The presence of free electrons and ions modifies the propagation speed of electromagnetic waves. Thus the apparent sea height measured by a radar altimeter such as that on board Geosat is influenced by passage through the ionosphere. We explore ionospheric effects using several methods. A model ionosphere for Westford, Massachusetts, is constructed from two frequency observations of Global Positioning System satellites. This model is compared with a theoretical calculation, Faraday rotation observations, and the ionospheric correction furnished with the Geosat geophysical data records. We discuss the influence of the solar cycle, which is important at the moment as a particularly active maximum is approached. Some general remarks on the ionospheric influence on the oceanographic signal observed by a satellite altimeter are also included.