Abstract:A parameter fitting algorithm was studied for the current broadcast ephemeris and the modernizing broadcast ephemeris which were used to analyze the orbit fitting accuracy and orbit extrapolating accuracy respectively. It was shown that the 18parameter algorithm is an order of magnitude higher than the 16parameter algorithm for the orbit fitting accuracy in the same fitting interval, and the 18parameter algorithm is better than the 16parameter algorithm for the 2hour orbit extrapolation. According to the current GPS broadcast ephemeris update strategy, the 4hour orbit fitting by the 16parameter algorithm could have the accuracy by 14cm, while the 3hour orbit fitting by the 18parameter algorithm could have the accuracy by 1cm.