This paper discusses the application of Maximum Likelihood techniques to the estimation of ionospheric Total Electron Content (TEC) from GPS receiver data. Estimators are derived for un-differenced dual frequency P-Code and cross-correlated P-Code data which require both code and carrier phase measurements. Cramer Rao lower bounds for the estimate variances are also defined. The techniques are applied to data obtained from a Turbo Rogue cross-correlation GPS receiver