What type of systems are included in external communications?

External communications encompass various methods and tools used to facilitate communication with entities outside of a vessel or command structure. Visual communications and publications like Pub 102 are critical components of this system.

Visual communications include signals, flags, and other means of conveying messages visually, which are particularly important when other forms of communication may be unavailable, such as during adverse weather conditions or failures in electronic systems. Pub 102 serves as a standard reference for these communications, providing guidelines on how to effectively execute visual signals.

The other options do not represent external communication systems in the same way. Messenger systems primarily deal with internal communication, ship's service phones are intended for intra-ship communication rather than external contact, and navigation guides pertain specifically to navigational aid rather than communication. Understanding the nuances of external communication systems helps ensure that a vessel can effectively relay information to nearby craft and installations, maintaining operational safety and coordination.