Currently, there are four basic technologies for position tracking and mapping in SE work: mechanical linkages, magnetic sensors, optical sensors, and acoustic sensors. SE systems are likely to include a mix of such systems, because each type of system has particular strengths and weaknesses and the requirements depend on the particular application. Although none of the inertial trackers currently available is adequate for SE applications, research is now under way to develop such trackers. As for many other kinds of devices, commercial specifications of position trackers and mappers are not reliable or consistent.

Mechanical trackers are relatively inexpensive, have very small intrinsic latencies, and can be reasonably accurate. Yet body-based linkage devices (called goniometers) may be cumbersome, whereas ground-based linkage devices (e.g., hand controllers) suffer from workspace limitations. The use of goniometers involves problems of fit and measurement related to alignment with joints, rigidity of attachments, calibration of linkages mounted on human limbs, and variations among individuals. The use of ground-based linkage devices involves the difficulty of tracking multiple limb segments and limb redundancies. Hybrid systems, in which body-based and ground-based devices are combined, are also likely to be required for some applications (e.g., both to track finger motion and to provide force feedback to the hand without causing forces to be applied to other portions of the body).

Magnetic trackers are commonly used because of their convenience, low cost, reasonable accuracy, and lack of obscuration problems. Significant current disadvantages that limit their usefulness include modest accuracy, short range, high latency (20-30 ms), and susceptibility to magnetic interference.

Optical sensing is one of the most convenient methods to use for certain kinds of tracking and is capable of providing accuracies and sampling rates that meet many VE requirements. The main drawbacks include visibility constraints and especially high costs.

Acoustic trackers are very attractive for VE because the costs are relatively modest and the accuracies and sampling rates are often sufficient. Efforts are being made to improve accuracies by taking into account atmospheric effects and by using echo rejection.

Inertial trackers, despite having played a distinguished role in the field of long-range navigation, have received little attention in the SE area. Their unique advantage is that they are unconstrained by range limitations, interference, and obscuration; also, latencies are low. Further reductions in sensor size and cost are needed to make inertial trackers a convenient and economical alternative to other trackers.

An ideal eye tracker would satisfy three requirements: linear response over a large range (roughly 50 deg), high bandwidth (1 kHz), and



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