The analysis of magnetic diagnostics in next generation thermonuclear reactors is carried out. Based on this analysis, the new approach of magnetic field measurement in hard radiation and temperature condition is presented. The concept is based on data fusion concept by integration of Hall sensor and coil. High precision of magnetic field measurement insures on periodic in-situ calibration, namely, forming by the coil test magnetic field of known magnitude and measuring the signals of the Hall sensor, which are due to this test field. According to the results of measuring the test signals, the coefficients of the measuring conversion function are calculated. Main calibration issues of functionally integrated devices on Hall sensors and coils are detailed. Measurement system consists a functionally integrated 3D probe on Hall sensor and coil, analog front-end and software. Currently six sets of such devices have been installed for trial operation in JET (Oxford, GB).