US Patent 7489308 Determining the location of the tip of an electronic stylus

An electronic stylus system includes an electronic stylus and base receiving unit. The electronic stylus includes a first ultrasonic transmitter, a second ultrasonic transmitter, an electromagnetic transmitter, and a writing tip. The base receiver unit includes a first ultrasonic receiver, a second ultrasonic receiver, and an electromagnetic receiver. The ultrasonic receivers of the base unit are operable to receive signals transmitted by the ultrasonic transmitters of the electronic stylus. Similarly, the electromagnetic receiver of the base unit is operable to receive signals transmitted by the electromagnetic transmitter of the stylus. The location of the tip of the electronic stylus relative to a given reference point is determined using the locations of two ultrasonic transmitters relative to the two ultrasonic receivers.