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.

Don't forget that DirecTV radiates a LOT more power than a typical deep
space probe. AND they often have a higher gain antenna. A geosync relay
satellite might have 96 TWTAs, each several hundred watts, on it,
feeding a very clever multiple feed dish which is many meters in
diameter (look at Thuraya, for instance).

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.

There's a moderately active Amateur DSN group that listens for things
like Chandrayaan or MRO using relatively small dishes (1-2 meters).

The choice of higher frequencies isn't because it carries more data.
It's because a higher frequency allows you to get more gain with the
same physical antenna size. Double the frequency, and your antenna gain
goes up by a factor of 4, at both ends of the link.. a total of 12 dB
improvement in SNR, for the same transmitter power and receiver noise
figure. And, there's more spectrum available up high.

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.

How high a data rate? If you're at 30 MHz (10m lambda), you're not
going to be pulsing at 10 MHz, or you're going to be generating a signal
that extends from 20 to 40 MHz (and then some). You need a low symbol
rate with lots of bits per symbol, which in turn means you'll need lots
of SNR.


On that other forum the data rate required for high def TV was given
as 256,000 bits per second.

HDTV, as carried on broadcast TV, is 19.8 Mbps. If you're happy with a
lower frame rate, or can do a lot of frame/frame compression, you can
get it lower.



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/


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.

Any one school only has a couple dipoles up.. the gain is quite low.

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?

How technical do you want to get? There's a book about space
telecommunications system design available for downloading from JPL
(http://descanso.jpl.nasa.gov/ somewhere on that site)

Here's some basic numbers you'll need:

Free space path loss in dB = 32.44 + 20*log10(distance in km) + 20
*log10(frequency in MHz)
That's between isotropic antennas (0dBi)..

Antenna beamwidth is 70 degrees/ (diameter of antenna in wavelengths)
Antenna gain is 27000/(beamwidth^2)

A typical receiver noise figure (after figuring in losses in
coax/waveguide, etc.) is probably 3dB.

kTB noise is -174 dBm/Hertz * 10*log10(bandwidth in Hz)





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:

Coherent combining would be a challenge, because of ionospheric
variability at HF, not to mention the other challenges.

Look up LOFAR or the SKA (Square Kilometer Array) for a fairly well
funded scheme.