The relative position and orientation between two subsea structures (210, 212) are measured by ultra-short baseline acoustic metrology. Dual mode master/slave (220, 222) stations comprising a multi-transducer array are fitted in known position and orientation relative to each structure. The first station (220) is operated in master mode to determine the position and direction of the second structure (222) relative to the first (220). The second station (222) is operated in master mode to determine the position and direction data from both operations the apparatus calculates relative position and orientation between the structures. The dual mode stations can be replaced by a master station and a slave station, swapping places. The application further provides for method and apparatus for docking an underwater vehicle to a subsea structure whereby a mobile metrology transducer is fitted to said underwater vehicle such that when docked together, said transducer is in a known positional relationship to said structure. The vehicle may dock with a succession of structures in turn.