The expansion of Finnish territorial waters in 1995 imposed more demands on marine surveillance. The new areas are mostly in the open sea, not covered by Finland's current surveillance. The expansion also increased the area in which submarines can operate, increasing the need for underwater surveillance.

The purpose of underwater surveillance is not only to detect enemy submarines but also to provide information on the surface situation. Underwater surveillance supplements and verifies information obtained by surface surveillance sensors.

The new underwater surveillance equipment designed in Finland can also be used on the open sea. It also greatly improves the ability to detect modern quiet submarines.

The surveillance unit of Sonac PTA equipment, which is used for mobile underwater surveillance, is located on board a vessel towing the antenna unit.