MAGSAT observed a geomagnetic sudden commencement at an altitude of 550 km on its dawnside orbit above the South Atlantic Ocean. The H-component of the magnetic field increased monotonically by 46 nT in about 4 minutes, while the D-component decreased westward by 10 nT in the first 1--2 minutes and then increased gradually. There was no significant deviation in the Z-component. The comparison with ground-based magnetograms showed that the negative impulse of the D-component provided the first experimental evidence of the global ionospheric current system for the preliminary impulse of the geomagnetic sudden commencement.