_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._