In order to take advantage of multispacecraft measurements that will be provided by the Cluster mission, we introduce a new method for analyzig the local geometry of a surface crossed by a group of satellites. This analysis relies upon the determination of the normal to the surface by each spacecraft. Assuming a rigid surface with constant curvature, we give a method to estimate its radii of curvature and its velocity. Theoretically, the method could give the three components of the velocity. But because of the sensitivity of the input data to uncertainties, the tangential velocity of the discontinuity must be provided by another method. ¿ American Geophysical Union 1994