[time-nuts] time nuttery in Space Communications

[Jeremy Nichols]

Some of the stories of deep space communication are really interesting.
Round-trip light times measured in hours, data rates measured in bits per
minute, remote transmitters with only a few watts. The knowledge is limited
to just a handful of people, I suppose.



On Sat, May 23, 2020 at 10:44 AM jimlux <jimlux at earthlink.net> wrote:

> One thing in all these DSN histories is that they don't make very much
> of the essential thing that separates Deep Space radio links from Near
> Earth radio links, and that's the timing: specifically the coherent link
> between the received signal and transmitted signal.  I suspect that's
> just because it's an "of course it's coherent" for everyone who does
> deep space comm, so it's of no great notice.
>
>
> One thing I find in folks building and using ground stations (and
> spacecraft) for LEO (vs beyond GEO) is that they essentially treat the
> communications path to the spacecraft (telecommand, in the lingo) as
> entirely separate from the communications path from the spacecraft
> (telemetry, in the lingo).
>
> This is also found in textbooks on communications systems at whatever
> level - you have a sender, you have a propagation path, you have a
> receiver.
> If there's a "connection" between the two directions, it's usually
> handled as a "network" thing (in the sense of ARQ or ACK/NAK, perhaps).
>
> Things like Doppler and oscillator stability are "nuisances to be
> compensated for" so that the receiver can be tuned to the right frequency.
>
> More than one ground station system for Earth orbit has no idea how long
> it takes for the signals to propagate through the chain - it doesn't
> matter - I put bits in at one end, and not too long (milliseconds, maybe
> seconds) the bits come out the other end.
>
>
> However, for those of us using the radio signals to track and navigate,
> or to do science, that "round trip light time" or "phase difference
> between receiving the signal at two different stations" is really,
> really important.
>
> And, a bit of codger, "get off my grass" sense (and I'm a relative
> newbie, having only done this for 20 years) - all those folks who are
> excited about going to the Moon are having to learn this all again.
> There's no GPS to give you your position, an omni antenna won't close
> the link, etc.
>
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-- 
Jeremy Nichols
Sent from my iPad 6.