is this used by military personnel?
Yes, tactical call signs
Call sign
Consistent call signs can aid in this monitoring, so in wartime, military units often employ tactical call signs and sometimes change them at regular intervals. In peacetime, some military stations will use fixed call signs in the international series.
what is apollo 9? It was a human spaceflight, the third in NASA's Apollo program. who were the crew members? McDivitt, Scott, and Schweickart did they use any call signs? Yes, "Gumdrop", "Spider" etc. where else are such signs used? In broadcasting and radio communications, and railroad stations
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Call sign [SEP] Military call signs
In wartime, monitoring an adversary's communications can be a valuable form of intelligence. Consistent call signs can aid in this monitoring, so in wartime, military units often employ tactical call signs and sometimes change them at regular intervals. In peacetime, some military stations will use fixed call signs in the international series. The United States Army uses fixed station call signs which begin with "W", such as WAR, used by U.S. Army Headquarters. Fixed call signs for the United States Air Force stations begin with "A", such as AIR, used by USAF Headquarters. The United States Navy, United States Marine Corps, and United States Coast Guard use a mixture of tactical call signs and international call signs beginning with the letter "N".