Existing surveillance sonar systems for anti-submarine warfare (ASW) use a pulsed sonar deployed at a low duty cycle. Continuously active sonar (CAS) is of special interest since the technique could provide better detection performance than conventional pulsed sonar, and it will provide the operator a continuous track. One can assume this will provide opportunities for improving the tracking performance. Furthermore, it is expected that CAS can be used for the suppression of false alarms, one of the main problems in shallow-water ASW. TNO has more than 20 years of practical experience with low-frequency active sonar (LFAS) for ASW. TNO has developed the interim removable low-frequency active sonar (IRLFAS) system demonstrator for the Royal Netherlands Navy, which uses the system onboard its operational platforms to gain operational experience. The feasibility of CAS has been successfully demonstrated with the IRLFAS system at sea in a shallow-water environment.