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.

Jim Lux wrote:

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).


Wow. 96 TWTs with several hundred watts each. From a satellite. And
what's the efficiency?

So these are powered by what? Small nuclear reactors?

Certainly not solar panels.

tom
K0TAR

On Jun 16, 8:06*pm, tom wrote:
Jim Lux wrote:

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).

Wow. *96 TWTs with several hundred watts each. *From a satellite. *And
what's the efficiency?

So these are powered by what? *Small nuclear reactors?

Certainly not solar panels.

tom
K0TAR

Not all will necessarily be on at the same time.
Typical narrow band coupled cavity TWTAs can get over 50% efficiency
(DC in to RF out)
Yes Solar Panels..10kW would not be unusual.

See, e.g., http://en.wikipedia.org/wiki/ASTRIUM_E3000 ... 14 kW of
power from 45 m^2 of solar panels and 4500kg of satellite..

This is so far beyond what is used in the scientific space program
it's mind boggling. But, hey, out of the $1-2B cost, the TWTAs are
probably only 5-10% of the total, and there are definitely quantity
discounts.

"Jim Lux" wrote in message
...


Snip

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.

Yes. OP said "near real time," which I take to mean "OK to drop some
frames," like the satellite video phones the reporters use from the
boondocks. Thus, high-def can be confined to a lot lower bandwidth if you
don't mind seeing compression artifacts as each frame is being built on the
screen.

I have a contemporary example: KABC-DT, Channel 7 Los Angeles is high-def
on 7-1 AND high-def on 7-2, with a service called Living Well. See
http://livingwell.tv/Welcome.html.

Living Well is apparently getting a skimpy bitshare, as compression
artifacts are obvious, especially on scene changes and motion, whereas ABC
programming on 7-1 is just beautiful. Living Well is very good, sharp HD,
but you can see details being "painted in" for a quarter-second after a
scene change.

"Sal"

Yes. *OP said "near real time," which I take to mean "OK to drop some
frames," like the satellite video phones the reporters use from the
boondocks. *Thus, high-def can be confined to a lot lower bandwidth if you
don't mind seeing compression artifacts as each frame is being built on the
screen.

I have a contemporary example: *KABC-DT, Channel 7 Los Angeles is high-def
on 7-1 AND high-def on 7-2, with a service called Living Well. *Seehttp://livingwell.tv/Welcome.html.

Living Well is apparently getting a skimpy bitshare, as compression
artifacts are obvious, especially on scene changes and motion, whereas ABC
programming on 7-1 is just beautiful. *Living Well is very good, sharp HD,
but you can see details being "painted in" for a quarter-second after a
scene change.


There's a fairly complex trade. For a lunar mission, the scene is
going to be pretty static, just shifted. (not like there's a baskeball
team doing a fast break in the field of view), so it should compress
well, given a suitable algorithm.

The challenge is that compression (especially good compression) takes
computational power. So you have a tradeoff: do you spend you joules
on compressing the images and radiate less RF energy, or do you
compress less, and use a bigger power amp. There's also a mass
tradeoff.. big amp or big antenna. The big antenna needs more
accurate pointing, which increases complexity. Or the trade of
frequency selection, higher frequency means more antenna gain, but
usually lower efficiency in the PA and higher NF in the receiver end,
as well as higher probabiliity of weather related fading.

And even there, because Moore's law means that semiconductor
technology is always advancing, the tradespace is shifting towards
more processing (because it gets cheaper in size, weight, power, while
power amps are pretty much at the physics limits)

This is, of course, "rocket science".. or more properly, spacecraft
system engineering. It's straightforward, for the most part, but non-
trivial. Pick your requirements, define the tradespace(s), try
configurations and see what happens.

Jim Lux wrote:
Yes. OP said "near real time," which I take to mean "OK to drop some
frames," like the satellite video phones the reporters use from the
boondocks. Thus, high-def can be confined to a lot lower bandwidth if you
don't mind seeing compression artifacts as each frame is being built on the
screen.



Here's a back of the envelope link budget for a 500E3 km link carrying 1
Mbps

Let's assume 2GHz for the working frequency (not necessarily a good
choice, but somewhere to start)

Free Space Path Loss from Moon to earth, between isotropic antennas =
32.44 + 20*log10(500E3) + 20*log10(2E3) = 32.44 + 114 + 66 = about 212 dB

Assume an antenna 2m in diameter at one end (moon end)
lambda for 2GHz is 15cm, so the antenna is 13 wavelengths in diameter
Beamwidth will be about 70/13 = 5 degrees.. OK, because Earth is 2
degrees wide from the moon, so you can just point at the middle of the
visible earth.

Assume an antenna 10m in diameter at the earth end. Beamwidth will be 1
degree, twice the visible lunar disc size, so you can just point at the
moon, generally.

Gains of antennas
2m @ 2GHz = 30dB
10m @ 2GHz = 44dB

Preceived at Earth = Ptransmitted +30 - 212 +44dB = Ptransmit -138dB.

Assume transmitting with 10 Watts or +40dBm..
Prec = -100dBm

Now, let's look at the receiver:
Power Spectral density of Noise is kT+NF.. kT is -174dBm/Hz and a decent
NF might be 2dB (allowing for some plumbing losses, etc.

-172 dBm/Hz

Eb (energy/Bit) = -100dBm -60dB (1Mbps) = -160dBmJ

So, Eb/No is about +12dB... If you allow 2dB for implementation loss,
that gets you to 10dB, which will get you a BER of 1E-6, which isn't
terrible. Coding will improve it, etc.

Take home message:

10W at 2Ghz with reasonably sized antennas at moon and earth can carry 1
Mbps.

Scales linearly with data rate.. You want 10Mbps, you need 100W. Or
bigger antennas.

On Jun 17, 11:32*am, Jim Lux wrote:
Jim Lux wrote:
Yes. *OP said "near real time," which I take to mean "OK to drop some
frames," like the satellite video phones the reporters use from the
boondocks. *Thus, high-def can be confined to a lot lower bandwidth if you
don't mind seeing compression artifacts as each frame is being built on the
screen.

Here's a back of the envelope link budget for a 500E3 km link carrying 1
Mbps

Let's assume 2GHz for the working frequency (not necessarily a good
choice, but somewhere to start)

Free Space Path Loss from Moon to earth, between isotropic antennas =
32.44 + 20*log10(500E3) + 20*log10(2E3) = 32.44 + 114 + 66 = about 212 dB

Assume an antenna 2m in diameter at one end (moon end)
lambda for 2GHz is 15cm, so the antenna is 13 wavelengths in diameter
Beamwidth will be about 70/13 = 5 degrees.. OK, because Earth is 2
degrees wide from the moon, so you can just point at the middle of the
visible earth.

Assume an antenna 10m in diameter at the earth end. *Beamwidth will be 1
degree, twice the visible lunar disc size, so you can just point at the
moon, generally.

Gains of antennas
2m @ 2GHz = 30dB
10m @ 2GHz = 44dB

Preceived at Earth = Ptransmitted +30 - 212 +44dB = Ptransmit -138dB.

Assume transmitting with 10 Watts or +40dBm..
Prec = -100dBm

Now, let's look at the receiver:
Power Spectral density of Noise is kT+NF.. kT is -174dBm/Hz and a decent
NF might be 2dB (allowing for some plumbing losses, etc.

-172 dBm/Hz

Eb (energy/Bit) = -100dBm -60dB (1Mbps) = -160dBmJ

So, Eb/No is about +12dB... *If you allow 2dB for implementation loss,
that gets you to 10dB, which will get you a BER of 1E-6, which isn't
terrible. *Coding will improve it, etc.

Take home message:

10W at 2Ghz with reasonably sized antennas at moon and earth can carry 1
Mbps.

Scales linearly with data rate.. You want 10Mbps, you need 100W. *Or
bigger antennas.

Thanks for the info. This at least should be doable with receiving
antennas operated by universities.


Bob Clark

On Jun 16, 9:43*pm, Jim Lux wrote:
...

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

LOFAR.
http://en.wikipedia.org/wiki/LOFAR

LOFAR like my proposal is to use many separate dipoles to detect long
wavelength radio waves. However, it is to have only 10,000 dipoles
whereas mine at the end will have ca. 1 billion dipoles.

The progenitors of the LOFAR project have argued in papers that it
could be used for the SETI search. However, this article by well known
SETI search scientist Seth Shostak argues LOFAR will be too weak to
detect Earth type radio transmissions at a distance of say 55 light-
years by a factor of 1 million:

Listening for ET’s Television.
November 9, 2006
by Seth Shostak, Senior Astronomer
http://www.seti.org/Page.aspx?pid=917

Then since my proposal will be about 100,000 times more sensitive
than LOFAR, it could detect Earth-like radio transmissions to about
1/3 the distance of 55 lightyears, or to about 18 lightyears way.
There are several star systems in that range. The Shostak article
notes you could get several hundred times better sensitivity by
listening to certain stars over months or years. Then my proposal
could detect such transmissions out to even 55 lightyears and further.


Bob Clark

On Tue, 30 Jun 2009 11:37:29 -0700 (PDT), Robert Clark
wrote:

The Shostak article
notes you could get several hundred times better sensitivity by
listening to certain stars over months or years.

And if you lost your keys at night, you might find them faster looking
under street lights.

Yet another troll.

73's
Richard Clark, KB7QHC