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Originally, the equipment itself was used for training. This was a waste of time on expensive equipment and in many cases led to damage. Nowadays simulation is so realistic that its fidelity is very close to the real equipment.

In the past, some front-line equipment was even written-off in training. This was accepted as a consequence of realistic training, but with hindsight this is surely quite unacceptable where there are alternatives. Mock-ups of the equipment were then used so that familiarisation with weapons and crew positions could be carried out without using the equipment itself. A Mock-up can be a simple representation of the equipment or a crew station with dummy switches, instruments and other controls so that checks can be carried out and repeated until they are learnt. More advanced mock-ups use instruments that work and incorporate correct responses to the use of switches and other controls. The word “Simulator” started to be used, which rolls off the tongue rather better than “Mock-up”.

Early Simulators

Dome screens such as this one of the Saab Full Mission GRIPEN fighter simulator have now become standard. (Photo: Saab)

However, the majority of these early “simulators” had no outside-world visual systems because technology did not exist for computer-based imagery of large areas. What did exist were so-called “Model Boards”. These were miniature representations of small areas of the real world that could be shown on screens at crew stations using small TV cameras that tracked across the model board in the same way as the real vehicle. Model boards work quite well for small areas, but are not suitable for large areas or where enemy action is to be trained. For aircraft, some model boards became large, indeed, very large. An example in Germany was a flight simulator model-board that extended from the base airfield all the way to potential targets several hundreds of kilometres to the East. In Army training, an equivalent to a model board is the “sand table” that can be used to model small areas of a range or battlefield, and some are many square metres in area.

XPI Simulation Imagery from Tank by Day (Photo: XPI) and with NVGs (Photo: XPI)

Meanwhile, aircraft cockpit trainers had existed since the 1930s, the most famous being the “Link Trainer” developed at Binghamton, NY, by Edwin Link. The Link Trainer was first purchased in 1934 by the US Army Air Force (USAAF), followed by the Imperial Japanese Navy and the UK Royal Air Force, then by many other air arms. The Link Trainer had a fully-enclosed cockpit with flight instruments that responded correctly to the pilot’s control movements. It also had real motion using pneumatic bellows for pitch and roll and an electric motor for yaw. Over 20,000 Link Trainers were built, later models being in service with some Air Forces until the early 1970s. They were fine for teaching basic instrument flying techniques to one pilot at a time, but in the 1950s, when complex jet aircraft came along, such as various types of US Bombers, the UK V-bombers, and civil jets like the Boeing 707 and DC-8, closer models of the real cockpit were needed. Early aircraft simulators had realistic cockpits and some were mounted on hydraulic-driven motion platforms. Although model-board visuals were available, they had limitations in fidelity and area. It was only when digital Image Generation technology became viable during the 1970s and 80s that simulator visual systems began to be effective training tools. Simulator imagery has to be created and stored in three-dimensions rather than just a simple flat image, so that the eye-point used in the simulation can be from any direction. Such a 3D data base needs very large storage compared to a 2D picture. Original simulator digital images were crude, limited to representations of large objects such as runways. However, advances in computer technology soon allowed 3D imagery to be improved and, equally important, to be stored. Another advance in image fidelity was the addition of “texture” within otherwise plain polygons. Texture within polygons started as simple contrast patterns but soon developed so that small images and even photographic detail could be applied to polygons in an overall visual scene. For instance, a skyscraper can be modelled using a series of so-called “texture maps”, each map containing just a few windows so that when many maps are put together, the whole building is seen on-screen without the need to model every detail separately.

Turning to night imagery, this includes light intensifiers such as Night Vision Goggles (NVGs) working in the near Infra-Red at a wavelength of about 1 micron. There is also the more expensive passive Forward Looking Infra Red (FLIR) which senses thermal contrast within the scene and works on the blackest of nights. Both NVGs and FLIR are easy to simulate by adding monochrome colours and appropriate texture to each polygon in the visual database. In real NVGs, a monochrome green colour is often used, but whatever colour is in the real equipment, it is easy to simulate. In any case, the picture can be changed from white-hot to black-hot at the touch of a switch, and is straightforward to simulate.

Simulator Display Systems

Nautical Imagery from Wärtsila Voyage (Photo: Wärtsila)

Turning now to simulator display systems, projectors can be used to show imagery on screens of various shapes and sizes. These can be used either in free-standing displays for group briefings, or to provide imagery at a simulator crew station. Simulator display technology also includes edge-blending between channels, and Head-Mounted Displays (HMDs). There is also what is known as “Collimation”, the word being derived from “co-linear” which implies parallel lines. Parallel lines in optics imply infinity-focus, rather than a focus at the screen distance of direct projection systems in which the lines converge. The essence of a collimated display is that the subject looks into a mirror that has vertical curvature, rather than at a screen. The amount of vertical curvature determines the focal distance, which can be set to what is best for the simulation, rather than always at precisely infinity. Single-screen collimated units are sometimes known as Wide-Angle Collimated (WAC), and several “WAC windows” can be placed side-by-side for a larger horizontal or vertical view. However, where two crew sit side-by-side, some imagery seen by one crew member cannot be seen by the other, leading to “black holes” in the display. The solution is to use a large mirror of wide horizontal extent, typically of mylar coated with a reflective surface. Imagery seen in the mirror is created on an intermediate screen above the mirror. This imagery is reflected in the mirror, and it is the reflection in the mirror that is seen by the subject. Typically, three or five projectors can be used, the screen height and width matching that of the mirror in which the crew view the scene. This system was first marketed in 1982 by Rediffusion Simulation of Crawley, UK (now split between L-3 Commercial and Thales), under the name Wide-angle Infinity Display Equipment (WIDE), with an unrestricted across-the-cockpit view of 150 x 40 degrees. Many other companies now make what are now called Cross-Cockpit Collimated Displays (CCCD) with cover up to 240 x 60 degrees.

Fighters and Helicopters

A Rockwell-Collins F-35 JSF simulator (Photo: Rockwell-Collins)

The above field of view is adequate for simulators for airliners and military transport aircraft, but what about fighters and helicopters? For some helicopters, there is a relatively simple solution – use a CCCD system for the main display but add two WAC Widows below the display for the downward look needed to hover. However, simulators for Attack Helicopters and Fighter aircraft need almost unlimited external view. One solution is to mount the simulator cockpit inside a large dome, and project the outside-world scene on the inside of the dome using projectors either inside or outside the dome. Visual domes are normally several metres in diameter, inside which can be placed the simulator cockpit, a Forward Air Control (FAC) or other station to be trained in the simulator, even, in large visual domes, a platoon of soldiers with shoulder-mounted SAM systems. For single-seat aircraft, instead of a dome, an array of back-projected flat screens can also be used to surround the pilot with the outside-world visual scene.

A specific example is the unique dome-based visual system used in the Full Mission Simulators for the F-35 LIGHTNING II fighter, a nine-nation programme with three variants of the aircraft. These are the F35A Air Force version, the F-35B Short Takeoff and Vertical Landing (STOVL) version for the US Marines and UK Royal Navy, and the F35C, a larger aircraft for the US Navy. The F35B is in service with the UK Royal Navy on board its new QUEEN ELIZABETH aircraft carrier. The prime contractor for training simulators for the F-35 A, B and C models is Lockheed Martin Training, Logistics & Sustainment in Orlando (LM-TLS). These use the Rockwell Collins WHOLE-EARTH database and the EP-X Image Generator with a resolution of 4 megapixels per channel. For outside-world and target imagery, 25 Rockwell Collins Zorro projectors are used. The simulator dome design is by the UK branch of Rockwell Collins (the ex SEOS company), and is unique in that it is only 1.6 metres in radius rather than several metres like other simulator visual domes. The small size means that the pilot has to strap in to the seat outside the dome, the seat then being slowly motored into the dome, after which it is raised to the correct pilot’s eye-point. Will this small radius dome set a standard for the future? I think not, because the only reason for its uniquely small size is that, many years ago in the original US Air Force Specification for the F-35 Full Mission Simulators, it was stated that they had to fit inside existing buildings that had already been built for an earlier generation of F-16 simulators. It would have been better and less expensive to use a new simulator building that could accommodate a conventional and cheaper visual system that would use one of the many full-size simulator visual domes that are readily available from many manufacturers. An example is the F-35B simulator at the British Aerospace factory at Warton in the north west of England, used for Research and Development of the B model. This simulator has been used for optimising deck landing techniques, linked to another simulator at Warton of the FlyCo station in the aircraft carrier itself. I have had the privilege of flying the Warton F-35 simulator with its big visual and 6-axis electric motion. Take-off using the UK carrier’s “Ski-jump” and hovering back to the deck at the end of the sortie, was extremely realistic, enhanced by the real motion that enable cues in a simulator to closely resemble those in the aircraft itself. This enables “handling fidelity” that is not possible with a visual-only simulator, particularly under conditions of night, poor visibility, or in situations where quick control movements have to be made.

Head-Mounted Displays

Model board and travelling camera 1970 (Photo: via author), First generation digital imagery (Photo: via author), CAE Medallion MR e-series imagery – October 2018 (Photo: CAE)

Head-Mounted Display systems (HMD) can also be used instead of larger displays. However, weight on the head and display resolution are critical. Although HMDs can show stereoscopic imagery, two separate image channels are required that must be set up very precisely if “simulator sickness” is to be avoided after prolonged use. However, in many roles it is not essential to use two different images because so called “optical infinity” is about 9 metres (30 ft), beyond which distance in the real world each eye sees the same picture. HMDs are useful if a training system has to be particularly compact or portable, because they give a wide outside-world view without the need for screens and projectors. Examples of HMDs in service include the Link AHMD in US Army systems such as the trailer-mounted AVCATT and in Reconfigurable Collective Training Devices (RCTDs) that are part of the “Flight School XXI” programme. Other HMDs are used in simulators for vehicle convoys and for shoulder-mounted weapons.

Summary

Computer-generated imagery is now available with nearly real-world resolution, including images of terrain by day and night, plus different weather conditions and the addition of forces both enemy and friendly. Imagery can be displayed in many ways, from TV screens, forward- and back-projected displays on screens or in domes of various sizes, mirror-based collimated systems with a distant focus, and Head-Mounted Displays. Some wide-angle displays are compatible with mounting on 6-axis motion platforms, giving unprecedented realism where training requires fidelity of handling rather than just a visual scene, such as in simulators for fixed-wing and rotary-wing aircraft, and for drivers and crew of ground-based vehicles such as tanks and other Armoured Fighting Vehicles (AFVs). Overall, modern simulator visualisation systems are very capable, and are now essential in modern training.

William Carter