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An interactive TV is requiring a more advanced remote controlling method than the traditional TV due to its diverse and complex functions. Many types of input technologies and interaction styles are being tested for remote pointing for an interactive TV. We introduce the SonarPen, an ultrasonic pointing device system developed for an interactive TV. It determines a cursor position from phase differences among sinusoidal signals received by three ultrasonic receivers. A common problem with phase difference methods is phase ambiguity, but we solved this problem by exploiting the directionality of an LED and discontinuity in phase difference signals. A performance evaluation of the SonarPen in comparison with other pointing devices showed that the SonarPen has a better throughput, while it has a relatively high error rate than others.