This paper presents an in-situ detector characterization and calibration method for differential I/Q Six-port radar systems. A separate calibration of the nonlinear detectors and the remaining linear RF front-end enables an independent analysis of errors caused by detector nonlinearity and by the measurement scenario. A linearity analysis of various detectors in Six-port radar systems shows the necessity of a detector calibration for accurate displacement measurements. The calibration is evaluated with three different Six-port based radar systems at 24 GHz in diverse measurement scenarios. An error reduction of up to 50% has been achieved.