Low frequency continuous active sonar

Existing sonar systems are commonly deployed at a low duty cycle, i.e. a short transmit signal is used followed by a long listening time. As a result, the target is only illuminated during a short time resulting in only one detection opportunity per ping. When a sonar system is capable of recording meaningful data during its transmission, one may consider increasing the duty cycle by transmitting long or continuous transmit signals. This increases the illumination time of the target and thus provides more detection opportunities. This is of special interest for manoeuvring targets with an aspect-dependent target strength. As a result of the increased target illumination, it is expected that continuous sonar will improve the probability of detection, and also the tracking performance. The principle, feasibility and performance of continuous active sonar (CAS) have been evaluated at sea in 2009 using the Netherlands Interim Removable Low Frequency Active Sonar (IRLFAS) system. This experiment revealed that with a dedicated transmit signal design and processing strategy, the received signal can be separated from the transmit signal. The detection performance observed in this experiment is similar to conventional pulsed transmit signals that were used as a reference signal. This result is promising since the CAS source level was reduced by 11 dB with respect to the reference signal. Thus, the principle, feasibility and prospects for CAS have successfully been demonstrated in the IRLFAS 2009 CAS experiment