A system is provided for determining the position of a mobile receiver unit in an environment. The system comprises: a transmission apparatus comprising a plurality of ultrasound transmitters configured to transmit a plurality of ultrasonic signals in different respective principal directions, each encoding a different respective direction identifier; a mobile receiver unit comprising an ultrasound receiver configured to receive the plurality of ultrasonic signals along a plurality of signal paths, at least one of which includes a reflection off an environment surface; and a processing system comprising a decoder arranged to decode the respective direction identifiers from the received signals. The processing system is configured to determine a respective time of arrival for each signal, and use location information relating to the transmission apparatus and the environment surface, together with the respective direction identifiers and times of arrival to calculate an estimated position of the mobile receiver unit.