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First of all, sorry for the general context of the question but I’m very inexperienced. I hope I'm not breaking the rules.

Simplifying, in the short sci-fi novel I’m writing there’s a super-intelligent alien probe that wants to communicate with us. It can intercept all of our communications leaving the Earth so it chooses to send a signal to the ESTRACK (or one of the other deep space networks).

Let’s say that the probe wants to send a simple “text” message, maybe a “do you copy?”. If it’s starting to learn English, to grease the wheels, how can it do it? What kind of... “format” would the signal have to be received and understood?

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    $\begingroup$ My own plan on the sister Worldbuilding site. $\endgroup$ – JDługosz Jun 8 '17 at 21:03
  • $\begingroup$ @JDługosz Thanks, I'll give it a look tomorrow morning, bed time now :) $\endgroup$ – Lupetto Jun 8 '17 at 21:21
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    $\begingroup$ deep space communications are done with very directional antennas, so unless the probe can plop itself between earth and the spacecraft, it'd have a hard time hearing both sides of a communication, or making itself heard to the ground network. $\endgroup$ – Jay Kominek Jun 8 '17 at 21:43
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That's a good & fun question; generally, there's a worldbuilding StackExchange sister site especially meant for people trying to build coherent universes. Check it out!

Anyway, your question can be reasonably interpreted as "if we have a sporadic transmitter that ground can't schedule, how would it communicate with ESTRACK"; and that's a serious SX question, so here we go:

There's ESA PSS-04-105, Issue 2.4 (November 1996) which defines the carrier frequencies, rates, bandwidths and modulation types of transmissions within ESTRACK.

For something like textual messages, that network wasn't actually meant – it's designed to primarily transport telemetry, and do some ranging, and nobody's expecting/decoding/guessing there'd be any English text among these numbers. But: guessing that if you send numbers that clearly aren't from normal operation, you'd be able to actually get an engineer to look for sense in the data received. Then they'd start to guess what it means. Chances are you're best off transmitting it as bits as used in the common (and old) ASCII encoding (which maps letters to binary numbers). At some point, people start recognizing that the numbers representing e and r are pretty common...

But: to first get the receiver to actually see data, you'd have to build a transmission that is "sound". Basically: Radio transmissions are enriched with what us communication engineers/ information theorists call "redundancy". Think of that as forms of checksums. For example, a common checksum would simply be the sum of all bytes sent (a byte can take values between 0 and 255), modulo 64. You just send your message and attach that checksum. The receiver checks whether it gets the same checksum as you sent, and if it did not, it knows something about the transmission is broken (and would ignore it). If you are cleverer than simple checksums, you can build codes that allow the receiver to "repair" a limited number of errors.

Space communication engineers are clever. The problem with that is that the codes can get very long and it can be very hard to guess how to build a valid transmission without their knowledge. So, that would probably pose a reasonable challenge for your aliens.

However, since that's all grounded in math, and math is the same all over the universe, and your aliens are advanced and definitely will have some form of codes since they need to communicate using radio waves themselves, that'd basically be a challenge in understanding how this crazy human mind understands number and ring theory, or, more generally, algebra. It will require that they either bring their mathematicians along, or that they have computers onboard that were programmed to solve such hard problems (us earthlings are desperate to find better and more codes than we know – basically, we have a rich, well-proven theory of what great codes must exist, but we simply know none of them, although there are very, very many), which makes it likely that they have some kind of quantum computer (just don't quote me on quantum computing – it's just that the computers we know aren't any good at solving that kind of problem, as our lack of code knowledge demonstrates). Which in turn means that it should be relatively easy for them, given enough recording of e.g. TV signals, figure out how to synthesize TV signals containing human speech/imagery. (not the picture, they've never saw what a TV screen shows, just the signal) Think of these funky deep neural networks transferring e.g. Van Gogh's style to your selfies. You can "deep dream" valid TV signals, too.

Thus, maybe they'd much rather directly interact with TV receivers on earth, sending disturbing images first, maybe things that look like strangely combined images that dominate TV transmissions, but get better the more they can actually observe what TV signals the humans reply with, once they figure out there's an extraterrestrial source of those.

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  • $\begingroup$ Thank for your reply, I'm going to read it calmly - I'm not an engineer, as I said nor a native English speaker :D BTW I wasn't actually sure whether to post on WB or here :) $\endgroup$ – Lupetto Jun 8 '17 at 13:12
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    $\begingroup$ "At some point, people start recognizing that the numbers representing e and r are pretty common..." or the "erronous" data is just viewed in a hex-editor, and the ascii translation can be read off the side ;) $\endgroup$ – Baldrickk Jun 8 '17 at 13:19
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    $\begingroup$ As someone who builds software defined radio applications: it will take me a while until I open up something that is clearly wrong data, but of which my receiver says it constantly passes decoding with little error, in a hex editor and look at the ASCII side of things. Me looking at things in a graphical view, as histogram, as entropy over position, as autocorrelation is much much much much much more likely. If I'm not expecting text, I won't look for it. That's as likely as if I send you a letter with a small bag of sand each day, and expect you to figure out you should count the grains.… $\endgroup$ – Marcus Müller Jun 8 '17 at 13:25
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    $\begingroup$ @Lupetto You cannot modify a radio message in transit (except if you're willing to, say, overpower it, but that's more likely to just turn both into garbage at the receiving end). You can however copy a message sent over radio, modify it to your heart's content, and transmit the new message in the same direction as the original was going. Or, since you presumably at that point know how the message is constructed, just construct your own! $\endgroup$ – a CVn Jun 8 '17 at 17:29
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    $\begingroup$ @Lupetto, Michael Kjorling is right about copying, re-transmitting, and modifying the message, but there's an additional caveat here: If you modify the contents (e.g. by replacing numbers with ASCII text), you will invalidate the checksums that Marcus has mentioned. At that point, the message looks exactly like a normal message that has been accidentally garbled - I think this strategy would make it less likely that engineers would notice the communication. $\endgroup$ – Bear Jun 8 '17 at 19:04
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Many ESA satellites use the Packet Utilization Standard (pdf) (PUS), published by the European Cooperation for Space Standardization (ECSS) for sending telecommands and receiving telemetry. This is essentially a transmission protocol, describing packet structure and the semantics of each item. It is based on eight bit octets.

In Chapter 5.4 you will find the telemetry packet structure. If the aliens are super-intelligent, they should have figured out the structure of PUS telemetry. And yes, there is a string type, usually with ASCII encoding. Knowing this, they should be able to craft telemetry that either appears to come from one of our satellites and be recognizable as "interesting", or they might craft telemetry triggering frantic activity because it appears to come from an "unknown" satellite or application on a satellite.

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  • $\begingroup$ Sounds promising. This is, more or less, the idea behind my second comment on the other answer? $\endgroup$ – Lupetto Jun 8 '17 at 15:51
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    $\begingroup$ @Lupetto Sort of. Your comment says modify a message in-flight. My thinking is along synthesize new telemetry message. $\endgroup$ – Jens Jun 8 '17 at 15:55
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I assume you have the probe orbiting the Earth or somehow following the Earth as it orbits the Sun.

If so the probe is unlikely to be able to send any response it gets from Earthlings, back to its home base, as the physics of Gaussian beams indicates that power spreads out over very large distances.

The probe would need to transmit Tetrawatts back to its home base for any usable signal to be read by them.

If the probe is not relatively close to the Earth, no readable signal could be picked-up by us.

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  • $\begingroup$ There could be relay spacecraft out of sight. The probe could have a larger, phased array flying in formation, or be using extremely short wavelengths (X-ray, gamma rays). Since none of this has been specified nor constrained, it's not possible to speculate this way. $\endgroup$ – uhoh Jun 9 '17 at 13:28

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