A system for determining the position of a mobile device, the system comprising: a transmitter device (4, 6) arranged to transmit an ultrasonic signal that encodes a binary identifier; a receiver device arranged to receive the ultrasonic signal, wherein one of the transmitter device and the receiver device is a mobile device; decoding means configured to decode the binary identifier from the received ultrasonic signal; and processing means configured to use the decoded binary identifier to determine information relating to the position of the mobile device, characterised in that: the encoding is such that each bit position in the binary identifier is associated with a respective pair of ultrasonic frequencies and with respective first and second time positions in the ultrasonic signal, with the value of the bit position in the binary identifier determining which ultrasonic frequency of the pair of ultrasonic frequencies is transmitted at the first respective time position in the ultrasonic signal, and with the other ultrasonic frequency of the pair of ultrasonic frequencies being transmitted at the second respective time position in the ultrasonic signal; and the decoding means is configured to decode the binary identifier by determining a value for each bit position in the binary identifier using information relating to the strength of the received ultrasonic signal at (a) the first ultrasonic frequency and first time position, in the received ultrasonic signal, associated with the respective bit position, (b) the first ultrasonic frequency and second time position associated with the respective bit position, (c) the second ultrasonic frequency and first time position associated with the respective bit position, and (d) the second ultrasonic frequency and second time position associated with the respective bit position. a transmitter device (4, 6) arranged to transmit an ultrasonic signal that encodes a binary identifier; a receiver device arranged to receive the ultrasonic signal, wherein one of the transmitter device and the receiver device is a mobile device; decoding means configured to decode the binary identifier from the received ultrasonic signal; and processing means configured to use the decoded binary identifier to determine information relating to the position of the mobile device, the encoding is such that each bit position in the binary identifier is associated with a respective pair of ultrasonic frequencies and with respective first and second time positions in the ultrasonic signal, with the value of the bit position in the binary identifier determining which ultrasonic frequency of the pair of ultrasonic frequencies is transmitted at the first respective time position in the ultrasonic signal, and with the other ultrasonic frequency of the pair of ultrasonic frequencies being transmitted at the second respective time position in the ultrasonic signal; and the decoding means is configured to decode the binary identifier by determining a value for each bit position in the binary identifier using information relating to the strength of the received ultrasonic signal at (a) the first ultrasonic frequency and first time position, in the received ultrasonic signal, associated with the respective bit position, (b) the first ultrasonic frequency and second time position associated with the respective bit position, (c) the second ultrasonic frequency and first time position associated with the respective bit position, and (d) the second ultrasonic frequency and second time position associated with the respective bit position.