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.
Electronic emissions can be used to plot a line of bearing to an intercepted signal, and if more than one receiver detects it, triangulation can estimate its location. Radio Detection Finding (RDF) was critically important during the Battle of Britain and reached a high state of maturity in early 1943 with the aid of United States institutions aiding British Research and Development under the pressures of the continuing Battle of the Atlantic during World War II when locating U-boats. One key breakthrough was marrying MIT/Raytheon developed CRT technology with pairs of RDF antennas giving a differentially derived instant bearing useful in tactical situations, enabling escorts to run down the bearing to an intercept. The U-boat command of Wolfpacks required a minimum once daily communications check-in, allowing new Hunter-Killer groups to localize U-boats tactically from April on, leading to dramatic swings in the fortunes of war in the battles between March, when the U-boats sank over 300 allied ships and "Black May" when the allies killed at least 44 U-boats—each without orders to exercise EMCON/radio silence.
Radio silence can be maintained for other purposes, such as for highly sensitive radio astronomy. Radio silence can also occur for spacecraft whose antenna is temporarily pointed away from Earth in order to perform observations, or there is insufficient power to operate the radio transmitter, or during re-entry when the hot plasma surrounding the spacecraft blocks radio signals.
Radio silence can be used 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.
Examples of radio silence orders
- Radio silencing helped hide the Japanese attack on Pearl Harbor in World War II. The attackers had used AM radio station KGU in Honolulu as a homing signal.
- On June 2, 1942, during World War II, a nine-minute air-raid alert, including at 9:22 pm a radio silence order applied to all radio stations from Mexico to Canada.
- Iraqi ships maintaining radio silence The Guardian
- Emissions Control 3 Mission Airforce Technology
- George Musser (July 14, 2015). "New Horizons Emerges Unscathed from Pluto Flyby". Scientific American.
- Jim Algar (November 15, 2014). "Philae Lander Historic Comet Mission Cut Short? Probe Goes on Radio Silence as Battery Depletes". Tech Times.
- Charles Q. Choi (June 16, 2015). "New Spaceship Antenna Prevents Radio Silence During Fiery Re-Entry". Space.com.
- U.S. Coast Guard, Radiotelephone Handbook, COMDTINST M2300.7
- Pearl Harbor National Geographic