linguistic:paraphrasing
Sensors

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 not 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.

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.

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.