A wireless coordinate input system for a display system includes a stylus that transmits ultrasonic energy to a plurality of ultrasonic receiving stations in a projection plane. In an embodiment, the stylus may include one ultrasonic transmitter used for determination of three-dimensional coordinates of the stylus relative to the projection plane. The stylus may also include a second ultrasonic transmitter controlled by a pressure-activated switch. When the stylus is pressed against the projection plane, the second transmitter turns on and is used for determination of two-dimensional coordinates of the stylus in the projection plane. The stylus may also include a higher frequency burst transmitter used to generate a time reference. One or more of the ultrasonic receiving stations may also include an ultrasonic transmitter for calibration.