Spread-spectrum steganography is the method of hiding a small or narrow-band signal (message) in a large or wide-band cover. This technology has been developed and used since the 1950s to provide a method of communication that is hard to intercept or jam. Similar to hiding a message within a picture, spread-spectrum steganography takes a smaller sound signal and mixes it in with a larger carrier signal. This makes this type of transmission very robust because there is so much extra cover signal that is surrounding the hidden message that it becomes like trying to find one person in a large crowd.
Another method of covert communication is the meteor scatter method. This method of communication is uniquely interesting. When a meteor travels through the Earth's ionosphere, it leaves behind an ionized trail. This trail can be used to reflect radio signals over great distances. As you might expect, special procedures are used when using the meteor scatter method. Typically, prearranged schedules are made outlining the time, frequency, and sequencing of the messages. Because the majority of the meteors falling allow a signal to be reflected for only a very short time, roughly a fraction of a second, a method called HSMS (High Speed Meteor Scatter) is often used. This method has been popular in Europe for many years and is now gaining popularity in North America.