Abstract. A series of aircraft experiments was performed using a specialized GPS receiver and a nadir-oriented left hand circularly polarized antenna. This apparatus received reflections of the GPS signals from water surfaces under a variety of sea states. The cross-correlation between the reflected signal and a reference pseudo-random noise code was recorded as a function of the relative time delay. The shape of this function showed a dependence on the roughness of the reflecting surface. This dependence generally followed that predicted by theory for bistatic scattering of rangecoded signals. Use of this information as a remote sensing technique for the determination of sea state is discussed.
System latency (time delay) and its visible consequences are fundamental virtual environment (VE) deficiencies that can hamper user perception and performance. The aim of this research is to quantify the role of VE scene content and resultant relative object motion on perceptual sensitivity to VE latency. Latency detection was examined by presenting observers in a head-tracked, stereoscopic head mounted display with environments having differing levels of complexity ranging from simple geometrical objects to a radiosity-rendered scene of two interconnected rooms. Latency discrimination was compared with results from a previous study in which only simple geometrical objects, without radiosity rendering or a 'real-world' setting, were used. From the results of these two studies, it can be inferred that the Just Noticeable Difference (JND) for latency discrimination by trained observers averages ~15 ms or less, independent of scene complexity and real-world meaning. Such knowledge will help elucidate latency perception mechanisms and, in turn, guide VE designers in the development of latency countermeasures.
User perceptual sensitivity to changes of system latency was tested in three simple virtual environments: one with only a foreground object, a second with only a background object, and a third that combined both of these elements. Prior psychophysical measurements of sensitivity, Just Noticeable Difference; and bias, Points of Subjective Equality, from our laboratory are confirmed with measurements in 13 subjects. Our measurements indicate that perceptual stability across a variety of virtual environments will require latencies less than 16 ms. We discount a possible explanation that the differences between our results and those from a study by Allison et al. could be related to a visual capture effect initially reported by L. Matin. Instead, the differences may be due to the type of psychophysical judgment rendered by the subjects and the degree to which subjects were instructed and practiced.
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