High-accuracy Total Electron Content (TEC) measurements are important for several applications, including GNSS time transfer with accuracies better than 10 ns. In addition, it is important to provide uncertainty estimates to algorithms that use the TEC measurements. Dual-frequency TEC errors are dominated by satellite inter-frequency biases, satellite inter-code biases, receiver inter-frequency biases, receiver noise and multipath, higher-order ionospheric errors, and receiver inter-code biases for certain receivers. To evaluate different algorithms and error mitigation methods, data were collected from four different GPS receivers in two locations at the Arecibo Observatory in Puerto Rico. Error mitigation methods include receiver hardware bias calibration, comparison with TEC maps provided by the Jet Propulsion Laboratory (JPL), receiver comparisons, TEC calibration during low TEC count as determined by the Arecibo radar, and multipath mitigation using the Code Noise Multi-Path (CNMP) algorithm [12]. The CNMP algorithm enables the definition of rigorous multipath error bounds on the TEC bias estimation.