What signal is used to impose silence on all stations when using RATT/NDPB?

The signal used to impose silence on all stations when using RATT (Radio Teletypewriter) or NDPB (Narrowband Digital Point-to-Point) is indeed "QRT SOS." This signal indicates that all stations must cease transmission immediately due to a specific operational requirement. It is a directive that ensures all stations return to silence, which is crucial for maintaining order and clarity in communication, especially in situations where important information needs to be conveyed or when there may be emergency conditions present.

The other signals listed serve different purposes. "PAN PAN" is used to indicate an urgent message that is not immediately life-threatening but requires attention. "MAYDAY" is the universal distress call indicating a life-threatening situation where immediate help is required. "SECURITE" is used for safety messages or warnings concerning navigation or a potential hazard. Each of these signals is designed to communicate specific levels of urgency and purpose in maritime and emergency communications, which highlights the importance of correct usage in maintaining effective and clear communication protocols.