We present terrestrial and space-based dual-frequency observations of a region of enhanced total electron content (TEC) over the southeastern United States at local nighttime during the geomagnetic storm of 29–31 October 2003. The apparently localized, large-amplitude, and nearly Earth-fixed midlatitude ionosphere disturbance contained about 10 m higher delay at Global Positioning System (GPS) L1 frequency than the nighttime background ionosphere TEC. Using the dual-frequency altimeter on board the Jason satellite, we show evidence that nearly all of the electron content was below its orbital altitude of 1300 km at 0000 local time on 31 October 2003. Dual frequency GPS measurements from the receiver on board the SAC-C satellite indicate that some portion of the electron content existed above the 700 km orbit altitude of SAC-C. We develop a horizontally piecewise constant regional model of the enhancement. We compare the model prediction of TEC with the SAC-C satellite GPS data to constrain the altitude of this enhanced TEC region. Our model indicates that the peak density of the anomalous region is at slightly higher altitude and greater in amplitude than that of the background. The TEC enhancement provides a concrete case study of an extreme scenario that both space-based and ground-based GPS augmentation systems must take into account in order to offer high-accuracy, high-integrity corrections to GPS for safety-of-life applications.