The integrated line-of-sight electron density within the ionosphere, known as the total electron content (TEC), is commonly used to quantify ionospheric propagation effects. In order to extrapolate single-point measurements of TEC to other locations and times, some characterization of the TEC spatiotemporal variation must be available. Using a four-channel receiver tracking coded signals fromthe NAVSTAR Global Positioning System satellites, estimates of both the mean variation and correlation coefficient have been made for the approximately 1200-km or 1-hour local time radius ionospheric region within view of a mid-latitude station. Results were obtained for morning and midday over a 4-week period near the autumnal equinox in 1989. The derived mean variation was found to be well characterized by linear functions of the local time and latitude separation between the ground site and the ionospheric penetration point of the signal. The correlation coefficient during midday was found to decrease linearly with latitude, longitude, and time separation, with values of about 0.91 for a 1000-km separation and 0.98 for a 1-hour separation. During morning hours the longitude and time coefficients were similar to the midday values, but the latitude coefficient was found to have a nonlinear dependence, with values as small as 0.70. The combined results suggest that the decorrelation is due primarily to longer term TEC fluctuations, such as day-to-day variation in the TEC spatial dependence, rather than to transient effects such as traveling ionospheric disturbances. The analysis provides a spatiotemporal characterization of TEC that can be used to extrapolate TEC values from single-point measurements ¿ American Geophysical Union 1993