Scrapbook 3: The Story of Telstar (5)
BOOSTING AND BOUNCING
TELSTAR has already been described as an ‘active’ satellite. It will now be useful to look at the two kinds of satellites that could be used for worldwide communications; that is, ‘active’ and ‘passive’ satellites.
An ‘Active’ satellite contains complex electronic equipment which receives signals from the ground station, boosts them and re-transmits them to the distant ground receiving station.
A ‘Passive’ satellite simply acts as a reflector. That is, it does exactly the same job as the natural reflecting layers in the ionosphere (see page 2). However, it is much more reliable than the natural layers, because the position of its reflecting surface is always accurately known. This is not so for the natural reflecting layers of the ionosphere. They are affected by other solar activity, which is why radio communications suffer from fading and distortion so badly at present.
An example of a passive satellite is the metalized balloon which was launched by the Americans in August, 1960, and called Project ‘Echo’. It is 100 feet in diameter and travels round the earth at a height of about 900 miles. In August, 1960, signals were bounced off the balloon and received by Post Office and Royal Radar Establishment engineers in England. This balloon still circles the earth and it can occasionally be seen in the night sky, looking like a fairly bright star.
The Americans are also interested in another kind of passive satellite — a very large number of fine metal needles about half-an-inch long — which they plan to scatter all round the earth, to form an artificial ionosphere. Their idea is to project a canister containing the needles into an orbit round the earth. The canister will then disintegrate and the needles will spread into a continuous belt.
A passive satellite only reflects signals. An active satellite fitted with a receiver and transmitter, picks up signals from a ground station, amplifies and re-transmits them.
This is Echo II — a passive satellite — being given rigidity tests. It is a 135 foot rigidized inflatable balloon. As you can see, a satellite is not always made of special metals in the well-known cigar-shaped form!