The first observation was by Sven Grahn in Stockholm with the TK beacon from Cosmos 114. Later, it emerged that the earliest beacon had been detected by Dieter Oslender of Bonn, Germany but he had not realised the significance of what he was hearing.

The first beacons detected by the Kettering Group were christened "TK" because of the resemblance to the Morse Code letters. The beacons came from the Zenit-4 flights and were transmitted at 19.995 MHz, the same frequency as the signals that were received during the mission.

It later transpired that Zenit-2 also had a beacon, TG. It had been missed because it was transmitted at a different frequency - 20.005 MHz, rather than the 19.995 MHz used while it was in orbit. The Zenit-2 beacons were finally detected at Kettering when a decision was made also to transmit them at 19.995 MHz.

Some of the later variants of Zenit used different letters (F and L ) but the common thread was the "T".

The reason for enclosing the beacon names in quotes is that, although the G, K, etc were Morse Code, the single dash of the "T" was longer than it should have been.

The over-long length of the "T" can be seen be seen in this plot created by Sven Grahn from an original TK beacon. A good description of the beacon signal can found at Sven's web site.

Out of deference to one of Korolyov's senior colleagues, maybe they should be re-christened KT, etc...... more on this below.

It was often given thought at the time, but other than letters being 'dot and dash' anagrams of each other, there was nothing really obvious. Of the Zenit beacons detected, K ( — ▪ — ) and G ( — — ▪ ) were made up of two dashes and a dot. The later F ( ▪ ▪ — ▪ ) and L ( ▪ — ▪ ▪ ) consisted of differing arrangements of a single dash and three dots. Transmissions of V ( ▪ ▪ ▪ — ) beacons that were detected at apparently-random times were probably from re-entering Yantar film-return capsules. It is a variation of F and L . The Recovery Beacon types for each Zenit version can be found at the foot of the page.

Based on the two observed dot-dash combinations, unused letters are W ( ▪ — — ) and B ( — ▪ ▪ ▪ ).

In standard Morse, a 'dash' is three times the length of a 'dot', and the gaps between dots and dashes within a letter are the same length as a dot. With the Zenit beacons, the "T" was equivalent in length to five dots rather than the conventional three dots.

All of the letters used have something in common - their lengths are equivalent to nine dots.

Looking at it as a dot or dash representing 'on' and a space being 'off', then the odd numbered positions are always 'on'. The letters are created by switching off a combination of the even numbered positions. The only proviso seems to be that it must create an acceptable Morse Code character, presumably to make the transmitted sequence easy to recognise by ear.

It brings up the possibility of one more combination where all of the even numbered position are off, the Morse Code for the number 5 ( ▪ ▪ ▪ ▪ ▪ ). That makes a total of three unused possibilities: TB, TW and T5.

Originally, the keying sequence for transmission could easily have been set up by a row of small switches, or soldered/printed links on a circuit board or commutator, simply turning-off the switches or omitting links in the positions where a space was required.

The fact that the chosen sequence was always a Morse letter may be down to a human factors, and there is no obviously-pressing need for it to vary between satellite types as it did. Perhaps the choice of a letter was simply to allow the recovery crews to differentiate between a weak signal from their target and extraneous background signals. On the very rare occasion two Zenit satellites re-entered within a few hours of each other, they were never the same Zenit type so the different beacons would have allowed listeners to determine which transmission was which.

There was an occasion where a variation on the "T" seems to have come into play. This is where we need to think of KT, KF, etc as mentioned earlier.

Soviet/Russian Acedemician Boris Chertok was a Deputy Chief Designer in Korolyov's team. In his memoirs "Rockets and People", he talks about the locator beacon from the Voskhod 2 spacecraft. His description is of a Morse Code sequence VN.... VN.... VN.... VN..... He describes it as a standing for the Russian words Все Нормально - "all normal" in English. In his personal account of descending inside Vostok, Yuri Gagarin also refers to transmitting "all normal" as a hand-keyed signal.

Of the two letters, V is a permissible variant of the beacon as demonstrated with its use by Yantar.

However, what of Chertok's "N" ?

Given the five-dot length of the "T" in the Zenit transmission, turning off the fourth position produces a standard Morse Code character of a dash followed by a dot.... the letter N :

With the close relationship between the origins of Vostok, Voskhod and Zenit, it is no real surprise that the beacons seem to be related.

One further type of Morse beacon, sounding very much like the Zenit transmissions was heard on a number of occasions from 1969 into the early 1970s. It was the alternating letters A and N transmitted at 20.005 MHz. This was reportedly a beacon carried by Soyuz and transmissions sometimes played out for many hours. The conclusion at the time was that it was associated with vehicle recovery practice or training sessions. There are no log entries for AN beacons near 20 MHz following re-entry at the end of actual Soyuz orbital missions, but the frequency is sometimes given as 10.003 MHz (half the value).

Soyuz still uses AN as a beacon but it is transmitted at the VHF international distress frequency of 121.5 MHz rather than the HF of the earlier days. It can sometimes be heard in the background during Russian coverage of Soyuz returns from the ISS when the broadcast microphone is near one of the ground crew radio receivers or when TsUP injects the signal directly into the TV feed.

AN can be formed by two of the long-style letter "T" with positions 2 and 4 switched off alternately ( ▪ —    — ▪ ) but that interpretation may be no more than fancy.

Zenit Version	First Mission	Beacon
Vostok (Piloted)	1961	VN
Zenit-2	1962	TG
Zenit-4	1963	TK
Voskhod	1964	VN
Zenit-2M	1968	TK/TG
Zenit-4M	1968	TK
Zenit-4MK	1969	TF
Zenit-4MT	1971	TL
Zenit-4MKT	1975	TK
Zenit-6	1976	TF
Zenit-4MKM	1977	TF
Resurs-F1	1979	TF
Zenit-8	1984	TF