AP identification and direction of motion were determined by comparing signals from the ultrasonic range sensors and piezoelectric ultrasonic transducers to thresholds. AP identification during walking, stopping, and turning occurred by comparing the piezoelectric ultrasonic receiver signal to a threshold (Figure 4). While walking, post-processed signals from the ultrasonic transducers are modulated as sine waves that are out of phase with each other, due to beacon angular motion at the hip (i.e., the beacon faces one transducer at a time, causing signal attenuation of the other transducers (Figure 4)).