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.
[Illustration: _Figure 1_]
satellite
horizon
elevation
azimuth
North
East
If we use a telescope, we note the time of the observation and we
usually take a photograph of the satellite. We locate the satellite in
terms of the two angles shown in _Figure 1_. One of these is the
_elevation_ angle—the number of degrees a telescope must be tilted above
the horizon to see the satellite. The second is the _azimuth_ angle—the
number of degrees between the plane in which we measure the elevation
angle and the north direction. Of course, we can also point a radar
antenna at the satellite in the same manner. The radar can receive a
signal transmitted by the satellite, or else it can send a signal to the
satellite and watch for the reflected waves that eventually return. In
the latter case, the satellite must have sufficient surface area to
produce an adequate reflected signal. These two kinds of precision
tracking were both possible with Echo I. Radar can also do something
that optical equipment usually can’t do: measure the distance out to the
satellite.