Mouser.. posted:

2

A VLF stands for Very Low Frequency transmission. When a submarine is about 30m under the water, they are able to send shallow signals to an antenna. These are primarily used as the backup to submarines as the majority of them now utilize laser to satellite as their primary means of transmission. It generally looks like a buoy or a wire that is trailing behind the submarine, kind of like that wire that hangs off your stereo and you have to tape it to the wall to get it to maintain good signal.

If you are diving deep then you have to use Extremely Low Frequency antenna to communicate with the surface, but this only good for burst transmissions like directing a submarine to close distance or to ascend to receive further instructions. This is why in Crimson Tide they spent the entire movie arguing about going shallow and whether they should nuke Russia. Because the buoy that would enable them to receive detailed VLF or EHF instructions was severed and they would have to ascend to shallow waters in order to raise the mounted VLF antenna, otherwise they would be utilizing an incredibly slow ELF antenna which would be like when you tried to download a high resolution picture off a 2400 baud modem.