The article presents a solution to the problem of determination of the air and ground velocities used in the technology of assessment of the air parameters' determination tools with the use of the satellite navigation systems when testing aircraft in unsteady flight conditions. The main factors of the speed calculation errors are presented; a metrological estimation of the suggested solution is calculated on the basis of the results of the analysis. In the flight testing methodology in aeronautics the questions of determination of the conventional true values of the air parameters play a significant role. The solutions to these questions evolved with the emergence of the satellite navigation systems and their usage in the flight tests. In recent years, with the use of the satellite navigation systems in Gromov Flight Research Institute the technology for assessment of the onboard air parameters' determination tools was developed and introduced into practice of the aircraft testing, which significantly improved the quality of the results of the flight tests. The article provides one particular solution to the problem of determination of the air and ground velocities used in the technology of assessment of the air parameters' determination tools with the use of the satellite navigation systems for testing aircraft in unsteady flight conditions. The main theses of the technology in general have already been presented in [2-4, 6-11]. The effectiveness of the suggested method was more than once demonstrated in the tests of the aircraft at high angles of attack. The presented results of the analysis explain the calculation algorithms and the factors of accuracy of the velocity determination, and justify the conditions of the method's efficiency. The results of the metrological assessment of the solution are important for development of the principles for assessment of the aircraft tests in unsteady flight conditions.