406-MHz emergency locator transmitters must not be activated for testing during the first five minutes after any hour, unlike 121.5-MHz ELTs (see AIM revised paragraph 6-2-5). Since 406-MHz ELTs use a network of satellites, switching them on for even a brief test alerts search-and-rescue officials almost instantly. Unless prior approval has been granted in writing by the National Oceanic & Atmospheric Administration, the preferred method is to have an avionics technician remove the ELT’s antenna and use special 406-MHz ELT test equipment to perform a full system check. Test equipment is available for sale or lease through several manufacturers and distributors.