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satellite beam acquisition tracking and pointing wikipedia
Post: #1

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.
Post: #2
An essential operation in satellite communication is the acquisition (location) of the satellite by the antenna of the ground terminal and the subsequent tracking of the satellite. The initial acquisition depends on an exact knowledge of the position of the satellite. In combination with the geographical location of the ground terminal, knowing the position of the satellite allows you to calculate accurate antenna signaling information. The degree of difficulty in locating and tracking a satellite is determined to a large extent by the type of orbit in which the satellite is located. The location and tracking of a synchronous satellite is relatively simple. This is because the satellite seems to be stationary. The location of an almost synchronous satellite is also relatively simple due to the slow relative motion of the satellite. However, the movement of an almost synchronous satellite is sufficient for precise tracking to be necessary to keep the narrow beam antenna pointing towards the satellite. Satellites in circular orbits of medium altitude or in elliptical orbits are more difficult to acquire and track due to rapid changes in position.

To acquire and track a satellite in space, ground antennas must be provided with very accurate pointing information. The pointing information of the antenna is based on the satellite's orbital prediction. 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 knowing the ephemeris data of a satellite, you can predict for any given location the apparent track of the satellite seen from that location. The constants that define an orbit are initially obtained through the tracking process. At launch, the rocket is tracked by the radar from takeoff to orbit and then until it is out of sight.

The tracking data obtained in this way are sufficient to make approximate predictions of the orbit. These predictions are made quickly with a computer and are sent to monitoring stations around the world. These other tracking stations watch the satellite during their first trip and record additional data. During the first week of orbit, tracking stations around the world obtain increasingly accurate data about the satellite. This information is placed on a computer where the corrections of the previous estimates of the orbit are made. Once the initial predictions are completed and the satellite link becomes operational, very few changes are made to these calculations. The orbits of a satellite will change slightly during a period of time; however, these changes are so gradual that the predictions will be precise enough to be used for weeks or even months without further corrections. When the orbits are known accurately, an ephemeris can be calculated for each satellite in the system.
 

Marked Categories : satellite acquisition and tracking, to ecquire knowleg, pointing and tracking satellites,

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