Abstract: The present invention relates to a positioning system for use in underwater/subsea hydrographic surveying and positioning applications The system comprises a submersible having a transponder formed and arranged for communication with a remote receiver. The transponder is formed and arranged to transmit a continuous water propagatable data signal, which signal includes a binary signal sequence, preferably a “gold code”, each of the transponder and the receiver being provided with an internal clock. The clock on the receiver and transponder are in substantial synchronisation. The remote receiver is provided with processing means for determining the position of the submersible by calculating the time taken for a signal to be transmitted by the transponder to be received by the remote receiver utilising the clock time of transmittal with respect to the clock time of reception at the remote receiver. Means for determining clock variation and resynchronising the clocks is described.