Total Electron Content (TEC) constitutes one of the key elements for observing the variable structure of the ionosphere. GPS provides a cost-effective alternative in TEC estimation through earth-based receivers. In this paper, one of the TEC estimation methods, namely Reg-Est, is investigated in detail in terms of its parameters and developed further to include improvements. Reg-Est estimates robust TEC using GPS measurements of 30 s time resolution. The method combines the vertical TEC computed from all the satellites in view over 10° horizon limit in the least squares sense through the minimization of a cost function which also includes a high pass penalty filter. Optional weighting functions and sliding window median filters are added to enrich the processing and smoothing of the data. In this study, the input to the Reg-Est is enlarged to include phase-corrected TEC. The best way of including the instrumental biases is investigated and the algorithm is updated to include the biases in the slant TEC computation. The effect of the thin shell height of the ionosphere in Reg-Est estimates is studied. It is concluded that the Reg-Est algorithm is very robust to the choice of thin shell height. The best weighting function to reduce the multipath effects and to minimize the non-ionospheric noise is selected. The improved Reg-Est algorithm can be used for all latitudes and for both quite and disturbed days of the ionosphere. The Reg-Est TEC are in excellent accordance with the estimates from IGS analysis centers.