I'm not sure if this still happens, so the question may be broader, but in the Apollo transcripts the astronauts would begin a call with something like "Houston, 13". I get that they may have been on an open mic so they would want to clearly identify that they were speaking to mission control. So starting with "Houston" makes sense. But... Who else would it be other than the astronauts? Why bother saying the number, or even the spacecraft name (as in "Houston, Aquarius")?

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    $\begingroup$ Because it is standard in radio communication to open with "(You), (me)" $\endgroup$ Apr 13, 2020 at 10:09
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    $\begingroup$ It would have been very important for the two capsules to state their mission numbers during the Gemini 6 & 7 rendezvous in 1965. $\endgroup$
    – Fred
    Apr 13, 2020 at 12:28
  • $\begingroup$ There was only one shuttle in space at a time but they still said "Houston, Atlantis" for example. @HagenvonEitzen why not add a reference and post that as the answer, it is correct. $\endgroup$ Apr 13, 2020 at 13:04
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    $\begingroup$ Even if the preamble words are not so informative, it is easier to understand the rest of the unknown message by hearing the lead familiar words. The listeners attune their hearing to the present timbre and noise level based on that preamble making the rest of the message easier to quickly understand. $\endgroup$ Apr 13, 2020 at 19:18

2 Answers 2


The standard protocol for any radio communication is to specify the intended recipient and the transmitter's call sign. This practice started in the earliest days of radio. Stations were sharing a common messaging medium, so there was a need for stations to identify each other; hence the invention of call signs. Even today, ham radio operators, broadcast radio stations, and television stations must obtain a call sign (e.g. Kxxx or Wxxx) from their government.

The use of call signs continued as the military, aviation, and then spaceflight adopted the use of radio communication. Even with their own dedicated frequencies, it is useful to have a way to distinguish between individual units. Astronauts through the Apollo program all had a pilot's license, and many of them came from the military, so the use of call signs was already ingrained.

The Gemini program did not give proper names to their spacecraft (see "Why weren't Gemini capsules given names?") but instead used their mission number as their call sign. Several Gemini missions involved the rendezvous of two spacecraft. When both spacecraft were manned, it was particularly necessary to use the call signs to distinguish the recipient of a transmission.

Apollo had two spacecraft -- the command/service module and the lunar module -- so again it was necessary to use call signs to distinguish the spacecraft. Many of the Apollo astronauts were involved in the Gemini program, so the protocols had already been established. Even though Apollo spacecraft had proper names, the Gemini tradition of using mission numbers was still common.

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    $\begingroup$ "Even though Apollo spacecraft had proper names, the Gemini tradition of using mission numbers was still common." The Apollo flights would use the mission number as callsign when all the crew were in the CM and there was no ambiguity about which spacecraft the call was coming from. When the LM was occupied and powered-up, the callsign would be the ship name rather the mission number. Interestingly, during the return flight of Apollo 13, even though the CM was shut down and all the communication was done from the LM, the callsign used was "Aquarius" rather than "13". $\endgroup$ Apr 13, 2020 at 15:14
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    $\begingroup$ @RussellBorogove I think, under the circumstances, that the Apollo 13 crew could be forgiven for not using the correct callsign when operating from the LM. $\endgroup$ Apr 13, 2020 at 18:35
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    $\begingroup$ The use of call signs here may even have been legally required under national and international regulations governing radio communications. Though, I can't say I'm familiar with the exact rules that would have applied. $\endgroup$ Apr 14, 2020 at 2:27
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    $\begingroup$ In aviation, not only is the "greeting" standardized, but the whole communication uses a special jargon composed of standardized phrases. This is called standard phraseology. For example, since the Tenerife runway collision, still the deadliest aviation accident, it is no longer allowed to use the term "takeoff" for anything but the actual takeoff clearance, i.e. the clearance to accelerate down the runway and take off. Any time before that, you must use the term "departure". If someone reports "ready for takeoff", a lot of people are going to be very nervous very quick! $\endgroup$ Apr 14, 2020 at 6:59
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    $\begingroup$ A minor aside, but as far as I know, broadcast radio and television stations using call signs is a mostly American thing; I've certainly never seen them in the UK. (Analogue radio stations often use their allocated frequency as part of their brand, and there were only ever 5 terrestrial analogue TV stations, all national.) $\endgroup$
    – IMSoP
    Apr 14, 2020 at 13:59

In addition to DrSheldon’s points, it’s worth noting that the communications between Houston and the spacecraft are only one part of what each controller in mission control is hearing. Rather than shout across the mission control room, each controller wears a headset that lets them monitor multiple communications channels ("loops"). Each controller can talk with their “back room” support team, without e.g. the guidance officer being distracted by the flight surgeon’s back room discussions.

Given this many-to-many communications system, the convention of starting each conversation with “addressee, [this is] speaker” gives a listener a chance to decide if they need to listen to the rest of the message, which takes some conscious effort when listening to multiple conversations at once. While it was usually clear from the audio quality which voices were from the spacecraft, it was still helpful to hear the preamble.

It’s instructive to listen to a recording of the combined flight controllers’ audio loop from such a mission and practice tracking different conversations. The podcast The Space Above Us has two of the most famous segments of Apollo program audio, with a brief introduction to each:

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    $\begingroup$ Each controller monitors many loops: pretty much always air-to-ground and the flight loop, their back room loops, and whoever else they are talking to. Learning to filter this stuff in your head is a large part of flight controller training. $\endgroup$ Apr 13, 2020 at 15:25
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    $\begingroup$ Not only to hear what is transmitted, but also to stop talking yourself so you don't talk over air-to-ground. (After hours of all those voices, I would be flying home and be hearing murmurs of ghost voices in the plane noise.) $\endgroup$ Apr 15, 2020 at 18:44
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    $\begingroup$ I maintain that shuttle Systems SCA instructors had it the worst of all in integrated sims: monitoring 3 flight controller teams (EGIL, EECOM, MMACS), the SMS instructor, a/g, and the flight loop. Throw in space-to-ground and the SSTF instructors for joint integrated sims, and it makes my head hurt just to remember it. $\endgroup$ Apr 15, 2020 at 19:40

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