Vanguard Magazine

vital flight and target tracking information to the pilot while allowing him to retain tactical SA by helping to keep his head out of the cockpit. The major limitation of the HUD was its limited field of view. The hel- met mounted display was a logical follow on to the HUD because it could duplicate most, if not all the HUD's functions but over a much larger field of regard. Helmet-mounted displays The use of a helmet-mounted display helps the pilot to designate a target outside the weapons field-of-view and pull lead on to the target or programme the missile to turn onto the target after launch. Fur- thermore, by interfacing the HMD with radar and JTIDS, steering information was possible via the use of cuing arrows. These enhancements were expected to al- low faster acquisition at greater ranges and faster weapons lock-on. Perhaps the best helmet-mounted-display application for improving situational awareness involved using the device to slave sensors, such as the radar, IR systems or a missile seeker, onto targets that have been acquired. By the early 1990s, ten companies had devel- oped prototypes of functioning helmet- mounted displays. Helmet-mounted-display optics are very similar to those of HUDs and perform two functions. First, they provide the pilot with an unobstructed view of the outside world and the cockpit. Second, they su- perimpose typical HUD symbology over the pilot's view. To achieve the dual dis- play the optics are designed so that they reflect or refract a very narrow waveband of light generated by the helmets' CRT, while allowing all other wavelengths to pass through. The display optics technolo- gies available are reflective and refractive optical sub-systems coupled with diffrac- tive combiners. To achieve reduced workload brought about by faster visual acquisition and weapon lock-on, determination of an ac- curate head position remained paramount. Virtual cockpit criterion for head position sensing systems was 1 milliradian. One de- gree is equal to 17.54 milliradians there- fore a required accuracy of one milliradian is equal to .057 degrees. Manufacturers therefore investigated four head position sensing systems: electro-magnetic, electro- optical, infra-red and acoustic. The electro-magnetic head position sensing system involved a small transmit- ter in the top of the canopy which gener- ated a three-axis magnetic field. The rela- tive strengths of the magnetic field were picked up by sensors on the helmet and processed by the helmet tracker. The ac- 34 APRIL/MAY 2021 www.vanguardcanada.com TECHNOLOGY maintained between symbols. As well, the use of coloured symbology would allow more rapid display interpretation when compared with traditional monochrome displays. The solution was envisaged as a large-area, high-resolution, three-dimen- sional, colour display. Cockpit Displays Cockpit displays were initially of the electro-mechanical vernier type. Increas- ingly sophisticated sensors and systems re- quired improved displays. With the advent of search-and-track radars, radar warning receivers, and forward-looking infra-red (FLIR) systems, each with their own dedi- cated CRT display, the cockpit panel be- came very cluttered. The multi-function CRT provided display improvements over the original dedicated sensor display. The most popular CRT used for MFDs was the shadow-mask CRT. The shadow mask CRT used three separate electron guns, a single vacuum tube and a phosphor coat- ing consisting of a series of arrangements of phosphor colour dots (usually red, green and blue). As the size of the display was increased, the angles at which the beams transit the shadow mask became greater and more electrons ended up being absorbed than was desirable. This was especially critical in a high ambient light environment since the shadow mask absorption reduced the brightness of the picture, impacting direct- ly on the contrast of the display and overall readability. Therefore, flat panel displays grew ever more popular. These display technologies included: thin film electro- luminescent, plasma display panels, light emitting diodes, and liquid crystal displays. The target technology for supercockpits was the active matrix liquid crystal display (AMLCD). Early development work on the Hor- net 2000 included consideration to equip the aircraft with a 10x10 inch, centrally mounted, tactical situation CRT display, two multi-function, colour CRT displays measuring 6x6 inches, and a fuel and en- gine situation display (either an LED or plasma display). The tactical situation dis- plays also incorporated a full digital mov- ing map display. The YF-22 Advanced Tactical Fighter and the European Fighter Aircraft were expected to incorporate simi- lar improvements in head-down display technologies. The heads-up display (HUD) provided Copyright Saab AB. Photo: Saab AB

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