The main objective of a sea surveillance sonar is the surveillance of marine activities below the waterline in selected areas like coasts, water ways, naval bases and oil platforms. The Sea surveillance can prove to be extremely beneficial in countering illegal activities that can strain a nation’s economy. These activities include sabotage, piratism, terrorism, smuggling and illegal immigration. A combined sonar consisting of a low frequency passive system and a high frequency active system can be used to detect, track and classify all subsurface contacts like submarines, divers and diver delivery vehicles within its assigned coverage area. In case of silent submarines a vertical transmitting array is supporting the detection of these targets in the low frequency range. The system is of a flexible open architecture so that it can easily be tailored to local needs. As operational support a sonar performance prediction system is integrated. The system has been tested. Experimental results will be shown.