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The FY4 geostationary meteorological satellite gets its orbit determination and prediction through ranging the distances from different ground stations to the satellite, so the coordinates of each station site are one of main factors which influence the accuracy of the orbit determination. The satelliteground distance is the distance from the antenna phase center of a ground station to that of the satellite in general, but the antenna phase center of the FY4 ranging station antenna cannot be used to determine the starting reference point of the ranging distance and site coordinate measuring point. On the basis of the theory of the Cassegrain antenna, the following are discussed: where the starting point of ranging is and how to fix the point of coordinates to measure in the condition of the special structure of the antenna. The correction algorithm is given from coordinate measurement point to the ranging start point. Taking the FY4B antenna for an example, the result of correction is analyzed according to the actual condition that five ground stations follow the satellites distributed in seven different sites of space. Finally, suggestions are provided for practical application.