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Radio transmissions consisting of only a single repeating Morse code letter

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 and began during the Soviet Union. (Some beacons sending Morse code letters are well known directional or non directional beacons for radio navigation. These are not discussed in this article.) Letter beacons have been referred to as:
SLB, or "Single Letter Beacons"
SLHFB, or "Single Letter High Frequency Beacons"
SLHFM, or "Single Letter High Frequency Markers"
Cluster beacons
MX — an ENIGMA and ENIGMA 2000 designation.

The single letter beacons can be classified into two groups, "cluster beacons" and "channel markers". A beacon "P" exists in both groups. A third group, called FSK beacons, is now extinct. The following sections list the beacons active as of December 2007, according to published listeners’ reports.

This group includes the "K" and "U" beacons, which are no longer active. They transmitted their Morse code single letter identification by shifting the frequency of the carrier by approximately 1000 Hz. This mode of "FSK CW" is designated F1A. The use of FSK indicated that the transmitter was suitable for FSK data transmissions, like radioteletype.

The purpose of the letter beacons is not yet known with certainty. Many theories have appeared in specialized publications, but none are based on documentary evidence. They have been postulated to be radio propagation beacons, channel markers, or beacons used in tracking satellites or for civil defense purposes. Some stations of this family, in particular the "U" beacon, have been implicated in deliberate jamming. According to ENIGMA, cluster beacons are used by the Russian navy (especially its submarine branch) to find the most suitable radio frequency for contact based on current radio propagation conditions. Robert Connolly also links "P" channel marker with communications facilities at the Russian naval base of Kaliningrad. "P" transmissions carrying Russian navy "XXX" (flash priority) Morse code messages with call signs RPM and RDL further support this view.

A few aero navigation non directional beacons also transmit single letter identification codes. They can be easily distinguished from letter beacons because they transmit in the allocated low frequency and medium frequency bands; most of them are listed in appropriate aviation handbooks and their transmission mode is A2A (full carrier with audio modulation).