The main civil voice frequency 121.5 MHz is monitored by most air traffic control towers, FSS services, national air traffic control centers, military air defense and other flight and emergency services, as well as by many commercial aircraft.
民用语音频率121.5 MHz受到大多数空中事务单位监控，包括空管塔台、FSS（英语：Flight service station）服务、国家空管中心、军事防空、其他飞行和紧急服务，以及许多商用飞机。