This paper compares TEC values calculated using a dual frequency GPS receiver TEC determination method and Incoherent Scatter Radar (ISR) measured ionosphere plasma density profiles. The GPS TEC method models the TEC along each satellite signal path as having its own vertical TEC whose major component is a reference vertical TEC with additional contributions due to the local TEC spatial derivatives. The reference vertical TEC, local spatial derivatives along latitude and longitude, and the receiver inter-frequency bias (IFB) are unknowns in the range equations [1]. Least squares solutions are obtained by solving these range equations over an extended period assuming that the IFB remains a constant during the period. This GPS TEC estimation method is vulnerable to the effects of multipath and code noise. In order to mitigate these effects, a modified version of the Code Noise Multipath (CNMP) algorithm currently used in WAAS reference stations is integrated into the GPS TEC estimation algorithm. The modified CNMP algorithm uses the code-minus-carrier (CMC) observable to reduce the pseudorange noise and multipath from range measurements. Making this correction introduces a bias due to the integer ambiguity in the carrier phase measurement which can be estimated [2]. This bias estimation has to be updated when a carrier cycle slip occurs and therefore, cycle slips detection is also implemented in the TEC estimation algorithm. This paper evaluates the accuracy and sensitivity of the integrated algorithm by comparing the estimated TEC with International GNSS Service IGS map and integrated electron contents from the electron density profiles collected by ISRs at the Arecibo Observatory in Puerto Rico, the Jicamarca Radio Observatory in Peru, and Poker Flat Research Range in Alaska. Simultaneous dual frequency GPS measurements and ISR electron density profiles at Jicamarca during March 2013, in January 2014 at Arecibo, and a number of days during July 2014 in Alaska were used in this study. The TEC computation method and results using the spatial derivative decomposition approach and comparisons with the TEC computed from simultaneous ISR measurements are presented in this paper.