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AN/APQ-181 Radar System

The world's stealthiest radar, the APQ-181 is a low probability of intercept, all-weather system that enables the B-2 Spirit stealth bomber to penetrate the most sophisticated air defenses. The AN/APQ-181 radar system enables the unique combination of stealth, range, payload, and precision weapons delivery capabilities of the U.S. Air Force B-2 Spirit stealth bomber. The radar currently employs a passive, two-dimensionally scanned antenna, with a mode suite that provides detection and precision engagement of time-critical targets.

While its AN/APQ-181 attack radar possesses a maritime search mode, the demise of the AGM-137 TSSAM denied the B-2A a suitable weapon for anti-shipping strike. While in theory it could be adapted to deliver naval mines in the manner performed by the B-52G/H and the B-1B, ensuring accurate deliveries requires a low altitude drop with concomitant potential for exposure, and a reduction in combat radius. The same difficulties would also arise with carrying the AGM-84 Harpoon, which would also require a weapon specific interface.

The AN/APQ-181, designed specifically for the Northrop Grumman B-2 Spirit stealth bomber, is a multimode radar system operating in the Ku-band (12.5 to 18 GHz). The radar features 21 separate modes for terrain-following and terrain-avoidance, navigation system updates, target search, location, identification and acquisition and weapon delivery. The APQ-181 is a completely redundant modular system which employs two electronically scanned antennas, sophisticated software modes and advanced Low Probability of Intercept (LPI) techniques that match the aircraft's overall stealth qualities.

The LPI approach used is understood to be a collection of individually effective design and operating techniques that, when integrated, greatly diminish the effectiveness of radio frequency emitter location sensor systems. LPI techniques include unique performance features built-in to the hardware and operated under radar mode control.

To meet reliability specifications and provide for operational redundancy, the APQ-181 provides two separate radar sets, each consisting of five Line-Replaceable Units (LRUs): antenna, transmitter, Radar Signal Processor (RSP), Radar Data Processor (RDP) and receiver/exciter, with all but the antennas able to function for either or both radar units. These LRUs weigh 955 kg and have a volume of 1.485 m3.The LRUs are installed in the aircraft in three zones. Each 260 kg antenna is mounted behind a large radome some 8 ft outboard of the aircraft centreline, just below the flying wing's leading edges. Antenna locations are marked by large, slightly darker, rectangular patches visible on the underside of the aircraft.

In 1991, the B-2 team (including Hughes, now Raytheon) was awarded the Collier Trophy, widely considered the most prestigious U.S. aviation award. The award was in recognition of the "design, development, production, and flight testing of the B-2 aircraft, which has contributed significantly to America's enduring leadership in aerospace and the country's future national security."

In 2002, Raytheon was awarded a contract to develop a new, Active Electronically Scanned Array (AESA) version of the APQ-181. This upgrade will improve system reliability, and will also eliminate potential conflicts in frequency usage between the B-2 and commercial satellite systems that also use the J band. The radar for the Northrop Grumman B-2 stealth bomber was likely to cause "significant damage" to commercial satellites, if the radar remained in its originally designed frequency range by the time the television and motion picture industries start using that frequency around FY07.

The B-2 System Program Office at Wright-Patterson, AFB, OH, negotiated with Raytheon and Northrop Grumman on a contract potentially worth $1.1 billion and $1.35 billion to modify the Raytheon AN/APQ-181 radar so that it does not conflict with commercial satellites. The Defense Information System Agency's (DISA) Joint Spectrum Center (JSC) completed a study which determined that the B-2 radar would likely cause significant damage to these satellites if operated in its current frequency range after FY07 and that the DoD would be liable for those damages under U.S. law and international treaty.

Since AN/APQ-181 would not be the primary user in the operational frequency band, the system would have to operate on a not-to-interfere basis, which could affect training and operations in the US. If the radar operations disrupted satellite transmissions or damagee commercial communications satellites, the U.S. Air Force likely would be liable.

Transitioned operations from J-band into Ku-band to reduce the effects of non-radar emitter interference. According to Raytheon, the U.S. Air Force selected the Active Electronically Scanned Array (AESA) option for its inherent reliability (largely because of its lack of moving parts) and also as an attempt at "future proofing" the system, which has substantial growth potential for future upgrades. The AESA design will also resolve a Ku-band spectrum compliance issue with the existing radar's operating frequency.

The B2 program began as a SCI program and not many in the spectrum community knew the specifics of its radar let alone the frequency bands it operated in. What they could have done differently was to ensure that the spectrum community was aware of its radar through appropriate channels to make sure its radar spectrum use was known, considered and protected. This demonstrated a lack of senior level government program office and contractor education and awareness of spectrum requirements. It also shows a lack of enforcement and adherence to the acquisition process by the milestone decision authority during the review process.

As part of the radar modernization program (RMP), Raytheon has designed, built and delivered an active array, based upon an advanced T/R module design offering improved performance and reliability. The RMP is currently in flight testing of the new antenna and will enter production in summer 2007, which will replace all the electronically scanned arrays currently in the fleet with the new active arrays. The new Active Electronically Scanned Array (AESA) provides the stepping stone to additional future radar enhancements.

Raytheon's active electronically scanned array (AESA) radar systems with groundbreaking performance and tactical advantages are the cornerstone of current and future aircraft. Throughout the past 35 years, Raytheon has remained at the forefront of radar revolution, AESA continues on in that tradition. After designing and building the world's first operational AESA fighter radar-the APG-63(V)2 for the F-15C, first fielded in 2000, Raytheon has continuously developed and refined its AESA technology. The culmination of this journey is the new F-15 and F/A-18 radars-the world's only and most advanced air-to-air, air-to-surface multimode AESA radars in production.

Providing incomparable detection, targeting, tracking, and self-protection capabilities, the new AESA systems are yet another hallmark in our evolving development of innovative radars in support of the men and women on the battlefield. The Raytheon AESA radar family may be tailored to fit any platform's size, space and requirements. When only the best and proven operational AESA capability and mission support is required, no one comes close to Raytheon's commitment to servicemen and -women worldwide. When a service member's life is on the line, only Raytheon's advanced AESA radars give pilots the edge in today's challenging scenarios. And it further enables their ability to operate in the increasingly net-centric battlespace of the future.

Raytheon's AESA radar systems have the ability to link fighters, bombers and surveillance aircraft, service members in the air and on the ground, integrate data from unmanned aerial vehicles and nontraditional ISR sources to provide the right information at the right time for the right mission. AESA radars are fielded on a variety of platforms, including the F-15 and F/A-18E/F, the B-2, and the ASTOR surveillance and reconnaissance aircraft. Adaptable to any platform, Raytheon's proven AESA technology gives pilots and aircrews the ultimate operational edge.



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