This paper presents a comparison between TEC estimated using a spatial gradient based approach for dual frequency receivers and using two Incoherent Scatter Radars (ISRs), one at the National Astronomy and Ionosphere Center (NAIC) near Arecibo, Puerto Rico and another at the Jicamarca Radar Observatory (JRO) near Lima Peru. The purpose of this comparison is to verify the accuracy of the spatial gradient TEC estimation method in measuring the overall electron content and small fluctuations caused by ionospheric structures such as sporadic-E in the receiver viewing area. A conventional dual frequency receiver calculates the ionosphere error by differencing the L1 and L2 pseudorange measurements. The results are contaminated by the difference in the two signals' propagation times through the satellite transmission system and the receiver, known as satellite and receiver differential code bias (DCB) respectively. While satellite DCBs are relatively stable and well monitored, the receiver DCB is dependent on the receiver environment, antenna and circuit design, and receiver signal processing algorithms making it difficult to calibrate or model. In this paper the ionosphere slant TECs and the receiver DCB are treated as unknowns in the range equations. Each slant TEC is related to the vertical TEC at the corresponding receiver-satellite ionosphere piercing point (IPP) through the mapping function. We model the vertical TEC at an IPP as having contributions from the receiver zenith vertical TEC and from the TEC spatial gradients. The paper compares the TEC estimated by our dual frequency GPS spatial gradient based method with TEC derived from ISR electron density profiles. The GPS and ISR data were collected simultaneously between March 25 and 27 2011 as well as January 23 and 26 2014 at NIAC using an array of four GPS receivers and from March 6 to March 11 2013 at JRO using a single GPS receiver.