A system for measuring a time offset of a sensor node relative to a reference time, the system comprising: a sensor node an acoustic communications unit arranged to transmit a first acoustic probe signal to the sensor node; a processing resource operably coupled to the acoustic communications unit and arranged to record a transmission time of the first signal, the transmission time being relative to the reference time; wherein the sensor node is arranged to receive the first signal and to transmit a second acoustic reply signal to the acoustic communications unit after a predetermined time delay, the second signal including time data indicative of a current internal time of the sensor node during the predetermined time delay; the acoustic communications unit is arranged to receive the second signal, the processing resource recording a receive time at which the second signal is received, the receive time also being relative to the reference time, and to calculate the combined time of flight of the first and second signals (42, 44), and using the combined time of flight and the predetermined time delay to calculate the time offset between the current internal time of the sensor node and the reference time; and the system is characterised by: a vessel comprising the acoustic communications unit, the acoustic communications unit being arranged to transmit the first acoustic probe signal through water to the sensor node, wherein the sensor node is a subsea sensor node the subsea sensor node being arranged to transmit the second acoustic reply signal through the water to the acoustic communications unit after the predetermined time delay; the vessel comprises a vessel motion compensation unit arranged to compensate, when in use, for differences in the time of flight between the first signal and the second signal. a sensor node an acoustic communications unit arranged to transmit a first acoustic probe signal to the sensor node; a processing resource operably coupled to the acoustic communications unit and arranged to record a transmission time of the first signal, the transmission time being relative to the reference time; wherein the sensor node is arranged to receive the first signal and to transmit a second acoustic reply signal to the acoustic communications unit after a predetermined time delay, the second signal including time data indicative of a current internal time of the sensor node during the predetermined time delay; the acoustic communications unit is arranged to receive the second signal, the processing resource recording a receive time at which the second signal is received, the receive time also being relative to the reference time, and to calculate the combined time of flight of the first and second signals (42, 44), and using the combined time of flight and the predetermined time delay to calculate the time offset between the current internal time of the sensor node and the reference time; and the system is characterised by: a vessel comprising the acoustic communications unit, the acoustic communications unit being arranged to transmit the first acoustic probe signal through water to the sensor node, wherein the sensor node is a subsea sensor node the subsea sensor node being arranged to transmit the second acoustic reply signal through the water to the acoustic communications unit after the predetermined time delay; the vessel comprises a vessel motion compensation unit arranged to compensate, when in use, for differences in the time of flight between the first signal and the second signal. a vessel comprising the acoustic communications unit, the acoustic communications unit being arranged to transmit the first acoustic probe signal through water to the sensor node, wherein the sensor node is a subsea sensor node the subsea sensor node being arranged to transmit the second acoustic reply signal through the water to the acoustic communications unit after the predetermined time delay; the vessel comprises a vessel motion compensation unit arranged to compensate, when in use, for differences in the time of flight between the first signal and the second signal.