[ 1] Measurements from the Global Positioning System (GPS) satellites provide a valuable source of information about the ionosphere in the form of integrations of electron concentration. The slant total electron content (TEC) through the ionosphere can be estimated for specific satellite-to-ground paths using the two GPS frequencies and knowledge of the dispersive properties of the ionosphere. However, these TEC values are in error because of the interfrequency biases of the satellites and receivers. In order to assess the accuracy of TEC in the ionospheric images, the determination of interfrequency biases must be studied. This paper addresses the determination of the magnitudes of these biases for individual GPS satellites paired with GPS receivers in Europe using the ionospheric imaging tool Multi-instrument Data Analysis System (MIDAS). This is done so that the accuracy of the TEC in the ionospheric images can be assessed. A simulation study was undertaken to verify the approach, then experimental results were compared with independent values of the biases calculated by the Center for Orbit Determination in Europe. Experimental results reveal that changes in the biases can be related directly to documented changes in receiver hardware. They allow an estimate of the receiver biases and hence the error in TEC estimation using GPS data.