HWU Rosnay: the French Navy's VLF transmitter on 20.9 kHz

What you hear

On 20.9 kHz, a broad, bright band of data on the waterfall and a steady, grainy tone in CW mode: the signature of minimum-shift keying, MSK, that every submarine transmitter in this guide shares. HWU is one of the loudest VLF signals in Europe, on a par with the German DHO38 on receivers in France and the Low Countries, and it is the station most likely to be the first thing a newcomer notices when a waterfall is scrolled down into the very-low-frequency band for the first time. The stream never stops and never changes character; it is encrypted, and no decoder can make text of it.

The station is not tied to a single frequency. Besides 20.9 kHz, the catalogue notes 18.3 and 21.75 kHz, and a listener looking at the whole band will often find HWU on more than one of them. Which frequencies are in use at a given time is decided by the navy, not announced, and the catalogue entry on 20.9 kHz is a starting point rather than a promise.

Who transmits and why

HWU belongs to the French Navy and stands at Rosnay, in the flat countryside of central France near Bourges, about as far from the sea as it is possible to get in that country. The distance does not matter: very long waves travel in the gap between the ground and the ionosphere and reach the oceans on either side, and they penetrate a few metres into sea water, which is how a submarine at shallow depth or trailing a wire antenna can receive a message without surfacing. France operates a fleet of submarines, including the ballistic-missile boats that must be reachable at all times, and Rosnay is the transmitter that reaches them. For that reason it runs around the clock.

What it sends is secret, and no listener will learn it. That the station exists, where it is and what frequencies it uses are public facts, and listening to it is only receiving.

When and where to listen

The stream is on the air at all hours, and VLF propagation is steady enough that the time of day matters little, with the usual slight strengthening and more variation at night and a wobble around sunrise and sunset. Receivers in France, Belgium, the Netherlands, Britain, Spain and Switzerland hear HWU at great strength; from Germany, Italy and Scandinavia it is strong, and from eastern Europe still readable on a quiet site. The receiver's antenna must work this low, and the site must be away from the mains harmonics and switching noise that fill the band near towns; a rural receiver is the one to pick.

The program tunes the station in CW with a passband of about a kilohertz, placing the stream as a tone in the middle of the audio. Look at the whole VLF stretch on the waterfall while you are there: HWU on one or more of its frequencies, GQD at 19.58, ICV at 20.27, DHO38 at 23.4 and NAA at 24.0 kHz make a line of submarine transmitters within five kilohertz of each other. If 20.9 kHz is quiet, check 18.3 and 21.75 kHz before concluding that the station is off.

In ShackDeck

Station catalogue → Mystery stations → HWU Rosnay (20.9 kHz). The program picks a public receiver whose range reaches 20.9 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. A receiver in France or Benelux shows the signal at its loudest, and the other frequencies of the station are a short retune away.