Aircraft That See In The Dark

The RAE’s Nightbird programme has shown that a relatively low cost, passive system, enabling a fast ground attack aircraft to be flown at night or in poor visibility by the same techniques as those employed during daylight, can meet this requirement. Moreover, it could increase the operational effectiveness of the aircraft by as much as 40%.

A tactical advantage of the system is that, being passive, it is undetectable. A terrain following radar (TFR) like that fitted to the Tornado and F-111, is an active system and therefore detectable, which makes it unsuitable for many of the roles undertaken by a ground attack aircraft.

Initially, the team examined three technologies: low light television (LLTV), night vision goggles (NVG) and forward looking infrared (FUR).

Trials were carried out with an LLTV camera mounted in an aircraft’s nose. The camera was light limited and operated in the visible and near infrared wavebands. The image was presented to the pilot on a head-up display (HUD) with 1:1 magnification, deployed to be superimposed on and corresponding with the view of the outside world. Normal HUD symbology for flight information – height, speed, directional setting and so on – was overlaid on the image.

Flying became possible in light levels down to¼ moon. But as the field of view window was only some 20 degrees, it could be effective only over benign countryside and when gentle manoeuvres were made.

Night vision goggles (NVG) became available in the early 1980s. Again, they are similar to LLTV: they are in the visible band, have a unity magnification and are fitted with image intensifier tubes. In appearance they are much like a lightweight pair of binoculars fitted to the pilot’s helmet, so that he can look directly through them. They are self-contained and need no aircraft installation.

The equipment used in the Nightbird programme had a 40 degree field of view, centred on the pilot’s eyes. By turning his head he could see in any direction. If NVGs are used, cockpit lighting must be confined to the blue-green end of the spectrum to which NVGs are insensitive, otherwise they would be dazzled. Instruments are viewed by the pilot, looking down below the goggles, which stand away from the eyes.

NVGs are sensitive to the red and near infrared wavebands, that part of the spectrum from which most starlight and moonlight energy emanates and where the reflectivity of earth and vegetation is good. In fact the equipment can operate down to starlight conditions.

Results from NVG trials showed that they could provide a good night flying capability. Also, the fact that the pilot could move his head and look around was a significant advantage in manoeuvring. If the equipment did not weigh more than 650 to 700 grams, it could be accommodated without undue discomfort, including the effect of ‘G’ forces for the duration of a normal mission.

But the ability of NVGs in the target acquisition mode was limited to that of the naked eye: it was not possible to see through, as it were, the camouflage of a small target such as a tank. Like the LLTV, the NVGs on their own failed to meet the requirement. Unlike the other two technologies, the third option was not light-limited. This was the infrared sensor fitted to the nose of a Hunter T7 trainer aircraft for flight trials. In this case, the image is displayed in the same way as by the LLTV installation and is also matched to the 20 degrees of the HUD field of view, with the same superimposition of flight symbology.

Being in the visible wavebands, it detects energy radiated from what it sees, rather than light reflected from specific objects, as with the LLTV. The images produced are excellent and interpretation has proved as easy as for the LLTV.

FLIR operates in total darkness and can penetrate industrial haze and mist, although it is limited by thick fog and rain. Objects with a high infra-red signature, like tanks, are easily discernable by infrared sensors and are difficult to camouflage in the thermal band. The trials demonstrated that most navigational features show up well: rivers, roads, lakes, buildings and trees are clearly visible. Yet, as with the LLTV, the limitations on manoeuvring still remained.

None of the technologies separately met the requirements; the answer lay in a combination of FUR and NVG. By adopting this technique, the pilot can use NVG to provide wide manoeuvring capability when flying under cloud or in little or no moonlight, while FUR gives him good forward penetration through mist and haze. FUR also presents him with the explicit targeting information he needs to complete his mission.

This principle of combination has proved so successful in practice that the RAF has contracted GEC Avionics for a system to equip the Tornado and the Harrier GR Mk5. The United States Marine Corps Harrier AV-8Bs will be similarly equipped.

The team is now considering ways of expanding an aircraft’s mission capability still further. An aircraft of the future will carry, in digital form, a terrain description of the area over which it is flying. Extremely accurate navigation is possible by comparing the outputs of onboard sensors with this digital database.

Given the terrain database and accurate aircraft position, a perspective view of the scene ahead of the aircraft can be generated in much the same way as a computer generates images for simulators.

Moreover, a simplified display without excessive clutter, showing ridge lines and major obstructions overlaid on the FUR image, could convey essential information necessary for safer low flying without TFR at night or in poor visibility. Other information, target zones, hazardous areas and so on could also be fed into the display as part of a mission planning system.

One of the RAE’s Hunters in now being equipped to provide a flight tool for research into these techniques. A Buccaneer is also to be used for further development of electro-optic sensors and the integration of these and other sub-systems using a digital data bus.