5 September 2005
Directed energy takes an unexpected turn and surfaces as a handy antimissile device that can be built into aircraft, ship and ground-based radars
Radar is emerging as one of the key weapons--nearly all of them still shrouded by secrecy--in the Pentagon's growing arsenal of nonexplosive devices.
Knowledge that radar can produce violent effects on electronic systems is not new. More than 20 years ago, bomber aircraft radars were capable of generating enough concentrated noise jamming to burn out the valve amps (tube amplifiers) in fighters attempting an interception. The emergence over the last few years of the active electronically scanned array (AESA) radar, and its ability to provide high average power for appreciable times, makes such electronically destructive devices all the more attractive and effective.
The weapons-effect utility of the AESA will provide a useful adjunct to other "nonkinetic"--not relying on explosives or impact--weapons research being done in the U.S. and in Europe. The intent is to develop an arsenal of weaponry that limits, or perhaps eliminates, collateral damage and unintended casualties, a particular concern with civilians in countries that coalition forces are trying to aid.
Arrays designed for carriage by aircraft appear to be focused on cruise missiles and self-defense against anti-radiation, home-on-jam and air-to-air missiles. The radars seem particularly effective against the latter categories because energy available to focus on the approaching missile increases as an inverse square as distance decreases.
With large ground- or sea-based AESA-type arrays, the targets are to include ballistic missile warheads, supersonic sea-skimming missiles or shoulder-fired surface-to-air missiles that threaten airport operations.
Some of the airborne AESA radars, designed for X-band frequencies, use thousands of small transmitters/receivers, each a couple of inches square, that allow the array to conduct many tasks simultaneously. Those include detection of small, even stealthy targets, tracking and communications. Along with the AESA's high average power, there are also bandwidth benefits and the ability to utilize flexible waveforms.
They can also be used for "jamming," a term now encompassing weapons effects on enemy electronics from the right combination of power, waveform, frequency and pulse length. Possible AESA techniques for attacking another radar could include burning through the target radar's antenna side-lobes, filter side-lobes, or other known features of the target system. Radar specialists suggest it is reasonable to suppose this capability is already available to some fielded systems.
The AESA transmitters can also be focused on other targets to deliver bursts of X-band radio frequency energy into the vulnerable electronics of missiles--the current focus--or enemy aircraft and helicopters or computer systems. Under such assault, computers become confused and missiles lose interest in their targets.
MOREOVER, THE AESA radar is related to high-power microwave (HPM) just beginning to emerge as missile defense systems. The primary difference is that AESA radars produce a sustained pulse for microseconds over a limited frequency range to create an effect while HPM produces a one-pico-sec. pulse of much greater power over gigahertz of frequencies, says a long-time Pentagon radar specialist. A laser beam, by comparison, would have to be held on the target for several seconds.
Some industry specialists say such descriptions oversimplify the technology because both AESA radars and HPM can produce a variety of pulse lengths and bandwidths. They contend the only difference between AESA radars and HPM systems are the waveforms and RF power levels. Both systems use the same electronics technologies and those electronics are optimized for the performance needed to achieve the desired radar or HPM effect. That goal is often to confuse or damage enemy electronics.
These capabilities aren't openly discussed. Moreover, those with knowledge of the technology offer many caveats.
"It's not wise to characterize all AESA radars as potential weapons," says an aerospace industry expert in advanced radars. "Most radars are for defensive purposes only." Also, "one does not need an AESA to turn a radar into a weapon. It can be done with other technologies. High power is required, but beyond that, it's mostly a software issue.
"Wide bandwidth is needed in order to find a vulnerable frequency for the target," he says. "Purpose-built HPM systems would be better in this than most AESAs."
THE NEWLY EMERGING "HPM [devices] come in different flavors," says a second industry specialist. "It can make it uncomfortable for a human being to be in the beam by relying on high average power to heat the skin. Another is used to confuse or burn out missile seekers. The level of peak and/or average power depends on the specific technique being used to counter the threat.
"In general, high peak power is not unique to HPM [devices]," he says. "Radars also use high peak power in long-range search modes, and AESA [arrays] are used for both. The discriminators are the waveform properties and techniques, which include power, duty cycle, pulse length, etc., to counter the various threats, not the technology that goes into the AESA itself. AESA [arrays] support HPM, radar, communications and electronic warfare applications. What makes them unique are the properties of the waveforms that are transmitted."
There are AESAs fielded that operate at HF frequencies (re-locatable over the horizon radar) to millimeter waves (communications and other radar applications). The transmitter/receiver (T/R) modules for these devices can range from several feet to less that 1/4-in. square. Moreover, many frequencies beyond Xband can be exploited to produce weapons effects.
AESA radars on fighter aircraft aren't particularly suited to create weapons effects on missiles because of limited antenna size, power and field of view, a senior U.S. Air Force official says. And, while weapons effects from radars are interesting, "There's no requirement yet for the capability," at least on smaller aircraft, he says. The military's primary concern for now is high-resolution radar with moving target and synthetic aperture capabilities.
While it's easier to plug the energy-hungry system into city electrical power grids as ground-based systems or the Navy's next generation of electric-propulsion ships such as DD(X), the capability is also quickly moving into airborne systems.
A weapons capability exists in a handful of F-15Cs modified with the APG-63(V)2 radars for cruise missile defense and the latest production F/A-18E/Fs. It will soon be part of the F/A-22 and B-2 as part of their radar upgrades, and AESA is also to equip the F-35 Joint Strike Fighter. HPM is eventually to be part of the Joint Unmanned Combat Air System payload, and the huge 4 X 21-ft. MP-RTIP AESA radar array is to fly on the E-10. An AESA array is also to be carried by the British Astor.
As to weapons effects, the AESA radar offers the best way to generate high, sustained power where countermeasures demand average power, the radar specialist says. However, weapons effects or countermeasures will vary depending on the target's sensitivities. Very high peak power, short duration pulse may be the best method for attacking enemy electronics. In that case, the attack might better and more cheaply be delivered by an HPM weapon. An AESA is best used where it can use its radar function to locate the target and focus its energy. It can then rapidly move to other targets and select how long it wants to dwell on each.
The radar's weapons effect is measured in watts/sq. cm. AESA arrays are more efficient and reliable since their RF and low-noise amplifiers are near the radiators so that very little energy is lost. The beam is produced by ganging the effects of thousands of lower-wattage T/R modules.
There are lots of similarities between ground-based HPM systems and AESA radars including the T/R modules. In fact, ground-based HPM is becoming affordable because the proliferations of AESA radars has driven the price of modules down.
"HPM and AESA radar are not much different," said the radar specialist. However, "HPM is not trying to be a radar. It's much simpler to concentrate the beam into a peak power pulse [since it's not busy detecting, tracking and identifying targets]. The instantaneous power it creates can burn out missile front ends [including focal planes and imagers]."
BY COMPARISON, AESA "radar uses more elaborate wave forms that focus on detection," he says. "They put more sustained energy onto enemy missiles and burn out the low noise amplifiers and receivers in a seeker." Other specialists caution this generalization isn't necessarily always true.
While HPM produces higher peak power, AESA often generates greater average power. That produces different operational and targeting strategies. For example, Raytheon's airport protection system uses infrared sensors to find the target and determine where to focus its beam. It also produces effects at longer range, possibly as much as 100 mi., because it produces powerful pulses of energy. AESA radar has the built-in ability to find and track a target, so it can be held on the target for the necessary additional microseconds needed to create its weapons effect.
Some HPM pulses are designed to be very broadband, covering "many gigahertz" of frequencies, so they are more likely to find any opening or vulnerability in a target, the radar specialist says. AESA radar has a narrower frequency range, but it uses its radar capability to identify a target, search a library for its vulnerable frequencies and then tailor the signal for the specific target.