Communication with submarines
Early submarines during World War II mostly travelled on the surface because of their limited underwater speed and endurance, and dived mainly to evade immediate threats or for stealthy approach to their targets.
A few nations have built transmitters which use extremely low frequency (ELF) radio waves, which can penetrate seawater to reach submarines at operating depths, but these require huge antennas.
VLF radio waves (3–30 kHz) can penetrate seawater to a few tens of metres and a submarine at shallow depth can use them to communicate.
Natural background noise increases as frequency decreases, so a lot of radiated power is required to overcome it.
This precludes submarines from transmitting VLF, but a relatively simple antenna (usually a long trailing wire) will suffice for reception.
Instead, someone who wishes to construct such a facility has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites), bury two huge electrodes in the ground at different sites, and then feed lines to them from a station in the middle, in the form of wires on poles.
As the ground conductivity is poor, the current between the electrodes will penetrate deep into the Earth, essentially using a large part of the globe as an antenna.
Owing to the limited bandwidth, information can only be transmitted very slowly, on the order of a few characters per minute (see Shannon’s coding theorem).
that the actual messages were mostly generic instructions or requests to establish a different form of two-way communication with the relevant authority.
From the surface, submarines may use naval frequencies in the HF, VHF, and UHF bands, and transmit information via both voice and teleprinter modulation techniques.
A recent technology developed by a team at MIT combines acoustic signals and radar to enable submerged submarines to communicate with airplanes.
In 2009, a US military report stated that "Practical laser-based systems for deep depths were unavailable because lasers operating at the right colour with enough power efficiency to be used in satellites did not exist.
If DARPA can demonstrate such a system under realistic conditions, it would dramatically change how submarines can communicate and operate, thereby greatly enhancing mission effectiveness, for example, in anti-submarine warfare.
