Ionospheric total electron content (TEC) data derived from dual-frequency Global Positioning System (GPS) signals from 30 globally distributed network sites are fit to a simple ionospheric shell model, yielding a map of the ionosphere in the northern hemisphere every 12 hours during the January 1--15, 1993 period, as well as values for the satellite and receiver instrumental biases. Root-mean-square (RMS) residuals of 2--3 TEC units are observed over the 20¿--80¿ latitude band. Various systematic errors affecting the TEC estimates are discussed. The capability of using these global maps to produce ionospheric calibrations for sites at which no GPS data are available is also investigated. ¿ American Geophysical Union 1995