Hybrid ground-body based systems are likely to be required; for example, finger motion should be tracked as well as hand motion.

  • Magnetic trackers Significant current disadvantages that limit usefulness include modest accuracy and high latency (20-30 ms). The high latencies are particularly troubling, as this limits their usefulness in real-time interaction. The accuracies are not competitive with most other tracking technologies. Furthermore, the influence of extraneous magnetic fields in the case of AC sensors makes it difficult to know the accuracy one is getting; there is no simple way of determining and compensating for interfering magnetic fields. It is an open question to what extent the accuracies and latencies can be improved.

  • Optical sensing Optical sensing is one of the most convenient methods, but has drawbacks due to visibility constraints. These drawbacks can be partially ameliorated by using multiple camera placement or target placement. The ability of passive stereo vision systems to process arbitrary environments is a long-range goal of the computer vision community; when eventually developed, stereo vision would represent an extremely attractive method for position tracking and mapping. In the meantime, developments in laser scanning and laser radar are promising, as sampling rates, fields of view, and accuracies are becoming quite reasonable.

    Laser interferometers are capable of the highest accuracy, which does not change with viewing distance. If the cost could be brought down, they would represent an attractive method of end-point or head-tracking. Because of the need for retroreflectors, it will be relatively difficult to track multiple limbs. Visibility constraints are a problem; if beams are ever interrupted, the absolute reference is lost. Relatively robust methods for establishing absolute references are required, perhaps through redundant sensing.

    In general, costs will have to be brought down for optical trackers to be more widely used.

  • Acoustic trackers These trackers have a definite role to play in VEs, because the costs are relatively modest and the accuracies are often sufficient. If in situ calibration of the speed of sound in air could be performed, or if ambient measurements could be taken that feed into a model, then the accuracy of acoustic trackers could be improved. Improvements in detection methods could probably reduce the effect of echos. By using multiple frequencies, it should be possible to track multiple markers simultaneously. Drift problems with phase coherent systems might be resolved by dead reckoning with time-of-flight measurements.

  • Inertial trackers Further reductions in sensor size and cost are needed to make inertial trackers a convenient and economical alternative to magnetic trackers. Hybrid systems combining inertial sensors with

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