Radio Frequency Countermeasures Suite Protects Aircraft, Maps Battlefield Threats

June 1999
By Frank Colucci

Post-Cold-War widely varied emitters require more discriminating detection.

The U.S. Army is developing new countermeasures to defeat smarter air defense threats, including systems that rely on radar targeting technology. The recently introduced suite of tools detects, identifies, locates and jams modern gun and missile radars.

On the AH-64D Longbow Apache and other battlefield helicopters, radio frequency countermeasures offer protection in a threat-rich environment. On the Army’s digital battlefield, the technology promises to classify and target threats for a true joint force networked on the tactical internet.

The development effort is being led by the Army aviation project office for aircraft survivability equipment, Huntsville, Alabama. ITT Industries, Clifton, New Jersey, delivered the second AN/ALQ-211 suite of integrated radio frequency countermeasures (SIRFC) engineering manufacturing development system to The Boeing Company earlier this year. Contractor flight tests began aboard the AH-64D in March. On the modernized Apache, SIRFC will undergo performance testing in the anechoic chamber at Edwards Air Force Base, California. Operational test and evaluation is planned for early next year, and production of the fully integrated countermeasures suite is scheduled for late 2000.

The SIRFC operational requirements document puts the ALQ-211 on the Army’s AH-64D Longbow Apache, the Special Operations MH-47E and MH-60K aircraft, and select UH-60 Black Hawks and CH-47 Chinooks. ITT is also under contract with Bell Textron to integrate the suite aboard the CV-22 Osprey tilt rotor of the U.S. Air Force Special Operations Command. A SIRFC test set was delivered to the CV-22 aircraft integration laboratory in February to verify system interfaces, and a full system for the CV-22 is due in the third quarter of 1999. The ninth engineering manufacturing development Osprey will be rebuilt as the CV-22 prototype with a full special operations avionics suite, including the AN/ALQ-211 SIRFC and multimission advanced tactical terminal. Together, SIRFC and digital communications give Army helicopters and the U.S. Air Force tilt rotor the tools to evade and potentially exploit battlefield threats.

For helicopter crews, air defense threats have grown more lethal and varied since the end of the Cold War. Instead of the well-defined Soviet threat, potential enemies now mix advanced air defenses from East and West and modernize them with unexpected capabilities. Those threats have adapted to the way army aviation fights. Helicopters fly nap-of-the-Earth to hide in terrain clutter, but newer threat radars are designed to extract helicopter signatures from that environment. Modern pulse-Doppler and monopulse radars have therefore outpaced today’s federated aircraft survivability equipment (ASE), which was designed to counter older, first-generation threats.

The SIRFC program aims to provide multifaceted passive and active protection against modern radio frequency threats. It analyzes the entire radar picture and responds with advanced jamming techniques to counter today’s second-generation threat radar as well as future threats.

Advanced development work on SIRFC, the new integrated radio frequency countermeasures suite for Army helicopters, was completed in 1993. In successful developmental and operational tests on a Black Hawk, prototype suite hardware flew against threat systems.

In 1995, situational awareness algorithms central to the countermeasures suite were also demonstrated on a Black Hawk as part of an advanced technology demonstration sponsored by the Army Communications-Electronics Command (CECOM), Fort Monmouth, New Jersey. The resulting AN/ALQ-211 combines situational awareness, radar warning and electronic countermeasures in an integrated architecture to detect, identify and jam multiple simultaneous threats.

Unlike any previous ASE suite, this broadband countermeasures set is designed to counter pulse, pulse-Doppler, continuous-wave and monopulse threats. The passive receiver portion of the system warns of emitters from C to M band and has the discrimination to identify threats with better than 90 percent certainty. Typically, radar-warning receivers look at just two signal parameters to identify enemy air defenders. By comparison, SIRFC checks 10 parameters to fingerprint enemy emitters with confidence. Threat characteristics are stored in the electronic order of battle that is loaded before each mission, and crews can call up threat data in flight.

Compared with today’s radar jammers, SIRFC transmits over a far wider frequency range. It also has the growth capability to reach lower microwave and millimeter-wave frequencies. The active jamming portion now includes transmitters to jam H- to J-band radars. It can expand to cover E- to H-band and J- to K-band threats in the future.

The countermeasures suite uses sophisticated range, velocity, angle and noise techniques, including those tailored to monopulse and pulse-Doppler threats. The product’s technique library includes modern jamming methods, and as with SIRFC hardware, software can be updated to counter future threats. Coordinated and integrated with warning receivers and jamming transmitters linked by reduced instruction set computer (RISC) microprocessors, SIRFC’s rapid response to the threat is appropriate and automatic. Integrated aircraft survivability equipment ensures that radio frequency protection is coordinated with infrared jammers, missile-warning receivers, laser-warning receivers, and flare and chaff dispensers. ASE also connects with precision navigators, digital maps and communications devices such as the improved data modem on the AH-64D and the multimission advanced tactical terminal on the CV-22.

Full integration also enables the AN/ALQ-211 to take advantage of sensor fusion. Onboard and offboard data sources update the electronic order of battle. For example, threats located by unmanned aerial vehicles and other intelligence sources can be datalinked to the host platform to update the suite’s electronic order of battle. The system also complements other onboard targeting sensors such as the AN/APG-78 Longbow fire control radar and AN/APR-48 radio frequency interferometer.

Army pilots have already demonstrated the power of the new integrated radio frequency survivability suite in simulation exercises. Helicopter crews flew with and without SIRFC in October and November 1997 at Fort Hood, Texas, in the battlespace integrated concept emulation program (BICEP). The networked simulation linked cockpits at Fort Hood to computer-controlled and man-in-the-loop air defenses from the threat systems management office in Huntsville.

BICEP was developed to provide training insights for distributed simulation, and it simulated the passive warning functions of the AN/ALQ-211. The exercise gave indications of SIRFC situational awareness and threat warning capabilities. Cockpit crews effectively used the system to evade and target SA-15 and 2S6M air defenders in a game area modeled on central Germany.

In simulations involving between eight and 12 aircraft, SIRFC threat warnings were available to two Apache, Black Hawk or Comanche crews through digital map displays. According to one participating ASE training manager from the directorate of training, doctrine and simulation, Fort Rucker, Alabama, “Every aircraft with SIRFC survived. The threats could pick us up, but because we could pick them up beyond their lethal engagement envelopes, we could bypass them.”

For battlefield helicopter crews, situational awareness is critical. Unlike the simple display on current radar-warning receivers, the AN/ALQ-211 provides true situational awareness. It enables a color digital map to display lethality zones that are determined by the type of threat system and the terrain. Tied to global positioning system/inertial navigation suites, the system is expected to geolocate threats within 10 percent of target range. “With SIRFC, you know exactly where the threats are. You get a positive identification on what the threats are. You see areas where you can fly to avoid engagement,” a BICEP pilot offers.

The effect on aircraft survivability was dramatic. In one BICEP simulation, a mine-laying Black Hawk with SIRFC evaded threats, while an escorting Apache with current radar-warning receiver technology was shot down. “The crew with SIRFC could use terrain masking and the intervisibility function of the digital moving map. They just found spots where the air defenders couldn’t hit them,” the pilot explains.

The AN/ALQ-211 shows emitters on the digital map with their current operating modes: dormant, acquisition, tracking or missile launch. With unambiguous threat warnings, aircrews can jam tracking radar or divert from mission plans only when necessary. “It makes a difference in the way you fly,” the simulation pilot says. “You can plan your route in flight and remain oriented on the objective.”

Today, helicopter pilots forced into terrain mask to evade air defenses lose sight of the threat when the radar-warning receiver loses line of sight to the emitter. By comparison, the AN/ALQ-211 stores the electronic order of battle before takeoff, updates the information in flight, and locates detected threats for the combat crew and others on the tactical internet.

In the BICEP exercise, an Apache with the AN/ALQ-211 passed the location of an air defense battery to a Comanche without SIRFC. The attack helicopter cued the armed scout to kill the threat. The same data burst to a rear-area tactical operations center could cue artillery, fixed-wing aircraft or other forces to targets.

With accurate geolocation information, pilots can suppress many enemy air defenses. The sensitivity and discrimination capabilities of SIRFC enable a Longbow Apache to pop up beyond the effective range of air defenses and target threats for hidden “shooters” or long-range artillery. Data transmitted through single channel ground and airborne radio systems, enhanced position location reporting systems, or other radios make aircraft equipped with AN/ALQ-211 a source of information to update the electronic order of battle. Real-time information shared on the digital battlefield can find enemy forces by their air defense signatures. An armored column protected by tactical air defense radar, for example, betrays its presence to SIRFC and the whole combined-arms team.

Passive warning is only one part of aircraft survivability, however. Active radio frequency countermeasures are also essential. Threat radar can remain inactive—that is, nonradiating—until a helicopter is within lethal range of the weapons system. Radio frequency countermeasures jam such pop-up threats. In addition, helicopters fly ing over water or the desert cannot hide from air defenses in defilade. Even with high-quality situational awareness, aircrews making a combat rescue or attacking a high-value target may have no sound route to evade radar threats. Active jamming confounds radar threats when there is no place to hide, and SIRFC is the only current integrated suite of passive warning receivers and active jammers packaged for helicopters.

To support a two-level maintenance scheme, SIRFC hardware is packaged in 17 standard electronic module-sized E (SEM-E) modules readily replaced in the field. The flexible, open architecture of the AN/ALQ-211 also enables adding or changing modules with varying missions and threats.

SEM-E modules also make the ALQ-211 compatible with other platforms. The mission equipment bays of the RAH-66 Comanche, for example, are configured for SEM-E modules and can accommodate SIRFC functions in their racks. Other aircraft will be able to use transmitter modules tailored to their radar cross sections.


Frank Colucci is an aerospace communications consultant.