THE PROBLEM
Before you can do anything with a communications satellite you have to know where it is at every instant of its motion around the earth. In other words, you need to know its orbit quite accurately. When you know this, you can predict when a particular station on the ground will be able to see the satellite and communicate with it. You also can tell when two or more stations can see the satellite simultaneously and communicate with each other. And you can estimate how many satellites will be needed to provide a group of ground stations with enough working time to maintain a communications service. This last, after all, is the ultimate goal of all our efforts in the communications satellite field.
In determining a satellite’s orbit, we find that we must do three things:
- We must obtain information on the position—and perhaps also the velocity—of the satellite whenever possible by observing it with scientific instruments.
- We must use this information to determine the satellite’s orbit at the time of the observations.
- We must be able to tell how this orbit changes between the times when we observe it, because a satellite’s orbit does not remain constant with the passage of time.
In the case of the Echo I satellite (see [page 16]), we engaged in the first and third of these activities. We had many chances to follow the satellite with our radars, and we could speculate how its orbit was changing through the months. In the case of the Telstar I satellite, we engaged in all three kinds of activity. We shall take a look at these problems in the sequence in which we came across them, for both the Echo and Telstar satellites.
How We Track Satellites
We collect on the ground most of the information to calculate a satellite’s orbit, using optical instruments or radar equipment. Following a satellite through the sky is called tracking; in the early days after the first Sputniks, some of this tracking was done with the naked eye or with very simple telescopes by the Moonwatch teams. Many of you may have observed Echo I on a clear night without any kind of instrument.
Figure 1