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The Apollo crews transmitted footage to Earth using SSTV, a ham radio technique that amateurs and pirates can use to send similar messages around the world.

Did NASA have any technical countermeasures in place to ensure that the signal they received from the Moon was legitimate? Or did they just assume that nobody would try to interfere?

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    $\begingroup$ A fair degree of interference protection would be implicit in the directional antennas required, also the receiving locations were not necessarily highly populated ones. And not that modulation technique has anything to do with interference or authenticity, but the "slow" scan used was 10 fps, which while slower than NTSC broadcast was much faster and spectrally wider than your typical ham SSTV. And then you have the whole building a convincing set problem - especially when the people you're trying to convince know more about what to expect to see than anyone else. $\endgroup$ – Chris Stratton Jun 13 '18 at 5:30
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    $\begingroup$ @ChrisStratton: Might as well just post an answer with that. $\endgroup$ – Nathan Tuggy Jun 13 '18 at 5:56
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    $\begingroup$ Agreed, the set of people both smart enough to pull this off and stupid enough to want to do this would be very small. $\endgroup$ – Hobbes Jun 13 '18 at 14:42
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    $\begingroup$ I would be stunned to find out that all the MSFN site operators were either complicit in a conspiracy, or so grossly incompetent that they could not tell the difference between a locally-generated spoof signal and a spot-source in the Sea of Tranquillity. $\endgroup$ – Martin James Jun 13 '18 at 20:24
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    $\begingroup$ @Uwe - actually the TV signal was separate from the other telemetry and the LM could only transmit telemetry or TV, not both at the same time. The CM could do both, using distinct transmitters, but the LM only had one which had to be mode-switched. Though that does mean that having real LM telemetry coming in at the same time as spoofed TV could be a giveaway... $\endgroup$ – Chris Stratton Jun 13 '18 at 20:40
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The signal from the Moon was received using giant parabolic antennas, e.g. the 64-m dish at the Parkes observatory. These have very good sidelobe rejection so they won't pick up any Earthbound signals.

Despite the space race, relations with their biggest enemy were good enough that the Russians shared Luna 15's flight plan with the Americans when this mission coincided with Apollo 11, so both could make sure there was no interference.

So there was no need for technical countermeasures.

To stop morons from interfering:

For a period of five days, from 17 to 21 July, the Observatory, Visitors' Centre, and all access roads for several kilometres from the telescope were closed to the general public. This was to prevent any outside interference from cars and other devices from disrupting the signal reception. Federal Police officers secured the site, and to gain access staff members were issued keys with "A 11" inscribed on a small aluminium key-tag.

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  • $\begingroup$ Perhaps you want to add information about the other four stations used for Apollo hq.nasa.gov/alsj/alsj-DSNLocations.html $\endgroup$ – Diego Sánchez Jun 13 '18 at 15:24
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    $\begingroup$ I wonder if any examples of that key-tag survive, pretty cool souvenir, not to mention super-rare. $\endgroup$ – Kelly S. French Jun 15 '18 at 16:19
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The difference between 10 W at 350,000km, and 1,000 W at 1km is 131 dB. If the pranksters on Earth used a directional antenna like the Ham radio operators shown below, the ratio would be even higher because that thing has much more gain than the Apollo antennas from the orbit and surface of the Moon.

It would only take a tiny bit of random or isotropic scattering from the secondary of the dish on the earth to reflect some sideways-propagating prankster signals into the feed horn of the receiver, so this would actually have been pretty easy to do.

Dishes don't have 130 or 150 dB of side lobe suppression as suggested here, so this kind of pranking would have been trivially easy.

However, in the late 1960's there was such near-universal enthusiasm, patriotism, exhilaration and excitement about the Moon landings that the idea of "pranking" them would probably never have occurred to anyone, and Ham radio operators are generally a noble lot as well, especially back then when there was mutual respect between the FCC and the public, unlike more recently.

EDIT: Thanks to @MartinJames's comments I've realized that while interference like this could certainly have posed a problem and resulted in what's better characterized as "jamming", it would not likely end up as successful "spoofing".

GPS spoofing works (for example) because the GPS receiver has a wide dynamic range and a teeny-tiny micro-controller, while in the case of the Apollo mission there was a crack team of attentive human specialists monitoring every aspect of the signal. If the strength were off by several tens of dB somebody would surely have said "hey, wait a minute!" and it would no longer be proper spoofing.

From the question Why such a large observed Doppler shift from Apollo 17 orbiting the moon?

Ham's setting up a 9 meter dish to receive signals from the Moon, and a Doppler shift measurement (offset) of the received signal at around 2287.5 MHz as the spacecraft orbited the near side of the Moon. From Tracking Apollo-17 from Florida.

Images from Sven Grahn's Tracking Apollo-17 from Florida

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    $\begingroup$ Re your 4th paragraph: theatlantic.com/technology/archive/2012/09/… $\endgroup$ – Organic Marble Jun 13 '18 at 12:18
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    $\begingroup$ @OrganicMarble That's an interesting read! "Consistently throughout the 1960s a majority of Americans did not believe Apollo was worth the cost..." is not at all inconsistent with "...enthusiasm, patriotism, exhilaration and excitement...". Thinking that something is too expensive doesn't necessarily mean you'd want it to fail. $\endgroup$ – uhoh Jun 13 '18 at 13:13
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    $\begingroup$ Agreed. I was more pulling on the near-universal enthusiasm in the late 60s. $\endgroup$ – Organic Marble Jun 13 '18 at 13:41
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    $\begingroup$ The doppler issue is an interesting one, though that can be modeled mechanically even (in doing narrowband coms with low orbit satellites continuous tuning is necessary). But strong locally sourced signals would be noticed - they'd be picked up without the fancy antenna, too. $\endgroup$ – Chris Stratton Jun 13 '18 at 14:11
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    $\begingroup$ @uhoh it's not just the signal strength, but also the way in which that signal varies with antenna alignment. I'm sure that the big dishes would need continual dynamic positioning to track the very weak signal from the LM, rovers etc. on the moon. Off by a tiny bit, and you lose reception completely. That tracking would be hopelessly thrown off by a local spoof signal, and the operators would very quickly realize that the signal profile indicated that the source was not on the moon. $\endgroup$ – Martin James Jun 14 '18 at 11:32
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My guess is that the RF hardware needed to do this would have been prohibitively expensive and hard to get working for people who didn't have a long history of cooperating with the US government (or Bell Labs). Cheap, capable open-source hardware and software would make something like this fairly straightforward now - but Moore's Law had just been coined in 1965, and it's never really pushed the capabilities of RF hardware like it did computers.

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    $\begingroup$ Actually HAMs did receive the signals directly from the moon. And there may have been more around then who knew how to generate a few watts on S-band than today. $\endgroup$ – Chris Stratton Jun 13 '18 at 20:45
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    $\begingroup$ Spreadsectrum WiFi, RFID tags and cell phone communications are close to magic to what was available in the 1970's. However the Apollo program mostly avoided magic in favour of reliable systems so the technology was not exotic, merely expensive. $\endgroup$ – KalleMP Jun 14 '18 at 7:35

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