wunder beta🔊 How submarines find each other in the dark
For seventy-one minutes the USS Greeneville could hear the ship it was about to kill. Hearing was never the problem. This is the gap between a sound and a place — bearings that are only lines, an ocean that bends sound into channels and bli
What you’ll learn
- The ocean is a listening roomExplain why hearing a sound underwater is only the first step towards knowing what and where it is.A hydrophone turns pressure into a signal and an array turns arrival times into a bearing — but a bearing is a line, not a place, and the Greeneville held one for seventy-one minutes without ever making it a position.
- Listen first, ping only when neededDistinguish passive from active sonar and work a range out of a round-trip time.Passive sonar buys concealment and gives up range; active sonar buys range and gives up concealment. Halve the round trip, multiply by a sound speed you have actually estimated, and then decide whether to believe the echo.
- The ocean bends the messageDescribe how the sound-speed profile creates channels and shadow zones, and why silence is ambiguous.Sound speed varies by under five per cent through the water column, and that is enough to refract sound over thousands of miles or hide a boat that is close by. The seabed is a participant too — clutter to a hunter, and a found submarine to a search team.
- Navigation without a windowConnect inertial navigation, its drift, and the cost of a fix to the problem of meeting another boat.Seawater blocks GPS, so a submerged boat computes its position and watches the error grow. Inertial systems took Nautilus under the pole in 1958; every way of checking them costs exposure, time, or a transmission.
- When submarines need to speakExplain why underwater communication is acoustics, and why identification needs evidence a contact cannot supply.The underwater telephone is designated a sonar set because that is what it is, and it leaks like one. A rescue vehicle homes on hull hammering with two hydrophones and a human listener — and a search team needs more than a return on a screen before it can name a wreck.
Questions this course answers
The Greeneville had the Ehime Maru on sonar for 71 minutes before the collision. What does that tell you?
The NTSB blamed inadequate interaction and communication among the combat systems team, and a failure to perform adequate contact analysis. The sound was there the whole time; turning it into a position is a separate job.
Why does a submarine have to change course to work out how far away a contact is?
One bearing gives a direction and nothing else. Bearing rate — how fast the direction changes as you move a known amount — is what separates a near contact from a far one.
Looking at the NTSB track chart, why did the Greeneville's manoeuvring make its contact picture worse rather than better?
The investigators found the too-rapid course changes resulted in no bearing rate change, which is necessary to identify a contact's course. Motion helps only if it is patient.
What is the real cost of using active sonar?
Active sonar buys a direct range measurement and pays for it with a transmission that travels outward to every listener in reach.
Match each term to what it actually does.
These are the four pieces the rest of the course is built from: two ways of asking, the sensor that hears, and the one measurement you get almost free.
An active pulse returns after 2 seconds. Roughly how far away is the object?
Sound covers roughly 1,500 metres per second in seawater, so 2 seconds is about 3,000 metres of travel — but that is out and back, so the object is about 1,500 metres away. The real calculation also needs a proper estimate of sound speed in that particular water.
Grounded in trusted sources
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