Abstract: First,the point positioning and all sorts of differential GPS techniques are discussed on GPS based geometric orbit determination in real time.Differencing pseudo range is proposed to use to determine the satellite geometric orbit in real time because it can weaken the effect of GPS satellite orbit error and clock error,and particularly effect of SA,and because it is not difficult to carry out.The result computed from practical observation proves that it can improve the precision of geometric orbit determination obviously.Then,robust estimation is tested to weaken or eliminate the effect of contaminated GPS based signal,and the result verfies that robust estimation can improve the precision of geometric orbit determination further.