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No main effects were found for visual scene level-of-detail manipulations, but there was a statistically significant interaction between level-of-detail and initial altitude (F(4,16) = 3.451, p = 0.032). Figure 66 illustrates this interaction. For the ascents, the final altitude more closely matches the desired doubled altitude as the level-of-detail becomes more constant. Only for the 21-ft initial-altitude descents did the high constancy level-of-detail database not result in the best repositioning performance. So, it appears useful to attempt to have the visual system mimic the level-of-detail changes, as one would experience in the real world.
Altitude-Rate Control Task. The performance measurement for the altitude-rate control task was mean absolute vertical rate during the climb or descent. Only data between vehicle altitudes of 10 and 40 ft were used, thus eliminating the initiation of either the climb or the descent.
Statistically, there were significant effects for the presence of vertical platform motion (F(1,4) = 78.846, p = 0.001) and for movement direction (F(1,4) = 14.806, p = 0.018). There was also a significant interaction between these two factors (F(1,4) = 12.379, p = 0.024). Figure 67 shows these effects. Note that the vertical rates were slower with motion than without motion, and that the vertical rates were slower when descending than when climbing. The presence or absence of platform motion had a stronger effect on performance than the movement direction.
Figure 68 shows the mean vertical speeds versus altitude for all of the pilots for the four combinations of move-ment direction and motion presence. These profiles illustrate that for climbs, vertical speed increased with increasing altitude, and that the increase was more pronounced when motion was absent. What seems to be occurring is that the presence of platform motion reduces the influence of optical flow rate changes on a pilot’s control of vertical speed. Optical flow rate when moving vertically (the angular rate at which objects move visually in elevation) is proportional to vertical speed divided by altitude (ref. 66). So, at constant vertical speed, optical flow rate continuously decreases during a climb. If pilots try to maintain a nearly constant optical flow rate, they will increase speed with increasing altitude.
The theory that pilots try to maintain a constant optical flow rate is supported by Johnson and Awe (ref. 67). They found that during a fixed-base simulation in which pilots were asked to maintain speed that the pilots often slowed as their altitude decreased. Since Figure 68 shows this same tendency, but less so with motion, it is believed that the acceleration cue mitigates the attempt to maintain a constant optical flow rate. Manipulations in level-of-detail did not affect the pilots’ ability to control vertical rate.
Summarizing the experiment discussed in this section, platform motion improved pilots’ accuracy in the altitude repositioning task, which was surprising. It is hypothe-sized that an integration of the visual and the motion cues is occurring, and that the integration affects a pilot’s estimate of altitude and altitude rate. The specifics of this integration are still unknown. That is, future work is needed to determine how many of the altitude and altitude-rate cues are derived from the visual and how many are derived from the motion system. Still, this study showed that one cannot ascribe any of the vertical states to a single cue. The overall conclusion is that for the control of altitude in simulation to be more like that of flight, vertical motion should be provided.
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