<P> Germany is working on the torpedo tube - launched short - range IDAS missile, which can be used against ASW helicopters, as well as surface ships and coastal targets . </P> <P> A submarine can have a variety of sensors, depending on its missions . Modern military submarines rely almost entirely on a suite of passive and active sonars to locate targets . Active sonar relies on an audible "ping" to generate echoes to reveal objects around the submarine . Active systems are rarely used, as doing so reveals the sub's presence . Passive sonar is a set of sensitive hydrophones set into the hull or trailed in a towed array, normally trailing several hundred feet behind the sub . The towed array is the mainstay of NATO submarine detection systems, as it reduces the flow noise heard by operators . Hull mounted sonar is employed in addition to the towed array, as the towed array can't work in shallow depth and during maneuvering . In addition, sonar has a blind spot "through" the submarine, so a system on both the front and back works to eliminate that problem . As the towed array trails behind and below the submarine, it also allows the submarine to have a system both above and below the thermocline at the proper depth; sound passing through the thermocline is distorted resulting in a lower detection range . </P> <P> Submarines also carry radar equipment to detect surface ships and aircraft . Submarine captains are more likely to use radar detection gear than active radar to detect targets, as radar can be detected far beyond its own return range, revealing the submarine . Periscopes are rarely used, except for position fixes and to verify a contact's identity . </P> <P> Civilian submarines, such as the DSV Alvin or the Russian Mir submersibles, rely on small active sonar sets and viewing ports to navigate . The human eye cannot detect sunlight below about 300 feet (91 m) underwater, so high intensity lights are used to illuminate the viewing area . </P>

What was the main advantage of the submarine in ww1