Letter beacons are radio transmissions of uncertain origin and unknown purpose, consisting of only a single repeating Morse code letter. They have been classified into a number of groups according to transmission code and frequency, and it is supposed that the source for most of them is Russia. (Some beacons sending Morse code letters are well known directional or non-directional beacons for radio navigation. These are not discussed in this article since spies don't use them.) Letter beacons have been referred to as: