A system for determining the position of a mobile receiver unit in an environment, the system comprising: a plurality of transmitter units (2, 3, 4, 5), each comprising a transmitter and configured to transmit a respective transmitter-unit identifier, phase-modulated on an acoustic carrier signal; a mobile receiver unit comprising a receiver and configured to receive an acoustic signal, from a transmitter unit of said plurality of transmitter units, encoding a transmitter-unit identifier; and a processing subsystem (205; 9) configured to: sample received signals to generate sampled data representing the received signals, wherein the transmitter-unit identifier spans a transmitter-unit-identifier-bearing portion of the sampled data; determine a time of arrival of the transmitter-unit identifier at the mobile receiver unit; obtain template data corresponding to the transmitter-unit identifier; decode the transmitter-unit identifier from the sampled data; and use said time of arrival and the decoded transmitter-unit identifier to determine information relating to the position of the mobile receiver unit in the environment, characterised in that the processing subsystem is further configured to: analyse the sampled data to determine a Doppler-induced phase deviation within the transmitter-unit-identifier-bearing portion of the sampled data; adjust the template data or the sampled data so as to change one or more phase shifts within the template data or within the transmitter-unit-identifier-bearing portion of the sampled data by an amount that depends on the determined Doppler-induced phase deviation; cross-correlate the template data with the sampled data after said adjusting; and use an output of the cross-correlation when determining said time of arrival and optionally when decoding the transmitter-unit identifier. a plurality of transmitter units (2, 3, 4, 5), each comprising a transmitter and configured to transmit a respective transmitter-unit identifier, phase-modulated on an acoustic carrier signal; a mobile receiver unit comprising a receiver and configured to receive an acoustic signal, from a transmitter unit of said plurality of transmitter units, encoding a transmitter-unit identifier; and a processing subsystem (205; 9) configured to: sample received signals to generate sampled data representing the received signals, wherein the transmitter-unit identifier spans a transmitter-unit-identifier-bearing portion of the sampled data; determine a time of arrival of the transmitter-unit identifier at the mobile receiver unit; obtain template data corresponding to the transmitter-unit identifier; decode the transmitter-unit identifier from the sampled data; and use said time of arrival and the decoded transmitter-unit identifier to determine information relating to the position of the mobile receiver unit in the environment, sample received signals to generate sampled data representing the received signals, wherein the transmitter-unit identifier spans a transmitter-unit-identifier-bearing portion of the sampled data; determine a time of arrival of the transmitter-unit identifier at the mobile receiver unit; obtain template data corresponding to the transmitter-unit identifier; decode the transmitter-unit identifier from the sampled data; and use said time of arrival and the decoded transmitter-unit identifier to determine information relating to the position of the mobile receiver unit in the environment, analyse the sampled data to determine a Doppler-induced phase deviation within the transmitter-unit-identifier-bearing portion of the sampled data; adjust the template data or the sampled data so as to change one or more phase shifts within the template data or within the transmitter-unit-identifier-bearing portion of the sampled data by an amount that depends on the determined Doppler-induced phase deviation; cross-correlate the template data with the sampled data after said adjusting; and use an output of the cross-correlation when determining said time of arrival and optionally when decoding the transmitter-unit identifier.