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Thursday, December 1, 2011

Keep One Eye Out

Tied to a monocular helmet display, the Distributed Aperture System (DAS) still promises Joint Strike Fighter pilots all-round situational awareness

By Frank Colucci

Stealthy, smart and sensor-rich, the F-35 Lightning II is the first tactical fighter with an all-round infrared visionics system that enables pilots to see through their own aircraft to track missile threats, opposing fighters and formation wingmen. The Northrop Grumman AN/AAQ-37 Distributed Aperture System (DAS) is flying on Lockheed Martin F-35 test aircraft AF-3, BF-4 and BF-5, and on two systems test beds. With its binocular Helmet Mounted Display System still in development, DAS will be integrated in 2012 with a monocular display compatible with night vision goggles. The passive sensor system still aims to provide the Joint Strike Fighter (JSF) with missile warnings for self-defense, however, “missile warning is really a very small part of the capability the DAS brings to the F-35,” said Phil Edwards, program manager for fifth-generation fighter improvements and derivatives at Northrop Grumman Targeting Systems Division in Baltimore. “We do think that once the system is onboard and we have young pilots in the iPad generation using it, other applications will be borne out of that.”

The DAS integrates six cooled, staring mid-wave infrared sensors on the F-35 topside nose and spine, cockpit sides and ventral fairing for full-time spherical coverage of the tactical environment. Infrared events are compared with a threat library, and the pre-processed DAS outputs go to the Lockheed Martin Integrated Core Processor (ICP) with precision angle-of-arrival information to cue the defensive suite, the Lockheed Martin electro-optical targeting system and Northrop Grumman’s AN/APG-81 active electronically scanned array (AESA) radar. The ICP generates DAS target symbology on the F-35 panoramic cockpit display and guidance cues for weapons.

“Adversary aircraft can’t sneak up behind the JSF without the aircraft and the pilot knowing it,” said Edwards. “It really is a ground-breaking technology.”

The broader utility of DAS was hinted at in June 2010 when the sensors on a BAC 111 test bed owned by Northrop Grumman tracked the first flight of the Falcon 9 commercial launch vehicle from Cape Canaveral. From 800 miles away, the test system followed the liquid-fueled rocket seamlessly across sensor boundaries, coasted through staging when target IR signature diminished and continued to track the second stage through its climb. A notional F-35 Scud Hunter or Wild Weasel could use the DAS to geolocate ballistic missile or surface-to-air missile launch points for its own attack or to cue networked forces.

This summer, the DAS demonstrated spherical situational awareness and target tracking capabilities at the Northern Edge exercise in Alaska. Tied to a surrogate helmet display aboard the Northrop Grumman test bed, the system saw nearby aircraft and terrain through poor visibility. The deployment demonstrated how an F-35 pilot in multi-fighter engagements could use DAS to track threat aircraft in an operational environment. “He can be free to maneuver; it keeps them in sight,” explained Edwards. “The DAS autonomously locks on and tracks those airborne targets. The JSF system will then use the information and decide how it presents the information to the pilot.” The infrared search and track function in DAS can potentially cue the seekers of AIM-9X infrared or AIM-120 radar missiles in the F-35 weapons bays.

To give the F-35 pilot an unobstructed view of the outside world, DAS video or derived symbology should be displayed on the head-tracking helmet mounted display system from Vision Systems International (VSI), in San Jose, Calif. ( Avionics, July 2010, page 20). The helmet mounted display system includes an electron bombarded active pixel sensor supplied by Intevac, of Santa Clara, Calif., as a standby night vision aid. However, a DAS visor display able to match the performance of the ITT AN/AVS-9 night vision goggles, now used by United States tactical jet pilots, is still in development. In October, Lockheed Martin chose BAE Systems in Rochester, U.K., to supply a night vision goggle helmet mounted display system for F-35 development. “Essentially, on the visor-projected helmet, the video source is not compatible with night vision goggles. To put the goggles on, you have to take that visor off,” said Paul Cooke, director, business development, defense avionics, at BAE Systems, in Johnson City, N.Y. In contrast, the BAE Q-Sight puts a holographic waveguide display close to one eye. “If you want it to be compatible with NVGs, you just clip them to the helmet and flip them down.” BAE has offered the joint program a follow-on binocular solution. VSI meanwhile continues to mature integrated night vision capability in the second-generation helmet mounted display system.

Stare, Stitch and Zoom

DAS technology for the joint-service, multinational F-35 started with the Navy Distributed Aperture IR Sensor (DAIRS) systems program in 1996. In 1999, Northrop Grumman patented algorithms to “stitch” staring cameras together in a seamless mosaic. The Navy funded the subsequent Multifunction Infrared Distributed Aperture System (MIDAS) advanced technology demonstration to integrate a helmet display and reduce DAIRS development risk for the Joint Strike Fighter. “The requirements for the F-35 and the requirements on the DAS system were extremely strenuous and very specific,” Edwards said. The JSF program wanted a DAS with high probability of threat detection, low false alarm rates and the ability to track multiple targets simultaneously. “That’s what allows DAS to track autonomously and allows the pilot to keep track of friendly aircraft and adversary threats in a dynamic environment. It’s really doing what two or three systems are doing on legacy aircraft plus capabilities no legacy aircraft has had before.”

MIDAS tests put four IR sensors on an Air Force F-16 and a Navy QF-4 to demonstrate the technology in a dynamic environment. Northrop Grumman subsequently integrated six DAS sensors on the BAC 111 test aircraft. The sixth sensor topside and aft on the F-35 is located in the BAC 111 cargo compartment. Testing the complete DAS with a blind sensor physically installed avoided software changes in full system development and qualification testing. “We didn’t have to provide different software for a five-sensor system,” said Edwards. “It thought the camera was there.” Exercise Northern Edge tested DAS with JSF Block 2 delivery software for the first time in an operational environment.

This year, Lockheed Martin integrated DAS with a representative F-35 cockpit display and helmet mounted display system on CATBird, the Cooperative Avionics test bed. The distinctive Boeing 737 has an extended nose, dorsal spine and ventral canoe, each with two DAS sensors. It enables Lockheed Martin engineers to test F-35 hardware and software in a dynamic environment.

As of October, the Distributed Aperture System has accumulated more than 245 flight hours on various platforms. At the Paris Air Show in June, Northrop Grumman showed imagery from an IR sensor with a 24-degree field of view. DAS sensors can be zoomed electronically within their field of view to let the F-35 pilot take a closer look at a selected target.

Second Sight

With the magnetically tracked binocular helmet mounted display system continuing development, Lockheed Martin intends to integrate DAS with an optically tracked monocular night vision goggle helmet mounted display. BAE is under contract to integrate its Q-Sight holographic display with a helmet shell similar to that worn by pilots of the Eurofighter Typhoon. “The helmet is capable of showing any video imagery that’s sent to it. It could be the DAS video or symbology,” said Cooke. “It was basically an internally developed and funded program that BAE’s been working on for awhile. It was selected for the U.K. Navy Lynx fleet and used with a thermal sight for the door gunner. We’ve continued development and qualification of the product making improvements to field of view, brightness, those types of things.”

The Royal Navy Q-Sight provides a 30-degree field of view. The F-35 version will cover 40 degrees and put JSF symbology or video about 25 mm before the wearer’s dominant eye. Night vision goggles can be flipped down just in front of the display to annotate goggle imagery with flight symbology. “The goggles are furnished by the customer,” said Cooke. “We don’t provide those as part of the solution; we’re just compatible with them.” The Q-Sight will give the first F-35 pilots a choice of DAS or annotated night vision goggle imagery at night. “I don’t think you’d ever want to mix DAS video and goggle video,” said Cooke. “It would be like watching two TV channels at once on a single screen.”

The night vision goggle helmet mounted display also substitutes the infrared LED head tracker of the BAE Striker helicopter crew helmet for the magnetic head tracker of the helmet mounted display system. “Because the JSF does not have a HUD, the accuracy requirements and low-latency requirements on the tracker are very severe,” said Cooke. “This one is very accurate, very low latency and very low risk.” The night vision goggle helmet mounted display with IR transmitter on helmet and receivers on the aircraft is unaffected by nearby metal, electrical noise and varying distance to the IR source. BAE has a Q-Sight hot bench in the U.K. and will deliver test helmets to Lockheed Martin in 2012.

Northrop Grumman has no upgrades planned for the F-35 DAS, but the company does see growth applications beyond the Joint Strike Fighter. “The sensor system is currently looking at any IR event it sees,” said Edwards. “We think it’s got some capability to detect rockets, mortars, RPGs, etc., and to provide warning to people on the aircraft and off that aircraft.” An all-round system able to track airborne targets may also give true sense-and-avoid capability to unmanned aircraft systems. “Much like radars were in the past (the DAS system), as it gets used and fielded, operators will have new sorts of applications we haven’t envisioned yet.”


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