06-08-2015, 07:58 PM
SATELLITE ACQUISITION AND TRACKING
An essential operation in communicating by satellite is the acquisition (locating) of the satellite bythe earth terminal antenna and the subsequent tracking of the satellite. Initial acquisition depends upon anexact knowledge of the position of the satellite. In combination with the geographic location of the earthterminal, knowing the position of the satellite enables you to compute accurate antenna pointinginformation. The degree of difficulty in locating and tracking a satellite is determined largely by whattype orbit the satellite is in.The locating and tracking of a synchronous satellite is relatively simple. This is because the satelliteappears to be stationary. Locating a near-synchronous satellite is also relatively simple because of theslow relative motion of the satellite However, the movement of a near-synchronous satellite is enoughthat accurate tracking is required to keep the narrow beam antenna pointed toward the satellite. Satellitesin medium altitude circular orbits or in elliptical orbits are more difficult to acquire and to track becauseof the rapid changes in position.Orbital PredictionTo acquire and track a satellite in space, the earth terminal antennas must be provided with veryaccurate pointing information. Antenna pointing information is based upon the orbital prediction of thesatellite. This information is derived from an EPHEMERIS table. This table provides the coordinates of asatellite or a celestial body at specific times during a given period. After you know the ephemeris data ofa satellite, you can predict for any given location the apparent track of the satellite as viewed from thatlocation.The constants defining an orbit are initially obtained by the process of tracking. At the time oflaunch, the rocket is tracked by radar from lift-off to orbit and then until it passes out of sight. Trackingdata obtained in this way is sufficient for making rough predictions of the orbit. These predictions aremade rapidly with a computer and sent to tracking stations all over the world. These other trackingstations watch for the satellite during its first trip and record additional data. During the first week oforbiting, tracking stations all around the world are obtaining progressively more accurate data concerningthe 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 changein 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 monthswithout further corrections. When the orbits are known precisely, an ephemeris can be calculated for eachsatellite of the system.
Antenna Pointing->
Antenna pointing instructions for each satellite must be computed separately for each ground stationlocation. A satellite that bears due south of station A at an elevation of 25 degrees may simultaneouslybear due southeast of station B at an elevation of 30 degrees. Antenna pointing instructions aredetermined by taking into consideration the orbital prediction and the latitude and longitude of eachground station.To establish radio contact with a satellite, the ground station needs to know the bearing and elevationof a satellite. This allows the antenna to be properly pointed.