Abstract: Error sources of velocity estimate have been changed since SA was off.Firstly influence quantities of all kinds of error sources of velocity determination were analyzed,the potential accuracy of GPS absolute velocity estimate was derived from both theory and field GPS data simulation.After that,two tests were carried on to evaluate the performance of GPS absolute velocity determination in the case of a static and an airborne GPS receiver and INS(inertial navigation system) device kinematic case.In static mode,the user velocity has been estimated to several mm/s with the carrier-phase derived doppler measurements,and several cm/s with the receiver generated doppler measurements.In kinematic mode,GPS absolute velocity estimates were compared with the synchronized measurements from the high accuracy INS.The root mean square statistics of the velocity discrepancies between GPS and INS comes up to dm/s,moreover it has a strong correlation with the acceleration or jerk of the aircraft movement.