Can DirectTV-type satellite dishes be used for SETI?

wrote:
I was interested to read this on the Seti League web site:

__________________________________________________ ________
Parasitic SETI
Dear Dr. SETI:
As a Satellite dish owner and a strong interest in SETI, I was
wondering if anything is available to allow the home satellite dish
owner to 'search' when he is not watching TV.

[snip]

Try doing some math:
Suppose the aliens are only 50 lt-yrs away, and they're broadcasting a
100Mhz signal with a 1MW transmitter. Calculate how many photons/sec
hit your dish. Is that enough to reconstruct an intelligible signal?
Paul Cardinale

In article .com,
Paul Cardinale wrote:

Try doing some math:
Suppose the aliens are only 50 lt-yrs away, and they're broadcasting a
100Mhz signal with a 1MW transmitter. Calculate how many photons/sec
hit your dish. Is that enough to reconstruct an intelligible signal?

Such a signal is almost certainly too weak for Arecibo.

However, it is not the photon count that matters; that will not be a problem,
but the sky and receiver noise [A].

For SETI@Home, you need 1GW at about 4 LY (as effective isotropic power)
and that is at a frequency where the sky noise is about 200 or 300
times less. The wavelength at 100MHz is incompatible with even domestic
C band dishes. Phoenix can do better.

Small dish SETI is possible, and given certain constraints can match
the performance of large dish SETI, in terms of volume of sky swept at
a given transmit effective radiated power per unit time. The advantages
it has is that it is economically possible to give simultaneous all sky
coverage (because the solid angle covered scales in the same way as the
gain) and the signal integration times in drift scan are more than 100
times those for Arecibo. If you do the maths, and especially if you
account for the fact that the effective diameters of Arecibo is more
like 100 feet, you should find you get close to break even, for signals
that have relatively short durations. All the strong signals we have
ever generated are short duration.

The sci.astro.seti FAQ gives link budgets for small dish SETI. It
doesn't go to the integration times that are really needed for break
even, but even with just 200 seconds, it suggests that a 12 foot dish
can detect Arecibo's transmit power at over 50 light years.

Incidentally, the sort of low noise pre-amplifier technology currently
used by SERENDIP/SETI@Home is available to amateurs for, in the region of,
US$ 100. The current, room temperature, system actually outperforms
the cryogenic ones that failed during the project.

[A] system noise temperatures of about 50K are achievable at 1.42GHz
(it's several 1000K at 100MHz, from the sky). That make the noise in
0.05Hz (a good SETI bandwidth) about 3.45E-16 ergs/s (sorry, my reference
book is mainly cgs, but the conversion factor is 1E-7). At 1.42GHz,
one photon is about 5.92E-17 ergs. Typically you use signal to noise
ratios of more than 10, to account for statistical variation in the
noise, so one is talking about 3.45E-15 ergs/s. The integration time for
this bandwidth is about 20 seconds, so one has to exceed 6.90E-14 ergs.
You are talking about 1,000 photons at 1.42 Ghz (regardless of the dish
size) to produce a signal reliably detectable above the system noise.
At 100MHz, you would need serveral million because each photon has 1/14.2
of the energy and the sky noise, noise will be several thousand K.

k3ym4st3r wrote:

a question, in order to use all the "civilian" dishes as an array they
should be placed in a predetermined shape right?

Not necessarily. Information consists of intensity and phase. The
former comes from surface area, the latter from extent. Configure to
match your target. Your two big problems are

1) Phase synchronization. If you want a big interferometer you
need to have all inputs exactly ganged in time and space. Nasty for
large separations - even if you locally record and centrally
synchronize.

2) Thermal noise. The Aricebo first amplifier is a ruby maser
sitting in liquid helium. They wouldn't do it if they didn't have to
do it. Atoms in quartz at room temp typically jiggle 3% of their bond
lengths, and it is anisotropic movement in space. Atoms in a
signal-carrying wire or conduit likewise. Thermal noise (static)
swamps the input signal.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

In article ,
Uncle Al wrote:

1) Phase synchronization. If you want a big interferometer you

This and funding are problems. However, with C band dishes, as noted,
about 100 unphased ones can give whole sky coverage and may outperform
Arecibo for signals of around 20 minutes duration. Multiple beam
phased arrays like the Allen telescope may shift the balance back to
the professionals.

2) Thermal noise. The Aricebo first amplifier is a ruby maser
sitting in liquid helium. They wouldn't do it if they didn't have to

The SERENDIP first amplifier is now a non-cryogenic semiconductor (HEMT,
I seem to remember) device and these sell on the amateur market for
around US$ 100. This is better than the original cryogenic amplifiers.
You've failed to track technology.

signal-carrying wire or conduit likewise. Thermal noise (static)
swamps the input signal.

The atoms don't matter, it is the thermal energy in the counduction band
electrons that matters. The nature of conductors is that there is very
little thermal coupling between electrons and atoms. The electrons in
modern low noise amplifiers (and in the feeder cables) are cooled by
radiations into space.

Incidentally, modern C band amateur systems are more sensitive than the
system that detected the Wow! event.

Thanks for the detailed response. Tell me what's wrong with this
argument about detectability posted to habitablezone.com :

__________________________________________________ _____________
Space Sciences
Sounded like a reasonable question to me
Posted by Robert Clark on 1/20/2005 5:16:35 AM

In Reply to: Sounded like a reasonable question to me posted by alcaray
on 1/19/2005 9:07:31 AM

We can get a rough estimate from the transmissions detected from the
Pioneer 10 spacecraft:

EARTH STRAINS TO HEAR PIONEER 10 SOME 7 BILLION MILES AWAY.
http://www.xs4all.nl/~carlkop/pioneer.html

The transmitter is 7.5 watts and the transmissions were detected using
NASA's 70 meter radio telescopes in its Deep Space Network. The
furthest detections occurred when Pioneer 10 was more than 10 billion
kilometers away (it recently stopped transmitting or the signal
strength dropped too low.)
Television broadcast stations send out transmissions at the megawatt
scale. So let's say an alien broadcast is a million times stronger than
the Pioneer 10 signal at transmission. The strength of a signal drops
by the square of the distance. So we could detect such a signal not a
million times further away than the Pioneer 10 signal, but only a
thousand times further away. So this signal could be detected 10
trillion kilometers away. This is the distance of 1 light-year!
If you wanted to be able to detect signals out to 10 ly, which includes
several stars, the signal strength would drop by a factor of 100, so
the collecting area would have to be 100 times as large, which means
the diameter of your telescope has to be 10 times as big. This gives it
a diameter of 700 meters. This is only twice as wide as the current
largest telescope the Arecibo radio telescope.
This is only a rough guess because we know the frequency Pioneer 10 is
transmitting on and we know what the signal is supposed to look like,
which is not the case with a supposed alien signal.

This link shows stars to within 30 ly:

The Closest Stars
http://www.dudeman.net/spacedog/const/close.shtml

One of these at 10.5 ly away is Epsilon Eridani. It was recently shown
to have an orbiting planet:

Epsilon Eridani.
http://www.solstation.com/stars/eps-erid.htm


Bob Clark
__________________________________________________ _____________

In article .com,
wrote:

We can get a rough estimate from the transmissions detected from the
Pioneer 10 spacecraft:

Please read the sci.astro.seti FAQ. It gives a lot of quantified ranges.
However also note that the SETI Institute believes that Allen array, which
has less effective area than Arecibo, will be able to detect nearby
TV carriers (probably because they can afford to look at a source for
much longer than they can do with Arecibo - a large time bandwidth
product - range doubles for each 16 times increase in observation time,
but volume covered doubles for every 4 times increase in observation
time).

The transmitter is 7.5 watts and the transmissions were detected using

The effective transmit power will be a lot more than this because of
the high gain antenna. (Effective isotropic radiated power - EIRP.)
If the data in the sci.astro.seti FAQ is correct, the Pioneer frequency
is 2.295 GHz and the EIRP is 1.6 kW. I'm not sure if you were quoting
detectable or usable ranges (the latter being much less). Also Pioneer
is known to exist, so one can use detection thresholds much closer to
the noise.

Television broadcast stations send out transmissions at the megawatt
scale. So let's say an alien broadcast is a million times stronger than

Television is used in the popularisation of SETI, presumably because it
is a concept that the man in the street can understand, but at best only
the carriers are detectable, and even they are not strong compared with
signals we can and do produce. Recovering the programme content is
a fantasy.

The leakage signals we produce that have really significant reaches
(best part of 1,000 LY) are things like planetary radar (very narrowband
and over 20TW EIRP). These don't, unfortunately, produce repeatable signals.
They do produce the sort of 15 minute duration signals that are optimised
for low gain simultaneous all sky searches!

Unfortunately, the general population has an expectation of SETI that
far exceeds what the SETI professionals' expect. Much of this has been
discussed over and over on sci.astro.seti.

David Woolley wrote:

In article ,
Uncle Al wrote:

1) Phase synchronization. If you want a big interferometer you

This and funding are problems. However, with C band dishes, as noted,
about 100 unphased ones can give whole sky coverage and may outperform
Arecibo for signals of around 20 minutes duration. Multiple beam
phased arrays like the Allen telescope may shift the balance back to
the professionals.

2) Thermal noise. The Aricebo first amplifier is a ruby maser
sitting in liquid helium. They wouldn't do it if they didn't have to

The SERENDIP first amplifier is now a non-cryogenic semiconductor (HEMT,
I seem to remember) device and these sell on the amateur market for
around US$ 100. This is better than the original cryogenic amplifiers.
You've failed to track technology.

KEWL! What does an organiker know?

(100 dishes) - (100 amplifiers)($100/amplifer)= $10K. Not so bad as
long as nobody wants to watch TV.

signal-carrying wire or conduit likewise. Thermal noise (static)
swamps the input signal.

The atoms don't matter, it is the thermal energy in the counduction band
electrons that matters. The nature of conductors is that there is very
little thermal coupling between electrons and atoms. The electrons in
modern low noise amplifiers (and in the feeder cables) are cooled by
radiations into space.

Incidentally, modern C band amateur systems are more sensitive than the
system that detected the Wow! event.

SETI to date has detected no presumptive hits within a 50+ lightyear
radius. Technological life is rare.

1) It may be intrinsically rare.

2) It may be culturally rare. Europe was that rare concidence -
weather, religion, clear glass, big balls - that moved out of its
cocoon to achieve sustainable heavy ****. China had it and lost it.
With ceramics but without clear glass, Chinese science stalled.
Philosophy plus bureaucracy doesn't build an Industrial Revolution.
Planets covered with subsistence agriculture and butterball priests
don't go anywhere interesting.

3) It may be transient. The First World will burn through physical
and social resources by 2050. Unless something drastically advances,
major upheaval and likely irreversible collapse is unavoidable. Ten
billon Third World animals will value life beneath all else and take
over. We'll never come back.

4) The necessary bit of sustaining knowledge may be lethally
difficult to discover or lethally expensive to engage.

5) We might be first in the neighborhood. It is then vital that we
grab it all and kill off any competition - what Europe did in the New
World and what we should do with the Third World.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

wrote in message
oups.com...
Thanks for the detailed response. Tell me what's wrong with this
argument about detectability posted to habitablezone.com :

One thing to consider is the bandwidth. Pioneer used a
transponder on the craft locked to a known uplink
frequency. You can get wide frequency coverage and
narrow-band detection using an FFT but the frequency
has to be stable, hence the need to search a range of
Doppler shift to compensate for the radial acceleration
of the source. Narrow band still means long integration
times and unless the aliens are targetting us, any
chance alignment is likely to sweep over our antenna in
a very short time. That relates to the beamwidth of the
transmitter which another consideration, a wider beam
(at their end) gives a longer period of illumination
but lower received power.

A third aspect is that you will only detect the carrier
this way, not modulation. For high power transmitters
it gives significant saving to use some form of
suppressed carrier scheme. Unles they are shining a CW
beacon at us to attract our attention (or they are very
stupid, highly advanced aliens), it is likely that only
a small fraction of the power would be in the carrier.

George



__________________________________________________ _____________
Space Sciences
Sounded like a reasonable question to me
Posted by Robert Clark on 1/20/2005 5:16:35 AM

In Reply to: Sounded like a reasonable question to me posted by alcaray
on 1/19/2005 9:07:31 AM

We can get a rough estimate from the transmissions detected from the
Pioneer 10 spacecraft:

EARTH STRAINS TO HEAR PIONEER 10 SOME 7 BILLION MILES AWAY.
http://www.xs4all.nl/~carlkop/pioneer.html

The transmitter is 7.5 watts and the transmissions were detected using
NASA's 70 meter radio telescopes in its Deep Space Network. The
furthest detections occurred when Pioneer 10 was more than 10 billion
kilometers away (it recently stopped transmitting or the signal
strength dropped too low.)
Television broadcast stations send out transmissions at the megawatt
scale. So let's say an alien broadcast is a million times stronger than
the Pioneer 10 signal at transmission. The strength of a signal drops
by the square of the distance. So we could detect such a signal not a
million times further away than the Pioneer 10 signal, but only a
thousand times further away. So this signal could be detected 10
trillion kilometers away. This is the distance of 1 light-year!
If you wanted to be able to detect signals out to 10 ly, which includes
several stars, the signal strength would drop by a factor of 100, so
the collecting area would have to be 100 times as large, which means
the diameter of your telescope has to be 10 times as big. This gives it
a diameter of 700 meters. This is only twice as wide as the current
largest telescope the Arecibo radio telescope.
This is only a rough guess because we know the frequency Pioneer 10 is
transmitting on and we know what the signal is supposed to look like,
which is not the case with a supposed alien signal.

This link shows stars to within 30 ly:

The Closest Stars
http://www.dudeman.net/spacedog/const/close.shtml

One of these at 10.5 ly away is Epsilon Eridani. It was recently shown
to have an orbiting planet:

Epsilon Eridani.
http://www.solstation.com/stars/eps-erid.htm


Bob Clark
__________________________________________________ _____________

Thanks for info. A carrier-wave is a pure sine wave isn't it? Wouldn't
that be in indication of intelligent origin?


Bob Clark

David Woolley wrote:
In article .com,
wrote:

We can get a rough estimate from the transmissions detected from
the
Pioneer 10 spacecraft:

Please read the sci.astro.seti FAQ. It gives a lot of quantified
ranges.
However also note that the SETI Institute believes that Allen array,
which
has less effective area than Arecibo, will be able to detect nearby
TV carriers (probably because they can afford to look at a source for
much longer than they can do with Arecibo - a large time bandwidth
product - range doubles for each 16 times increase in observation
time,
but volume covered doubles for every 4 times increase in observation
time).

The transmitter is 7.5 watts and the transmissions were detected
using

The effective transmit power will be a lot more than this because of
the high gain antenna. (Effective isotropic radiated power - EIRP.)
If the data in the sci.astro.seti FAQ is correct, the Pioneer
frequency
is 2.295 GHz and the EIRP is 1.6 kW. I'm not sure if you were
quoting
detectable or usable ranges (the latter being much less). Also
Pioneer
is known to exist, so one can use detection thresholds much closer to
the noise.

Television broadcast stations send out transmissions at the
megawatt
scale. So let's say an alien broadcast is a million times stronger
than

Television is used in the popularisation of SETI, presumably because
it
is a concept that the man in the street can understand, but at best
only
the carriers are detectable, and even they are not strong compared
with
signals we can and do produce. Recovering the programme content is
a fantasy.

The leakage signals we produce that have really significant reaches
(best part of 1,000 LY) are things like planetary radar (very
narrowband
and over 20TW EIRP). These don't, unfortunately, produce repeatable
signals.
They do produce the sort of 15 minute duration signals that are
optimised
for low gain simultaneous all sky searches!

Unfortunately, the general population has an expectation of SETI that
far exceeds what the SETI professionals' expect. Much of this has
been
discussed over and over on sci.astro.seti.

In article .com,
wrote:

Thanks for info. A carrier-wave is a pure sine wave isn't it? Wouldn't
that be in indication of intelligent origin?

That's generally what the SETI literature means when they say they are
searching for narrow band signals. Being narrowband is the primary
indicator of intelligence for most current SETI searches, but in
itself it doesn't exclude human sources.

However, a sine wave is only a carrier if there is also modulation
present, so you will never see a pure carrier (you may see a beacon
that is a pure sine wave and you may see planetary radar like that).
Old technology, particularly AM broadcast, actually results in most of
the power going into the carrier and the rest being spread in frequency,
so it is possible to detect signals that are less than 1 Hz wide with
a lot of power. Modern technology puts very little carrier frequency
power into the signal.

The received signal will be distorted in by noise and propagation
disturbances, and chirped by the relative acceleration between the sender
and receiver (a SETI beacon may well be compensated to an inertial frame,
for the intended transmission direction, though).

In many detection scenarios, the signal will be modulated by the
beam patterns of sending or receiving antennas.