17-11-2017, 11:13 AM
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.
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.