A subsea measurement system comprising: a first reference beacon disposed at a first known location; a second reference beacon disposed at a second known location; a third reference beacon disposed at a third known location; and a USBL acoustic communications module for coupling to a subsea element to be monitored, the USBL acoustic communications module being capable of moving, when in use, relative to the first reference beacon, the second reference beacon and the third reference beacon; wherein the USBL acoustic communications module comprises a processing resource and is arranged to determine first range-related data to the first reference beacon in response to receipt of a first signal by the USBL acoustic communications module from the first reference beacon, second range-related data to the second reference beacon in response to receipt by the USBL acoustic communications module of a second signal from the second reference beacon, and third range-related data to the third reference beacon in response to receipt by the acoustic communications module of a third signal from the third reference beacon; characterised in that the USBL acoustic communications module is further arranged to determine first angle data to the first reference beacon in response to receipt of the first signal by the USBL acoustic communications module from the first reference beacon and second angle data to the second reference beacon in response to receipt of the second signal by the USBL acoustic communication module from the second reference beacon; and the system further comprises a further processing resource arranged to calculate at least one location-related parameter for the subsea element to be monitored comprising an estimated position and an estimated attitude associated with the USBL acoustic communications module, the estimated position and the estimated attitude being in three axes, respectively, using the first, second and third range-related data and the first and second angle data, respectively. a first reference beacon disposed at a first known location; a second reference beacon disposed at a second known location; a third reference beacon disposed at a third known location; and a USBL acoustic communications module for coupling to a subsea element to be monitored, the USBL acoustic communications module being capable of moving, when in use, relative to the first reference beacon, the second reference beacon and the third reference beacon; wherein the USBL acoustic communications module comprises a processing resource and is arranged to determine first range-related data to the first reference beacon in response to receipt of a first signal by the USBL acoustic communications module from the first reference beacon, second range-related data to the second reference beacon in response to receipt by the USBL acoustic communications module of a second signal from the second reference beacon, and third range-related data to the third reference beacon in response to receipt by the acoustic communications module of a third signal from the third reference beacon; characterised in that the USBL acoustic communications module is further arranged to determine first angle data to the first reference beacon in response to receipt of the first signal by the USBL acoustic communications module from the first reference beacon and second angle data to the second reference beacon in response to receipt of the second signal by the USBL acoustic communication module from the second reference beacon; and the system further comprises a further processing resource arranged to calculate at least one location-related parameter for the subsea element to be monitored comprising an estimated position and an estimated attitude associated with the USBL acoustic communications module, the estimated position and the estimated attitude being in three axes, respectively, using the first, second and third range-related data and the first and second angle data, respectively.