We compare total electron content measurements made using the Arecibo Incoherent Scatter Radar (ISR) with those made using the Global Positioning System (GPS). The ISR measurements have a limited range for these observations, up to 1500 km. We extend these profiles to GPS heights of 20,200 km with the aid of a numerical model. We use a GPS receiver on St. Croix, which has been calibrated using JPL's Global Ionospheric Mapping (GIM) technique. In addition, we also use the ISR to calibrate GPS measurements made at Isabela, PR and see how the calibration holds up on the next day. The GIM technique gives very good results on both a quiet night and a night with a severe ionospheric depletion. Normalizing the Isabela receiver to the ISR also gives good results and shows promise as a way to independently calibrate nearby GPS receivers in the future. Finally, we give evidence that the severe depletion observed by the ISR on the night of June 25/26, 1998 was associated with an elongated TEC depletion. The structure may be related to a disturbance originating in the southern hemisphere. ¿ 2000 American Geophysical Union