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The CDT uses SONAR. We know when it is at the bottom because as it goes down, it makes a 3500 kHZ "ping" (ping sound file) every few seconds. Since we know the speed of sound in seawater is about 1500 meters per second, we can calculate how far the CDT is above the bottom by measuring the amount of time between each ping and its returning bottom echo. This is how all SONAR systems work.

Sound is transmitted by water in an amazing way. While all electromagnetic waves, such as light and heat are quickly absorbed by seawater, sound can travel thousands of miles underwater along the SOFAR channel. Although it is quite black 200 meters down, you could say that sound is the light of the sea. All sea creatures use sound in some way to communicate or to escape predators. The sea is far from silent!

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