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 <jim...@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. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > -- Jeremy Nichols Sent from my iPad 6. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.