CIS Time Signals on VLF

Locations, callsigns and starting times of the received VLF time signal stations, 25 kHz

On January 10, 2020, I did a round-up of VLF time stations from the Commonwealth of Independant States (CIS). They are controlled by the Russian Navy and start their main transmission on 25.0kHz. Then they change to a couple of four other VLF channels. See here for some detailed information in Russian. The diagram below shows a panorama of all received station (Khabarovsk in the Far East missing, as they skip transmission on the 10., 20. and 30. each month) on all frequencies versus time and signal level.

Five locations, six transmissions, five frequencies – this diagram puts it all together.

The diagram features a time resolution of 1s and has a resolution bandwidth of about 0.12 Hz. It is part of a 24h session, made with Winradio’s Excalibur Sigma SDR, active dipole MD300DX (2x5m) and Simon Brown’s software SDRC V3. This software delivers also the values for level over time, which were visualized and combined with QtiPlot software.

Only seemingly, Vileyka and Krasnodar are transmitting on two channels at the same time (from 07:06 UTC/11:06 UTC). This is not the case, but their transmitters show a bit wider signal in their first part of the transmission. Thus, the much weaker (ca. -30dB) “signal” at the same time, but 100Hz up, is some kind of sideband, but not the carrier!

You will see some variation of the carrier power, especially following sign on, but also during the transmission. This can bee seen with tenfold time resolution (i.e. 100ms) and magnifying the dB-scale, see diagram at the bottom as just one example. Fading can be largely excluded for several reasons, artificial characteristic of changes and VLF propagation during short periods among them.

Under the microscope: This rise of 1.5dB of the carrier is part of the workflow of switching on/tuning the transmitter. There are many such details, and they may differ from transmitter, location and performance. Such details might be used for “fingerprinting”.

P.S. The map at the top was made with free software Tableau Public. The locations are geo-referenced, and a satellite map as background will you lead directly to the antennas. Please try this here.

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