Assessment of the RFI environment in key passive microwave bands for Earth observation

Abstract. Radio Frequency Interference (RFI) is spreading worldwide, affecting numerous Earth Observation (EO) instruments. Among these, microwave radiometers play an essential role, providing critical measurements for climate monitoring, weather forecasting, and numerous other applications. In order to plan for future satellite missions and fully exploit currently available measurements, it is crucial to study the contamination levels at bands where radiometers operate. This work presents the Earth Observation RFI Scanner (EORFIScan), an RFI detection system for EO products that combines multiple RFI detection techniques in order to reduce missed detections. This software has been used to assess several passive microwave bands from 6 up to 200 GHz, including both exclusive and shared bands. Analysis and validation of this method is presented for the year 2022. The resulting RFI probability maps show significant contamination in the bands up to and including 18.7 GHz; thus for the assessment, we mainly focus on bands from 6 to 18.7 GHz, where the most widespread contamination is found. A few brightness temperatures in the range of 350–400 K have been observed at 23.8 GHz and one at 36.5 GHz, which suggest the presence of man-made emissions. At higher frequencies, RFI contamination is not clearly visible in the analysed data. Comparisons with simulated radiances from a numerical weather prediction model are presented as a way to evaluate the RFI detection, finding that flagged observations are typically warmer than model simulations, as would be expected for RFI. It is clear from the results presented that RFI is already a concern for users at lower frequency passive microwave bands, and it is recommended that real-time monitoring systems are developed to keep an eye on the evolving threat of RFI in EO bands.