larson@unix.SRI.COM (Alan Larson) (08/30/88)
In article <6878@umn-cs.cs.umn.edu> haque@umn-cs.UUCP (Samudra E. Haque) writes: >It was mentioned that the aliens (Big Green WoMen?) could >receive TV carriers at a distance of 100 LY. > >They would have to have tremendously good recievers for that feat. > >Also, once they received such a "carrier" signal from the >electromagnetic spectrum, what would they do with it? They coudn't >possibly know the modulation parameters (i.e., bandwidth, modulation >techniques <am, fm, fsk, pcm, etc. etc.> or even chrominance and >luminance coding mechanisms <pal, secam, ntsc> in that TV signal once >they get it - if at all they do. When the author said they would receive the carriers, he no doubt realized that the carriers contain far more concentrated energy than is availiable in the modulation. While they may be able to detect the carriers, it is unlikely that they could receive the modulation. Those parts of the signal are quite a bit weaker. (Most of the 'power' is in the carrier.) A quick calculation indicates that a TV station would need about 10E9 watts effective power before it would have a chance of being seen on the moon with a normal antenna. A 100 foot dish would probably be required for a normal quality picture. At 93E6 miles, the dish would have to be about 1000 feet in diameter just to 'sort of' see the picture. These assumptions have been based on a 10 BILLION watt TV station on channel 14. There are not many TV stations that come close to 10^10 watts Effective Radiated Power. >What I'm trying to get it that SETI don't stand a chance of getting >ANY useful information from our radio/tv carriers What I am pointing out is that the carriers are all they might have. The modulation will have faded into the noise. By the way -- to the person who suggested using the doppler on the carriers to determine things about us -- it may be possible, but it may be that the carrier frequency stability is not that good over the long term. Alan