I worked on a system for the U.S. Navy back in the early 80s that used lower frequencies than that. The transmission rate was several characters a minute, encrypted, to be received by Submarines.<p>One interesting thing about this was that were were able to be the first people to use &quot;Software Defined Radio&quot; because the frequencies were low enough to deal with on a 20 MHz CPU. We used the Fairchild 9440&#x2F;9445, which was a Nova4 on a chip. 20MHz CPU was insane at this time. There was also special custom hardware on the front end digitizing things.<p>I can&#x27;t really tell you much more about the system ;-) but it was one of the most interesting projects I&#x27;ve ever worked on.