The Sentinel system is designed for use in typical harbour environments, where there will be reverberation from structures, and interfering noise from machinery etc. The use of FM replica correlation and beamforming give large processing gains against interference, and the system has been shown to work reliably in these environments. However we also provide two main methods to monitor the effects of the environment on the performance of the sonars. First we provide an advanced acoustic performance prediction tool which gives an estimate of performance based on measured sound speed profiles and background noise. Secondly, the system has a built-in performance monitoring system which measures the interfering noise and reverberation, and indicates which directions may be seeing high levels of interference.