Abstract: Static transmitter stations are used to determine the position of a mobile receiver unit. Each transmitter station transmits an ultrasonic signal including a transmitter-specific phase-shifting signature. The receiver unit receives a signal and identifies a transmitter station by its signature. It uses the received signal and the identity of the source transmitter station to determine the position of the mobile receiver unit. The signature may include two patterns phase-shift-key (PSK)-encoded on respective carrier signals of the same frequency but different phase, with the patterns being offset from each other by a transmitter-specific offset. The signal from the transmitter station may also include a PSK-encoded message.