Saturday, July 31, 2010

Receiving the Extraterrestrials - Radio Communications

Artificial satellites intended for broadcasting and communications are included and the scope of the chapter extends to the basics of moonbounce and interplanetary radar experiments and radio astronomy. For each of these subjects there is an enormous wealth of information and whole books are devoted to each one. The present text provides only an outline of each with some suggestions for detail on each subject. The common factor about almost all signals from space is that they have traveled over a very long path so all are very weak. (The only exceptions are radio noise from the Sun and occasional signals from the planet Jupiter.) For example, the signal strength available at ground level from a 30-W transponder on a geostationary satellite is liable to be significantly less than 1 pW/m 2 ; expressed in terms of decibels below 1 W/m 2 , figures in the range of 125 to 130 are common. (A picowatt is 120 dB less than a watt.) Signals used by radio astronomers and the returns from interplanetary radar experiments are liable to be yet another 60 to 80 dB weaker. The most sensitive possible receiver is essential, and highly directional aerials are commonly used, but there are no really strong signals coming from those directions so great dynamic range is not required.

Within the time scale of a human lifetime the fixed stars that form the background to the celestial sphere may be regarded as stationary, but, in fact, the whole Milky Way Galaxy is rotating about its center which is in the direction of the constellation Sagittarius. Our Solar System, as a whole, is part of that rotation and is moving at about 20 km/s in a direction toward the constellation Hercules. That plane of rotation about the center of the galaxy forms another frame of reference which is of great interest to professional radio astronomers. Artificial satellites which move in orbit around the Earth can be tracked in relation to figures for azimuth and elevation as seen from particular points on the Earth’s surface or they can be related to positions in declination and right ascension against the background of the fixed stars. Both systems of measurement can be used, and there are most appropriate uses for each one. To translate from one to the other can be complicated and must give due allowance for the rotation of the Earth and its associated time considerations. A satellite in geostationary orbit, for instance, maintains the same azimuth and elevation figures in relation to the Earth for very long periods but is moving in relation to the fixed stars so that it is carried around the celestial sphere once each day.

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