HWU transmitter is a French facility for transmitting orders to submerged submarines of the French Navy. HWU transmitter, situated near Rosnay at 46°42'47"N, 1°14'39"E, is one of the largest radio transmitters in France and is clearly visible on satellite pictures. It uses an antenna carried by thirteen guyed masts (the tallest one 357 m, 6 of 310 m, 6 of 270 m), which are, together with the masts of Allouis longwave transmitter, the tallest structures in France.
HWU transmitter works on 18.3 kHz, 20.9 kHz and 21.7 kHz. Its transmissions can be easily received in Europe when the transmitters are active by using a PC soundcard connected to a receiving coil and FFT software. Although the signals are very easy to receive, it may be impossible to decipher them without knowing the cipher and key used in the communications.