Abstract: A low-orbit satellite with precise and complete dynamic model and 500-km altitude is simulated. The precise coordinates of 27 GPS satellites in the same time span are calculated by using Chebyschev polynomial interpolation method. Based on simulated low earth satellite orbit and GPS precise ephemeris, P-code observations with different magnitude errors are simulated. By means of least square patch processing method, the biases of initial state parameters are calculated using double-frequency P-code ionospheric-free combinations of GPS simulated measurements. Last, double-frequency P-code observations with random error (standard error σ=2.7m) and GPS clock error (maximum bias=5m) in a motion period (95min) are simulated. By using ionospheric-free combination measurements, the short-arc orbit determination in a period is fulfilled. The results reveals that 6~9 GPS satellites can be traced at the same time using 5° cut-off angle and the biases between estimators of coordinate parameters and simulated true values are at centimeter level and those of velocity are at mm/s level.