David Woolley wrote:

In article <3F2776AC.1080505@hccnet.nl>,
Ejo Schrama <e.schrama@hccnet.nl> wrote:

But if the alien signal were a spread spectrum modulated then the chances of detection are suddenly very small. GPS signals are such and

Whilst that is generally true, and the most efficient signals are the
most undetectable.

if you wouldn't know the PRN key you couldn't even tell there was a 500 watt transmitter flying 10000 km overhead. Also you can show that a fair

This is not actually true.  An earth based amateur SETI system can detect
the carrier, see <http://www.setileague.org/photos/hits.htm#GPS_qrm>,
however it is much weaker than for an intentional beacon and for AM or
(during silence) analogue FM broadcasts.

Cute, didn't know that.

I'm not sure how much the detectability is deliberate (to allow the receiver
to acquire the satellite) and to what it a by-product of using an odd length
PRN sequence which will have an excess of one phase state.

There is a clear access PRN code that is publically available, this is what all commercial GPS receivers use to boot.

amount of directed electromagnetic energy is required to span such vast distances in space. That's why I've always been sceptical about a success of the seti project.

I think the professionals would agree that it is more likely that current
projects will set a lower uppper bound on the number of signals, rather
than confirm one, but, nonetheless, the link budgets to relatively near
stars are far from unworkable at the moment, and the SETI Institute
reckon that their one square kilometre array would be able to detect
leaked analogue TV carriers from several stars.  Our radar observations
of asteroids (about half of which are narrow band CW) are detectable,
with current technology, to very large distances (upwards of 500 LY),
although they would fail a repeatability test.

Maybe that would be already a goal, to find an upper lower band on the number of signals. Within a 500 LY distance a directed SETI search could then be more efficiently because no more than approx 10^4 to 10^6 solar systems will have to be checked. But also in this field I haven't heard about a hit.

Currently operating transmitters and receivers are theoretically capable
of achieving 10kb/s data rates to the nearest star, with suitable signal
processing.

That might be true, but apply now the drake equation to just 10^6 solar systems, maybe 1 remains and it could be us.

X-Accept-Language: en-us, en

No Dutch, and you really prefer US to British English?  Google might
surprise you if you improve this.

Not sure whether I can change this.....

Ejo