###### Abstract  
A microwave radiometer onboard a geostationary satellite can provide for the continuous atmospheric sounding of rapidly evolving convective events even in the presence of clouds, which has aroused great research interest in recent decades. To approach the problem of high-spatial resolution and large-size antennas, three promising geostationary microwave (GEO-MW) solutions-- geostationary microwave radiometer (GMR) with a 5 m real aperture antenna, geostationary synthetic thinned aperture radiometer (GeoSTAR) with a Y-shaped synthetic aperture array, and geostationary interferometric microwave sounder (GIMS) with a rotating circular synthetic aperture array--have been proposed. To compare the potential impact of assimilating the three GEO-MW sounders to typhoon prediction, observing system simulation experiments (OSSEs) with the simulated 50-60 GHz observing brightness temperature data were conducted using the mesoscale numerical model Weather Research and Forecasting (WRF) and WRF Date Assimilation-Four dimensional variational (WRFDA-4 Dvar) assimilation system for Typhoons _Hagitis_ and _Bualoi_ which occurred in 2019. The results show that the assimilation of the three GEO-MW instruments with 4 channels of data at 50-60 GHz could lead to general positive impacts in this study. Compared with the control experiment, for the two cases of _Bualoi_ and _Hagitis_, GMR improves the average 72 h typhoon track forecast accuracy by 24% and 43%, GeoSTAR by 33% and 50%, and GIMS by 10% and 29%, respectively. Overall, the three GEO-MW instruments show considerable promise in atmospheric sounding and data assimilation. The difference among these positive impacts seems to depend on the observation error of the three potential instruments. GeoSTAR is slightly better than the other two GEO-MW sounders, which may be because it has the smallest observation error of the 4 assimilation channels. Generally, this study illustrates that the performance of these three GEO-MW sounders is potentially adequate to support assimilation into numerical weather prediction models for typhoon prediction. 20221453  
view relative to the Earth's surface, a microwave sensor operating in geostationary orbit would overcome that obstacle, producing continuous microwave brightness temperature observations in a fixed domain [7]. The possibility of a microwave radiometer sounding the atmospheric state from a geostationary satellite has long been studied [8; 9; 10]. However, achieving adequate spatial resolution from 36,000 km is a long-standing problem. Several instrumental concepts using different technologies paved the way for future investigations. Early pioneering projects such as the Geosynchronous Microwave (GEM) sounder [11] and Geostationary Observatory for Microwave Atmospheric Sounding (GOMAS) [12] were based on traditional real aperture (RA) radiometer technologies similar to those used for low earth-orbiting (LEO) satellites. GEM used a 2 m diameter steerable Cassegrain reflector antenna, and GOMAS increased it to 3 m [13]. However, GEM and GOMAS could not achieve the required spatial resolution of -50 km at the lowest atmospheric sounding frequency band of 50-60 GHz [14]. Recently, the Shanghai Aerospace Electronic Technology Institute (SAETI) designed a geostationary microwave radiometer (GMR) with a 5 m real aperture antenna as one possible implementation in the framework of the Chinese Fengyun geostationary satellite series FY-4 [15; 16]. A prototype was manufactured, and the system design was verified through laboratory and imaging tests. Another alternative approach is synthetic aperture interferometric radiometer (SAIR) [17; 18]. Active research is ongoing on this subject with two concepts under investigation: geostationary synthetic thinned aperture radiometer (GeoSTAR) [19] and geostationary interferometric microwave sounder (GIMS) [20]. GeoSTAR is a geostationary microwave (GEO-MW) sensor developed by the Jet Propulsion Laboratory (JPL) to provide high-spatial resolution soundings of the Earth's atmosphere.