3
$\begingroup$

For want of a better term, is it possible crews will use 'non-digitally modulated' radio to communicate with earth?

Apollo missions had very effective communication, with a ~2.5 second 'round trip' time through analog S-band radio.

Modern digital encoding/decoding generally introduces a level of delay and the potential of packet loss. It is possible to hear radio static in Apollo communications, but voices remain largely understandable - arguably more understandable than a degraded digital signal.

Could Artemis return to a similar 'analog-era' S-band voice configuration to Apollo, without digital encoding?

$\endgroup$
  • 2
    $\begingroup$ Ultimately the architecture will depend on who wins. Gateway (if functional) may be a comm relay point. Orion has X and S band, using the DSN. Data will be modulated and have FEC, so it's already a little different from unified s-band. Too early to answer... $\endgroup$ – mothman Oct 12 '19 at 4:12
2
$\begingroup$

Modern digital encoding/decoding generally introduces a level of delay

Um, you mean modern digital encoding/decoding introduces error coding that allows us to use channels that are way too bad for analog comms?

and the potential of packet loss.

Um, you mean if even then the SNR is too bad, it might happen that we lose data, but not as much as for the analog voice communication that wouldn't even work to begin with?

We can put math to this. In fact, we've been putting math to the problem of "how much intellegible information can I transport over a noisy channel" for 80 years now.

The result is that we use channel coding, which we only know halfway decent implementations of for digital transmissions. Hence, making the probability of having a useful communication higher than in analog communications is the whole motivation, and spoiler: it works, and that's why literally any communication system that's not stuck with compatibility does that.

You really want to learn about information theory if you want to understand why – it's a fun subject, and the results are pretty clear! (pun intended.)

It is possible to hear radio static in Apollo communications, but voices remain largely understandable - arguably more understandable than a degraded digital signal.

That's a misconception.

The Apollo voice channel had fantastic SNR by modern standards. From NASA's APOLLO EXPERIENCE REPORT - VOICE COMMUNICATIONS TECHNIQUES AND PERFORMANCE:

On the basis of this previous work, the NASA and the prime contractors mutually agreed on a signal-to-noise standard for the Apollo voice channels of 14 decibels for 90-percent word intelligibility and 4 decibels for 70-percent word intelligibility when both the sig­ nal and the noise powers are expressed in root-mean-square (rms) values.

14 dB is a pretty nice thing, and they were using 3.5 kHz channels. To compare this to a modern bidirectional voice standard that does about the same "target understandability", FreeDV, they'd use 1.2 kHz (so, about one third of the bandwidth) and intelligibility works down to 4 dB SNR – in other words, if we just expand the FreeDV bandwidth to 3.5 kHz (which is pretty trivial, thanks to the modern multicarrier digital structure), we'd get three times the SNR, and the thing would work down basically to 1.75 dB. The "70% intelligibility" at 4 dB is way, way, way worse than FreeDV.

Oh by the way, FreeDV is structured like it is because it also has to deal with multipath propagation – a problem spacecomms will usually not have. Under such idealized conditions, things become waaaaay easier.

I'm sorry I'm a bit verbose on this, but I hear this so often from amateur radio people. I actually work in this field and it is totally beyond me how people that take that hobby seriously can still claim something like that, 40 years after commercial and military reality has proven them wrong.

Could Artemis return to a similar 'analog-era' S-band voice configuration to Apollo, without digital encoding?

Certainly not, because that would be incredibly bad.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Thanks for backing up with data on why modern digital encoding is used! $\endgroup$ – mothman Oct 12 '19 at 20:17
  • $\begingroup$ Thank you for the great sources. I apologize about my ignorant assumptions. Reading over Information theory, it's obvious why digital is the clear option. Would you agree that "speed of light" communication is still an advantage of analog modulation though? Seems feasible there could be some unforeseen time-sensitive situation where a fast response is more important than bit-perfect response. The validity of that balances on just how much delay digital processing introduces though, which I know little about. $\endgroup$ – S. Low Oct 15 '19 at 10:36
  • $\begingroup$ @S.Low high latency truly is one of the banes of the math behind channel coding – namely, you can only really achieve channel capacity (i.e. get as much data through without error as theoretically even possible) if you had an infinitely long code, and hence, infinite delay. So, for low-latency systems, the amount of error correction you can do is limited by the latency of that correction system, indeed! For space comms, this is less relevant: in Apogee, the moon is 405,400 km away from earth, so light takes a roundtrip time of about 2.7 s. compare: couple dozen milliseconds in processing delay $\endgroup$ – Marcus Müller Oct 15 '19 at 11:28
  • $\begingroup$ @S.Low but the latency is indeed one of the reasons why stage microphones are only now beginning to become predominantly digital; it was pretty hard to compress the digital signal and channel code it within the bandwidths given within a time that a serious professional studio / event recording would consider negligible. And that leads to acoustically interesting effects like that in the studio mix, the vocal position might seem different than on stage… $\endgroup$ – Marcus Müller Oct 15 '19 at 11:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.