A callsign is a pseudonym that radio operators such as CAPCOM will use to refer to a specific person or group of people.
- The callsign for Mission Control is "M.C." (pronounced "emm-cee").
- The callsign for the Habitat is "Habitat", though "Hab" is also acceptable.
- The callsign for Earth Station 1 is "Earth" or "ES1" (pronounced "ee-ess-one").
In recent times, callsigns have come to be used as friendly nicknames, and have entered the general Spacesim culture. Some examples from the Mission Daedalus Astronauts are "Angel" for Lt. Christian Angel, "Africa" for Lt. Cmdr. Peter Macdonald and "Euro" for Lt. Euan Wheaton.