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SATELLITE ACQUISITION AND TRACKING
An essential operation in communicating by satellite is the acquisition (locating) of the satellite by
the earth terminal antenna and the subsequent tracking of the satellite. Initial acquisition depends upon an
exact knowledge of the position of the satellite. In combination with the geographic location of the earth
terminal, knowing the position of the satellite enables you to compute accurate antenna pointing
information. The degree of difficulty in locating and tracking a satellite is determined largely by what
type orbit the satellite is in.
The locating and tracking of a synchronous satellite is relatively simple. This is because the satellite
appears to be stationary. Locating a near-synchronous satellite is also relatively simple because of the
slow relative motion of the satellite However, the movement of a near-synchronous satellite is enough
that accurate tracking is required to keep the narrow beam antenna pointed toward the satellite. Satellites
in medium altitude circular orbits or in elliptical orbits are more difficult to acquire and to track because
of the rapid changes in position.
Orbital Prediction
To acquire and track a satellite in space, the earth terminal antennas must be provided with very
accurate pointing information. Antenna pointing information is based upon the orbital prediction of the
satellite. This information is derived from an EPHEMERIS table. This table provides the coordinates of a
satellite or a celestial body at specific times during a given period. After you know the ephemeris data of
a satellite, you can predict for any given location the apparent track of the satellite as viewed from that
location.
The constants defining an orbit are initially obtained by the process of tracking. At the time of
launch, the rocket is tracked by radar from lift-off to orbit and then until it passes out of sight. Tracking
data obtained in this way is sufficient for making rough predictions of the orbit. These predictions are
made rapidly with a computer and sent to tracking stations all over the world. These other tracking
stations watch for the satellite during its first trip and record additional data. During the first week of
orbiting, tracking stations all around the world are obtaining progressively more accurate data concerning
the Satellite. This data is put into a computer where corrections of earlier estimates of the orbit are made.
Once the initial predictions are complete and the satellite link becomes operational, very little change
in these calculations is made. The orbits of a satellite will change slightly over a period of time; however,
these changes are so gradual that predictions will be accurate enough to be used for weeks or even months
without further corrections. When the orbits are known precisely, an ephemeris can be calculated for each
satellite of the system.
Antenna Pointing
Antenna pointing instructions for each satellite must be computed separately for each ground station
location. A satellite that bears due south of station A at an elevation of 25 degrees may simultaneously
bear due southeast of station B at an elevation of 30 degrees. Antenna pointing instructions are
determined by taking into consideration the orbital prediction and the latitude and longitude of each
ground station.
To establish radio contact with a satellite, the ground station needs to know the bearing and elevation
of a satellite. This allows the antenna to be properly pointed.
