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Underwater acoustic communication
Acoustic underwater telephony is a method for underwater communication, especially with submarines. It is also known by its original code name Gertrude.
The range is very limited and depends on the water depth and temperature. Gertrude does not work through the thermocline. Simply put Gertrude is a powerful speaker that emits sound waves directly into the surrounding water. The passive sonar sensors of another boat or ship can absorb these waves.
Although water sound to today\’s communication needs sufficient only very inadequately, electromagnetic waves, depending on the turbidity of the water in transmission lines over distances of 10 to 300 m beyond practically unsuitable, the water sound that is without alternative. The speech signal is shifted over a modulation method in a higher frequency range in order to achieve a better signal to noise ratio.
As early as World War II was the underwater telephone, UT or Gertrude used. It was an analog voice transmission in SSB technology in the upper sideband with 9 kHz as carrier and the common from the former telephony frequency range 300 Hz to 3 kHz. In particular, in the shallow waters of the reception due to multipath transmission was extremely poor. It was only this one frequency channel.
Meanwhile transmission method of modern mobile technology can be used that allow because of poor dispersion conditions and the low available bandwidth (mainly frequencies between 5 kHz and 40 kHz), but only small amounts of information and ranges. The day appointed by the NATO system achieves ranges up to 10 km, the successor system Deep Siren promises some 100 km range.
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