directorate will provide the enabling technology for work underway at the AFRL Munitions Directorate at Eglin Air Force Base, Florida.

Considerable work remains to achieve effective ATR, Hanson cautions. Potential ground targets are diverse and not easily distinguished from harmless civilian counterparts. They also can be partially obscured in a variety of urban or rural environments, which adds complexity to any solution. Issues include higher resolution to discern detail on the target, identification of fixed and mobile targets, detection despite obscurement or camouflage, all-weather day/night identification and advanced data processing by improved algorithms.

The directorate’s countermeasures effort, which Hanson describes as “equally important” to sensor development, comprises several programs. Among the most important is the laser infrared flyout experiment, or LIFE. This program aims to develop a laser-based countermeasure system to protect large, less maneuverable aircraft such as the C-17 transport. The LIFE system would help defeat infrared guided missiles, which are effective against these larger aircraft during takeoff and landing.

The laser is not powerful enough to cook the sensors of an attacking missile. Rather, it is designed to confuse the missile seeker that is trying to attack the aircraft. It effectively would jam the seeker to break its lock and would ensure that the missile veered away from the original track. This would be achieved by modulating the intensity of the laser beam that is shining on the seeker head, which will confuse the seeker with its internal modulation. Live missile tests with a cable-mounted moving target will ensue this year.

Another countermeasure concept under consideration is called Medusa, after the mythological figure upon whose sight men turned to stone. This principle would be applied in the form of destructive countermeasures against enemy radars. The goal, as Hanson relates, is to guarantee that any foes who set their radar sights against an Air Force vehicle would be turned to vapor. The moment a threatening radar signal is detected, the signal’s identity and location would be verified and supplied to an onboard missile or another aircraft for a destructive response. Hanson notes that this largely will require passive sensors.

A related activity involves overcoming enemy countermeasures. Antiradiation missiles designed to home in on enemy radars tend to lose accuracy when the target radar is turned off. While some missiles can continue on their flight path, the directorate’s goal is to guarantee that a missile will destroy a target radar with only the briefest exposure to its transmissions. This antiradar missile or precision guided munition could be guided by global positioning system data, Hanson offers, and its capability would not be limited to mission-specific aircraft.

“If we could put systems on every fighter aircraft that would allow the aircraft, when illuminated [by enemy radar], to get some information on where that illumination came from—and pass that information onto other aircraft—that would be a very robust capability,” he states. Currently the Air Force has mission-specific aircraft, such as Rivet Joint, that are dedicated to analyzing these enemy signals. Adding the capability to all other aircraft would create a host of new sensor platforms that can help locate enemy radars. “We would have a lot more eyes and ears out there, and that just increases our capability to locate these radars tremendously,” he adds.

For the more distant future, the directorate is looking at concepts that would destroy a hostile missile sensor. Again, this capability would have to include a means of actually diverting the missile away from its original track. Scientists in Hanson’s directorate are working on this “jammage,” or mixture of jamming and damage, to ensure that a hard-charging missile is diverted without possibility of target reacquisition. The ultimate jammage device would be a directed energy beam that actually knocks the missile out of the sky, Hanson notes.

Detection of active signals is well established, but the Sensors Directorate is working to detect passive sensor systems when they are tracking Air Force aircraft. One approach is to discover a passive optical tracking system by using a cat’s eye reflection principle. Hanson explains that most optical systems feature a glass lens that reflects when illuminated by exterior light. While detecting these types of sensors is much more difficult than detecting active ones, the directorate is pursuing several concepts to spot these optics. Operational systems may lie at least 20 years ahead, he notes.