Old And New Sensors Drive for High Resolution

This article first appeared in Aviation Week & Space Technology.

Airborne intelligence, surveillance and reconnaissance (ISR) sensors continue to change rapidly, with much of that driven by operator needs. In particular, the level of detail required to track a suspect individual, identify a vehicle or find traces of a planted improvised explosive device (IED) is much finer than that needed to track a formation of tanks.

Airborne radar has made great advances in recent years, but faces ultimate limits imposed by the laws of physics. As in other areas, some developers are responding by looking higher up the frequency spectrum. Lidar is laser-based radar, which uses optical frequencies; since wavelength is a major factor in determining resolution, Lidar systems can achieve higher resolution than radar. Short pulses and good resolution mean that Lidar can map objects in three dimensions, and the combination gives Lidar a decisive edge in airborne imaging.

However, despite the technology's utility in ISR, Lidar sensor development in many areas is being driven by concerns for helicopter safety. Accidents due to loss of visibility have caused more helicopter losses in Iraq and Afghanistan than enemy fire. This has driven the development of the Air Force Research Laboratory's Multifunction Laser Radar System, which commenced in 2008. The system is intended to give timely alerts about power lines, poles and guy wires. It also provides an accurate assessment of a landing zone even in conditions of complete “brownout,” where visibility is lost due to dust. The Lidar can see in darkness and bad weather, and its imagery gives the pilot a stable reference to avoid drifting.

The 50-lb. Multifunction Lidar is being developed by H.N. Burns Engineering Corp. Mounted on a gimbal on the nose of the helicopter, it uses an infrared fiber laser and has a claimed accuracy of 1 cm (0.4 in.). It is integrated with the Brown-Out Symbology System to provide an easily comprehensible visual display of a landing zone in false color. The ground is shown in green and hazards are colored according to height. Boulders and other objects projecting 18 in. from the ground that could affect landing gear are shown as yellow, and objects 6 ft. or more from the ground which are a rotor hazard are red.

In an initial test in 2010, an EH-60 Blackhawk fitted with the Multifunction Lidar achieved a 77% landing rate in complete brownout conditions, with the other attempted landings ending in a safe go-around. The system is now being refined to work better in a cluttered landing area, for follow-on testing in 2013.

Lidar's high resolution may also provide new identification capabilities in ISR. Bridger Photonics is collaborating with Montana State University to design a Multimode Lidar for the Air Force, giving the F-35 the ability to identify targets at long range using a frequency-modulated, continuous-wave Lidar. The system is based on 3-D feature-specific imaging.

This technique, also known as compressive sensing, means it does not produce a camera-like picture but has software which pulls out the key signatures that identify a target, in particular the patterns of vibration associated with an aircraft. The developers believe it should be possible to provide 6-in. resolution at 20 km (12.5 mi.) in all visibility conditions. The design is due to be completed this year, and if validated should lead to low-cost Lidar sensors built using existing components.

Lidar vibrometry may also have potential for identifying various other targets, including ground vehicles, and even locating tunnels.

Lidar can be developed using solid-state electronics, with all the benefits this brings in miniaturization and lower costs. Radar systems can be shrunk to a couple of pounds, but future Lidar may be smaller. Vescent Photonics is working on a matchbox-sized infrared (IR) micro-Lidar. Its key innovation is to replace the mechanical beam-scanning system with an electronic one with no moving parts. When a ray of light passes through a material, the amount by which the beam is bent depends on the index of refraction of the material. The index of refraction of some liquid crystals can be changed by applying an electric current. Vescent's beam director has a waveguide with liquid crystal cladding which shifts simply by changing the applied voltage. By using the right materials and design, the device can scan an 80-deg. field of view in less than a millisecond.

The micro-Lidar combines electronic scanning with a type of laser diode known as a vertical cavity surface-emitting laser. These have limited power but can be mass-produced at low cost, and being solid-state can be integrated into a chip-scale system. Vescent's prototype micro-Lidar for the Air Armament Center should be completed this year. It is part of a larger Air Force program which aims to develop a range of compact Lidar systems, some costing less than $500 per unit and with a range of at least 500 meters (1,640 ft.).

This type of Lidar could be carried by even the smallest UAVs, providing precise 3-D imaging for collision and hazard avoidance, terrain following, landing, and target detection and tracking. The low cost and compact size would also make it suitable for new guided munitions with sophisticated target identification.

Lidar technology is still relatively immature compared to radar. Although it already provides inch-perfect topographical mapping in Afghanistan and elsewhere, there are few tactical systems. In June USAF awarded a $32 million Lidar contract to Science Application International Corp., including basic research into sources and sensors. At present it is being developed for niche applications, but these are likely to widen as the technology evolves. The pace of development suggests that Lidar's potential is only beginning to emerge.

The ISR world is also seeing the growing use of compact, relatively short-range but very-high-resolution radar. As one Israeli engineer pointed out earlier this year, radar used to be regarded as a substitute for visible light or IR sensors in bad weather—something to be used if necessary, but a poor stand-in due to its low resolution. What is now becoming appreciated is the ability of radar data to augment the visual picture, literally adding a third dimension when geo-registered and fused with IR or visual imagery. Radar is naturally good at detecting moving targets, and human-made objects—even camouflaged—often have a distinctive radar signature.

The U.S. has recently released some new radar technologies for export and announced contracts for others. Lockheed Martin has just been cleared to sell the AN/APY-12 X-band synthetic aperture radar (SAR) to a number of new customers, including Taiwan, Italy, Sweden and South Korea. A product of the Phoenix-based Lockheed Martin division that developed the first SARs in the 1950s (then Goodyear, and later Loral), the APY-12 was developed for the U.S. Army's RC-7B Airborne Reconnaissance Low surveillance aircraft and Japanese F-15 fighters carry a podded version.

APY-12 is claimed to produce “photographic quality” images, while tracking surface vehicles, taxiing aircraft and hovering helicopters in ground moving-target indication (GMTI) mode. It also has a wide-area GMTI mode and improved geolocation capability that makes it possible to search a large area for moving targets and overlay that data on a map.

Another goal for radar developers is “dismount detection”—extending GMTI to detect moving people. Raytheon was awarded a contract in July to produce four pod-mounted radar systems for the USAF, designed to be carried by the MQ-9 Reaper unmanned aerial vehicle (UAV) and intended for dismount detection.

A third relatively new U.S.-made radar is the Northrop Grumman ASQ-236, a pod-mounted ISR radar that started development in the late 1990s and is now in service on Air Force F-15Es. Information on what the 1,000-lb. pod actually does is classified, but it is identified as a Ku-band system, with an active, electronically scanned array (AESA) antenna, that “provides detailed maps for surveillance, coordinate generation and bomb impact assessment purposes.” It is the only known U.S. radar that uses a rotating “repositioner” like that built into Selex Galileo fighter radars.

One clue may be found in the USAF's comment that the ASQ-236 “leverag(es) the technology development associated with the F-22 Raptor.” The Air Force and U.S. industry have explored the use of ultra-high-range resolution techniques for target classification and identification, and these would likely be used on the F-22 radar. Applied to a Ku-band radar, the result would be extremely high resolution in range, making it possible to detect such things as disturbed earth due to IED placement.

With all this technology, it is interesting to know that one of the USAF's most valuable sensors still uses wet film—2 mil. of it per sortie. The Lockheed U-2 is at least the third carrier for the KA-80 Optical Bar Camera (OBC), which was designed in the 1960s by Itek (now part of Goodrich). Its purpose was to acquire 1-ft.-resolution imagery of a 1,700 X 40-mi. swath of China, on a single overflight by the Compass Arrow stealth UAV. Today, there is no known in-service digital sensor that does exactly what the OBC does.

Credit: U.S. Air Force

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