The development of undersea defence technologies such as sonar relies heavily on the availability of high quality acoustic data. However, data acquisition is particularly expensive as sea trials involve experienced manpower and costly high-tech equipment. Also, at sea, the environment remains unpredictable and many disturbing effects are combined. Therefore, in the underwater technology department of TNO, the simulator SIMONA was designed to help developing and testing new algorithms for advanced array processing, at low cost, in a specific and controlled environment. SIMONA generates realistic hydrophone data for sonar arrays thanks to a modular structure. The various simulation modules for platform motion, active and passive target responses, reverberation and background noise have been verified with validated models and operational examples. SIMONA has proved an excellent tool in a wide range of sonar applications such as anti submarine warfare, torpedo defence, marine mammal localisation and mine countermeasures.