Cross-Talk between Altitude Changes and Speed Control during Simulated Low-Altitude Flight

Abstract

Simulations of flight over a planar environment create an ambiguous optical stimulus in which changes in altitude can potentially induce inappropriate control of speed. Previous research has shown that changes in altitude produce changes in global optical flow rate that lead to erroneous judgments of speed and increased RMS error in speed control. However, from these studies it is unclear whether erroneous speed control was directly due to misperceiving changes in altitude as changes in speed or simply a response to more complex optical flow masking the optical effects of speed changes. Our experiment used a speed maintenance task during simulated flight over a planar surface to examine the effect of changes in altitude on control of speed. We found that power in participants' speed control input increased at the specific frequencies of the simulated altitude changes relative to power at those same frequencies when no altitude disturbance was present. These results clearly show that controllers respond to altitude changes as if they are changes in speed during simulated flight over a planar environment. Hence, the optical effects of altitude changes appear to be perceived as changes in speed and induce systematic errors in speed control.

Further Evidence for Ground Dominance in Control Of Speed During Low Altitude Flight

“…Our second experiment examined whether color and contrast differences between the cloud and ground textures contribute to ground dominance. This experiment replicates the stimuli used by Wotring et al (2008) but reverses the texture assignments for the ground and cloud surfaces. If equivalent speed-altitude cross-talk is found for both the single and dual planar stimuli in this experiment it suggests that differences in color and contrast do not play a significant role in ground dominance.…”

“…For flight at constant forward speed over a planar surface, decreases in altitude result in increases in GOFR, while increases in altitude will result in decreases in GOFR. Hence, if GOFR is used to perceive and control forward speed, cross-talk between altitude changes and speed control will occur-a result found by Wotring et al (2008).…”

“…More importantly, Snowden, Stimpson, and Ruddle (1998) and Shaudt (2006) have shown that these differences can affect speed control. Hence, an additional control experiment is needed to test whether the differences in luminance contrast in the displays of Wotring et al (2008) and Adamic et al (2010) contribute to the ground dominance effect.…”

“…Low-altitude fixed-wing flight presents a unique challenge to pilots due to a potentially deadly combination of a narrow margin for error in controlling speed and altitude and the potential for cross-talk between altitude changes and control of speed (Wotring, Dyre, & Behre, 2008). The leading explanation of altitude-speed cross-talk is that the visual system estimates speed primarily from global optical flow rate (GOFR): formally defined as forward speed divided by altitude or eye-height (Warren, 1982).…”

“…Our first experiment examined whether the visual system can indeed integrate optical flow across multiple surfaces for speed control. Using a speed maintenance task, this experiment simulated "flight" relative to one or two planar surfaces (see Figure 1) oriented vertically on either one or both side(s) of the simulated viewpoint and parallel to the direction of movement-essentially replicating the stimuli of Wotring et al (2008) and Adamic et al (2010) with the planes rotated about the roll axis by 90 degrees. For these stimuli, there is no ground defined, so if observers demonstrate speed-"altitude" (in this case, "altitude" translates to horizontal, rather than vertical, distance) cross-talk for both the single and dual plane conditions, it would suggest that ground dominance results, in part, from an inability of the human visual system to integrate optical flow across multiple surfaces.…”