Very Low Frequency (VLF) Transmitters
For the SID monitors to be able to track VLF (very low frequency) radio signals, somebody has to be transmit those VLF signals. Lucky for us -- several nations have VLF transmitters they use to communicate with their submarines. Why? Because only VLF signals have a long-enough wavelength (like 12 kilometers or 8 miles) that will penetrate into the ocean. There are over 30 of those transmitters around the world. The older SID monitors could only listen to a single station. The newer SuperSID instruments can listen to 6 or 8 stations.
Depending upon the signal strength of the transmitter, many of the VLF signals can be picked up all over the world. However, you will generally receive stronger signals from transmitters nearest your location.
Because the VLF waves are long, the transmitters are big. Usually their wires run from mountain top to mountain top!