Good morning everyone,

This is the first of two questions for today (so far...).

I assume SETI systems can determine signal (antenna) polarity, horizontal and vertical, if a signal is one or the other?

Dave

Only if the signal is analyzed. It's not clear how many signals, if any, have been analyzed for form or possible content.

Hi William,

Thanks for the quick reply! That caveat works for my situation - the beauty of writing fiction. ; ) Overall, what I'm working on will portray SETI as accurately as possible while still telling the story of an actual discovery. Maybe someday it'll actually be made into a film starring (insert name here).

Dave

Over the vast distances involved the source polarisation will have been lost, which is why most radio telescope use circular polarisation, which is better at small signal detection at UHF/microwave frequencies

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Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

...
I assume SETI systems can determine signal (antenna) polarity, horizontal and vertical, if a signal is one or the other?

Dave

The seven feed horns of the ALFA array at Arecibo do have detectors at right angles, each detector provides separate signal so we get 14 channels of recorded data. But "horizontal" and "vertical" only make sense when transmitter and receiver have a common ground plane, so the channels are identified as Beam 0 Polarity 0, Beam 0 Polarity 1, Beam 1 Polarity 0, etc.

If e.t. were sending a plane wave transmission, it could arrive with any angular relationship to the detectors so might be either equal or unequal for the two polarities. If the transmission were circular polarization it would tend to be equal.

                                                                  Joe

Hi Bob,

Dang, a wrench in my gears! ; ) Actually, maybe not... what about right- and left-handed polarization? (Bob or anyone else...)

Dave

Thanks for the reply Joe!

It sounds like if a beam with Polarity 1 and Polarity 2 were received, the two could be distinguished regardless of orientation to the detectors (at least most of the time)?

Hi Bob,

Dang, a wrench in my gears! ; ) Actually, maybe not... what about right- and left-handed polarization? (Bob or anyone else...)

Yes, you can have "clockwise" or "anticlockwise" circular polarization.

That is often used for satellite communications...

And despite the vast distances of space, a specific polarization will remain distinct until the signal is eventually lost in the cosmic cacophony of noise.


Keep searchin',
Martin

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See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)

Thanks for the reply Joe!

It sounds like if a beam with Polarity 1 and Polarity 2 were received, the two could be distinguished regardless of orientation to the detectors (at least most of the time)?

Err... Is not each beam listed as polarity 0 and polarity 1?...

For any strong enough signal, a plane polarized signal will be picked up at anything between 0dB in one polarity and -3dB in the 90deg-off other polarity, or equally across both polarities if 45deg to both. Similarly, a circularly polarized signal will be picked up equally by both polarities.


Now for a question:

Does the s@h signal processing make any attempt at determining signal polarization?

We only process only one channel at a time so I suspect not.


Keep searchin',
Martin

---

See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)

Polarization of the Cosmic Microwave Background has been used to detect gravitational waves by the Bicep2 telescope at the South Pole. But the receiver was supercooled, which is not the case at Arecibo or Green Bank.
Tullio

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Thanks for the reply Joe!

It sounds like if a beam with Polarity 1 and Polarity 2 were received, the two could be distinguished regardless of orientation to the detectors (at least most of the time)?

Err... Is not each beam listed as polarity 0 and polarity 1?...

For any strong enough signal, a plane polarized signal will be picked up at anything between 0dB in one polarity and -3dB in the 90deg-off other polarity, or equally across both polarities if 45deg to both. Similarly, a circularly polarized signal will be picked up equally by both polarities.

The signal at 90deg-off is much less than -3db (theoretically zero), the signal at 45% is -3dB. Think sine of the angle.

Now for a question:

Does the s@h signal processing make any attempt at determining signal polarization?

We only process only one channel at a time so I suspect not.

It is actually quite easy to combine the two signals and therefore ensure signals that are not line with with either polarization as we receive them will not be rejected if they are slightly below threshold.

Keep searchin',
Martin