USBL (ultra-short baseline, also known as SSBL for super short base line) is a method of underwater acoustic positioning.
A USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder or responder on the seafloor, on a towfish, or on an ROV.
The time from the transmission of the initial acoustic pulse until the reply is detected is measured by the USBL system and is converted into a range.
[1] USBLs are used in "inverted" (iUSBL) configurations, with the transceiver mounted on an autonomous underwater vehicle, and the transponder on the ship/shore that launches it.
In this case, the "topside" processing happens inside the vehicle to allow it to locate the transponder for applications such as automatic docking, target tracking, and the exchange of text messages.