Abstract: A Doppler-based radar system used in monitoring the speed of moving vehicles includes a self test circuit and related method for independently and remotely testing the operability of the entire radar system including the critical microwave components. The self test circuit includes a low frequency modulation diode positioned adjacent the antenna horn. The modulating diode causes a change in the voltage standing wave ratio seen by the radar system(s), thereby modulating the energy of the reflected radar test signal. The modulated test signal is compared to the original test signal and a resultant control signal is generated. The radar system is combined with a retarder system controller and a control system computer to provide an industrial control application, such as for controlling railway cars in a hump yard. Any number of additional radar systems/retarder system controllers can be added to expand the application.