The Combined Radar/Radiometer (ComRAD) is a ground-based microwave instrument system mounted on a 19-m hydraulic boom truck. This instrument package is an outgrowth of a network analyzer-based L, C, and X band polarimetric radar system developed jointly by NASA/GSFC and George Washington University which has provided reliable calibrated radar data in soil moisture field campaigns across the United States since the early 1990s. The system has been upgraded with the addition of a dual-polarized 1.4 GHz total power radiometer. A novel broadband stacked-patch dual polarized feed resonates at both the 1.4 GHz radiometer and 1.25 GHz radar frequencies, enabling the L band radar and L band radiometer to share the same 1.22-m parabolic dish antenna. Having the radar and radiometer utilize the same antenna with the same 12° field of view greatly simplifies the development of active/passive microwave retrieval algorithms. External calibration of the radiometer is achieved using cold sky and ambient microwave absorber targets during each measurement run, while radar calibration is achieved using known microwave reflectors (flat disks and dihedral) at the beginning and end of each extended measurement series. In addition to supporting basic science on microwave remote sensing of hydrological parameters like soil moisture, ComRAD can also function as a ground-based instrument simulator for L band space missions such as SMOS, Aquarius, and SMAP.