How was it possible for astronauts to send back tv/radio signals to earth from the moon? I have just been teaching my son about waves behavior and understand that radio waves are reflected back to earth from the ionosphere. As far as I know microwaves were not used for communication purposes back then. How did the Apollo team then manage to send TV/ Radio signals back from space?
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1$\begingroup$ Interestingly, as you probably have read about, the iPhone has something like 2 million times the storage of the 1969 Apollo 11 spacecraft computer. $\endgroup$– Alexanders WilliamCommented Oct 9, 2015 at 21:28
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$\begingroup$ Page 2 in the PDF (page 41 of Aviation Week & Space Technology, January 20, 1969) of Apollo 8 Proves Value of Onboard Control has a diagram showing the Earth-bound and Earth-orbiting systems used to communicate with Apollo spacecraft and crew. $\endgroup$– userCommented Aug 15, 2016 at 14:48
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$\begingroup$ Aren't EM waves reflected due to total internal reflection at the ionosphere? In which case there's not much of a problem sending EM-waves straight through from the outside. Am I missing something? $\endgroup$– user2705196Commented Apr 8, 2021 at 12:22
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3 Answers
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It can be seen through this Wikipedia article that all communication with the Apollo missions was done at about 2.2 GHz, which is well above the frequency that reflects on the Ionosphere (No higher than about 30 MHz).
FYI, the Gemini radio system also used frequencies in the UHF range, although it included some VHF, and even some HF. The HF/ VHF signals were either used for voice communication, or nearby operations (Recovery, acquisition). HF was more used then than it is today, and allowed for somewhat of an over the horizon effect. Depending on propagation, it can be difficult, but the 15 MHz used has minimal interference.
Microwaves are considered those with a higher frequency than 300 MHz, so technically, they did use microwaves for the Apollo program. These signals work great to penetrate the atmosphere.
answered Oct 8, 2015 at 13:58
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1$\begingroup$ "Microwaves are considered those with a higher frequency than 300 MHz" I'm not so sure. At least in modern amateur radio, even 1300 MHz isn't generally considered "microwave". I think the best boundary between longer-than-microwave and microwave that I've seen is where waveguides and parabolic dishes become the preferred methods of coupling RF into the air, which happens somewhere in the range 1.5-2.0 GHz or so. Of course, terms have changed over the years; a hundred years ago, 3 MHz was "high frequency", and a little earlier, radio frequencies that high were considered utterly useless. $\endgroup$– userCommented Oct 25, 2015 at 13:56
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$\begingroup$ I remember that on at least 1 Apollo mission, they announced that they were able to receive radio signals directly from the space suits. One of the advantages of using higher frequencies is that some of the components, including the antennas are smaller at those frequencies. $\endgroup$ Commented May 5, 2016 at 1:27
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1$\begingroup$ Good info. In addition, if I recall, those frequencies that do bounce off of the ionosphere only do so for certain angles. It is possible for me in Arizona, on the 40M band, to easily talk to someone in Wyoming, via ionospheric bounce, or to someone in my antenna's line of sight 10 miles away, but very difficult to talk to someone 100 miles away in my own state. At that acute angle, the radio waves go straight through the ionosphere rather than bouncing back to Earth. $\endgroup$ Commented Jan 21, 2019 at 12:50
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http://novosti-kosmonavtiki.ru/mag/2005/1045/24532/ image of the Earth rising above the lunar horizon, taken on a television channel from one of the Apollo, intercepted by the Soviet tracking station in Crimea ("Shkolnoye").
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2$\begingroup$ while interesting, this does not provide an answer to this question. It would be a good answer to space.stackexchange.com/questions/33359/… $\endgroup$– HobbesCommented Jan 21, 2019 at 8:18
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$\begingroup$ In the text: "The TNA-400 was equipped with a low-noise receiver operating in the 13 cm range (S band, in which the transmitters of the Apollo lunar modules worked))." $\endgroup$ Commented Jan 23, 2019 at 5:09
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$\begingroup$ consider adding this to your answer to Hijacked space data, notable instances of recovering images or other goodies from someone else's space mission? I think it's a perfect fit! $\endgroup$– uhohCommented Jul 22, 2019 at 2:50
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1 Unified S Band 2.1 Ghz TV Signal / Voice / Telemetry / Medical Moon to Earth / Earth to Moon <*
2 Two UHF Frequencies 225-380Mhz band used on the moon <*
3 Various radar 8.1 Ghz
4 C-Band 5.6 GHZ
5 2 x Intelsat Satelites for round world & USA TV coverage
6 890mhz tracking
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1$\begingroup$ Welcome on the Space SE! Could you please send some references? $\endgroup$– peterhCommented Jul 22, 2019 at 6:35