On Tue, 16 Jun 2009 15:57:38 -0700, Robert Clark wrote:

On another forum there was debate about whether the requirement of "near
real time" high definition video transmissions was achievable for a such
a low-cost mission.
It would certainly be doable if the receiving antennas on Earth were
the large radio antennas used for space communications with
interplanetary probes or those radio antennas used for radio astronomy.
This is evidenced by the fact that the Kaguya(Selene) lunar orbiter
mission was able to send high definition video to a large receiving dish
radio antenna. And also by the fact that DirecTV sends high definition
video to only 2 foot size antennas from geosynchronous orbit; so 10
times larger antennas would be able to receive such signals from a 10
times larger distance at the Moon.
However, I was wondering if it would be possible to detect this using
amateur sized equipment at such a large distance. Usually for receiving
high data rates you used transmissions at very high frequencies, as
higher frequencies can carry more data. For instance both Kaguya and
DirecTV transmit the high def video at gigahertz frequencies.
However, for the system I'm imaging I'm thinking of using much lower
frequencies, and necessarily longer wavelengths. What I wanted to do is
transmit at decametric wavelengths. High data transmissions rates would
be achieved by making it be pulsed in an on-off fashion at high
intensity but at a rapid rate.
On that other forum the data rate required for high def TV was given
as 256,000 bits per second. So I wanted to make these transmissions be
pulsed at this rapid rate at wavelengths of a few tens's of meters.
My decametric wavelength requirement was because of the fact that
high schools and universities have programs for detecting radio
emissions from Jupiter at these wavelengths:

NASA's Radio JOVE Project.
http://radiojove.gsfc.nasa.gov/

The Discovery of Jupiter's Radio Emissions. How a chance discovery
opened up the field of Jovian radio studies. by Dr. Leonard N. Garcia
http://radiojove.gsfc.nasa.gov/libra...discovery.html

These school and university receiving antennas on Earth consist of
dozens to hundreds of vertical dipoles of lengths at the meters scale to
correspond to the radio wavelengths. Some questions I had: how intense
would the pulse have to be on the Moon to be detectable from the Moon
above background noise for a detector on Earth of say a few dozen
dipoles? Could this be done for the transmitter of power of say a few
hundred watts for a low cost, low weight lander mission? Could the
transmitter antenna on the moon be only a few meters size for the low
weight requirement?
A secondary purpose I had in mind was a pet project of mine involving
linking these many school receivers to form a global telescope at
decametric wavelengths:

From: (Robert Clark) Date: 23 May 2001 11:15:06
-0700
Subject: Will amateur radio astronomers be the first to directly detect
extrasolar planets?
Newsgroups: rec.radio.amateur.space, rec.radio.amateur.antenna,
sci.astro, sci.astro.seti, sci.space.policy
http://groups.google.com/group/sci.a...se_frm/thread/
c0018b68662c14e9

The long wavelengths should make the requirements for accurate
distance information and timing synchrony between the separate detectors
easy to manage even for amateur systems. Using this method might make
the detection achievable even if the power or transmitting antenna size
requirements are not practical for a low cost, low weight lander on the
Moon for an individual detector on Earth.
The recent achievement of real-time very long baseline interferometry
should make it possible to integrate these separate detector signals in
real-time as well:

Astronomers Demonstrate a Global Internet Telescope. Date Released:
Friday, October 08, 2004 Source: Jodrell Bank Observatory
http://www.spaceref.com/news/viewpr.html?pid=15251


You will never get uncompressed HD video transmitted from the lunar
surface. And really, there is no need for it if the compression is
handled right. Only a few people I know can do that part.

Since the image is mostly repetitive, a low bitrate can be achieved which
should allow for a very good signal level path budget. this would make
for a higher energy per bit and a more reasonable earth station within
the budget of amateurs. (if thats the goal) To achieve a very low
bitrate, such things as Pre/post-distortion to utilize less bits, (black
gamas) using extremely long GOP structures and since the action of the
video is extremely slow and repetitive, a slow frame rate such as 1fps.
These can be counteracted at the receive station with software without
effecting the total image resolution.

The image resolution is where the wow factor is anyway! :-)

My *guess* is that whomever would put a spacecraft on the lunar surface
would only have one high speed datapath back. The HD transport stream
would be muxed in with the other data elements of the spacecraft on a
transmission system without consideration for amateur reception. Perhaps
encrypted if a commercial entity is paying for the broadcast rights.