A system for determining the position of a mobile receiver unit (3, 3a, 3b) in an environment, the system comprising: a transmission apparatus (6, 6a, 6b, 6c), comprising a plurality of ultrasound transmitters (9, 10, 11, 12, 13) and being configured to transmit a plurality of ultrasonic signals in different respective principal directions from the transmission apparatus (6, 6a, 6b, 6c), each ultrasonic signal encoding a different respective direction identifier; and a mobile receiver unit (3, 3a, 3b), comprising an ultrasound receiver and being configured to receive the plurality of ultrasonic signals along a plurality of signal paths, wherein at least one of the signal paths includes a reflection off a surface (15, 16, 17) in the environment; and a processing system, wherein: the processing system comprises a decoder and is arranged to decode the respective direction identifiers from the received ultrasonic signals; the processing system is configured to determine, for each of the signal paths, a respective time of arrival of the respective ultrasonic signal at the mobile receiver unit; and the processing system is configured to use (i) location information relating to the transmission apparatus (6, 6a, 6b, 6c), (ii) location information relating to the surface (15, 16, 17), (iii) said respective direction identifiers, and (iv) said times of arrival, to calculate an estimate of the position of the mobile receiver unit (3, 3a, 3b). a transmission apparatus (6, 6a, 6b, 6c), comprising a plurality of ultrasound transmitters (9, 10, 11, 12, 13) and being configured to transmit a plurality of ultrasonic signals in different respective principal directions from the transmission apparatus (6, 6a, 6b, 6c), each ultrasonic signal encoding a different respective direction identifier; and a mobile receiver unit (3, 3a, 3b), comprising an ultrasound receiver and being configured to receive the plurality of ultrasonic signals along a plurality of signal paths, wherein at least one of the signal paths includes a reflection off a surface (15, 16, 17) in the environment; and a processing system, wherein: the processing system comprises a decoder and is arranged to decode the respective direction identifiers from the received ultrasonic signals; the processing system is configured to determine, for each of the signal paths, a respective time of arrival of the respective ultrasonic signal at the mobile receiver unit; and the processing system is configured to use (i) location information relating to the transmission apparatus (6, 6a, 6b, 6c), (ii) location information relating to the surface (15, 16, 17), (iii) said respective direction identifiers, and (iv) said times of arrival, to calculate an estimate of the position of the mobile receiver unit (3, 3a, 3b). the processing system comprises a decoder and is arranged to decode the respective direction identifiers from the received ultrasonic signals; the processing system is configured to determine, for each of the signal paths, a respective time of arrival of the respective ultrasonic signal at the mobile receiver unit; and the processing system is configured to use (i) location information relating to the transmission apparatus (6, 6a, 6b, 6c), (ii) location information relating to the surface (15, 16, 17), (iii) said respective direction identifiers, and (iv) said times of arrival, to calculate an estimate of the position of the mobile receiver unit (3, 3a, 3b).