RWM: the Russian time and frequency standard from Moscow on 4996 kHz

What you hear

RWM has no speech and no music, which is why the catalogue sets CW: with the beat oscillator on, the unmodulated carrier becomes a steady note and the pulses become beeps. In AM you would hear almost nothing for minutes on end. The cycle repeats every half hour. From the top of the hour and from the half hour there are eight minutes of bare carrier, a pure tone with nothing on it. Around the eighth minute the station sends its callsign, RWM in slow Morse, over and over for about a minute. From the tenth minute to the twentieth comes one pulse per second, a tidy beep each second with a longer one at the minute; the lengths of some of these pulses are coded to carry DUT1, the difference between UTC and the time given by the Earth's rotation. From the twentieth to the thirtieth minute the pace rises to ten pulses per second, which sounds like a buzz, again with a marker at each minute. Then the carrier returns and the cycle begins afresh.

On the waterfall the station is a razor-thin line in the carrier minutes and a dotted one in the pulse minutes, which makes it easy to spot even when the audio is buried in noise.

Who transmits and why

RWM is the shortwave arm of the Russian state time and frequency service, run by VNIIFTRI, the national metrology institute outside Moscow. The transmitter stands at the Taldom site about a hundred kilometres north of the capital. Its job is calibration: the carrier is derived from the national standard, so a laboratory, a plant or a broadcaster anywhere in the country can check its oscillators against it and set its clocks from the pulses. Satellite timing has taken over most of that work, but the station keeps going as the reference that needs no satellites.

The same pattern goes out on 4996, 9996 and 14996 kHz. The odd "996" is deliberate: 4 kHz below the round 5, 10 and 15 MHz where WWV, BPM and other time stations sit, so that the signals do not collide, yet a receiver tuned to the round frequency with a wide filter sees both the Russian pulses and the American or Chinese ticks side by side.

When and where to listen

4996 kHz is the night and winter frequency. After dark it covers European Russia, Scandinavia, the Baltic states and Poland with a strong signal and usually reaches all of Europe; by day it is heard only a few hundred kilometres from the transmitter. Receivers in Finland, the Baltics and Poland hear it like a local station, those in Germany, the Netherlands and Britain steadily through the night. For the daytime there is 9996 kHz, and 14996 kHz for long distances when the sun is high.

CW is the right mode for a bare carrier, but there are alternatives: some listeners set USB with the dial 1 kHz below the carrier and get the same note with a wider filter, which helps when the receiver drifts a little.

In ShackDeck

Station catalogue → Time signals → RWM Moscow 4996. The program picks a public receiver that hears 4996 kHz, tunes it in CW with a narrow passband so that the carrier comes out as a clean tone, and shows the waterfall where the pulse minutes appear as a line of dots. No receiver, antenna or account of your own is needed. The daytime frequency, RWM Moscow 9996, is in the catalogue as well.