About ET radar..

I have been wanting to write this for a long time. I will create a web-site
for this at some time, but for now, I'd just like to get your feedback.

There have been many questions in the newsgroup as to how far away our
earthly transmissions are detectable. Also the seti FAQ page deals with this
subject, and concludes that a seti@home - style receiver system cannot
detect much more than just the very strongest signals from earth if located
far outside our solar system.

In this posting, I want to put some formula's behind this, and basically
analyze the following question :

If an ET transmitter is designed to reach a receiver at a certain range, how
far does that range need to be so that we can detect it here on earth, say
100LYs away from the transmitter.

Basically, I split the sort of ET signals into two target applications :
(1) point-to-point : this would be any signal intended for a receiver of
which the location is known (to ET)
(2) radar : this would be any signal intended to detect a passive object
that will reflect some of the power, so that it can be detected in a
receiver (located close to the transmitter).

In the analysis below, I conclude that it is impossible for us to detect an
ET transmission which was intended for ET's planet, or close to the planet.

The only application that we have a chance of detecting is RADAR.
More specifically, we can only detect ET radar which attempted to detect
asteroids or other 'near-ET-planet' objects. Even if, and maybe especially
if, ET has a technology more advanced than ours.

All this might not come as a surprise to anyone, but I feel that it is
important to understand what sort of signals we are likely to receive (apart
from beacons, which apparently are not present around the water-hole). Also
the formula's might be of help to some of you determining the likelihood
that we will detect signals from extra-terrestrial origin. Also, it would
make sense to consider this analysis if we would start looking for 'leakage'
signals from ET.

Another conclusion is that if we would have a square-mile antenna for SETI,
this would not change much about the sort of signals we can expect to
detect. There will be more radar signals, but it would still be very
unlikely that we would detect any point-to-point ET communication signals.

As a side-note : IF there are ETI's out there within a 100LY range, AND they
use asteroid or high-resolution radar, then we WILL be able to detect these
radar signals... That puts the recent discarding of signal SHGb02+14a (which
can easily be a radar signal) in a whole new light....

Take this for what it's worth. Your comments are appreciated..

Rob


----------------------------------------------------------------------------
-----------------------------------------
Point-to-point transmissions :

Define :
R : range to the intended target receiver (meter)
rt : effective radius of the transmitter antenna used (meter)
rr : effective radius of the receiver antenna used (meter)
lambda : wavelength (c/f) used (meter)
P : line power of the transmitter (Watt)
SNR : signal/noise ratio in the receiver
BW : bandwidth used in the receiver (Hz)
kT : Bolzmann constant times system temperature
pi : 3.141...

The following formula is basically the one from the seti FAQ page :

(1) SNR = P * (2*pi*rt / lambda) * (rr / (4*R^2)) * 1/(BW*kT)


Radar :
ro : effective radius of the object (or resolution) that the radar
attempt to detect

(2) SNR = P * (2*pi*rt / lambda) * ( ro^2 / (4*R^2) ) * (rr / (4*R^2)) *
1/(BW*kT)

See here the R^4 drop-off of radar... This is due to reflection at the
target object.
The factor ( ro^2 / (4*R^2) ) might require some explanation.
I am assuming here that the object that the radar attempts to detect is
reflecting the power that it receives omnidirectionally. Essentially, if 10W
of power would hit the object, I assume that it absorbs 5Watt and reflects
the other 5 in the 180degree cone back to the direction of the transmitter.
An ideal spherical, metal object would create a factor 2*(ro^2 / 4R^2)
because it would reflect all incoming power back into the 180degree cone it
came from.
An ideal mirror has a totally different factor : pi*ro^2, because it
reflects all incoming power back. That power obviously would only be
directed back to the receiver if the mirror is in one exact position.
This (mirror) would destroy the R^4 drop-off which is typical for radar.
That is why stealth fighters have very 'flat' surfaces. Any incoming radar
signals will be reflected to different directions than the original incoming
radar signal, so nothing gets detected in the receiver (which is typically
at the same location as the transmitter).
Any reflection factor should have a 1/R^2 factor in it, so the factor
ro^2/(4*R^2) seems reasonable for radar, and if you want to, you can
multiply that by a factor (alpha) between 0 and 1, for different materials
of the target object...


Important for the next deduction are not the absolute numbers, but the ratio
of them :

Radar signals must be a factor 4* (R/ro)^2 stronger than any point-to-point
transmission, to obtain the same SNR for the same target range.

So what does this mean for us as external, far-away, eavesdropping observers
?
It means that radar signals can be detected a factor 2*R/ro further than
point-to-point transmissions.
That is in fact a very rough estimate, and requires more detailed analysis
(further down), but roughly this is correct.

How big is R/ro ?
Lets look at some radar applications on earth. I don't know much about the
specifications of various radar applications, but the following numbers seem
to make sense :

- air traffic surveillance radar : 1m resolution at 100km range : R/ro =
100,000
- Arecibo asteroid radar : 100km resolution at 10 light-minutes :
1.8*10^11 / 1*10^5 = 1,8M.

I'm not sure if these numbers are true and accurate, but it is clear that
radar systems would actually penetrate 100k to 1M times deeper into space
than do transmissions which are intended for a certain defined target
(point-to-point communication).
In other words, they are a factor 1*10^10 to 1*10^12 times stronger in power
(R/ro)^2)...

----


Now lets ask another question, and see if we can come up with a reasonable
answer :

If we, at say 100LYs distance, are listening to such ET signals, what would
the target range have to be of either a ET point-to-point transmission or a
ET radar transmission, so that we can still detect it ?

Obviously, if we want to achieve the SNR that ET intended for its
application, and we are using a similar receiving antenna, and receiver
sensitivity, then the range would need to be 100LYs. That would only be true
for beacons, but I do not want to discuss that here in this posting.

So where can we achieve advantage over ET's receiver, so that we can detect
her signal further than the intended range ? Here are some ideas with
ballpark estimates :

a) We can settle with a lower SNR ration. Most applications require 30dB
or so SNR, and we might settle for just 10db, so we can gain 20dB here
(power factor 100, distance factor 10).

b) We can use a bigger receiving antenna. The power factor we can gain is
(r/rr)^2, where r would be our antenna radius, and rr is ETs receiving
antenna for the application. Arecibo has a 100m effective radius (correct me
if I'm wrong), which can easily be 10x larger than what ET is using. So we
might gain 20db with this. A square-mile receiver (as is in the planning)
might add another 15db to this.

c) We can integrate the signal we receive. Here it gets complex, but
important. Seti@home uses very narrowband (0.1Hz?) 'channels' and integrates
any signal with this. Integration typically enhances a signal by a factor
sqrt(BW*t) in SNR (power factor). t (time) must be restricted for us to
about 10sec (0.1Hz minimum bandwidth) or so, since any strongly beamed ET
transmission not intended for us would not be aimed at earth much longer
than that. Bandwidth could be small (for high-resolution asteroid radar), or
very wide (for broadband communication transmissions). Overall. I think that
a BW*t factor of 10 (for very narrowband, high-resolution radar or
communication) to 1G (for 100MHz broadband communication) is reasonable to
assume. That means that we can gain 5-35db with integration.

d) We can improve the sensitivity of the receiver (system temperature).
Assuming that ET is not stupid, we cannot gain much here. 100K is reasonable
for any application, and thus the max we can gain here is about 5dB or so
(for a 30K receiver).

Overall, we should be able to obtain 50db (for narrowband transmissions) to
80dB (for broadband) improvement of SNR, with a seti@home style receiver
system on Arecibo. That translates to a distance-factor of about 300-10K.
That's not a lot, compared to the distance factor of 100K-1M of radar versus
point-to-point communication....

So, to answer the question : The target range for ET transmissions,
detectable at seti@home at 100LYs would be the following :
- factor 100LYs / 10K = 0.01 LY = 650 AU for broadband point-to-point
communication
- factor (100LYs / 300) / 1M = 0.17 Light-minutes = 3*10^6 km. for
narrowband radar.
- factor (100LYs / 10K) / 1M = 94*10^3 km for broadband radar

Broadband radar is not really an option to consider, since for radar, the
resolution can be much better improved by narrowing the bandwidth. But who
knows what ET thinks about this...

So what conclusions can we draw from this ?
Seems that it is out of the question that we would ever detect any ET
point-to-point communication signal, since it would have to be targeting
something or somebody far out into or outside of ET's star system.
However, we DO have a chance to detect radar signals from ET. There is not
much of a chance to detect surveillance radar, targeting ET's terrestrial
objects, but there IS a chance to detect ET's narrowband asteroid radar, or
even high-resolution (military?) radar for ET's planet's close-proximity
(incoming missiles?) analysis. And there is a minute chance to find some
broadband radar signals from ET which were intended for objects on or near
ET's planet.

Interesting to note here, is that a square-mile antenna would not change
much about this analysis. The target range of ET transmissions would reduce
by a factor of 10 at most, but that still requires an absurdly large target
range for point-to-point transmissions (in the range of 65 AU). Only an
interplanetary radio-hub would become detectable, but still very faintly. So
overall, even if we increase the size of the receiver from Arecibo to a
square-mile antenna, we still would only be able to detect ET radar
signals.....

It might be time to start listening for radar signals...

I'm sorry, an error slipped into my formula's when I copied them from my
notes.
Here are the correct ones :

point-to-point :
(1) SNR = P * (2*pi*rt / lambda)^2 * (rr/(2*R))^2 * 1/(BW*kT)

radar:
(2) SNR = P * (2*pi*rt / lambda)^2 * ( ro^2 / (4*R^2) ) * (rr/(2*R))^2 *
1/(BW*kT)

The factor between them ( ro^2 / (4*R^2) ) (or (ro/(2*R))^2 if you will)
remains the same

Rob

"Rob Dekker" skrev i meddelandet
. com...

....
So, to answer the question : The target range for ET transmissions,
detectable at seti@home at 100LYs would be the following :

It's probably a bit optimistic expecting ET to be within 100 LY.

The Hiparcos catalog lists 2473 stars within 100.1 LY (32.5 mas) If the
stars last 10 billion years and a technical civilisation evolve on planets
around all of them, the technical civilisation has to last 4 million years
if one should always be detectable.

Maybee you should assume a distance of 1000-5000 LY.

....

"Bjorn Damm" wrote in message news:euA8d.159$fQ5.22@amstwist00...

"Rob Dekker" skrev i meddelandet
. com...

...
So, to answer the question : The target range for ET transmissions,
detectable at seti@home at 100LYs would be the following :

It's probably a bit optimistic expecting ET to be within 100 LY.

The Hiparcos catalog lists 2473 stars within 100.1 LY (32.5 mas) If the
stars last 10 billion years and a technical civilisation evolve on planets
around all of them, the technical civilisation has to last 4 million years
if one should always be detectable.

Maybee you should assume a distance of 1000-5000 LY.

...

1000LYs should indeed give a much larger audience..
How many stars are there withing 1000LY's ? About a million or so ?

Any way, the only application that would be detectable at that distance
would be narrowband radar, with a target range of 1.7 light minutes or more :

- factor (1000LYs / 300) / 1M = 1.7 Light-minutes = 3*10^7 km. for
narrowband radar.

That would probably just be asteroid radar....
Unless an ETI spends a significant amount of energy (proportional to their
overall energy usage) on some form of asteroid radar, we will not hear very much
from 1000LYs away...

In article ,
Rob Dekker wrote:

In the analysis below, I conclude that it is impossible for us to detect an
ET transmission which was intended for ET's planet, or close to the planet.

That's only true of signals that approximate optimal coding. Modern system
do have that characteristic, but there are large numbers of signals out there
that have a massive carrier signal that was there to allow simple receiver
designs. Those carriers have a very high power spectrum density and
are much more detectable than the information that accompanies them.
Even spread spectrum leak carrier, typically in inverse proportion to
the code length.

Typical old technology levels do still represent a problem for drift
scan parasitic SETI, like SERENDIP, and its offshoot, SETI@Home.

The only application that we have a chance of detecting is RADAR.
More specifically, we can only detect ET radar which attempted to detect
asteroids or other 'near-ET-planet' objects. Even if, and maybe especially
if, ET has a technology more advanced than ours.

We don't use radar for this purpose. We use passive optical methods for
detection and only use radar in tracking mode, to examine targets with
already fairly well known orbits.

that we will detect signals from extra-terrestrial origin. Also, it would
make sense to consider this analysis if we would start looking for 'leakage'
signals from ET.

We are not avoiding looking for leakage signal. The problem is that
they tend to be either undectable or unverifiable.

Another conclusion is that if we would have a square-mile antenna for SETI,
this would not change much about the sort of signals we can expect to
detect. There will be more radar signals, but it would still be very
unlikely that we would detect any point-to-point ET communication signals.

The SETI Institute believe that the Allen telescope will be able to
detect analogue TV carriers from nearby stars even though it has a
smaller effective capture area than Arecibo; that's a result of analogue
carriers being a very ineffective use of the fraction of a Hertz that
they occupy and of the Allen telescope allowing extended SETI observations.

As a side-note : IF there are ETI's out there within a 100LY range, AND they
use asteroid or high-resolution radar, then we WILL be able to detect these

Asteroid, but not high resolution, and we will not be able to verify the
detection. It is the one off nature of asteroid radar detections that
means we could have observed many signals, but never twice from the same
source, at which point they become indistiguishable from statistical
noise fluctuations.

radar signals... That puts the recent discarding of signal SHGb02+14a (which
can easily be a radar signal) in a whole new light....

As noted before, the very fact that this is detected at multiple times
is a contra-indication to radar. The other problem with this signal is
the wide range of chirps. Whilst our CW planetary radar is pre-chirped for
the round trip. The acceleration along the line of site of interesting
objects is likely to be rather smaller than the earth rotation component,
so one would expect quite close to a signal half compensated for planetary
rotation. As that would tend to compensate for our rotation, to a first
approximation you would expect a chirp that was approximately zero and,
in any case, rather less than the maximum rotation chirp of about 0.15Hz/s.
(There is a small risk of actually rejecting it because it is too close to
zero chirp.)

For a low earth orbit satellite as the radar system, you wouldn't expect
more than about +1.5 Hz/s as pre-chirp. The target would have to be
extremely fast and passing very low for a police siren type chirp to
require the amount of chirp observed.

Radar :
ro : effective radius of the object (or resolution) that the radar
attempt to detect

It is very easy to find the idealised range for our planetary radars; they
are exactly the high power signal values for Arecibo, in the FAQ, as the
only reason that Arecibo is able to transmit such powers is because it is
used for planetary radar. Also, in practice, feed point power is limited
by available cast off technology, i.e. military radar transmit powers.

An ideal spherical, metal object would create a factor 2*(ro^2 / 4R^2)
because it would reflect all incoming power back into the 180degree cone it
came from.

Even one much larger than the wavelength wouldn't do that. it would
scatter more back towards the sender than any other direction, but near
the limbs, it would scatter almost in the original propagation direction.
The overall formula may or may not be right - I'd have to set up the
integral to check that - but the logic is wrong.

radar signal, so nothing gets detected in the receiver (which is typically
at the same location as the transmitter).

Although not so for planetary radar, which is often done with Goldstone
as uplink and Arecibo as downlink, and probably not true of modern
air defence radars.

Important for the next deduction are not the absolute numbers, but the ratio
of them :

But as noted elsewhere, we have absolute numbers for earth based planetary
radars.


Radar signals must be a factor 4* (R/ro)^2 stronger than any point-to-point
transmission, to obtain the same SNR for the same target range.

But this is the noise after despreading the signal. That's true for
both communications and radar (CDMA mobile phone signals are spread),
but is particularly important for strategic military radar, where you
want range as well as direction, but you are limited by technology as to
the maximum instaneous power. In that case, you send a pseudo random
sequence towards the target, at constant power, but spread over about
a MHz. Unless you have prior knowledge of the spreading code and rate,
a third party cannot achieve anything even approaching the sensitivity
that the intended receiver will achieve.

Furthermore, for military radar, detectability and predictability are
undesirable. Detectability within the useful operating range is probably
unavoidable for long range radars, but predictability makes jamming easier
and can be avoided; I don't know if it is avoided for radar - it is for
the military channels on GPS.

About 50% of asteroid tracking radar is done CW, but that will be strictly
in tracking mode for a known target. Otherwise, CW radar is limited to
low power, closer range, uses, like radar speed traps.


- air traffic surveillance radar : 1m resolution at 100km range : R/ro =
100,000

Most air traffic radar is SSR (secondary surveilance radar) which is
relatively low power (same as ground to air voice, and operating in the
same VHF frequency band) because it uses a transponder on the aircraft
which receives and actively retransmits the signal. There will be
some primary radars, but I'm not sure how many of them there will be.
The really high power primary radars are military, and predictability
of the signal characteristic for these is undesirable, because it makes
jamming easy.

Primary radar is likely to be operated very much at the limit of its
sensitivity envelope when detecting light aircraft.

I wouldn't be surprised if the only primary radars attempting this sort
of range and sensitivity were military and coast guard systems.

- Arecibo asteroid radar : 100km resolution at 10 light-minutes :
1.8*10^11 / 1*10^5 = 1,8M.

As noted above, the figures for this are those given in the FAQ. The
Arecibo transmitter is 1MW feed point power; I presume that the 22TW
EIRP is a correct reflection of the antenna gain applied to this power.

Whilst detectable, it is uncomfirmable, because followup studies are
very unlikely to see a repeat event.

You missed one from the FAQ, weather radar, which has to be primary
radar, and has to be high power because it is relying on relatively low
efficiency back scatter, but uses very straightforward pulse doppler
techniques. I suspect that this reflects the strongest non-military
transmissions that are commonly active.

a) We can settle with a lower SNR ration. Most applications require 30dB
or so SNR, and we might settle for just 10db, so we can gain 20dB here
(power factor 100, distance factor 10).

Planetary radar is going to operate at the lowest possible SNRs, because
of the cost of creating better ones. They are likely to be less than those
required for SETI, because the target is known to exist. SETI has to use
relatively high SNR's to keep the number of false positives in check.
S@H uses an SNR of 13.4dB, for spikes, but gets one or two false positives
per work unit; that's because of the large number of different parameters
tried. I believe that Phoenix use something more like 9dB, because they
can do an immediate followup (but at the cost of requiring dedicated
telescope time).

Also, broadband signals will require a higher signal to noise ratio because
of not knowing the background noise levels (for narrowband you can use adjacent
channels as a reference).

I don't think that primary radar or weather radar use anything like
30dB.


b) We can use a bigger receiving antenna. The power factor we can gain is
(r/rr)^2, where r would be our antenna radius, and rr is ETs receiving
antenna for the application. Arecibo has a 100m effective radius (correct me
if I'm wrong), which can easily be 10x larger than what ET is using. So we

Not for planetary radar, the only type with any reasonable chance of
detection. That uses Arecibo class antennas. Actually it uses a larger
effective area on Arecibo, because it uses the Gregorian optics, which
better illuminate the dish.

might gain 20db with this. A square-mile receiver (as is in the planning)
might add another 15db to this.

c) We can integrate the signal we receive. Here it gets complex, but
important. Seti@home uses very narrowband (0.1Hz?) 'channels' and integrates
any signal with this. Integration typically enhances a signal by a factor

S@H doesn't do any integration for most observations, and doesn't
do any straightforward integration. Most of the time in S@H is spent on
spikes, which are processed with a time-bandwidth product of unity, i.e.
no integration. The gaussian detection can be considered as a form of
integration, but it is not straightforward. The pulse detection also
involves integration, but of non-continuous signals.

However, asteroid radar does integrate, over several minutes. (On the
other hand, the asteroid will spread the frequency of the returned signal,
so reduce the detectability of a narrow band signal and force integration
for the same observation time.)

sqrt(BW*t) in SNR (power factor). t (time) must be restricted for us to
about 10sec (0.1Hz minimum bandwidth) or so, since any strongly beamed ET

Typical transit times are more like 25 seconds, for a drifting,
Arecibo sized, transmitter. Planetary radar holding on a distant
object might manage 15 to 30 minutes, but would require targetted
operation by us to take advantage.

transmission not intended for us would not be aimed at earth much longer
than that. Bandwidth could be small (for high-resolution asteroid radar), or

It can be small, but about 50% of it is 10 to 100kHz because of deliberate
spreading.

very wide (for broadband communication transmissions). Overall. I think that
a BW*t factor of 10 (for very narrowband, high-resolution radar or
communication) to 1G (for 100MHz broadband communication) is reasonable to
assume. That means that we can gain 5-35db with integration.


But for narrowband planetary radar you may lose about 10dB more than you
gain because you are not integrating as long as the intended receiver.

d) We can improve the sensitivity of the receiver (system temperature).
Assuming that ET is not stupid, we cannot gain much here. 100K is reasonable
for any application, and thus the max we can gain here is about 5dB or so
(for a 30K receiver).

Asteroid radar uses radio astronomy quality receivers. I can't imagine that
strategic radars use cheap receivers either. Deep space probes may
be conservative and old, and satellites may be conservative, so uplink
signals may be designed for higher noise levels. Downlink signals are
being beamed at and scattered by the earth, and for deep space will
use very low noise receivers.


Overall, we should be able to obtain 50db (for narrowband transmissions) to
80dB (for broadband) improvement of SNR, with a seti@home style receiver

Only for communication satellite uplinks. Earth based communications aren't
really intended to escape to space, so will not produce an optimal signal
in that direction.

For CW planetary radar, you will be near break even, or negative. For
pseudo-random asteroid radar, you will be 15 to 30dB negative.

Broadband radar is not really an option to consider, since for radar, the
resolution can be much better improved by narrowing the bandwidth. But who
knows what ET thinks about this...

But broadband radar is typically interested in range resolution, which
drives to wider bandwidths.


So what conclusions can we draw from this ?
Seems that it is out of the question that we would ever detect any ET
point-to-point communication signal, since it would have to be targeting
something or somebody far out into or outside of ET's star system.

You mean like the more than half a light day to which we target?

objects, but there IS a chance to detect ET's narrowband asteroid radar, or
even high-resolution (military?) radar for ET's planet's close-proximity
(incoming missiles?) analysis. And there is a minute chance to find some

Typically 1MHz bandwidth.

broadband radar signals from ET which were intended for objects on or near
ET's planet.

Interesting to note here, is that a square-mile antenna would not change
much about this analysis. The target range of ET transmissions would reduce

The advantage they give is the ability to do full time targetted SETI and
to observe in multiple directions at once.

by a factor of 10 at most, but that still requires an absurdly large target
range for point-to-point transmissions (in the range of 65 AU). Only an

We have point to point systems operating over more than 90AU.

interplanetary radio-hub would become detectable, but still very faintly. So
overall, even if we increase the size of the receiver from Arecibo to a

But might well operate as optical frequencies with no overspill beyond
the limb of the target planet.

square-mile antenna, we still would only be able to detect ET radar
signals.....

It might be time to start listening for radar signals...

We are looking for CW radar signals. But any detections will likely
be unconfirmed; the lack of modulation wouldn't even allow you to use
secondary indicators of intelligence for a one off detection. You can't
look for chirped beacons without looking for CW.

Basically it is the weakness of most communication signals and the
non-repeatability of radar that make beacons the most likely candidate
for a signal that can be distinguished from statistical noise variations
and independently verified. But what we look for is independently
verifiable signals which look artificial, and have a very low probability
of being statistical artefacts when all the evidence is taken into account
(although individual observations will almost certainly fail the last
test).

Hi David,

I appreciate your sceptical response, and additional notes

I kind of claimed that, if ET is smart, that asteroid (or
out-of-home-planet)
radar is pretty much the only application which we might be able to
detect at a distance of 100 or 1000LYs.

You added weather radar and deep-space probe comm-links to that,
and corrected some of the detection factors (making the target range
of detectable ET radar larger than I assumed).

Notes below on some interesting issues in this continued quest
to speculate on the most likely ET signals...

Rob


"David Woolley" wrote in message
...
In article ,
Rob Dekker wrote:


[....]
The SETI Institute believe that the Allen telescope will be able to
detect analogue TV carriers from nearby stars even though it has a
smaller effective capture area than Arecibo; that's a result of analogue
carriers being a very ineffective use of the fraction of a Hertz that
they occupy and of the Allen telescope allowing extended SETI
observations.

Although I totally encourage (an fund) the Allen telescope project,
I dont think we should have any hopes for detecting Analog TV carriers.
Apart from the fact that we (as our only example) generate most
analog TV carriers below 1GHz (where gallactic cyclotron radiation makes
interstellar detection unlikely), there is another convincing reason
which reduces chances of finding analog TV carriers :

According to Drake's formula and the most reasonable assumptions
of its variables, if analog TV carriers exist for only 100 years in a ET
civilisation, there would be only 100 existing in the Galaxy today.
So the nearest one would be at 5,000 LYs or so.
Not much chance of detecting that.

I indeed assumed that ET is more intelligent that we are, and thus
creates near-optimal signals for their applications (radar or
point-to-point)
for remainder of their (hopefully 10's of thousands of years, otherwize
we will never find them) existence.

[.....]

We are not avoiding looking for leakage signal. The problem is that
they tend to be either undectable or unverifiable.


I'm not sure about the latter.
I don't think we actually made an effort to set-up detection algorithms
which would focus on radar (or not-often-repeating) signals.

One thing with radar signals (and also energy-efficient beacons) is
that they dont appear very often, but they do on average, increase the
detection probability of 'some' signal in the direction of origin.
I dont think that with seti@home we covered each star often enough,
or covered enough bandwidth, to actually talk about probability density
of detected signals, but a repeating signal in a certain direction
should at least trigger interest...

[......]
radar signals... That puts the recent discarding of signal SHGb02+14a
(which
can easily be a radar signal) in a whole new light....

As noted before, the very fact that this is detected at multiple times
is a contra-indication to radar. The other problem with this signal is
the wide range of chirps. Whilst our CW planetary radar is pre-chirped
for
the round trip. The acceleration along the line of site of interesting

Still, why do you think that ET would want to pre-chirp ?
There my very well be another reason to 'modulate' a radar signal
with some frequency modulation.

We (humans) are not sending out beacon signals (around
the water hole), but still we look for them. Yet we ARE sending out radar
signals (quite a few of them), but if we detect one which is not
exactly pre-chirped, we discard it as a Rio 0. And 'because' it repeats
it cannot be a radar, and if it would not repeat, it would not even
be logged as anything interesting.

This seems a self-fulfilling prophecy, targeted as finding a single
possible ET application (a constantly transmitting beacon).
That unnecessarily reduces our chances of finding ET signals.

We should really think about how to prove or disprove that/if
the galaxy is filled with radar (or other not-often-repeating) signals.

[....]
An ideal spherical, metal object would create a factor 2*(ro^2 / 4R^2)
because it would reflect all incoming power back into the 180degree cone
it
came from.

Even one much larger than the wavelength wouldn't do that. it would
scatter more back towards the sender than any other direction, but near
the limbs, it would scatter almost in the original propagation direction.
The overall formula may or may not be right - I'd have to set up the
integral to check that - but the logic is wrong.

I actually thought that the logic was quite creative 8-)

It might indeed not be absolutely correct logic for the pure metallic spere,
(although I'm pretty sure the formula still remains correct),
but it must be correct for a 'fractel'-like rough surface.

The integrals would become to complex to calculate, so I just looked
at a real-life rough surface which reflects EM radiation. The moon.

I found that the moon, wether full or almost crecent, still radiates
the same amount of light per exposed surface. So each surface
area on the moon must be an omnidirectional transmitter of light.

That would also be the only explanation for the R^4 radar signal
power drop-off.

If the object behaved as a mirror, instead of omnidirectional,
it would show only a R^2 drop-off.

[...]
Also, broadband signals will require a higher signal to noise ratio
because
of not knowing the background noise levels (for narrowband you can use
adjacent
channels as a reference).

I never understood this remark (you made it several times). Isn't it true
that any
signal, broadband or narrowband, always has 'adjacent channels' ?
I mean if a signal is 1Hz wide, and we conclude that because the adjacent
1Hz
channels show lower (noise) power, then is that not similar to a signal of
1MHz with
adjacent channels of 1MHz showing lower (noise) power ?

Why would broadband signals require higher SNR ?

Same thing with time (pulses are detected by checking adjecent time slots
showing
lower energy..).

[.....]
For CW planetary radar, you will be near break even, or negative. For
pseudo-random asteroid radar, you will be 15 to 30dB negative.


That's too bad. Means that the assessments of radar range go up to
real astroid radar (from near-home-planet radar).

Still, I'm not discarding broadband military radar as you do : there
has to be a way to obtain a better ratio for these signals.
After all, since they are broad-band, the receivers will be
broad-band, which lets in a lot of noise, so power requirements
have to go up (for same target range)...
That is all incorporated in the formula I gave, which is why broadband
radar is still positioned as the most likely one to be detectable. Just
because of it's massive power requirements.

I DO agree that with pseudo-random codes you need lower SNR's, but
energy/bit remains the same. Military broadband radar just wants to
detect a lot more than one bit, and that is why they go broadband...

[....]
by a factor of 10 at most, but that still requires an absurdly large
target
range for point-to-point transmissions (in the range of 65 AU). Only an

We have point to point systems operating over more than 90AU.


I assume you refer to the Pioneer 10 comm-links.
Indeed, these should be detectable with a square-mile telescope at 100LYs.
And because the Pioneer 10 receiver/antenna is rather small, the
transmission
should be rather powerfull, so ET can probably detect it with an
Arecibo-sized
dish at 100LYs (with all tricks described above). But not much further than
that....

Problem is of course that these (space probe) comm links do not happen
very often (unless ET has massive amounts of probes which need constant
instructions). But if they do occur, they will be probably be narrowband
(slow
data transfer is acceptable), and accurately targeted.
So they should remain in our antenna beam for extended time,
They will not (have to) be 'pre-chirped' though, assuming some intelligence
in the probe's receiver....)

[....]
Basically it is the weakness of most communication signals and the
non-repeatability of radar that make beacons the most likely candidate
for a signal that can be distinguished from statistical noise variations
and independently verified. But what we look for is independently
verifiable signals which look artificial, and have a very low probability
of being statistical artefacts when all the evidence is taken into account
(although individual observations will almost certainly fail the last
test).

I second that.

Still, there should be more ways to analyze the data we receive.
Maybe probability analysis (of non-recurring signals from a certain
direction)
could tell us more about the possibility of ET transmissions.
That's just one blunt idea. I'm sure there are smarter ways to find
statistically interesting directions....

Bottom line : We humans are sending out many thousands
of times more radar signals (detectable at 100's of light years) than
we are sending beacon signals. Why would ET do differently ?

Thanks David !

Rob

In article ,
Rob Dekker wrote:

I never understood this remark (you made it several times). Isn't it true
that any
signal, broadband or narrowband, always has 'adjacent channels' ?
I mean if a signal is 1Hz wide, and we conclude that because the adjacent
1Hz
channels show lower (noise) power, then is that not similar to a signal of
1MHz with
adjacent channels of 1MHz showing lower (noise) power ?

Using adjacent channels as a reference works for narrowband signals, because
there are no natural narrow band signals. It doesn't work well for broadband
signals as, on that scale, natural signals could be contained within the
bandwidth, so you cannot assume that the in channel noise is identical to
the adjacent channel noise.