Active-array radar development is picking up pace, as the promises of unheard-of reliability and undreamt-of capability begin to become reality

There is a generation change under way in airborne radars, with traditional mechanically scanned antennas giving way to active electronically scanned arrays (AESAs). Active-array radars offer increased capability, reduced weight, lower cost and higher reliability. Near-simultaneous operation in different modes is only one of the advantages attracting more aircraft designers and operators to AESAs.

Raytheon has just begun testing the most advanced AESA yet, the APG-79 multimode radar for the Boeing F/A-18E/F Super Hornet. Over the next year, Northrop Grumman will begin flight testing its APG-80 Agile Beam Radar for the Lockheed Martin F-16 Block 60, and ground testing its MESA active-array radar for Boeing's 737-based airborne early warning and control (AEW&C) aircraft. Working together, the two companies are developing a family of airborne ground-surveillance AESAs under the Multi-Platform Radar Technology Insertion Programme (MP-RTIP).

The US Air Force, meanwhile, plans to retrofit more Boeing F-15s with an AESA upgrade of Raytheon's APG-63(V)1 radar, and Northrop Grumman's active-array APG-77 for the Lockheed Martin/Boeing F/A-22 is to be updated with AESA technology under development for the Lockheed Martin F-35 Joint Strike Fighter. Raytheon is under contract to upgrade the Northrop Grumman B-2's radar with an active array.

In an AESA, a fixed array of solid-state transmit/receive (T/R) modules replaces the mechanically steered antenna. The first benefit is a reduction in weight by eliminating the antenna drive system. Second is the elimination of signal losses incurred by ducting outgoing radar energy via wave-guides from the single transmitter to the rows of beam-forming slots in the flat-plate antenna, and incoming energy from the antenna back to the single receiver.

The radar beam is formed and steered electronically, by controlling the phase of each of the hundreds of T/R modules making up the array. A beam can be formed and pointed in a fraction of a second, allowing near-simultaneous operation of different radar modes, whether air-to-air or air-to-ground. And because the number of times the radar can illuminate a target is no longer determined by its antenna scan rate, the energy on target can be increased and both range and resolution improved.

Module failure

Perhaps outweighing even the benefits of "inertialess" beam steering is the fact that up to 10% of the modules in an array can fail before the radar's performance degrades below acceptable levels. Based on initial operational experience with AESAs, says Raytheon, this suggests the aircraft's airframe will wear out before the radome need ever be removed for array repairs.

Radar engineers talk in terms of generations of active arrays, depending on the technology in the T/R modules. First-generation AESAs are made up of relatively large single modules, called bricks. The latest-generation arrays have smaller, lighter, cheaper T/R modules, packaged in groups, or tiles, that share power supplies and electronics.

The APG-79 AESA uses sixth-generation T/R modules, says programme manager Tom Kennedy. "We have gone from bricks to tiles to even more affordable, compact and lighter sixth-generation tile modules." Packaging multiple T/R modules into one tile reduces the number of chips and results in a lightweight array. "This AESA is a quarter of the weight of an equivalent [earlier generation] active array," he says.

Kennedy says the APG-79 is Raytheon's "first totally new fire-control radar in 30 years". In addition to the AESA, the radar has a new back end, consisting of the receiver/exciter and common integrated sensor processor. The receiver/exciter generates the waveforms that drive the transmit modules, and converts the receive module signals from analogue to digital for the sensor processor, which performs both signal and data processing using multiple parallel PowerPCs linked by a high-speed, 1GHz, Fiber Channel databus. "It's probably 10 times more capable than current fire-control radars," says US Navy APG-79 programme manager Capt Dave Dunaway.

Testing has begun, with an APG-79 engineering development model in Raytheon's El Segundo "roof house" radar integration laboratory producing real-beam images of Catalina Island, 50km (30 miles) away off the California coast. "We have proved in the laboratory that the software can command the complete radar," says Kennedy. "It can generate the waveforms and code the pulses, which are amplified by the antenna transmit modules. It can bring the signal back via the receive modules to the receiver, convert it from radio frequency to digital, put it through the processor and produce a map. We have proved the brand-new radar is functional."

Flight testing will begin in June 2003 in an F/A-18F. Initial operational capability on the Block 2 Super Hornet is scheduled for September 2006. The US Navy plans to forward fit and retrofit 411 of its 460 planned F/A-18E/Fs with the APG-79, and the multi-function AESA is a key part of the proposed EA-18G electronic-attack variant, 90 of which are planned if funding can be found.

Technology from the APG-79 will also find its way into the F-15. The US Air Force has 18 F-15Cs equipped with the APG-63(V)2, a version of Raytheon's improved APG-63(V)1 fitted with a first-generation "brick" AESA to gain operational experience with active arrays. The USAF now plans to equip additional F-15Cs, and Raytheon is offering to upgrade the APG-63(V)1 with a new "tile" AESA, to produce the APG-63(V)3. A scaled-up version of the APG-79 is being offered to upgrade F-15Es. An active-array upgrade for the F/A-18C/D's Raytheon APG-73 radar is also planned, says Kennedy.

AESAs are also finding their way on to other platforms. Northrop Grumman has rolled out the first MESA radar antenna for the 737 AEW&C and, after ground testing, will deliver the active array to Boeing next May for installation on the aircraft, with flight testing to begin in early 2004.

Optimum coverage

The MESA is housed in a 10.7m (35ft)-long "top hat" radome and is the first electronically scanned AEW radar capable of simultaneous 360° coverage, says Bill Adams, Northrop Grumman vice-president, airborne surveillance systems. There are three steerable arrays, two side-emitting and one fore/aft-looking, and 288 large book-size T/R modules in 48 radiating columns constructed of composites for lightness.

Adams says the top hat design was a trade-off between aerodynamics and cost. Conformal arrays distributed around the airframe generate less drag, but require duplication of T/R modules. Radar gain with the top antenna is not as good as with side arrays, but the housing requires only 22 retaining bolts and weighs just 2,300kg (5,000lb) compared with 11,000-18,000kg for a rotodome-type installation.

The top hat's design also lends itself to growth, and Northrop Grumman sees the MESA as a building block for the US Air Force's planned Multi-sensor Command and Control Aircraft (MC2A) Spiral 2 replacement for the Boeing E-3 Airborne Warning and Control System. Pre-planned improvements include a higher-power module, larger aperture and addition of ground moving-target indicator and synthetic-aperture radar (SAR) modes.

AESAs can solve problems as well as increase performance, and the active-array upgrade of the B-2's Raytheon APQ-181 will move the Ku-band radar's centre frequency to overcome interference, as well as improve range and resolution. The APQ-181 has two redundant transmitter/receiver/processor strings cross-coupled to two passive electronically scanned arrays - scanned electronically in elevation and electromechanically in azimuth.

"Twenty years ago that was all that was available," says Andy Johnson, Northrop Grumman radar integrated product team leader. "We have electromechanical devices going off all the time, creating wear and tear and maintenance requirements." The upgrade will drop in two AESAs, and reduce array weight. The processors and the radar's 21 air-to-ground and air-to-air modes will be unchanged, although growth plans include high-resolution SAR imagery and increased range.

The component advanced development phase will run to August 2004, and be followed by system development and demonstration. The first B-2 flight with the new radar is expected in late 2005. Plans call for an initial six aircraft to be upgraded, with the remaining 14 B-2s to be retrofitted by 2010 under a $900 million programme.

Source: Flight International