September 03, 2012
Devid Hambling London and Bill Sweetman Washington
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.