Multi-Sensor Microwave Soil Moisture Remote Sensing: NASA's Combined Radar/Radiometer (ComRAD) System

This paper describes the development of a ground-based radar/radiometer system called ComRAD. The system under development is an outgrowth of a network analyzer-based L, C, and X band polarimetric radar system developed jointly by NASA/GSFC and the George Washington University. The system is mounted on a 19-m hydraulic boom truck and has provided reliable calibrated radar data in soil moisture field campaigns across the United States since the early 1990's. The truck instrument system is now being upgraded with the addition of a dual polarized 1.4 GHz total power radiometer. The new system includes a temperature monitoring and control system to keep the radiometer at a constant temperature. All front-end components have been placed on an aluminium plate whose temperature stability has been achieved by using Peltier cells. Periodic switching to the internal noise sources is used to reduce the influence of gain variations in the output voltage. A novel broadband stacked-patch dual-polarized feed resonates at both the 1.4 GHz radiometer and 1.25 GHz radar frequencies, enabling both the radar and the radiometer to share the same 1.22-m parabolic dish antenna. Having the radar and radiometer utilize the same antenna with the same 12deg field of view greatly simplifies the development of active/passive microwave retrieval algorithms. With these new capabilities, ComRAD will be an important tool in examining Earth science research topics such as the synergistic effects between active/passive microwave sensors for soil moisture estimation in the presence of vegetation