In article <Uqu8d.24335$QJ3.14580@newssvr21.news.prodigy.com>,
Rob Dekker <rob@verific.com> 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).