É chamado de guarda porque todos devem ouvir / guardar a frequência apenas no caso de alguém ter algum problema.
Definições de proteção:
- a state of caution, vigilance, or preparedness against adverse circumstances
- watch over in order to protect or control.
O OBJETIVO 6-3-1 (h) (1) diz ( ênfase minha):
121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, FSSs, and radar facilities. Normally ARTCC emergency frequency capability does not extend to radar coverage limits. If an ARTCC does not respond when called on 121.5 MHz or 243.0 MHz, call the nearest tower or FSS.