Letter beacon

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.
Letter beacons have been referred to as:
Transmission locations.
These radio transmissions were discovered in the late 1960s. Their presence became known to the wider amateur radio community in 1978, when beacon "W" started transmitting on 3584 kHz, in the 80 meters band. There is indirect evidence that this particular transmitter was located in Cuba.
In 1982 there were also reports, supposedly based on HF direction finding by the US military, that beacon "K" transmitting on 9043 kHz was located at , near the city of Khabarovsk in the USSR. A few years later, it was suggested that the "K" beacons were actually located at Petropavlovsk on the Kamchatka Peninsula and the "U" beacons were located at the Barents Sea coast, between Murmansk and Amderma.
According to D.W. Schimmel, in 1986 the Federal Communications Commission (FCC) released the following HF direction finding results for single letter beacons, all of which indicate locations in the USSR:
The link with the USSR and, more recently, Russia is further supported by the existence of single letter beacons transmitting letters existing only in the Cyrillic Morse code alphabet.
The ENIGMA group also accepted these locations for cluster beacons "C", "D", "P" and "S", adding Vladivostok for beacon "F".
A recent source (2006) regarding locations was published on the Web by Ary Boender. This publication also contains an extensive list of letter beacon frequencies, both current and historical. The following locations are given for cluster beacons:
For solitary beacons and markers, Boender suggests these locations:
Transmissions of the "P" beacon in December 2007, even on medium frequency (420 and 583 kHz), suggest the Russian naval base at Kaliningrad as a possible source. Kaliningrad officially uses the ITU registered call sign "RMP".
Types.
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.
Cluster beacons.
A group of radio beacons with single-letter identifiers ("C", "D", "M", "S", "P", "A", "M" and "K") have been regularly reported near 3594, 4558, 5154, 7039, 8495, 10872, 13528, 16332 and 20048 kHz. The term "cluster beacons" is frequently used for them, as they transmit in parallel on frequencies only 0.1 kHz apart. These beacons transmit only their single-letter identifier in standard CW (A1A) using Morse code.
The following call signs and frequencies of cluster beacons were reported between September 2007 and November 2008 in the Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:
Occasionally, some cluster beacons (especially "F" and "M") have been reported transmitting on frequencies different from their regular channel for short periods.
Solitary beacons and channel markers.
A second family of letter beacons includes all those operating outside the clusters. For this reason, they are often called "solitary beacons" or "solitaires". They also transmit their single-letter identifier in standard CW (A1A) using Morse code.
A few solitary beacons, like "R" on 4325.9 and 5465.9 kHz, operate exactly like the cluster beacons, sending only their single letter identifier.
However, the majority of solitary beacons, most notably "P" on various MF and HF frequencies, transmit their single-letter identifier in Morse code. Sometimes the routine transmission is interrupted and brief messages are sent in fast Morse code or in an FSK digital mode. Therefore, a more appropriate term for these beacon-like single-letter transmissions is "channel markers", as their purpose is to occupy and identify a particular HF transmission channel when no traffic is transmitted. There is no evidence that the cluster beacon "P" and the solitary beacon "P" are directly related.
It was reported in "Numbers and Oddities", issue 142, that beacon C on 8000 kHz also transmitted messages under the regular call sign RIW, which is allocated to a Russian naval communication station in Khiva, Uzbekistan.
There are also a few oddities transmitting signals with poor modulation and irregular timing, like "V" on 5342 and 6430.7 kHz.
The following call signs and frequencies of solitary beacons and markers have been reported recently (September 2007 to September 2009) in Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:
FSK beacons.
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.
ENIGMA designation.
ENIGMA devised a naming scheme for all stations in their sphere of interest. In the original scheme, the following identifications were issued to letter beacons:
ENIGMA-2000, the Internet-based ENIGMA successor group, revised the original ENIGMA designators. The current designations for letter beacons are the following (since 2007):
Applications.
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.
Similar systems.
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).