By a spherical harmonic expansion of the magnetic potential the geomagnetic Sq variation is shown to have peak-to-peak amplitudes of a few tens of nanotesla at 400 km altitude. It is shown, in particular, that for harmonics of degree n up to about 4, with which Sq can be well represented, the vertical component of the magnetic field at altitudes to about 400 km is virtually the same as on the ground. Even though the NASA's Magnetic Field Satellite (Magsat), scheduled for launch in September 1979, is planned to be placed in a twilight sun-synchronous orbit, Sq over certain areas of the earth can be of comparable magnitude as magnetic anomalies of geological origin sought in the data. Due consideration should be given to Sq as well as to the effects from magnetospheric sources in analyzing the Magsat data. The Sq effect is expected to be large during the period of Magsat operation which is near the predicted solar maximum.