The propagation of radio waves broadcast to the Earth by a satellite is slowed by the total electron content (TEC) of the ionosphere. Spaceborne radar altimeters, like those aboard the Seasat and Geosat satellites, rely on the time delay between the transmission of a radar pulse and its reception, after reflection from the ocean surface, to estimate the range to the surface. The slowing of the electromagnetic wave by the ionosphere will increase the apparent distance betwen the satellite and the surface. Variations of the TEC of the ionosphere on horizontal spatial scales less than 300 km could potentially be confused with or mask the presence of sea surface height changes associated with mesoscale ocean circulation. Using dual-frequency broadcasts from Global Positioning Systems (GPS) satellites, the spatial variability of the ionosphere is estimated. It is shown that for altimetric monitoring of mesoscale ocean circulation, ionospheric variability is not a significant problem. For altimetric studies of ocean circulation on ocean basin scales, compensation for ionospheric variability may be necessary. ¿American Geophysical Union 1991