Can DirectTV-type satellite dishes be used for SETI?
rgrego...@yahoo.com
__________________________________________________________
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
Scott
the post it is mentioned about sweeping the dish across the sky and the
answerer mentions C Band. DirecTV dishes ordinarily are not set up to
"sweep" across the sky, nor are they operating at C Band. So, is
anybody using DirecTV dishes for SETI work? If old C Band stuff can be
used for SETI listening, that would be great since (at least around me)
there are many old C Band setups unused in yards these days. Many could
probably be had for free to anyone who would be willing to removed the
big old ugly beasts from back yards.
Scott
N0EDV
Mark Dunn
smaller.
"Scott" <acep...@bloomer.net> wrote in message
news:hcmdnUBM-vh...@bright.net...
Androcles
<rgrego...@yahoo.com> wrote in message
news:1106126272.1...@z14g2000cwz.googlegroups.com...
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.
Alfred A. Aburto Jr.
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
Uncle Al
>
> 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.
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
k3ym4st3r
should be placed in a predetermined shape right?
Aso Merapi
> > 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.
Aso Merapi
"k3ym4st3r" <k3ym...@yahoo.com> wrote in message
news:1106151097.1...@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.
Martin Brown
> 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
Paul Cardinale
rgregorycl...@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.
[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
nightbat
nightbat
That's about a little over hundred years too late, for Tesla was
building his mega communication tower but the bank interests tore it
down and sold it for pennies on the dollar. Anyway who needs towers or
large disks now to contact ET because now we have amazing net UFO pocket
detectors and arriving cosmic star folk Darla and crew?
the nightbat
David Woolley
Paul Cardinale <pcard...@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.
Uncle Al
>
> 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.
David Woolley
Uncle Al <Uncl...@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.
rgrego...@yahoo.com
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
_______________________________________________________________
Uncle Al
>
> In article <41EEE0B3...@hate.spam.net>,
> Uncle Al <Uncl...@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 shit. 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.
George Dishman
<rgrego...@yahoo.com> wrote in message
news:1106227400.4...@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
David Woolley
rgrego...@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.
rgrego...@yahoo.com
that be in indication of intelligent origin?
Bob Clark
Tom
Much SETI work occurs near 1.4 GHz, which is a frequency
at which earth's atmosphere has very low attenuation (it's been nicknamed
the 'water hole'). One of the smallish DirecTV type dishes, which
is designed for Ku bnad (12 GHz) reception is going to be far too
small to be effective down in the 1+ GHz range. It is probably too
small even to be effectively illuminated by any kind of feed horn
in that range. Thus, it would not make a very good antenna at the
most useful SETI frequencies.
-- Tom
<rgrego...@yahoo.com> wrote in message
news:1106126272.1...@z14g2000cwz.googlegroups.com...
Alfred A. Aburto Jr.
> The article appears to be referring to C-band (4 Ghz) satelite dishes.
> Much SETI work occurs near 1.4 GHz, which is a frequency
> at which earth's atmosphere has very low attenuation (it's been nicknamed
> the 'water hole'). One of the smallish DirecTV type dishes, which
> is designed for Ku bnad (12 GHz) reception is going to be far too
> small to be effective down in the 1+ GHz range. It is probably too
> small even to be effectively illuminated by any kind of feed horn
> in that range. Thus, it would not make a very good antenna at the
> most useful SETI frequencies.
>
> -- Tom
>
But if you work SETI at 12GHz then the 18inch (~0.5m) dish antenna will
have a beamwidth (theoretical beamwidth) of about 3 degrees. This is
just slightly bigger than the 2.5 degree beamwidth expected from a 5m
dish antenna working at 1420.406MHz!
So, if one works SETI at the higher frequency (12000MHz) then the
performance will be entirely adequate for amateur SETI.
Al
[snip]
Rob Dekker
<rgrego...@yahoo.com> wrote in message news:1106260782.6...@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
David Woolley
rgrego...@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.
David Woolley
Alfred A. Aburto Jr. <abu...@sbcglobal.net> wrote:
> > Tom wrote:
> > The article appears to be referring to C-band (4 Ghz) satelite dishes.
> > Much SETI work occurs near 1.4 GHz, which is a frequency
> > at which earth's atmosphere has very low attenuation (it's been nicknamed
It's also very low attenuation at 12GHz. In fact, the low attenuation
of the atmosphere extends from about 0 to 20GHz. The ionosphere limits
the lower bound to around 30MHz. The effective lower bound for SETI is,
however, set by galactic synchrotron radiation, at around 1GHz. 1.4GHz
has a significant excess of noise, so is not at the bottom of the waterhole.
> > the 'water hole'). One of the smallish DirecTV type dishes, which
The low noise/low attenuation waterhole is from about 1GHz to about 20GHz.
The reason that 1.4GHz is used is that, given that you have limited
resources, where should you concentrate them. 1.4GHz is used because
it is a critical frequency for radio astronomy (mapping insterstellar
hydrogen) which means radio astronomers in the whole universe are likely
to observer it and there is a lot of equipment already optimised for
that frequency at radio telescopes. 'Waterhole', in this context,
is a metaphor based on where do you look for wild animals.
There is a third definition of waterhole, based on trying to restrict to
a limited range based on radio astronomy. This is from the hydrogen
hyperfine line at around 1.4GHz to a group of hydroxyl ion lines at
around 1.6GHz.
> > is designed for Ku bnad (12 GHz) reception is going to be far too
> > small to be effective down in the 1+ GHz range. It is probably too
It is too small, but there are aguments for going to considerably
higher frequencies than 1.4GHz. E.g. our planetary radar is at
about 2.3GHz and the frequency where frequency spreading by the
insterstellar medium is lowest is even higher. I think that in
Carl Sagan's "Contact" story, he used pi * (hydrogen hyerfine) as
the first detection frequency.
Some SETI is done at optical frequencies.
> But if you work SETI at 12GHz then the 18inch (~0.5m) dish antenna will
> have a beamwidth (theoretical beamwidth) of about 3 degrees. This is ...
> So, if one works SETI at the higher frequency (12000MHz) then the
> performance will be entirely adequate for amateur SETI.
The sensitivity depends on the capture area. You would need a larger
transmit gain to compensate. Using the same transmit aperture as for
1.4GHz would be enough to compensate.
Motuwojjal
news:1106126272.1...@z14g2000cwz.googlegroups.com...
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.
Dan Mckenna
Rob Dekker wrote:
> <rgrego...@yahoo.com> wrote in message news:1106260782.6...@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
Rob Dekker
"Motuwojjal" <mowwarka...@bounretyon.com> wrote in message news:UssId.129527$K7....@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.
>
Robert Clark
spent on the Square Kilometer array with completion expected by 2015:
Radio Astronomy Will Get a Boost With the Square Kilometer Array.
http://www.universetoday.com/am/publish/radio_astronomy_boost_ska.html?1592004
As I stated below the primary extra components to be added to the
DirecTV dishes to be used for radio astronomy are the feedhorn and the
preamp. The feedhorn is simply a metal pipe of little cost. The
SetiLeague site lists sellers of specially made preamps for SETI
search at $150. But these are handmade. From the low number of circuit
elements, I estimate that if mass produced at the millions of items
level the price could come down in the range of $10 each.
This site states the rate of growth of satellite TV subscribers is at
4 million per year:
Satellite TV Basics.
http://www.satellitetv-hq.com/hqguides/satellite-tv-basics.html
This means the *receivers* for the SKA system if using these
satellite dishes could be installed in 1 year at an extra cost of $10
x 4,000,000 = $40 million dollars. Note that the feedhorn and the
preamp could be attached at manufacture would not cost more on
installation labor. The installation is already paid for by the
satellite TV subscribers. To pay for the extra cost for the added
equipment you could simply add $1 extra per month to the subscriber
rate. Then these 4 million, .5 meter wide dishes would have a total
collecting area of a disk 2000 x .5 m = 1000m = 1km wide, the total
collecting area expected for the SKA. Moreover this would have the
advantage that an additional square kilometer of collecting area would
automatically be added every year over several years going by the
present growth rate.
You could also attach the extra equipment to the 25 million satellite
systems already installed in perhaps 4 or 5 years. The number of
satellite TV subscribers worldwide was 60 million in 2003 and is
expected to grow to 100 million by 2008:
Digital Satellite TV Platforms Continue to Gain Subscribers, and
Profits are on the Rise.
http://www.instat.com/press.asp?ID=1171&sku=IN0401236MB
If this many .5 meter antennas were networked together, they would
have the collecting area of a single antenna 10,000 x .5 m = 5 km
wide.
Note that the idea of using over 50 million separate, stationary
elements is one of the proposals being considered for the SKA
architecture:
Aperture Array (AA)
http://www.skatelescope.org/pages/design_nl.htm
This method of keeping the receiving antennas fixed while detection
directions are determined electronically is called the phased array
approach and has the advantage that many separate targets can be
observed simultaneously. It also has the advantage that interfering
local signals can be suppressed. However, the Aperture Array has
antennas close together in a predetermined configuration with the
positions precisely determined. How could this work for the randomly
positioned satellite dishes?
Methods of differential GPS and carrier phase synthesis now have the
capability of determining position to within millimeters. The method
compares the GPS signal between a precisely known site and an unknown
site to locate the unknown site to within centimeters. Then a
comparison is made in the actual phase of the signals received at the
two sites to locate the unknown site to within millimeters:
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 should be sufficient for keeping the signals for the millions of
antennas in phase up to perhaps 3 cm wavelength, 10 Ghz frequency.
Timing synchronization can be obtained by synchronizing from the
common signal received by the dishes from the satellite.
The Argus telescope at Ohio State University (this is different from
Project Argus operated by The Seti League) may provide a model for how
sensitive such a system can be operating from noisy populated areas:
Newsgroups: sci.astro.seti
From: Bob Dixon <dixo...@osu.edu>
Date: Wed, 19 May 2004 12:49:02 -0400
Subject: The Argus Telescope
http://groups-beta.google.com/group/sci.astro.seti/browse_frm/thread/54ddf1f29f629f20/
Argus Expands the Search For Life.
By Daniel Sorid
posted: 03:30 pm ET
09 June 2000
http://www.space.com/searchforlife/seti_argus_000609.html
Note though that the continent wide satellite dish system will have
an advantage over Argus in dismissing unwanted signals in that such
signals would only be detected by a local group of antennas not the
continent wide system.
In mentioning an estimated price for this system, I emphasized the
estimate was for the *receiving* part of the system. But of course for
such a system of separate receivers, it is just as important to
combine and process the signals.
In the thread for DirecTV being used for SETI, someone mentioned you
might need to transfer 1 Gbps from each antenna for detections at 12
Ghz. I seem to recall that analog signals can be tranferred in greater
density than digital signals. Perhaps the signal received by each
antenna can be transmitted in analog form with a stamp indicating its
location and time of origin.
For examples of the data density required we could look at some
examples of systems of separate antennas that have been used to give
combined signals in *real time*:
Astronomers Demonstrate a Global Internet Telescope.
Date Released: Friday, October 08, 2004.
http://www.spaceref.com/news/viewpr.html?pid=15251
This produced data of 32 Mbits/second for each telescope for
observations at 1.6 Ghz. So at 16 Ghz perhaps 320 Mbps might be
expected for each antenna. The data was sent over a high-speed
internet network available to universities that operates at gigabits
per second. Within a few years, the data transfer rate is expected to
reach tens of gigabits per second.
And:
Prototype SETI Antenna Array Will Help Radio Astronomers Too.
Date Released: Wednesday, June 07, 2000.
http://www.spaceref.com/news/viewpr.html?pid=1992
This is of the Argus telescope at Ohio State University. The 64
antennas here detect signals from 400 to 2000 Mhz. The antennas
together produce 2.56 gigabytes per second, or 20.48 gigabits per
second. So each telescope produces 320 Mbps. This article states that
no physical connection could economically carry that much data over
distance however this was written in 2000. Ultra wideband technology
(mentioned below) now has that capability.
For processing the data for the proposed SKA system, I expect the
distributed computing system used by Set@Home to be used, wherein
millions of computers take part in the calculations. As for how the
data can be sent by the individual antennas, there are a few possible
ways the signals could be combined.
1.)DirecTV offers a two-way broadband satellite internet service
called DirecWay. This allows signals to be sent from the home antennas
back up to the transmitting satellite. However, this system currently
has only a 100,000 subscribers in place. I want to use the millions of
subscribers using the satellite TV systems. I think a minor low-cost
modification of the current TV antennas would also allow them to
transmit to the satellites used for broadband internet service. (I
don't think the satellites used for TV service can be used to receive
signals.)
2.)Another possibility for transferring the data from each antenna
might be to use military satellites currently used for surveillance on
radio transmissions, perhaps using satellites that were decommissioned
and are no longer used for sensitive military tasks.
3.)Possibly the techniques used with amateur packet radio could be
used. Here radio links are used to setup data networks analogously to
how the internet sets up data transfer networks using the TCP/IP
protocols:
N6GN's Microwave Link Page
http://www.sonic.net/~n6gn/uwavelink/uwv.html
INEXPENSIVE MULTI-MEGABAUD MICROWAVE DATA LINK
http://www.sonic.net/~n6gn/hr89/uwvarticle.html
4.)Ultra wideband (UWB) promises gigabit data transfers over both
cable and wireless connections and should be available this year
(2005):
New chipset promises gigabit broadband on cable and wireless.
Rupert Goodwins
ZDNet UK
May 11, 2004, 15:20 GMT
http://news.zdnet.co.uk/communications/0,39020336,39154271,00.htm
Ultrawideband in 2005, but only in America
Rupert Goodwins
ZDNet UK
February 19, 2004, 09:45 GMT
http://news.zdnet.co.uk/communications/wireless/0,39020348,39146644,00.htm
Ultrawideband: Wireless Whoopee.
08:34 AM Oct. 09, 2004 PT
"SAN FRANCISCO -- Think of it as Wi-Fi on steroids. On its way to U.S.
living rooms and maybe even automobiles is a new type of high-speed
wireless connection that promises downloaded data rates of up to 1
gigabit per second -- roughly 18.5 times the speed of Wi-Fi -- to
personal computers and other devices.
"This ultrawideband technology, which could become available in the
next two years, also allows the devices to send data upstream to a
network at 480 megabits per second."
http://www.wired.com/news/technology/0,1282,65297,00.html?tw=wn_tophead_3
5.)Some public utilities now collect their meter readings from radio
transmitters attached to their meters. The data is collected by
receiver on utility poles and then transmitted to a central site. This
method could be adapted to work for collecting the data from the
separate antennas.
6.) The above methods would require that the data transmissions be on
specified frequencies that will not be used for detections. However,
another method might not have this limitation:
Broadband Over Power Lines?
01:15 PM Feb. 09, 2003 PT
"ST. LOUIS -- Coming to a home or office near you could be an electric
Internet: high-speed Web access via ubiquitous power lines, of all
things, making every electrical outlet an always-on Web connection."
http://www.wired.com/news/technology/0,1282,57605,00.html
This is a new technique already being tested in small markets to
provide interent service over power lines. The speed of transmission
can be ramped up to 1 gigabits per second using ulta wideband
technology.
B.)This last leads me to another proposal for large scale separated
antennas for radio astronomy: using the electrical wiring in
households as radio antennas. Here's a post to
rec.radio.amateur.antenna discussing this:
==========================================================================
From: Ed Hare, W1RFI (w1...@arrl.net)
Subject: Re: ISO info about using house wiring as a TV antenna
Newsgroups: rec.radio.amateur.antenna
Date: 2000-12-29 15:33:06 PST
Richard Friday <tgi...@bigfoot.com> wrote in message
news:92j4h7$2b2$1...@bob.news.rcn.net...
> I know this post might be off-topic but could find no other newsgroup that
> had "antenna" in its name. I'd be most appreciative if someone could point
> to a more suitable discussion or other source of information.
> I've seen advertised a device that claims to allow you to use the
> electrical wiring of your house as a tv antenna. You plug this device
> into an outlet, and then use connections it provides as your tv antenna.
> I'm trying to find out if this actually works but have not been able to
> find any reviews.
This device will receive some signals. However, house electrical
wiring is
not a very good VHF antenna system for a couple of reasons:
First, it is very difficult to predict the direction that the house
wiring
will best receive from. It is quite likely that the antenna pattern
will
have all sorts of peaks and nulls, sort of as if you had a rotatable
TV
antenna that was pointing in several directions at once. This may not
pick
up much of the TV signal you want to pick up or may have multiple
responses,
resulting in ghosts.
Also, an electrical power line can be a very noisy place. All sorts of
electronic devices on the line, from power-line equipment itself to
every
motor or power supply plugged in near you may create noise that will
interfere with the signal you want to receive.
If you have no other antenna choice, that device may be useable, but I
don't
think it will work as well as a good set of "rabbit ears" on top of
your TV.
Ed Hare, W1RFI
==========================================================================
The disadvantage of the electrical wiring going in several different
directions may actually be an advantage in regards to a SETI search
since you would want the detections to be omnidirectional. If there
are 100,000,000 homes which average 10 meters across then this would
result in a collecting area of 10,000 x 10 meters = 100 km across.
Since you would want to include large commercial establishments, the
size would actually be larger than this.
Bob Clark
rgrego...@yahoo.com wrote in message news:<1106126272.1...@z14g2000cwz.googlegroups.com>...
Teage
Rob Dekker
I don't want to sound negative, but your plan is really not an alternative or even competition for the SKA.
Other people already commented on many yet unresolved issues :
- Bandwidth is restricted to the data rate of getting signals from the dishes to the central place.
- A non-phased array suffers significantly on sensitivity AND on narrow-beam forming
w.r.t. a phased array (such as SKA and ATA)
- Static DirectTV dishes diminish aiming the beam (one of the downsides of Arecibo too).
- Cost is not only in the receiving elements. Cost calculation would need to include the entire system.
Rob
"Robert Clark" <rgrego...@yahoo.com> wrote in message news:832ea96d.05020...@posting.google.com...
Joseph Lazio
RD> I don't want to sound negative, but your plan is really not an
RD> alternative or even competition for the SKA. Other people already
RD> commented on many yet unresolved issues :
RD> [...] - Cost is not only in the receiving elements. Cost
RD> calculation would need to include the entire system.
Indeed, there is considerable concern about the 12-m dishes proposed
by the U.S. and Indian groups for exactly this reason. The 12-m
dishes and associated electronics have costs that are similar to other
proposals. There are some people, though, who worry that it will be
impossible to process the data from an array composed of 12-m dishes
because there won't be enough computing power, even in 2020.
--
Lt. Lazio, HTML police | e-mail: jla...@patriot.net
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
Robert Clark
response via email on this proposal which he has granted permission to
post here:
============================================================
Date : Tue, Feb 08 09:47 AM EST 2005
>From : "Dr. H. Paul Shuch" <****@setileague.org>
To : Robert Clark <****@****.widener.edu>
Reply To : "Dr. H. Paul Shuch" <****@setileague.org>
Subject : Re: Could DirecTV satellite dishes be used for the Square
Kilometer Array - and a more radical proposal.
Robert Clark wrote:
> Hello Dr. Shuch. The following was posted to sci.astro.
Many thanks, Bob. This is an interesting thread, and I appreciate you
sharing it with me. I can see two possible drawbacks to the proposal
(and you have my permission to post my comments to the usnet group):
(1) Fequency coverage. There is a practical limit to the frequency
range over which a parabolic reflector can efficiently be used. The
upper limit is set by surface smoothness (deviations from the parabolic
curve should not exceed about 1/10 wavelength at the highest operating
frequency). There is also a lower limit; the diameter of the dish
should not be less than about 20 wavelengths at the lowest operating
frequency. In the case of 0.5 meter diameter DBS dishes, this means
the
shortest practical operating wavelength is 2.5 cm, equating to a lowest
practical operating frequency of 12 GHz. (I know, many hams use these
dishes successfully at 10,368 MHz, but their efficiency is lower than
you would expect, and the pattern not as sharp as it should be.) It is
no coincidence that 1/2 meter diameter dishes are used for a satellite
band that extends from 12 to 14 GHz! If used for SETI, we would lose
access to most of the interesting microwave spectrum.
(2) Spatial coverage. Antennas used for satellite TV reception spend
their life pointed at communications satellites. Those satellites park
in a specific arc (the Clarke, or geosynchronous orbit, belt). As
viewed from Earth, at different latitudes, all antennas pointed at the
Clarke belt subtend a narrow range of declinations (from about +10 to
-10 degrees Dec.) So, even neglecting limited right ascension coverage
(the subtended RA will vary as the Earth rotates on its axis), a
network
of DBS dishes doing parasitic SETI will miss about 90% of the sky.
Now,
we might get lucky, and find ETI lurking on or near the ecliptic. But
out to maybe 1000 LY, stars are uniformly distributed in declination,
with respect to Earth. And even more distant stars, such as those
dense
populations at the galactic centre, are at declinations very different
from the Clarke satellite belt (the galactic centre is in Saggitarius,
some -27 or so degrees in declination). Thus, unless we point those
millions of dishes somewhere other than at the direct broadcast
satellites, we'll probably miss ETI.
Not to throw cold water on the subject; I would certainly encourage
such
a collaborative venture. Only, don't think it will solve all the SETI
problems.
Yours for SETI success,
Paul
--
H. Paul Shuch, Ph.D. Executive Director, The SETI League, Inc.
URL http://www.setileague.org
"We Know We're Not Alone!"
============================================================
Rob Dekker
"Joseph Lazio" <jla...@adams.patriot.net> wrote in message news:ll3bw46...@adams.patriot.net...
>>>>>> "RD" == Rob Dekker <r...@verific.com> writes:
>
> RD> I don't want to sound negative, but your plan is really not an
> RD> alternative or even competition for the SKA. Other people already
> RD> commented on many yet unresolved issues :
> RD> [...] - Cost is not only in the receiving elements. Cost
> RD> calculation would need to include the entire system.
>
> Indeed, there is considerable concern about the 12-m dishes proposed
> by the U.S. and Indian groups for exactly this reason. The 12-m
> dishes and associated electronics have costs that are similar to other
> proposals. There are some people, though, who worry that it will be
> impossible to process the data from an array composed of 12-m dishes
> because there won't be enough computing power, even in 2020.
>
Mmm. That would surprise me (that there would not be enough computing power).
The amount of computing power to create (compute) a phased single beam from an
array roughly linear with the amount of elements to be 'phase-correlated'.
The ATA is planned to have 350 elements (of 6m dish).
The SKA would plan to have 3300 elements (of 12m dishes) .
So that is only a factor of 10 more elements than the ATA.
In terms of Moore's law, a factor 10 in computing power is achieved in roughly 2.5 years.
This means that if SKA is completed 2 1/2 years after ATA, the system w.r.t.
computing power would cost the same as the same system on a full operational ATA.
ATA is planned to have cost about $30-40M upon competion, so I don't see that
the computing power available for the SKA would be a limiting factor (either technology or cost).
Rob
Greg Hennessy
But a factor of 100 more baselines.
Steve Willner
> RD> I don't want to sound negative, but your plan is really not an
> RD> alternative or even competition for the SKA. Other people already
> RD> commented on many yet unresolved issues :
> RD> [...] - Cost is not only in the receiving elements. Cost
> RD> calculation would need to include the entire system.
In article <ll3bw46...@adams.patriot.net>,
Joseph Lazio <jla...@adams.patriot.net> writes:
> Indeed, there is considerable concern about the 12-m dishes proposed
> by the U.S. and Indian groups for exactly this reason. The 12-m
> dishes and associated electronics have costs that are similar to other
> proposals. There are some people, though, who worry that it will be
> impossible to process the data from an array composed of 12-m dishes
> because there won't be enough computing power, even in 2020.
Joe is an expert and can correct me if I have something wrong here,
but I think it might be useful to point out some of the qualitative
issues.
For an interferometer, the point-source sensitivity depends only on
collecting area, not on the number of antennas. That would favor
large numbers of small antennas, but... well, there's always a "but,"
isn't there?
First of all, in order to calibrate the interferometer, you have to
detect bright point sources ("calibrators") _on every baseline_.
That means with every pair of antennas. And you have to do it faster
than the atmospheric phase changes, which is typically tens of
minutes, depending on frequency. This sets a minimum size for your
antennas, typically several meters depending on operating frequency.
Second, receivers aren't free, and every antenna has to have one.
For a major facility such as the SKA, you want to work at multiple
frequencies, so every antenna needs several receivers. I haven't
priced receivers lately, but I'd guess prices in the 10's to 100's of
thousand dollars might be in the ballpark. This pushes you to larger
antennas, but larger antennas cost more per unit collecting area than
smaller ones. One estimate I've seen is that the antenna cost goes
as the 2.5 power of diameter. This isn't a law of nature, but the
practical number won't be too far from that. If the receiver price
is A, and the antenna price is B*d^2.5, the price per unit area goes
like
A/d^2 + B*d^0.5
This function rises at large and small values of d and has a minimum
at some intermediate value. That cost minimum is the size you want
to make your antennas, independent of how many of them you buy. Of
course receiver performance and cost aren't fixed numbers. You could
buy cheap receivers, accept poor performance, and make up for it by
buying lots more antennas and receivers. This is another
optimization problem, but in all the studies I've seen, it turns out
best to pay more for good receivers. Observing time goes as the
square of system temperature, and having crappy receivers makes the
calibrator problem worse.
A third issue is the one RD and Joe mention: correlator cost. Cost
ought to go roughly linearly with number of baselines, which for N
antennas is N*(N-1)/2. This pushes towards fewer but larger
antennas.
A fourth issue is primary beam size. The size of the field of view
you can map at one time with a single receiver on each antenna goes
inversely as the size of the antennas. This favors small antennas
and more of them.
There are also technical issues such as sidelobes and shielding. You
don't want any part of the antenna beam to "see" the ground. I am no
expert but would expect these issues to favor large antennas or at
least ones larger than a few meters in size.
As you can see, the tradeoffs are complicated. I expect the
interferometer designers know about cheap, mass-produced antennas and
will use them if they represent the best solution. It seems to me
unlikely that they do.
--
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
Rob Dekker
"Steve Willner" <wil...@cfa.harvard.edu> wrote in message news:421115a4$1...@cfanews.cfa.harvard.edu...
[....]
> Joe is an expert and can correct me if I have something wrong here,
> but I think it might be useful to point out some of the qualitative
> issues.
>
[....]
> A third issue is the one RD and Joe mention: correlator cost. Cost
> ought to go roughly linearly with number of baselines, which for N
> antennas is N*(N-1)/2. This pushes towards fewer but larger
> antennas.
I'm not a total expert in this field, but know enough to be dangerous..:)
If I'm not mistaken, to phase correlate N elements, you do not need
to phase-correlate all N^2 baselines, but (after time/phase-adjusting each of the
individual N signals to intended beam direction), you can simply create a correlator tree
of 2->1 phase correlators. The tree would then consist of N correlators,
which counts for linear complexity of computation power.
I could be off with this, but I seem to remember this from college (long ago).
Question for Joe : Which part of the SKA system would be the bottleneck in
computational effort, and create a problem even in 2020 ?
Rob
Matt Giwer
> 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 shit.
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
rgrego...@yahoo.com
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
Joseph Lazio
RD> Thanks for a good overview of trade-offs for phased arrays Steve !
RD> "Steve Willner" <wil...@cfa.harvard.edu> wrote in message
RD> news:421115a4$1...@cfanews.cfa.harvard.edu... [....]
>> Joe is an expert and can correct me if I have something wrong here,
>> but I think it might be useful to point out some of the qualitative
>> issues.
RD> [....]
Steve's post certainly captured most of the essential details. In
particular, the US proposal for the SKA involves 12-m dishes because,
with current knowledge, these are thought to be at the minimum in the
"cost curve." That is, a 12-m dish provides a good balance between
the costs of obtaining antennas and obtaining receivers.
>> A third issue is the one RD and Joe mention: correlator cost. Cost
>> ought to go roughly linearly with number of baselines, which for N
>> antennas is N*(N-1)/2. This pushes towards fewer but larger
>> antennas.
RD> I'm not a total expert in this field, but know enough to be
RD> dangerous..:)
RD> If I'm not mistaken, to phase correlate N elements, you do not
RD> need to phase-correlate all N^2 baselines, but (after
RD> time/phase-adjusting each of the individual N signals to intended
RD> beam direction), you can simply create a correlator tree of 2->1
RD> phase correlators. The tree would then consist of N correlators,
RD> which counts for linear complexity of computation power.
No, one does have to multiply the signals from all N antennas
together, which is roughly an N^2 problem. The issue in radio
astronomical interferometry is that the "visibility" or "correlation
coefficient" is a measure of the Fourier transform of the sky
brightness. Every unique pair of antennas gives one "visibility," so
one wants to obtain all possible visibilities from the N antennas.
RD> Question for Joe : Which part of the SKA system would be the
RD> bottleneck in computational effort, and create a problem even in
RD> 2020 ?
Actually, the correlator, as difficult as it is, may not be the
limiting factor for the SKA computation. Some people think they know
today how to build an SKA-level correlator.
The most prominent concern has been how does one make an image? The
SKA is envisioned as having a large field of view (e.g., 1 deg^2 at 1
GHz, and some people are concerned that that's not large enough). In
order to make high-quality images, one probably has to image that
entire field of view with something approaching 0.1 arcsecond
resolution. Thus, a single image would be something like 10,000 x
10,000 pixels in size, and that's only for a single frequency. One
probably has to utilize multiple frequencies simultaneously, in order
to defeat some other effects. Moreover, current processing of radio
interferometric images is an iterative process, in which one makes an
image, corrects it for certain effects, makes a new image, improves
the corrections, makes a new image, ....
There's a memo floating around discussing all of these aspects of
processing. IIRC, the conclusion was that the processing power
required to make high-dynamic range images scaled as the dish diameter
(which sets the field of view) as something like D^6 or D^8. Thus,
all other things being equal, just changing from the 25-m VLA dishes
to the proposed 12-m US SKA dishes would increase computation by a
factor of 64 to 256, and that's even without taking into account the
much larger number of antennas.
I should say that not everybody agrees with this analysis, but it is
recognized that the computational aspects of the SKA may be
significant.
David Woolley
Matt Giwer <ma...@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.
Matt Giwer
> In article <ZgiQd.33788$pc5....@tornado.tampabay.rr.com>,
> Matt Giwer <ma...@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
Matt Giwer
> 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
David Woolley
Matt Giwer <ma...@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.
Matt Giwer
> In article <jRBQd.79846$JF2....@tornado.tampabay.rr.com>,
> Matt Giwer <ma...@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
Rob Dekker
> RD> I'm not a total expert in this field, but know enough to be
> RD> dangerous..:)
>
> RD> If I'm not mistaken, to phase correlate N elements, you do not
> RD> need to phase-correlate all N^2 baselines, but (after
> RD> time/phase-adjusting each of the individual N signals to intended
> RD> beam direction), you can simply create a correlator tree of 2->1
> RD> phase correlators. The tree would then consist of N correlators,
> RD> which counts for linear complexity of computation power.
>
> No, one does have to multiply the signals from all N antennas
> together, which is roughly an N^2 problem. The issue in radio
> astronomical interferometry is that the "visibility" or "correlation
> coefficient" is a measure of the Fourier transform of the sky
> brightness. Every unique pair of antennas gives one "visibility," so
> one wants to obtain all possible visibilities from the N antennas.
Might it be that we are talking about two different correlations..?
I was talking about a simple phase-correlator : accumulate signals
from all elements. In hardware, that is done with 'phase-splitters'.
(At least that's how I hooked my 4 loop-yagi's together back in the old days).
Antenna phase-splitters are passive devices which simply 'add' the electrical
field of multiple feed-points (or split them if you are transmitting).
For a (digitally sampled) antenna signal, these would be simply be 'adders'
which need to 'add' the signal value for each element for each sample.
For one beam, for 10Ghz sample rate (5 GHz bandwidth), and N elements,
you would need to do 10*10^9 * N additions per second (pretty large
amount of digital adders), but it is linear w.r.t. N.
I think you might have been talking about a smarter technique to do this work.
>
> RD> Question for Joe : Which part of the SKA system would be the
> RD> bottleneck in computational effort, and create a problem even in
> RD> 2020 ?
>
> Actually, the correlator, as difficult as it is, may not be the
> limiting factor for the SKA computation. Some people think they know
> today how to build an SKA-level correlator.
>
> The most prominent concern has been how does one make an image? The
> SKA is envisioned as having a large field of view (e.g., 1 deg^2 at 1
> GHz, and some people are concerned that that's not large enough). In
> order to make high-quality images, one probably has to image that
> entire field of view with something approaching 0.1 arcsecond
> resolution. Thus, a single image would be something like 10,000 x
> 10,000 pixels in size, and that's only for a single frequency. One
> probably has to utilize multiple frequencies simultaneously, in order
> to defeat some other effects. Moreover, current processing of radio
> interferometric images is an iterative process, in which one makes an
> image, corrects it for certain effects, makes a new image, improves
> the corrections, makes a new image, ....
Interesting...
Joseph Lazio
RD> "Joseph Lazio" <jla...@adams.patriot.net> wrote in message [....]
RD> If I'm not mistaken, to phase correlate N elements, you do not
RD> need to phase-correlate all N^2 baselines, but (...), you can
RD> simply create a correlator tree of 2->1 phase correlators. The
RD> tree would then consist of N correlators, which counts for linear
RD> complexity of computation power.
>> No, one does have to multiply the signals from all N antennas
>> together, which is roughly an N^2 problem. The issue in radio
>> astronomical interferometry is that the "visibility" or
>> "correlation coefficient" is a measure of the Fourier transform of
>> the sky brightness. Every unique pair of antennas gives one
>> "visibility," so one wants to obtain all possible visibilities from
>> the N antennas.
RD> Might it be that we are talking about two different
RD> correlations..?
Potentially. Your comment about a "tree" perhaps should have clued me in.
RD> I was talking about a simple phase-correlator : accumulate signals
RD> from all elements. In hardware, that is done with
RD> 'phase-splitters'. (At least that's how I hooked my 4 loop-yagi's
RD> together back in the old days).
I'd call that a beam former.
The Very Large Array (and other interferometers of its ilk) work by
combining the signals from all unique pairs of antennas.
Specifically, one takes the signals from antennas #1 and #2, multiples
them together, and then integrates for a short time. That's the
measured quantity, often called a "visibility" or "correlation
coefficient." One then repeats this for antennas #1 and #3, antennas
#1 and #4, #1 and #5, #1 and #6, ..., until one has made all unique
combinations from 27 antennas. (Those who enjoy math problems can try
to figure out how many such combinations there are. :)
To make it even more complicated, some of the designs for the Square
Kilometer Array (and the Low Frequency Array and the Long Wavelength
Array and others) contemplate using both beam formers and correlators.
Thus, one would start with a bunch of dipoles. The signals from a
"small" number of dipoles (e.g., 100) would be combined in the manner
you described. This produces a signal typically called a "station"
beam or "pod" beam. The signals from various stations (numbering
perhaps 100) are then combined by a correlator.
Robert Clark
come with the satellite dishes can even detect frequencies other than
the 12 Ghz the satellites broadcast at.
In regard to the wavelengths that the .5 m antennas could usefully
collect, I've seen references that a dipole antenna should be a quarter
of the wavelength. Is this different for parabolic antennas?
I'm thinking perhaps you could simply attach flat metal dipoles to the
surface of the .5 meter antennas so they could collect megahertz
frequencies. Then at a quarter wavelength you could collect down to 2
meter wavelengths or 150 Mhz.
While we're at it, the thought of using dipoles, like TV antennas,
raises another possibility. You need receivers for these frequencies.
Where could we get large numbers of receivers already existing at these
frequencies? Inside TV's! Assuming we could solve the problem of
tranmitting the data to a central site (ultra wideband, broadband over
power-lines, amateur packet radio, etc.), we could have all new TV's
come installed with a circuit that transmits received signals to the
central site. The number of new TV's sold yearly worldwidde is 90
million. This would result in markedly larger numbers of possible
receivers even above the satellite TV approach.
There might be privacy issues with this idea however.
Bob Clark
Joseph Lazio
RC> Another possible problem is that I'm not even sure the receivers
RC> that come with the satellite dishes can even detect frequencies
RC> other than the 12 Ghz the satellites broadcast at. In regard to
RC> the wavelengths that the .5 m antennas could usefully collect,
RC> I've seen references that a dipole antenna should be a quarter of
RC> the wavelength. Is this different for parabolic antennas?
The rule of thumb is that the dish should be more than about 6
wavelengths across, or diffraction starts to become really important.
Thus, for instance, the VLA (25 meter diameter), when operated at 74
MHz (4 meter wavelength), is right on the hairy edge.
Jan Panteltje
<rgrego...@yahoo.com> wrote in
<1109017460.4...@f14g2000cwb.googlegroups.com>:
I am just a humkble electronics person with several years experience with
sat dishes.. and software (I write).
2 things bother me about the proposal (topic)
1)
The smaller the dish, the worse the signal to noise (lower signal level),
and you mostly will be in the noise.
2)
polarization, sat dish LNBs come hor or vert polarized, and perhaps circular.
What will it be.
If you only use the dish and your own preammp what will it be made of?
10 Ghz = 3 cm wavelength.
Remember the dishes are used to receive from only 40000km or a little more
away, and then get 'acceptable' signal to noise.
The sats transmits several watts of power.
For astronomy you will likely want to be a few orders of magnitude more
sensitive.
Liquid cooled parametric preamps for the amateur?
What am I missing here?
Schweinkolben
"Robert Clark" <rgrego...@yahoo.com> wrote in message
news:1109017460.4...@f14g2000cwb.googlegroups.com...
> Another possible problem is that I'm not even sure the receivers that
> come with the satellite dishes can even detect frequencies other than
> the 12 Ghz the satellites broadcast at.
True, 12 gig only.
> In regard to the wavelengths that the .5 m antennas could usefully
> collect, I've seen references that a dipole antenna should be a quarter
> of the wavelength. Is this different for parabolic antennas?
Yes, it is different.
> I'm thinking perhaps you could simply attach flat metal dipoles to the
> surface of the .5 meter antennas so they could collect megahertz
> frequencies. Then at a quarter wavelength you could collect down to 2
> meter wavelengths or 150 Mhz.
nope, dish too small.
Dish needs to be 10 wave lengths or bigger.
@150 that is too big
Flat dipoles on surface are basiclly shorted out
> While we're at it, the thought of using dipoles, like TV antennas,
> raises another possibility. You need receivers for these frequencies.
> Where could we get large numbers of receivers already existing at these
> frequencies? Inside TV's!
Noise figure is no good.
>Assuming we could solve the problem of
> tranmitting the data to a central site (ultra wideband, broadband over
> power-lines, amateur packet radio, etc.),
None of those have enough bandwidth/range to work.
> we could have all new TV's
> come installed with a circuit that transmits received signals to the
> central site. The number of new TV's sold yearly worldwidde is 90
> million. This would result in markedly larger numbers of possible
> receivers even above the satellite TV approach.
TVs create considerable RF noise and will cover the signals you want to
recieve.
> > > 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
Not true at all.
complete fiction,
how do you handle the pointing errors of 1,000,000 dishes?
You have to phase lock all the received signals to combine them, a
nd 0.5 meter dish works poorly at 12 GHz, and below.
rgrego...@yahoo.com
this:
Who's been using your PC?
"From the user's point of view, surfing the web is simple - you type
a URL (uniform resource locator) into a web browser and, most of the
time, the correct web page appears on the screen. Behind the scenes
however a sophisticated process regulated by layers of complex
protocols is responsible for finding, checking and delivering the page
that has been requested. In the latest issue of Nature, Barabasi et al.
describe a way of 'hijacking' this communication infrastructure and
creating a distributed network made up of unwitting web servers."
http://www.nature.com/nature/fow/010830.html
Parasitic computing.
Albert-Laszlo Barabasi*, Vincent W. Freeh², Hawoong Jeong*
& Jay B. Brockman²
* Department of Physics; and ² Department of Computer Science and
Engineering,
University of Notre Dame, Notre Dame, Indiana 46556, USA
"Reliable communication on the Internet is guaranteed by a
standard set of protocols, used by all computers1. Here we show
that these protocols can be exploited to compute with the communication
infrastructure, transforming the Internet into a distributed
computer in which servers unwittingly perform
computation on behalf of a remote node."
NATURE | VOL 412 | 30 AUGUST 2001 | p. 894-897
http://www.nd.edu/~parasite/nature.pdf
Bob Clark