Science > Physics > Can DirectTV-type satellite dishes be used for SETI?
| Topic: |
Science > Physics |
| User: |
"" |
| Date: |
19 Jan 2005 03:17:52 AM |
| Object: |
Can DirectTV-type satellite dishes be used for SETI? |
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. I do a bit of programing
and would love to help make it so home dish owners could do this. Is it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
Bill T.
The Doctor Responds:
Absolutely, Bill! Parasitic SETI with a home satellite TV dish is not
only feasilble, it's widely practiced. A second feedhorn and preamp
assembly are mounted next to the C-band horn/LNB at the apex of the
dish (see Figure 2 of this article). This assembly feeds the rest of a
SETI system (see our online Tech Manual). You can then sweep out the
sky, as described here. And yes, a million participants would be nice,
but our goal is a more modest 5000 stations.
__________________________________________________________
http://www.setileague.org/askdr/parasite.htm
I believe they are referring to the 6 ft. backyard type antennas,
judging from the linked images on the page. But could the roof mounted
DirectTV and Dish Network type antennas be used for SETI?
The mentioned extra equipment are an extra feedhorn and a
preamplifier. The feedhorn can made cheaply but the preamp seems
expensive. If these preamps were mass produced for this purpose could
their per item cost be brought under $50?
I'm envisionig a government agency such as NSF, or a scientically
interested billionaire, paying satellite TV companies to attach this
extra equipment to their satellite dishes. Say $100 million is
earmarked for the program. Then you would want the extra cost to be
under $100 for each dish for say 1,000,000 subscribers. Judging from
the diagram in the online Tech Manual linked to on the page, the other
equipment should be doable by the equipment that comes with the
satellite TV system. Computer processing would be done separately at a
central location.
If you had a 1,000,000 of these .5 meter wide antennas it would have
the detection sensitivy of a single antenna 500 meters wide.
Bob Clark
.
|
|
| User: "Motuwojjal" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
22 Jan 2005 07:23:00 AM |
|
|
<rgregoryclark@yahoo.com> wrote in message
news:1106126272.160248.294240@z14g2000cwz.googlegroups.com...
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. I do a bit of
programing
and would love to help make it so home dish owners could do this. Is
it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
Bill T.
The Doctor Responds:
Absolutely, Bill! Parasitic SETI with a home satellite TV dish is
not
only feasilble, it's widely practiced. A second feedhorn and preamp
assembly are mounted next to the C-band horn/LNB at the apex of the
dish (see Figure 2 of this article). This assembly feeds the rest of
a
SETI system (see our online Tech Manual). You can then sweep out the
sky, as described here. And yes, a million participants would be
nice,
but our goal is a more modest 5000 stations.
__________________________________________________________
http://www.setileague.org/askdr/parasite.htm
I believe they are referring to the 6 ft. backyard type antennas,
judging from the linked images on the page. But could the roof
mounted
DirectTV and Dish Network type antennas be used for SETI?
The mentioned extra equipment are an extra feedhorn and a
preamplifier. The feedhorn can made cheaply but the preamp seems
expensive. If these preamps were mass produced for this purpose
could
their per item cost be brought under $50?
I'm envisionig a government agency such as NSF, or a scientically
interested billionaire, paying satellite TV companies to attach this
extra equipment to their satellite dishes. Say $100 million is
earmarked for the program. Then you would want the extra cost to be
under $100 for each dish for say 1,000,000 subscribers. Judging from
the diagram in the online Tech Manual linked to on the page, the
other
equipment should be doable by the equipment that comes with the
satellite TV system. Computer processing would be done separately at
a
central location.
If you had a 1,000,000 of these .5 meter wide antennas it would have
the detection sensitivy of a single antenna 500 meters wide.
Bob Clark
Apart from the technical aspects, there's also a logistics problem.
Who is going to coordinate who will scan what segments & when they'll
do it.
.
|
|
|
| User: "Rob Dekker" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
24 Jan 2005 05:07:30 PM |
|
|
"Motuwojjal" <mowwarka.wiollie@bounretyon.com> wrote in message news:UssId.129527$K7.4738@news-server.bigpond.net.au...
[...]
If you had a 1,000,000 of these .5 meter wide antennas it would have
the detection sensitivy of a single antenna 500 meters wide.
Bob Clark
Apart from the technical aspects, there's also a logistics problem.
Who is going to coordinate who will scan what segments & when they'll
do it.
Not to mention the logistics of information transfer :
To transmit even a moderate 1GHz bandwidth (from the 12GHz band) to the
coordinator, each dish would need to have at least a 1Gbps data connection.
Scanning a much smaller bandwidth would greatly diminish usability for the array.
Some fiber-optic cable would do, but most people don't need satellite TV any more
if they have a fiber-optic cable coming into their house.
.
|
|
|
|
|
| User: "Androcles" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 08:16:02 AM |
|
|
<rgregoryclark@yahoo.com> wrote in message
news:1106126272.160248.294240@z14g2000cwz.googlegroups.com...
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. I do a bit of programing
and would love to help make it so home dish owners could do this. Is
it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
Bill T.
The Doctor Responds:
Absolutely, Bill! Parasitic SETI with a home satellite TV dish is not
only feasilble, it's widely practiced. A second feedhorn and preamp
assembly are mounted next to the C-band horn/LNB at the apex of the
dish (see Figure 2 of this article). This assembly feeds the rest of a
SETI system (see our online Tech Manual). You can then sweep out the
sky, as described here. And yes, a million participants would be nice,
but our goal is a more modest 5000 stations.
__________________________________________________________
http://www.setileague.org/askdr/parasite.htm
I believe they are referring to the 6 ft. backyard type antennas,
judging from the linked images on the page. But could the roof mounted
DirectTV and Dish Network type antennas be used for SETI?
The mentioned extra equipment are an extra feedhorn and a
preamplifier. The feedhorn can made cheaply but the preamp seems
expensive. If these preamps were mass produced for this purpose could
their per item cost be brought under $50?
I'm envisionig a government agency such as NSF, or a scientically
interested billionaire, paying satellite TV companies to attach this
extra equipment to their satellite dishes. Say $100 million is
earmarked for the program. Then you would want the extra cost to be
under $100 for each dish for say 1,000,000 subscribers. Judging from
the diagram in the online Tech Manual linked to on the page, the other
equipment should be doable by the equipment that comes with the
satellite TV system. Computer processing would be done separately at a
central location.
If you had a 1,000,000 of these .5 meter wide antennas it would have
the detection sensitivy of a single antenna 500 meters wide.
Bob Clark
Since it is quite clear that a dish is used for highly directional
communication,
and SETII stands for Search for Extra Terrestrial INTELLIGENCE by
IDIOTS....
Androcles.
.
|
|
|
|
| User: "Uncle Al" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 09:59:54 AM |
|
|
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. I do a bit of programing
and would love to help make it so home dish owners could do this. Is it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
[snip]
1) What is the diameter of a home satellite dish? What is the
diameter of the Aricebo dish? Intercepted amplitude varies as the
square of that ratio.
2) What is the temperature of a home satellite dish amplifier?
What is the temperature of the Aricebo first amplifier?
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
.
|
|
|
| User: "k3ym4st3r" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 10:11:37 AM |
|
|
a question, in order to use all the "civilian" dishes as an array they
should be placed in a predetermined shape right?
.
|
|
|
| User: "Aso Merapi" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 10:23:11 AM |
|
|
"k3ym4st3r" <k3ym4st3r@yahoo.com> wrote in message
news:1106151097.158262.300540@z14g2000cwz.googlegroups.com...
a question, in order to use all the "civilian" dishes as an array they
should be placed in a predetermined shape right?
no they have to be phased though.
So the received signal adds in phase from all the dishes.
That is what the big arrays do.
.
|
|
|
|
| User: "Uncle Al" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 04:35:31 PM |
|
|
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
.
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 01:35:53 AM |
|
|
In article <41EEE0B3.6E0ED937@hate.spam.net>,
Uncle Al <UncleAl0@hate.spam.net> 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.
.
|
|
|
| User: "Uncle Al" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 01:30:50 PM |
|
|
David Woolley wrote:
In article <41EEE0B3.6E0ED937@hate.spam.net>,
Uncle Al <UncleAl0@hate.spam.net> 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
.
|
|
|
| User: "tj Frazir" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
21 Jan 2005 10:48:25 AM |
|
|
125000 light year galaxy.
the nearest sun is 1000 light years.
You then must look at the same age on the next arm of the galaxy 30,000
light years away.
the digital image must be downloaded inless than 1 second .
You wount find a radio station out there.
ALL you will find is 1 second bits or milibits.
Its fast and digital.
Seti is not digital and is a waist.
Download a movie via satelite in one second.
Or biuld a real fast recorder and play it to your PC real slow .
Seti is not biult to download ,
,,when they do they will find out just how many planets are sending
digital information.
lucy ball lasted 100 years.
digital last 100,000 years.
One supperconductor photon detector and a high speed burner will
downrecord so you can download.
the didgital data will be between 30,000 and 120,000 light years old.
A laser beamed data only neads te photon detector and no dish .
A dish wount work with a laser.
seti is not digital .
the future is boadcast via digital laser .
If you want to hear ET you must build HIS radio
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
ET uses digital laser .
He is constantly sending you data waiting till your smart eouph even
thow he wount know if or when you do.
We can beam back but they must wait 30000 years to hear if we exsist.
They saw our planet 30,000 years ago.
They are expecting us .
the sun near the tip of the next spiral arm on our galaxy .
30,000 years out farther on thier arm than us.
The same raduis the earth will be around the senter of galaxy in 30
million years.
They are 30 million years more advanced.
OPPs. I said too much
.
|
|
|
|
|
|
| User: "tj Frazir" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 11:22:26 PM |
|
|
seti is a waist of money .
125,000 light years is our galaxy.
we are near the outside ege of our galaxy.
1000 light years is next door ,,next sun.
Dont look for radio waves .
Look for digital images that download real quick.
Radio has a short eveolution .
If your looking to hear anything from another world , it wount be on
the radio.
It will be a digital image broadcast in a milisecond . A more evolved
system .
They would use a satelite and supperconductor recivers with out any
dishes.
radio is week and the evolution of radio was a short few years.
600,000 light years to the next gallaxy ..forget ever hearing anything
from there ,
Ill bet there is life we can hear 30,000 light years away.
They cant hear us ,,but we can see thier pages and play thier movies
from thier page .
It will nead to download the movie in a few miliseconds
No alians could ever get here ,,but in 30,000 years they may send a
machine thaat will take 300,000 years to get here.
Its only posible to download an image but thats all .
You cant ask a question.
You could download evry page .
a trillion earths in 125000 light year galaxy .
300 billion suns in our galaxy .
a trillion posible earths in our own back yard.
But te galaxy is 14 billion years old.
as old as the univrse .
at .01 C it has turned 180 million light years of rotation wile at
the same time 180 million light years from the wall of the balloon of
the big gang.
This is an old galaxy and we are out on the oldest part. 15 light
years from its ege.
the next spiral arm of the galaxy is 30,000 light years at the same
point out 15 light years from the ege.
Ill bet you can download from that arm .
.
|
|
|
|
|
| User: "Martin Brown" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 10:48:07 AM |
|
|
k3ym4st3r wrote:
a question, in order to use all the "civilian" dishes as an array they
should be placed in a predetermined shape right?
You can choose your shape and suffer the resulting sidelobes.
But you do have to determine and compensate for the baseline geometry to
within fractions of a wavelength. All the signals from the target must
add in phase if you are to get any benefit from using multiple aerials.
This is non trivial if you want to track a celestial object across the sky.
A large phased array at low frequencies discovered the first pulsar. You
will need a few acres of toy dishes dedicated to the task to stand any
chance at all. Try the sun and Jupiter instead - you should see them OK.
Regards,
Martin Brown
.
|
|
|
|
|
|
| User: "Paul Cardinale" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 11:16:05 AM |
|
|
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
.
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 03:50:59 PM |
|
|
In article <1106154965.613364.173750@f14g2000cwb.googlegroups.com>,
Paul Cardinale <pcardinale@volcanomail.com> 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.
.
|
|
|
| User: "" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 07:23:20 AM |
|
|
Thanks for the detailed response. Tell me what's wrong with this
argument about detectability posted to habitablezone.com :
_______________________________________________________________
Space Sciences
re: 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
_______________________________________________________________
.
|
|
|
| User: "George Dishman" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 03:06:15 PM |
|
|
<rgregoryclark@yahoo.com> wrote in message
news:1106227400.489449.102950@c13g2000cwb.googlegroups.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
re: 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
_______________________________________________________________
.
|
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 07:23:20 AM |
|
|
In article <1106227400.489449.102950@c13g2000cwb.googlegroups.com>,
rgregoryclark@yahoo.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.
.
|
|
|
| User: "" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
20 Jan 2005 04:39:42 PM |
|
|
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 <1106227400.489449.102950@c13g2000cwb.googlegroups.com>,
rgregoryclark@yahoo.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.
.
|
|
|
| User: "Rob Dekker" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
21 Jan 2005 10:06:50 PM |
|
|
<rgregoryclark@yahoo.com> wrote in message news:1106260782.697981.215390@c13g2000cwb.googlegroups.com...
Thanks for info. A carrier-wave is a pure sine wave isn't it? Wouldn't
that be in indication of intelligent origin?
Yes. A carrier is a pure sine wave (very, very narrowband), and
Yes, this would be an indication of artificial (intelligent) origin.
Nature's most narrowband signals (certain masers) are wider than 300Hz bandwidth,
so carriers will stand out from natural signals.
Problem with carriers is that they are actually a 'waste' of energy, since they
contain no information. So the assumption is that carriers have a limited lifetime
in a technologically advancing civilisation. For us, since we are moving away from
analog and to digital transmissions (where carriers are not really needed), the
lifetime of carriers for communication applications is almost reached, and was
thus about 100 years.
This realization has triggered some news articles in the past month that
"Earth is disappearing from the radar screen", which was readily picked up by
SETI critics that there is no use to continue SETI. This is a somewhat strange
conclusion, since SETI was so-far focused on finding intended signals (beacons)
as opposed to unintended (TV/radio/radar) signals, and also because it only applies to
communication signal technology.
Other applications (such as the planetary radar that David mentioned) and
also SETI beacons, are much more likely to continue to use carriers (CW signals),
even for much more advanced technologies, since their narrow bandwidth character
by itself is very advantageous for sensitivity.
So technological lifetime for carrier signals can still be much longer than lifetime
of only analog TV.
Rob
.
|
|
|
| User: "Dan Mckenna" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
22 Jan 2005 01:44:34 PM |
|
|
Rob Dekker wrote:
<rgregoryclark@yahoo.com> wrote in message news:1106260782.697981.215390@c13g2000cwb.googlegroups.com...
Thanks for info. A carrier-wave is a pure sine wave isn't it? Wouldn't
that be in indication of intelligent origin?
A carrier, if transmitted from the surface of a planet would have very
useful information if it was continuous:
1) rotation rate of planet
2) orbital velocity of planet around star
If you could see the parent star then you would know the spectral type
and distance to the star. This would let us know the orbit, distance
from the star and based on the spectral type, the stellar flux from the
star at the surface of the planet.
These parameters would go a long way to understanding the distribution
of life in the galaxy.
I would think.
Dan
It's only a hobby
Yes. A carrier is a pure sine wave (very, very narrowband), and
Yes, this would be an indication of artificial (intelligent) origin.
Nature's most narrowband signals (certain masers) are wider than 300Hz bandwidth,
so carriers will stand out from natural signals.
Problem with carriers is that they are actually a 'waste' of energy, since they
contain no information. So the assumption is that carriers have a limited lifetime
in a technologically advancing civilisation. For us, since we are moving away from
analog and to digital transmissions (where carriers are not really needed), the
lifetime of carriers for communication applications is almost reached, and was
thus about 100 years.
.
|
|
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
21 Jan 2005 03:54:50 PM |
|
|
In article <1106260782.697981.215390@c13g2000cwb.googlegroups.com>,
rgregoryclark@yahoo.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.
.
|
|
|
|
|
|
|
|
|
| User: "Matt Giwer" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
15 Feb 2005 02:20:09 AM |
|
|
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. I do a bit of programing
and would love to help make it so home dish owners could do this. Is it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
I talked about this months ago. Forming a beam, antenna gain, requires knowledge of the phase angle
differences between the sensors.
The bottom line is you cannot aggregate dishes without both
1) relative location of all dishes to each other to a fraction of a wavelength
1a) data transmission phase lag = 0 after compensation
2) tolerances in the receiver phase being to equally high tolerances
It is feasable if the combination of 1 and 2 do not result in more than about 1/36th wavelength
error. The more it deviates from that the less useful. 1/36th would cover about a 10 degree circle
in the sky. And that would indicate the array gain, roughly log(180^2/10^2). At 1/4 wavelength error
it is log(180^2/90^2) and a 1/2 wavelength error it is an antenna gain of log(1).
Maybe when the European GPS goes up and if it can be used seamlessly with the present US system it
might be possible to get accuracies of a few inches which is way to great. But even if perfect
sending the date to be aggregated depends upon the delays along the way from sender to inches in
difference in the cable and fiber lines being known to the fraction of a wavelength. If installers
do not have the exact measure of the length of cable and fiber used it all goes to *****.
And then there is the impossible job of calibrating them all.
--
Security at presidential press conferences is so good
a man can use a fake name and pass the background
check. Or are there no background checks?
-- The Iron Webmaster, 3380
.
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
15 Feb 2005 03:03:20 PM |
|
|
In article <ZgiQd.33788$pc5.5440@tornado.tampabay.rr.com>,
Matt Giwer <matt@tampabay.REMover.rr.com> wrote:
It is feasable if the combination of 1 and 2 do not result in more than about 1/36th wavelength
That's about 6 times better than the requirements for dishes [A]. I don't
know where you got that figure from, but it is wrong.
error. The more it deviates from that the less useful. 1/36th would cover about a 10 degree circle
Er! 1/36th in the beam direction would cause a deviation of the order of
(in radians) 1/(36 * <baseline length in wavelengths>) in the direction
of the individual fringes. The direction in which the fringes are
intended to coincide would only see a small reduction in in-phase
signal.
Whilst I strongly suspect that differential and carrier phase will not
solve the problem, it is nothing like as bad as you suggest.
in the sky. And that would indicate the array gain, roughly log(180^2/10^2). At 1/4 wavelength error
If the peak to peak error is 1/4 wave, in the worst possible configuration,
the array gain will be degraded by 3dB.
[A] Generally the profile accuracy suggested for dishes is 1/10th to 1/12th
of a wavelength, but the phase error is doubled by the reflection.
.
|
|
|
| User: "Matt Giwer" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
16 Feb 2005 12:35:59 AM |
|
|
David Woolley wrote:
In article <ZgiQd.33788$pc5.5440@tornado.tampabay.rr.com>,
Matt Giwer <matt@tampabay.REMover.rr.com> wrote:
It is feasable if the combination of 1 and 2 do not result in more than about 1/36th wavelength
That's about 6 times better than the requirements for dishes [A]. I don't
know where you got that figure from, but it is wrong.
A dish is summing power. You don't care much. It could be less if the receiver at the focal point
were larger but at some point the receiver blocks too much incoming signal.
In a distributed array you must have phase information so you can determine the direction it is
pointing. If we assume these dishes are all pointing with zero error at the satellite 32,000 miles
up from all over the US (for example) they are all pointing at a different point at infinity. So if
you want to look at a point at astronomical distances you have to correct for the satellite aim point.
Consider a dozen widely spaced antenna pointed at the same satellite. They go on to point to widely
different points in space as they "pass through" the satellite. That gives you no gain at all, it
does not not improve the signal to noise ratio.
To take the data from all of them and sum the data from one point in the sky, Tau Ceti the odds on
favorie, you have to phase shift the data from all of them such that you are only considering the
part of the signal that corresponds to that direction.
Because television sensors all converge on a very local point 32,000 miles away (36,000 mile orbit
from the center and varying by latitude) the "aim point" rapidly diverges beyond the satellite. To
recombine that noise into useful data you have to know how to "point" it to a single direction to
combine signal to exceed the noise.
Given the reality that only the dishes in a very small town can be considered to be pointing to
roughly the same point in deep space there is no gain of interest as the smaller the town the
smaller the deep space area and the lesser the gain.
Two dishes one mile apart exactly pointing to the same point 32,000 miles up point to greatly
different places in deep space.
--
If you sue and act as your own attorney while the other side
hires Alan Derschowitz, only winning is noteworthy.
-- The Iron Webmaster, 3356
.
|
|
|
| User: "David Woolley" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
16 Feb 2005 04:09:15 PM |
|
|
In article <jRBQd.79846$JF2.31490@tornado.tampabay.rr.com>,
Matt Giwer <matt@tampabay.REMover.rr.com> wrote:
A dish is summing power. You don't care much. It could be less if the receiver at the focal point
Mathematically (and because of the linearity of EM fields with respect to
superimposition, dishes sum field (volts/metre) not power. In fact dishes
can be treated as the limiting case of phased arrays, with infinitesimal
elements (and free space phasing lines); that's how you work out the beam
pattern to get the approximate gaussian (the actual one for a circular
aperture is, I believe, a Bessel function). Using analogue delay lines
for phasing is standard for stacked arrays and, with switchable loops,
even for some electronically steerable ones. (Dishes do combine power
overall, but so do delay line phased arrays, and you can only work out
beam patterns by considering the phased integration of the fields.)
However, one doesn't need to think about dishes to demonstrate that even
quarter wave peak to peak along the beam direction only compromises by
3dB. (Perpendicular to the beam, there is no effect.)
The worst case configuration is with half exactly an eighth wave high and
half an eighth wave low. The in-phase components follow a cosine pattern
and are + and - 45 degrees. Cos (45 degrees) is sqrt (2). The out of phase
components follow a sine pattern and cancel. Squaring to get power, one
gets a factor of 2, i.e. 3dB. Note that I haven't made any assumptions
about the array size, here, except that the individual element capture
areas don't overlap.
Whilst I have severe doubts as to the ability to phase to even this accuracy,
it is 9 times coarser than the 1/36th you were claiming, and has only
halved the effective number of elements.
.
|
|
|
| User: "Matt Giwer" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
18 Feb 2005 09:43:54 PM |
|
|
David Woolley wrote:
In article <jRBQd.79846$JF2.31490@tornado.tampabay.rr.com>,
Matt Giwer <matt@tampabay.REMover.rr.com> wrote:
A dish is summing power. You don't care much. It could be less if the receiver at the focal point
Mathematically (and because of the linearity of EM fields with respect to
superimposition, dishes sum field (volts/metre) not power.
Correct. Sonar background myself. Never did quite learn to think in E-M terms for posting purposes.
In fact dishes
can be treated as the limiting case of phased arrays, with infinitesimal
elements (and free space phasing lines); that's how you work out the beam
pattern to get the approximate gaussian (the actual one for a circular
aperture is, I believe, a Bessel function). Using analogue delay lines
for phasing is standard for stacked arrays and, with switchable loops,
even for some electronically steerable ones. (Dishes do combine power
overall, but so do delay line phased arrays, and you can only work out
beam patterns by considering the phased integration of the fields.)
However, one doesn't need to think about dishes to demonstrate that even
quarter wave peak to peak along the beam direction only compromises by
3dB. (Perpendicular to the beam, there is no effect.)
The worst case configuration is with half exactly an eighth wave high and
half an eighth wave low. The in-phase components follow a cosine pattern
and are + and - 45 degrees. Cos (45 degrees) is sqrt (2). The out of phase
components follow a sine pattern and cancel. Squaring to get power, one
gets a factor of 2, i.e. 3dB. Note that I haven't made any assumptions
about the array size, here, except that the individual element capture
areas don't overlap.
Whilst I have severe doubts as to the ability to phase to even this accuracy,
it is 9 times coarser than the 1/36th you were claiming, and has only
halved the effective number of elements.
And again correct. I was thinking in terms of a linear sensor array not an area sensor array. Area
would proportional to the square the linear accuracy required in the lesser direction. My error was
a hangover from my earliest work.
Regret the delay in responding.
--
TV networks cover news with the same stories in the same
amount of time and in the same order. There is no
evidence they are independent.
-- The Iron Webmaster, 3365
.
|
|
|
|
|
|
|
| User: "" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
15 Feb 2005 02:58:35 AM |
|
|
Matt Giwer wrote:
rgregoryclark@yahoo.com 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. I do a bit of
programing
and would love to help make it so home dish owners could do this.
Is it
possible? What would it take? Does the dish have to follow a spot
or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
I talked about this months ago. Forming a beam, antenna gain,
requires knowledge of the phase angle
differences between the sensors.
The bottom line is you cannot aggregate dishes without both
1) relative location of all dishes to each other to a fraction of a
wavelength
1a) data transmission phase lag = 0 after compensation
2) tolerances in the receiver phase being to equally high
tolerances
It is feasable if the combination of 1 and 2 do not result in more
than about 1/36th wavelength
error. The more it deviates from that the less useful. 1/36th would
cover about a 10 degree circle
in the sky. And that would indicate the array gain, roughly
log(180^2/10^2). At 1/4 wavelength error
it is log(180^2/90^2) and a 1/2 wavelength error it is an antenna
gain of log(1).
Maybe when the European GPS goes up and if it can be used seamlessly
with the present US system it
might be possible to get accuracies of a few inches which is way to
great. But even if perfect
sending the date to be aggregated depends upon the delays along the
way from sender to inches in
difference in the cable and fiber lines being known to the fraction
of a wavelength. If installers
do not have the exact measure of the length of cable and fiber used
it all goes to *****.
And then there is the impossible job of calibrating them all.
--
Security at presidential press conferences is so good
a man can use a fake name and pass the background
check. Or are there no background checks?
-- The Iron Webmaster, 3380
Methods of fixing your position within millimeters using GPS exist as
long as you can make a comparison to the GPS signals received at a site
whose position is known within millimeters. This precisely known site
can be kilometers away. This is used in surveying for example:
CARRIER-PHASE TRACKING
"Carrier-phase tracking provides for a more accurate range resolution
due to the short wavelength (about 19 centimeters for L1 and 24
centimeters for L2) and the ability of a receiver to resolve the
carrier phase down to about 2 millimeters. This technique has primary
application to engineering, topographic, and geodetic surveying and
may be employed with either static or kinematic surveys. There are
several techniques that use the carrier phase to determine a station's
position. These include static, rapid-static, kinematic, stop-and-go
kinematic, pseudokinematic, and on-the-fly (OTF) kinematic/Table 8-4
lists these techniques and their required components, applications,
and accuracies."
http://cartome.org/FM3-34/Chapter8.htm
This would suffice for centimeter wavelengths.
Bob Clark
.
|
|
|
| User: "Matt Giwer" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
16 Feb 2005 12:46:15 AM |
|
|
wrote:
Methods of fixing your position within millimeters using GPS exist as
long as you can make a comparison to the GPS signals received at a site
whose position is known within millimeters. This precisely known site
can be kilometers away. This is used in surveying for example:
This is a good start. Is it affordable to calibrate every installation? These sites are by
definition on houses and such and installed by people who know only how to do that. So each has to
be properly calibrated.
CARRIER-PHASE TRACKING
"Carrier-phase tracking provides for a more accurate range resolution
due to the short wavelength (about 19 centimeters for L1 and 24
centimeters for L2) and the ability of a receiver to resolve the
carrier phase down to about 2 millimeters. This technique has primary
application to engineering, topographic, and geodetic surveying and
may be employed with either static or kinematic surveys. There are
several techniques that use the carrier phase to determine a station's
position. These include static, rapid-static, kinematic, stop-and-go
kinematic, pseudokinematic, and on-the-fly (OTF) kinematic/Table 8-4
lists these techniques and their required components, applications,
and accuracies."
http://cartome.org/FM3-34/Chapter8.htm
This would suffice for centimeter wavelengths.
BUT the transmission of the data back to the processing source has to be perfect. It cannot be by
raw cable as that would require a separate network and if it existed people would use cable for TV
not satellites. It has to be processed data going back and then we have local computers and data
packets and delays all piling up. Most of those can be overcome. Even if there were raw data fibers
going back to the processor, the length of the fiber path has to be known exactly to preserve phase
information.
--
What is a conspiracy theorist when the US government believed
in an insane conspiracy by Iraq against the US?
-- The Iron Webmaster, 3366
.
|
|
|
|
|
|
| User: "Alfred A. Aburto Jr." |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 08:17:07 AM |
|
|
rgregoryclark@yahoo.com 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. I do a bit of programing
and would love to help make it so home dish owners could do this. Is it
possible? What would it take? Does the dish have to follow a spot or
can it sweep the sky from a fixed position? If this is possible it
could add a million listeners to the system.
Bill T.
The Doctor Responds:
Absolutely, Bill! Parasitic SETI with a home satellite TV dish is not
only feasilble, it's widely practiced. A second feedhorn and preamp
assembly are mounted next to the C-band horn/LNB at the apex of the
dish (see Figure 2 of this article). This assembly feeds the rest of a
SETI system (see our online Tech Manual). You can then sweep out the
sky, as described here. And yes, a million participants would be nice,
but our goal is a more modest 5000 stations.
__________________________________________________________
http://www.setileague.org/askdr/parasite.htm
I believe they are referring to the 6 ft. backyard type antennas,
judging from the linked images on the page. But could the roof mounted
DirectTV and Dish Network type antennas be used for SETI?
Sure, they'd work. The dishes are smaller (1 ft diameter or so) but the
same principles apply as for the larger dishes (8-12 ft).
The only thing is that you'd need to work at much higher frequencies.
You'd need to do SETI at the 12GHz band.
Al
The mentioned extra equipment are an extra feedhorn and a
preamplifier. The feedhorn can made cheaply but the preamp seems
expensive. If these preamps were mass produced for this purpose could
their per item cost be brought under $50?
I'm envisionig a government agency such as NSF, or a scientically
interested billionaire, paying satellite TV companies to attach this
extra equipment to their satellite dishes. Say $100 million is
earmarked for the program. Then you would want the extra cost to be
under $100 for each dish for say 1,000,000 subscribers. Judging from
the diagram in the online Tech Manual linked to on the page, the other
equipment should be doable by the equipment that comes with the
satellite TV system. Computer processing would be done separately at a
central location.
If you had a 1,000,000 of these .5 meter wide antennas it would have
the detection sensitivy of a single antenna 500 meters wide.
Bob Clark
.
|
|
|
| User: "Aso Merapi" |
|
| Title: Re: Can DirectTV-type satellite dishes be used for SETI? |
19 Jan 2005 10:19:21 AM |
|
|
I believe they are referring to the 6 ft. backyard type antennas,
judging from the linked images on the page. But could the roof mounted
DirectTV and Dish Network type antennas be used for SETI?
Sure, they'd work. The dishes are smaller (1 ft diameter or so) but the
same principles apply as for the larger dishes (8-12 ft).
The only thing is that you'd need to work at much higher frequencies.
You'd need to do SETI at the 12GHz band.
Al
only problem is the signal loss, the 1/(d^2) loss from alien planet to Earth
assuming you keep your antenna directly pointed at his for at least 15
min(rather difficult since neither know where the other is).
If he is 20 light years away, or about 1 E+13 miles away the signal loss is
about -400 dB (at C band)
What this means is that the alien will have to convert a small moon
completely into microwave energy pointed towards earth.
I doubt the aliens could get funding to do it, because they would have to
choose a direction to aim their 1/10 degree beam too.
Listening is the low cost EZ part.
If SETI was really serious they would be building a huge microwave
transmitter.
.
|
|
|
|
|
|
Related Articles |
|
|
