HWU transmitter
The HWU transmitter is a French facility for transmitting orders to submerged submarines of the French Navy. Situated near Rosnay at 46°42'47"N, 1°14'39"E, it is one of the largest radio transmitters in France and is visible on satellite pictures although it is completely blurred out. 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 the Allouis longwave transmitter, the tallest structures in France. The 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. See also 

 Sainte-Assise transmitter
 List of masts
 List of tallest structures in France
 Torreta de Guardamar
 Anthorn Radio Station
 Skelton Transmitting Station

Military radio systems
Towers in France
French Navy technology
Buildings and structures in Indre
Transmitter sites in France