Abstract: The simulation of GPS observation on a moving low-earth-orbiter(LEO) is to simulate the measured signals that would be received by a GPS receiver on a LEO.Since a LEO and GPS satellites are in a three-dimensional coordinate system,the values of various errors and observation noise level can be well calculated and known in advance,so using GPS simulation of the observations of a onboard GPS receiver can be of great help for judging some important parameters in orbit determination such as ambiguity resolution method,Kalman filtering technique and so on.In this paper,the method of simulating GPS observations of an onboard GPS receiver is systematically studied.The emphasis is first put on principle and mathematical model.Then,the GPS dual-frequency P codes and carrier-phase observations of potential onboard GPS receivers are simulated by employing some elaborately selected error modes.The results show that the simulated observations of CHAMP satellite obtained from software developed by the author are very close to the real measurements in-situ and such is observation noise level,therefore the simulation method used in this paper can meet the needs of different users of GPS data.