Civil Alert System Broadcast is a radio signal in Fallout 4. It can be received after extending the satellites at relay tower 0BB-915. the message is a pre-recorded Civil Alert Message, warning the general public of the falling bombs.
The origin of the broadcast is a ham radio in a military bunker, which can be accessed from a hatch at the base of the southwestern satellite at the Fort Hagen satellite array. Note that this is NOT the same bunker as the one involved in the quest line of Automatron. It can be accessed via Military Bunker entrance located in base of array dish opposite to one that contains Automatron (add-on) bunker.