LetвЂ™s check some of them more detailed: Underwater TelephoneДЄnВ underwater telephoneВ sometimes called Gertrude is also used to communicate with submersibles. There are a lot of underwater communication methods in use/under development. Going even further down the spectrum, signals in the extremely low frequency (ELF: 3 Hz to 30 Hz) band are capable of penetrating 120 meters of seawater, which is deep enough for any submarine to maintain its stealth. While the high frequency (HF: 3 MHz to 30 MHz) and low frequency (LF: 30 kHz to 300 kHz) bands are perfectly capable of reaching across the globe thanks to ionospheric refraction, the high conductivity of seawater rapidly attenuates signals in these bands.Дialing down the spectrum a bit, the very low frequency (VLF: 3 kHz to 30 kHz) band starts to exhibit decent penetration of seawater, down to a depth of perhaps 20 meters. To keep the subs safely below the surface, naval commands began exploring the very lowest end of the radio spectrum. Transmitting messages to these submarines is an active area of research. In the event of a nuclear war, submergedВ ballistic missile submarinesВ have to be ordered quickly to launch their missiles. U-212 Class SubmarineДuring theВ Cold War nuclear-powered submarinesВ were developed that could stay submerged for months. Submarines can surface and raise anВ antennaВ above the sea level, then use ordinary radio transmissions, however this makes them vulnerable to detection byВ anti-submarine warfareВ forces. BecauseВ radio wavesВ do not travel well through goodВ electrical conductorsВ likeВ salt water, submergedВ submarinesВ are cut off fromВ radio communicationВ with their command authorities at ordinary radio frequencies.

Communication with submarinesВ is a field withinВ military communicationsВ that presents technical challenges and requires specialized technology.

Us navy vlf receiver