RSDN-20 Alpha: the Russian VLF navigation system on 12.649 kHz

What you hear

The lowest frequency in the catalogue, and the strangest-looking signal on it. On the waterfall around 12.649 kHz there are no steady ribbons like the submarine transmitters' but short, bright pillars: pulses a fraction of a second long, appearing in a fixed sequence, vanishing, appearing again, with the same pattern repeating every few seconds. Look wider and the same pillars appear on 11.905 and 14.881 kHz, keeping time with each other. In CW mode each pulse is a clean tone burst, and the sequence sounds like a slow, patient rhythm, more a clock than a message.

This is RSDN-20, better known by its name Alpha, and the pulses are not communication at all. They are a measuring signal, and the only information in them is their timing.

Who transmits and why

Alpha is a Russian radio navigation system that has been in service since 1962. A small number of very powerful transmitters, at Krasnodar in the south, Novosibirsk in Siberia, Khabarovsk in the Far East and Revda in the far north-west, take turns sending pulses on the three frequencies. Because VLF propagation is so stable, a receiver thousands of kilometres away can measure the phase of the pulses from different stations against each other, and from the phase differences compute where it is. It is the same idea as the Omega system the West ran until the 1990s, and the same idea, with much more precision, that satellite navigation later made universal. Alpha is a forerunner of GPS, and unlike Omega it was never switched off: it is still used by ships and aircraft that need a fix when satellites are unavailable or untrusted.

There is no secret content here. The pulses are what they are, a timing signal that anyone can receive. The order of the stations within the cycle is fixed; the article does not list it, because it is easier to see than to read.

When and where to listen

The transmitters are strong, and VLF propagation is steady by day and stronger but more variable at night, so Alpha can be seen at any hour. Receivers in Finland, the Baltic states, Poland and Scandinavia see the Revda and Krasnodar stations clearly; from central and western Europe the pillars are fainter but visible on a quiet receiver, and receivers in Russia and Asia are closer to the other stations. Each station has its own slot in the cycle, so on a receiver that hears several, the pillars on one frequency come in a staggered pattern rather than all at once, which is how you tell them apart.

At 12.6 kHz the receiver is everything. Many public receivers do not go this low, or have antennas that do, but with a wall of mains harmonics beneath. Pick a rural receiver, let the waterfall run for a minute to see the full cycle, and widen the view to take in all three frequencies. The program tunes the catalogue frequency in CW with a passband of about a kilohertz, so each pulse arrives as a short tone burst; there is nothing to decode, and the pattern is the point.

In ShackDeck

Station catalogue → Mystery stations → RSDN-20 Alpha (12.649 kHz). The program picks a public receiver whose range reaches 12.649 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. If the pulses are not visible, try a receiver in Finland, Scandinavia or the Baltic states, where the northern stations are strongest.