Earth <--> Moon comms

In article ,
Keith F. Lynch wrote:
For a moonbase, there is probably no great problem in installing a
10-50m antenna, ...

Why bother? Earth is close enough that even a non-directional antenna
will work fine.

Yes, but you pay a big price in data rate per kilowatt. Directional
antennas speed up your links, or reduce your power requirements, or both,
by rather a lot.

... The net result is that you need about +-7deg of tracking on each
axis, if I recall correctly.
That's a small enough motion that you might well be able to do it
by moving the dish's feed assembly above a stationary dish...

Or just by having an antenna with a low enough gain that it's viewing
the whole region the earth is ever in at once.

Possible. That was more or less done for the ALSEPs. But for high-speed
work, you are taking a power penalty of a factor of 250 or more by having
a beam that's circa 16deg wide rather than 1deg or less. (To a reasonable
first approximation, in such cases the receiving antenna has to collect
the same energy per bit, so 100x the illuminated area requires 100x the
power.) This is unlikely to be appealing for a base. For mobile
operations with modest data rates, it might be of interest.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
But for high-speed work, you are taking a power penalty of a factor
of 250 or more by having a beam that's circa 16deg wide rather than
1deg or less. (To a reasonable first approximation, in such cases
the receiving antenna has to collect the same energy per bit,
so 100x the illuminated area requires 100x the power.) This is
unlikely to be appealing for a base. For mobile operations with
modest data rates, it might be of interest.

What are the actual numbers? If it's 250 milliwatts instead of one
milliwatt, so what?

You get one bit per second per Hz if your signal to noise ratio is
unity. Just how noisy is the sky behind the earth? Behind the moon?
With modern receivers, we can disregard noise produced *within* the
receiver.

And just how many bits per second do we want to send?

As for how many Hz we have, bascially, all of them, if there are no
other Earth -- Moon channels operating. We can easily avoid the
noisier bands. And, by making profligate use of spectrum space,
you can get high data rates even with a poor signal to noise ratio.
--
Keith F. Lynch - - http://keithlynch.net/
I always welcome replies to my e-mail, postings, and web pages, but
unsolicited bulk e-mail (spam) is not acceptable. Please do not send me
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Keith F. Lynch wrote:
Henry Spencer wrote:
But for high-speed work, you are taking a power penalty of a factor
of 250 or more by having a beam that's circa 16deg wide rather than
snip
What are the actual numbers? If it's 250 milliwatts instead of one
milliwatt, so what?

You get one bit per second per Hz if your signal to noise ratio is
unity. Just how noisy is the sky behind the earth? Behind the moon?
With modern receivers, we can disregard noise produced *within* the
receiver.

And just how many bits per second do we want to send?

It depends on what you'r wanting.
If it's a 'flags and footprints' mission, like apollo, 32kbits/sec
is probably adequate for practically everything - near real time
video, voice, or maybe even just text/low quality voice at 1kbit/sec or so.

However, if you'r actually trying to work up there, you may need lots
more.
For every person, it may well speed up things if they have a remote
PA on earth, that can see and hear everything they do (at work) to help
them, schedule, ...
This can easily use a few hundred kbits/sec/person.

Making stuff 100% reliable with complete failsafes may be more massive
and expensive than sticking a webcam or two on it, and having people
watching it earthside able to turn things off, if not fully teleoperation.

Say 20-30 cameras for each selenite, and you'r getting to
megabits/second/person.


As for how many Hz we have, bascially, all of them, if there are no
other Earth -- Moon channels operating. We can easily avoid the
noisier bands. And, by making profligate use of spectrum space,
you can get high data rates even with a poor signal to noise ratio.

You are somewhat limited by atmospheric absorbtion, and local interference
to the earthbound recievers.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
Things a surgeon should never say:
Better save that for the autopsy.

In article ,
Keith F. Lynch wrote:
But for high-speed work, you are taking a power penalty of a factor
of 250 or more by having a beam that's circa 16deg wide rather than
1deg or less...

What are the actual numbers? If it's 250 milliwatts instead of one
milliwatt, so what?

Depends on how much data you want to send. It's likely to be many watts,
kilowatts probably, given that you'll have a lot of data and you'll want
to use modest antennas on Earth. (You can't monopolize DSN's 70m dishes
for ongoing base operations.) While not huge, it's not going to be trivial
either, and a performance penalty that big can't just be brushed aside as
unimportant.

If it was as easy as you think, spacecraft communications systems would be
a whole lot simpler than they really are. Being able to get decent data
rates from deep space with milliwatts of power would vastly simplify some
missions that are near and dear to my heart...

You get one bit per second per Hz if your signal to noise ratio is
unity. Just how noisy is the sky behind the earth? Behind the moon?

Depends on the frequency. It's not entirely trivial, especially at the
higher frequencies where Earth's atmosphere stops being entirely
transparent.

With modern receivers, we can disregard noise produced *within* the
receiver.

Not so. It's low but not negligible.

And just how many bits per second do we want to send?

Depends on what your lunar base is doing, but it could be quite a lot.
Multiple video channels, for sure -- one very effective "force multiplier"
for a lunar base is to do routine tasks (e.g., driving a bulldozer) by
teleoperation from Earth, and reserve on-the-spot labor for trickier
things like maintenance.

As for how many Hz we have, bascially, all of them, if there are no
other Earth -- Moon channels operating.

Sorry, wrong. There are specific frequency bands assigned to deep-space
communication, and they are not all that generous, and there are other
demands on them.

(Outside the amateur bands, in fact, frequency assignment and coordination
for space use is a monumental headache.)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
If it was as easy as you think, spacecraft communications systems
would be a whole lot simpler than they really are. Being able to
get decent data rates from deep space with milliwatts of power would
vastly simplify some missions that are near and dear to my heart...

The moon isn't deep space. Voyager 1 is thirty thousand time further
than the moon, meaning signals are about a *billion* times weaker.
And it transmits with just 25 watts and a fairly small dish antenna.
This signal has been picked up by *amateurs*.

In other words, an identical dish antenna -- one which could easily be
mounted on a rover -- on the moon could send an equally strong signal
to earth with a power of just 25 *nano*watts. A standard 9 volt Radio
Shack battery could power it for over ten thousand years (assuming the
battery had that long a shelf life). Power is *not an issue*.

If I knew how noisy space was at various frequencies, I could easily
calculate how much power it would take as a function of bandwidth and
antenna size. Does anyone know where I can find that information?
Thanks.

But from just back-of-the-envelope doodling, signals between earth and
the moon are trivial. Between earth and the edge of our solar system
are challenging. Between earth and other solar systems are possible
with today's technology, but only with the largest antennas, the
strongest transmitters, the most sensitive receivers, and low
bandwidths. Between earth and other galaxies are possible in
principle, but not with anything close to today's technology.
--
Keith F. Lynch - - http://keithlynch.net/
I always welcome replies to my e-mail, postings, and web pages, but
unsolicited bulk e-mail (spam) is not acceptable. Please do not send me
HTML, "rich text," or attachments, as all such email is discarded unread.

How about laser communications? Its possible to hit a target on the Moon's
surface using a laser based on Earth. An even better possibility is to place a
laser in orbit around Earth that receives radio or microwave signals from Earth
and converts it to laser. The laser hits a laser receiver on a fixed location
on the Moon and converts it to radio signals to remotely control equipment. the
idea is to use tight beams where ever possible and not broadcast
omnidirectional except locally.

Tom