S M Shahrokhy
Remote Sensing Division
Surveying and Geomatics Department Iran
M R Saradjian
In order to receive the transmitted microwave signal from a satellite in maximum power, the receiving antenna must be oriented exactly towards the satellite. Some of important parameters to track a satellite are satellite’s rising time, setting time, overhead time, visibility duration and maximum elevation angle (indicating the closeness of the satellite path above the station). These parameters are sometimes difficult to extract directly. In this study, a look up table for satellite tracking is constructed which also overcomes the problem of direct calculation of the mentioned parameters. The look up table construction is performed in two stages. In the first stage, satellite position is determined in every desired moment. In this study, orbital elements are used to determine the satellite position. These elements are propagated for most remote sensing satellites in a daily basis by NASA in TLE format. There are a variety of orbital models such as circular, Elliptical, Brouwer-Lyddane, SGP and SDP to predict the satellite position. Although simpler, the elliptical (keplerian) model is sufficient if it is used along with the compensation of orbit perturbation. However if the orbital elements are too old, a more sophisticated model such as SGP is required.
The second stage is involved with the calculation of pointing parameters to the satellite position (from the ground station antenna). Once the satellite position is determined, pointing parameters with respect to the ground antenna position are calculated using spherical trigonometry. According to the results output from the two stages, a table containing the pointing parameters versus time is constructed which is then used directly by the antenna servo motors to track the satellite.