Thank you. That is what I was hoping to hear. Low frequency Towed sonar is indispensable for effective ASW. If I am not mistaken HIZIR had low frequency passive array, should it be enough to listen for Submarine at a more significant distance than Mid-frequency hull mounted active sonar?
Or active low frequency emission is required hence Dufas is needed? I mean, Submarine themselves never ping.
Submarines emit low frequency noise from their wakes or propellers, or from auxiliary noise (machinery, vibration, crew walking around, HVAC units, other mechanical sources, etc.) and/or when they switch on active sonar for various reasons. So a passive array sonar would still work for detection depending on the scenario.
An active towed sonar can change depth to ping at a depth where submarines can dive - using underwater currents, high gradients in salinity/density and temperature for reflectivity and concealment*. Or a low-frequency sonar can emit high sound power at low frequencies that are less susceptible to damping by distance**, allowing targets to be picked up/detected at greater distances. So, yes, it adds another dimension to ASW, as does a variable depth sonar. Also, compared to hull-mounted sonars, towed array sonars have less interference from the ship's own noise and can be built larger due to the flexibility of space, or can be used to form multistatic configuration with one vessel.
*Depending at the conditions there exists an underwater sound channel which sound is entrapped and travels with less lossess, so VDS may opt to listen in here or the dipping sonar may opt to be here.
**These properties highly vary by the sea, season. That is another reason why Navies have a high interest in hydrograpghy, oceaonagraphy and survey vessels, or they fund scientific researches in these field, or pay great interest in them to be supported by the state.
An ASW mission currently includes;
-Detection
MTI-SAR / Surface Search Radar***
Sonobuoys + MAD
Helicopter + dipping sonar
Hull mounted sonar
Towed array (passive unit)+ Variable depth (active unit) sonar
-Engagement
LWT from MPA / Helicopter / ASW asset
VL-ASROC / Depth Chargers
These are applicable for high seas ASW. As I said, I believe it becomes complex for both sides in shallow seas and even commercial fishing - high frequency sonar can be used to hunt submarines at those conditions since the maneuverability is highly restricted.
*** Once a submarine surfaces the periscope, it is morelikely to be detected by radars of surface assets, or MTI-SAR of MPA assets than being picked out by MAD or sonars. A Submarine utilizes periscope for engagement or Intelligence missions, or near surface navigations.
