Radio silence

In telecommunications, radio silence is a status in which all fixed or mobile radio stations in an area are asked to stop transmitting for safety or security reasons.

The term "radio station" may include anything capable of transmitting a radio signal.

Military
An order for Radio silence is generally issued by the military where any radio transmission may reveal troop positions, either audibly from the sound of talking, or by radio direction finding. In extreme scenarios Electronic Silence ('Emissions Control' or EMCON) may also be put into place as a defence against interception.

In the British Army, the imposition and lifting of radio silence will be given in orders or ordered by control using 'Battle Code' (BATCO). Control is the only authority to impose or lift radio silence either fully or selectively. The lifting of radio silence can only be ordered on the authority of the HQ that imposed it in the first place. During periods of radio silence a station may, with justifiable cause, transmit a message. This is known as Breaking Radio Silence. The necessary replies are permitted but radio silence is automatically re-imposed afterwards. The breaking station transmits its message using BATCO to break radio silence.

The command for imposing radio silence is:
 * "Hello all stations, this is 0. Impose radio silence. Over."

U.S. practice, if an individual is heard transmitting inappropriate information over an unencrypted radio network, or simply one that is not authorized for the type of information being given, involves the BEADWINDOW codes. BEADWINDOW is not used if calling it out would draw attention to the violation. Used by Australia, Canada, New Zealand, the United Kingdom, the United States, and other nations working under their procedures. Standard BEADWINDOW codes (e.g., "BEADWINDOW 2") include:
 * Position: E.g., disclosing, in an insecure or inappropriate way, "Friendly or enemy position, movement or intended movement, position, course, speed, altitude or destination or any air, sea or ground element, unit or force."
 * Capabilities: "Friendly or enemy capabilities or limitations. Force compositions or significant casualties to special equipment, weapons systems, sensors, units or personnel. Percentages of fuel or ammunition remaining."
 * Operations: "Friendly or enemy operation – intentions progress, or results. Operational or logistic intentions; mission participants flying programmes; mission situation reports; results of friendly or enemy operations; assault objectives."
 * Electronic warfare (EW): "Friendly or enemy electronic warfare (EW) or emanations control (EMCON) intentions, progress, or results. Intention to employ electronic countermeasures (ECM); results of friendly or enemy ECM; ECM objectives; results of friendly or enemy electronic counter-countermeasures (ECCM); results of electronic support measures/tactical SIGINT (ESM); present or intended EMCON policy; equipment affected by EMCON policy."
 * Friendly or enemy key personnel: "Movement or identity of friendly or enemy officers, visitors, commanders; movement of key maintenance personnel indicating equipment limitations."
 * Communications security (COMSEC): "Friendly or enemy COMSEC breaches. Linkage of codes or codewords with plain language; compromise of changing frequencies or linkage with line number/circuit designators; linkage of changing call signs with previous call signs or units; compromise of encrypted/classified call signs; incorrect authentication procedure."
 * Wrong circuit: "Inappropriate transmission. Information requested, transmitted or about to be transmitted which should not be passed on the subject circuit because it either requires greater security protection or it is not appropriate to the purpose for which the circuit is provided."
 * Other codes as appropriate for the situation may be defined by the commander.

Non-military radio silence orders
Radio silence can also be maintained for other purposes, such as for highly sensitive radio astronomy, or in nautical and aeronautical communications to allow faint distress calls to be heard (see Mayday). In the latter case, the controlling station can order other stations to stop transmitting with the proword "Seelonce Seelonce Seelonce". (The word uses the French pronunciation of the word silence, "See-LAWNCE."). Once the need for radio silence is finished, the controlling station lifts radio silence by the prowords "Seelonce FINI."

Disobeying a Seelonce Mayday order constitutes a serious criminal offence in most countries.

The aviation equivalent of Seelonce Mayday is the phrase or command "Stop Transmitting - Distress (or Mayday)". "Distress traffic ended" is the phrase used when the emergency is over. Again, disobeying such an order is extremely dangerous and is therefore a criminal offence in most countries.

In the USA, CONELRAD, EBS and EAS were also a way of maintaining radio silence, mainly in broadcasting, in the event of an attack.

Radio silence orders

 * Radio silencing occurred after Japan attacked Pearl Harbor in World War II using AM radio station KGU in Honolulu as a homing signal.
 * On June 2, 1942, during WWII, a nine-minute air-raid alert, including at 9:22 pm a radio silence order applied to all radio stations from Mexico to Canada.