After looking at the software and hardware requirements for my Project Argus station, and mulling over such technical questions as integration time constant and Doppler shift correction, I have come to the following epiphany:
I must look for the most obvious signal - and that is the signal that I would choose to send myself, if I had the money to do so.
What that means (and it seems obvious once put on paper) is that I must look for myself. This is necessary to make the signal recognizable to us when finally detected.
Any ETI that I might hope to detect must be more like than unlike me, in most basic ways. Not to put too fine a point on it, but, for example, I think this ET would think in the same time frame as we do. Not at the speed of a glacier or at the speed of bullet, but somewhere near our 'thinking speed'. ET's physical makeup would have to be about the same as ours. Not as small as a bacterium or as large as one of the rolling hills I can see from my window, but somewhere in-between. This would give him the same type of control over his environment, and the same capability as I have to construct the needed transmitter, which could produce a signal which I can recognize. Not all ETI need be like me; only those who I have a realistic chance of detecting.
ETI's transmitter must be an RF signal generator. Some other, more exotic form of communication may well be in use, but since I can't construct a receiver to detect exotica, it's not worth considering. This leaves optical SETI open - but not for me. I know nothing about the optics required on that scale. As a microwaver, I'll stick to the area where I have a shot at SETI success.
The signal must be a deliberate beacon. That's the only type I, and most other Argus stations, would have a ghost of a chance of hearing. Leakage detection seems less likely, if only because of the transmit power requirements needed to show up on my system. Detecting planetary radar also seems unlikely, because it seems that it would only be sent for short periods. Once a radar echo was recovered, the transmitter would most likely be turned off or pointed somewhere else. The modulation scheme needed for an effective planetary radar might also make it difficult to recognize on this end.
I would set my beacon up in the waterhole to maximize its chances of discovery. I would want to be heard, and that is the most obvious place to start. The hydrogen line is at 1420 MHz and the hydroxyl line at 1662 MHz. I would transmit at exactly halfway between the two, at 1541 MHz. (One could also make a case for the geometric mean of the hydrogen and hydroxyl lines, which is 1536 MHz. But we're splitting hairs here.) I would expect ETI to similarly transmit somewhere near the middle of the waterhole, if he wants me to detect him. Unfortunately, my Project Argus system (receiver and filter) can't tune this frequency, but if I were to make changes to my system, that is where I would choose to monitor.
An ideal interstellar beacon should be narrow band to concentrate the transmit power, and to make it distinguishable from natural sources. It must be directed at our star. This is necessary to conserve power, and to make possible reception over huge distances. So a directed beacon is what I am looking for. I can see ETI pointing such a beacon at each candidate star, one at a time, sending the beacon for some length of time, and then moving to the next star.
The above targeted beacon strategy implies that Earth rotation Doppler compensation is a minimum requirement of our Project Argus receiving stations, if only to exclude local signals. Correcting for the Doppler shift due to our travel around the Sun is also a requirement. I have the Earth rotation Doppler chirp running now - the other compensation is an unknown quantity to me at this point, but something which Project Argus participants should be working on.
My hypothetical interstellar beacon would be locked onto each star for about a year at a time. We may have missed ETI's signal already, and may have to wait another 300 million years for it to show up again. Or, it may be starting tomorrow. Since we just don't know, we may as well assume that it starts tomorrow.
If I were sending a beacon, its transmitter frequency would be Doppler-adjusted to the Galactic center of rest. Since the purpose of a beacon is to be seen against a background of other signals, this would make it clear to anyone receiving it that it was an intentional signal. Again, I have no idea how to design this correction into my receiver chirp. If it's small (less than about 0.01 Hz/sec), no matter where I point my antenna I can't use it anyway, because my 10Hz/Bin resolution and planned 30-minute integration time constant make such small Doppler rates moot. If the compensation for the Galactic center of rest is a sizeable fraction of a Hz per second, I'd better figure out how to implement it!
My beacon would be a CW signal on/off modulated in a regular way. I might send Morse code in a repetitive pattern, and I would send it at a speed slow enough to allow integration of each character, but not so slow as to allow the signal to drift across many bins during a given key-down period.
If I concentrate on looking for myself, I may well miss signals sent by those not like me. But I know that creatures who think like me exist (if only by Earth's own example.) Designing our search around those not like us involves pure speculation, and may reduce our chances for SETI success.
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this page last updated 4 January 2003
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