The HWU transmitter is a French facility for transmitting orders to submerged submarines of the

French Navy The French Navy (, , ), informally (, ), is the Navy, maritime arm of the French Armed Forces and one of the four military service branches of History of France, France. It is among the largest and most powerful List of navies, naval forces i ...

. 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 b
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

sound card A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio ...

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 In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

and key used in the communications.

See also