“Acoustic radar graphic input device” by de Bruyne

Accurate X,Y position determining digitizers presently require a precision array of wires embedded in the tablet. In rear-projection tablets these wires tend to limit the optical quality of images projected on the screen. Principles of acoustic radar are applied in a device which achieves better resolution, at a lower cost and without the use of wires, either in a tablet or a cursor. The function of acoustic reflectors for automatic calibration is described, achieving an accuracy of + mn; 0.01% of the distance to the sensors. In the case of small tablets, a hand-held stylus may be tracked at the rate of 900 locations a second.

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