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 sank at least 44 U-boats—each without orders to exercise EMCON/radio silence. Other uses 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. In the USA, CONELRAD and EBS (which are now discontinued), and EAS (which is currently active) are also ways of maintaining radio silence, mainly in broadcasting, in the event of an attack. 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. See also Dead air Guard band The Zone of Silence Radio quiet zone CONELRAD References Military communications Radio communications Spacecraft communication Emergency Alert System Civil defense Silence