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 The head-up display (HUD) is a collimated, transparent display medium upon which graphical information, or superimposed symbology, can be presented. Since it is transparent, the main feature of the HUD is that it is located above the instrument panel, allowing the pilot to simultaneously view the out-the-window scene and the superimposed symbology, without refocusing the eyes or making large eye-scan movements. HUDs have been shown to be a superior presentation method for flight path symbology over that of traditional flight director displays (Boucek, Pfaff & Smith, 1983).
Fixed-Location Symbology - The Display Location Effect
Fixed-location HUD symbology appears to lead to attentional tunneling, which reduces the pilot's ability to maintain awareness of instrument information and information in the far visual scene (Foyle, Sanford & McCann, 1991). With the conventional head-down configuration, attentional tunneling may be disrupted by the eye and head movements necessary to scan back and forth between the panel display and the far visual scene, so joint awareness is improved (Weintraub, Haines & Randle, 1984; Sanford, Foyle, McCann, & Jordan, 1993).
 In our laboratory facility, we have developed an experimental testbed to explore various possibilities for reducing or eliminating attentional tunneling. The testbed consists of a PC-generated graphic representation of the outside visual scene, on which HUD imagery is superimposed. A flight simulation task is used to evaluate the influence of fixed superimposed symbology location on information integration. Subject pilots are simultaneously required to follow a ground track and to maintain an altitude of 100 ft. Altitude information is available via the out-the-window (terrain) visual cues only, or by a superimposed digital altitude indicator (i.e., simulated HUD symbology). The HUD symbology can be presented at various distances from flight-relevant terrain path information. Root mean squared error (RMSE) altitude and RMSE heading are measured.
The results of these simulation studies indicate that subject pilots were not able to attend simultaneously to both the fixed-location HUD information and the outside world terrain when directly superimposed over the center of flight path. Directly superimposed placement did not result in information integration, and in fact, encouraged attentional fixation. Simultaneous processing of both the HUD information and the outside world information was best in conditions that encouraged attentional or visual scanning, specifically when the symbology was at least 8 degrees away from the center of the flight path.
"Scene-linked" Head-Up Display (HUD) symbology.
 "Scene-linked" symbols are drawn, and move, as virtual objects in the out-the-window scene. As the aircraft moves through the world, the scene-linked symbols undergo the same visual transformations as real objects. There are no differential motion cues to cause the visual system to interpret the virtual symbols as separate from the world. In the absence of these cues, attentional tunneling should be prevented, enhancing the ability to process scene-linked HUD symbology in parallel with real-world information
Our research has found that when the HUD altitude indicator is converted into a "scene-linked" or virtual indicator, attention tunneling no longer occurs, but out-the-window information processing actually improves. Studies indicate two findings, extending the results and conditions of scene-linked HUD symbology. The first part-task simulation showed that the placement of virtual indicators do not require any special placement in the environment in order to facilitate efficient simultaneous processing of cockpit displays and the out-the-window environment. A second simulation optimized scene-linked route symbology on the HUD, an inherently limited field-of-view (FOV) display device. Results indicated that scene-linked symbology showing the route edges is insufficient, and that additional route markers must be added that are offset from the route effectively cueing the route edges that are not visible due to the limited HUD field of view.
Examples of Scene-Linked Symbology:
- Scene-Enhancements (of real objects)
- Scene-Augmentations(virtual objects)
- Virtual Instruments (Projecting virtual instrument gauges in world)
Go to: Head-Up Display and Attention Publications Available for Download
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