Because of the extreme difficulty of building transmitters that can generate such long waves, frequencies in this range have been used in very few manmade communication systems. However, SLF waves can penetrate seawater to a depth of hundreds of meters. Therefore in recent decades the U.S. and Russian military have built huge radio transmitters using SLF frequencies to communicate with their submarines, with India being in the process of constructing one to be commissioned by 2015. The U.S. naval service is called Seafarer and operates at 76 hertz. It became operational in 1989 but was discontinued in 2004 due to advances in VLF communication systems. The Russian service is called ZEVS and operates at 82 hertz.
The requirements for receivers at SLF frequencies is less stringent than transmitters, because the signal strength (set by atmospheric noise) is far above the noise floor of the receiver, so small, inefficient antennas can be used. Radio amateurs have received signals in this range using simple receivers built around personal computers, with coil or loop antennas connected to the PCs sound card. Signals are analysed by a software fast Fourier transformalgorithm and converted into audible sound.