On JET, a key source of calibrated electron temperature Te profiles is from the measurements of the full ECE spectrum made by a Fourier Transform Spectrometer (FTS). It is absolutely calibrated by using a calibration source inside the vacuum vessel. High spatial and temporal resolution ECE Te profiles are obtained using a 96 channel heterodyne instrument which is cross calibrated on each JET pulse against the data from the FTS system. Residual systematic frequency dependant errors at the 5-10% level can then be evaluated and corrected for, using specific discharges in which the toroidal field is varied while keeping the shape of the Te profile constant. This improvement in the calibration method has been systematically applied at JET for the first time improving both the smoothness and the symmetry of the Te profiles. The consequences of this improvement are discussed. In addition, it is shown that no deviation occurs in the FTS calibration for more than 8 years, which is relevant for ITER.