Communication With Submarines - Very Low Frequency

Very Low Frequency

VLF radio waves (3–30 kHz) can penetrate seawater to a depth of approximately 20 meters. Hence a submarine at shallow depth can use these frequencies. A vessel more deeply submerged might use a buoy on a long cable equipped with an antenna. The buoy rises to a few meters below the surface, and may be small enough to remain undetected by enemy sonar / radar.

Due to the low frequency a VLF broadcast antenna needs to be quite big. In fact, broadcasting sites are usually a few square kilometers (or miles). This prevents such antennas being installed on submarines. Submarines only carry a VLF reception aerial, and do not respond on such low frequencies. So a ground-to-submarine VLF broadcast is always a one way broadcast, originating on the ground and received aboard the boat. If two-way communication is needed, the boat must ascend to periscope depth (just below the surface) and raise a telescopic mast antenna to communicate on higher frequencies (such as HF, UHF or VHF).

Because of the narrow bandwidth of this band, VLF radio signals cannot carry audio (voice), and only transmit text messages at a slow data rate. VLF data transmission rates are around 300 bit/s - or about 35 8-bit ASCII characters per second (or the equivalent of a sentence every two seconds) - a total of 450 words per minute. Simply shifting to 7-bit ASCII increases the number of transmitted characters per time unit by 14%. An additional shift to a 6-bit or a 5-bit code (such as the baudot code) would result in speeds of more than 600 and 700 words per minute.