This paper describes an experiment involving a professional downhill skier equipped with GPS and MEMS-IMU, as well as a triad of magnetometers and a tactical-grade inertial unit. The experiment illustrates the navigation performance of GPS/MEMS integration compared to high-quality GPS/INS integration challenged by the environmental conditions and dynamics of ski-racing. The MEMs sensors'output is directly compared to the reference signals. This information is used to validate the error models of the inertial and magnetic measurements and to select the appropriate integration algorithms.