Ocean Sonics offers a Sync GPS which allows the icListen to be synchronized by receiving a GPS pulse per second (PPS).

The system is very low power, small and can be mounted on a mast above water or on shore. Typical PPS can synchronize the seconds to a resolution of nanoseconds.

However, the Sync GPS modulates the trailing edge of the PPS signal to encode the date & time as well. This guarantees that all instruments are fully synchronized to GPS time after two minutes.