[Laser] Re: Laser digest, Vol 1 #231 - 7 msgs

[laser@mailman.qth.net]

In addition to the laser power/beam divergence problem that others have commented on,
there is a potentially more serious problem for such a system.  For approximately two weeks
every month (28 days), the transmitter will not be in sunlight.  The Moon is tidally locked to 
the Earth, such that it presents the same face to the Earth.  However, this means that
during its 28 day orbit of the Earth, a particular point on the Moon's surface is in
darkness [1].  Thus, any type of transmitter designed to operate while on the surface
of the Moon will have to either accept the fact that solar power cells will not work
for half the time, or be designed to provide an alternative power source [2].

[1] This may not be true for certain locations near the poles of the Moon.  However,
it is very difficult to launch anything into a Lunar polar orbit, and, the polar areas of
the Moon are not well mapped, which implies that the landing may be a bit hazardous.

[2] A side effect of the light to dark (and vice versa) transition is that the temperature
of the transmitter will vary greatly.  This can be quite hard on the electronics.

Another point of consideration is that the most attractive time for a Lunar based camera
to take pictures of the Earth is when the Moon is located between the Earth and the Sun.
In this position, the maximum amount of the Earth's surface that is visible to the Moon is
illuminated.  Unfortunately, this is also when the transmitter on the Moon is in darkness,
and is unable to use solar power.  Additionally, this is also when an observer on the
Earth looking toward the Moon will be (partially?) blinded by the Sun.  

As another poster has pointed out, Earth pictures are avilable from some satellites in 
geosynchronous orbit (I think some of the GOES satellites may provide these 
images.).  Landing a functional transmitter on the moon, and aligning it adequately
may not be an easy task.  For an automated system, this could become quite expensive,
due to the requirement for soft landing the package, and the requirement for the
(auto?) alignment system.  For a manned landing, there would be safety concerns 
about the process, and the training budget alone would probably make the idea
impossible.

I'm not real sure what political and legal considerations would have to be considered
in placing such a transmitter on the Moon.  I seem to remember that there are some
international treaties (which the US hasn't signed?), which detail what operations
are possible on the Moon.  These would have to be researched (and that, alone
would probably keep a staff of lawyers busy for quite some time).

Then, there are the "environmental" considerations.  Someone even once went
so far as to propose changing the Moon into a giant billboard, and there was a 
huge cry of protest.  Do we really want to go there again  (although an infrared
laser might not cause nearly as much protest since it would be invisible to most
people)?

Plus, given that it's a one-way transmission system, would there really be that much
interest?

I seem to also remember that one of the OSCAR satellites had a laser downlink 
on it, although I'm not sure of what the status of that is.

Some of the other ideas that have been floated have been things like a 
resurrection of Project Echo, where a huge aluminized mylar balloon was orbited, 
with the idea being to use it as a passive reflector.  The original Project Echo
was to use it as a passive radio wave reflector, although a similar idea should
work for lasers.  Of course, the reason that Project Echo didn't succeed is that
the orbital decay for the large, light balloon was much higher than predicted, 
and it decayed from orbit after a relatively short time.  

Another variation on that might be to orbit a corner reflector.  This could function
as a passive laser reflector, and allow communications over a narrow to medium
sized footprint.  Of course, one of the trade-offs that would be involved would be
the altitude at which to orbit such a device.  A lower orbit would mean that a narrower
footprint, and, thus, stronger signals, at the sacrafice of a smaller communications
distance, and with additional tracking problems as the reflector traveled overhead.
On the other hand, a higher orbit would ease tracking problems, with the ideal case
of a corner reflector orbiting in a geosynchronous orbit.  I haven't worked out the
power requirements for such a system yet, but it probably exceeds reasonability.

Dave
WA4QAL

-----Original Message-----
From: TWOSIG@aol.com
Date: Sun, 22 Feb 2004 11:30:24 EST
To: laser@mailman.qth.net
Subject: [Laser] pictures from the Moon
Reply-To: laser@mailman.qth.net

The headlines about returning to the Moon (and maybe a manned mission to 
Mars) seem to be slowing.  Here's an idea that might stir some public interest in 
going to the Moon and developement of laser communication receivers.

If a camera is set on the Moon and pointed at the Earth, the pictures could 
be sent down to on a laser beam.  The pictures would run from interesting ( 
showing the current phase of the Earth and changing weather patterns, even the 
rotation of the globe ) to spectacular ( the Moon's shaddow on the Earth or the 
Earth moving across the Sun during a Lunar eclipse ). The Moon is tide locked 
to the Earth so that once aligned, neither the camera nor the downlink 
antennas would need to be adjusted.   A package of experiments could send data down 
with the pictures.

OK, so that's not such an original idea.  But think of it this way.  If the 
laser downlink transmitter had a 2 milliradian beam width.  At a distance of 
239,000 miles, the spot would be 478 miles across, and the Earth would be 
turning under it.  If I set it to New York City, the spot will cover North into 
Maine and South beyond Washington, DC.  If the beam can be received with a 
telescope of 6 inches, anyone under that spot who can see the Moon, can receive the 
pictures and data with the right equipment.  ( Someone else juggle the numbers 
about how much power is needed for x amount of beam width and y amount of 
aperture on the telescopes.)

Since the Earth is turning under the spot, in about an hour it will shift to 
Chicago.  In another hour, to Denver.  Later Salt Lake City, then San 
Francisco.  Still later it will move across the pacific to Tokyo, Beijing, Ankara, 
Athens, Naples, Madrid and Lisbon.  About twenty-five hours later back to New 
York.  Maybe experimenter in each of those cities could work together.

Take look at a world map and you can see that very different countries share 
Latitude.  By using multiple lasers aligned to differing Latitudes, on camera 
/ experiment site could service all interested sites.   Makes for some 
interesting choices.  If New York wanted a stronger signal so that it could be 
received by smaller telescopes, they could narrow the beam, which they could share 
with Beijing and Madrid, but not Boston or Tokyo.  Of course that might also 
shorten the viewing time.  London could choose a wide beam that  could be viewed 
almost from Moon rise until setting.  The telescope needed to collect the 
data would need to be larger, but the spot would cover all cities from the North 
Pole to southern England.

Another idea:  If the laser beam comming down used multiple encoding, what 
you get from the system will depend on the instrument you use.  A small 
telescope might look a the Moon and not see the beam at all.  A slightly larger unit 
might see the light from the beam and be able to read Morse code identification 
and a coded status or data "word".  A still bigger telescope, might get the 
Morse as an electronically detected MCW tone set.  Still bigger telescope might 
detect that each pulse of the MCW tone is actually PWM with the picture data. 
 

Now for the serious Astronomer, the light from the Moon makes serious sky 
viewing difficult, so I do not see why they would object to a little more light 
from the downlink lasers.  And if they are looking at the Moon, I think the 
changing sun glint from a permanent manned base would be more trouble than known 
laser links at a known schedule.

James
N5GUI