A system for online relative phase calibration is provided. The system includes at least one excitation source configured to generate multiple excitation signals. The system also includes at least two sensors coupled to respective ones of the at least one excitation source via a transmission line, wherein the two sensors are configured to receive respective ones of the excitation signals. The system further includes at least two phase detectors configured to receive at least two reflected signals from the two sensors via the transmission line, wherein each of the two phase detectors are configured to output a respective voltage representing a phase difference between respective ones of the reflected signals and respective ones of the excitation signals. The system also includes a switch coupled to the sensors and the phase detectors, the switch configured to switch the phase detectors between the sensors at a pre-determined switching interval. The system further includes a calibration module coupled to the phase detectors, wherein the calibration module is configured to receive and process the respective voltage from the at least two phase detectors to generate respective calibrated voltage signals, wherein the calibration module is further configured to and match the calibrated voltage signals of each of the phase detectors.