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 about 30 of those transmitters around the world, but each SID monitor only listens to one. And the only transmitters we listen to are:
Although we listen to transmitters in the US, they can be picked up all over the world. That's the point of using VLF radio waves!
- NAA - located in Maine, USA
- NLK - located in Washington, USA
- NML - located in North Dakota, USA
Because the VLF waves are long, the transmitters are big. Usually their wires run from mountain top to mountain top!