In-situ methods for the calibration of the Correlation Reflectometry (CR) Systems on the JET Tokamak are presented. These methods consists of introducing a modulation in the path length of the wave or in the control voltage of the microwave Local Oscillator (LO) in order to produce fringes at the output of the CR systems. One of these methods is implemented by introducing small changes in the waveguide path length of a few millimeters. Using this procedure, all four reflectometers could be calibrated simultaneously and their phase deviations, amplitude imbalances and DC offsets routinely corrected. In another alternative method, an external wave generator modulates the voltage input of the LO with frequencies up to 1MHz. In both methods, the same experimental assembly for the plasma discharge measurements, i.e. the same microwave systems, microwave components and waveguides, electronics and electrical connections is used. These techniques also allow system enhancements to be introduced in a controlled way and comparisons of the correlation analysis, between signals in calibration mode and during plasma discharges, to be performed straightforwardly.