# Passive Microwave Imagers, Their Applications, and Benefits: A Review
## 1 Introduction  
This review primarily examines the applications of Passive Microwave Imager (PMWI) data across various domains, rather than focusing on technical methodologies or algorithms. It highlights the significant role of PMWIs aboard satellites in advancing the understanding of Earth's atmospheric and surface processes. Operating in the microwave region of the electromagnetic spectrum and observing brightness temperatures (Tbs) to monitor atmospheric water subsistence and surface variables over ocean and land [1], PMWI sensors provide critical data for weather forecasting [2; 3; 4], climate monitoring [5], and environmental research [6]. Key programs and organizations such as the Defense Meteorological Satellite Program (DMSP) by the Department of Defense (DOD) [7], the National Aeronautics and Space Administration (NASA) through the Global Precipitation Measurement (GPM) mission [1; 8], the Japan Aerospace Exploration Agency (JAXA) with its Global Change Observation Mission--Water (GCOM-W) [9], the National Oceanic and Atmospheric Administration (NOAA) with its Joint Polar Satellite System (JPSS) [10], the Indian Space Research Organisation (ISRO) and French Centre National d'Etudes Spatiales (CNES) with the Megha-Tropiques satellite [11], and the China Meteorological Administration (CMA) with its Fengyun (FY) satellite series have been at the forefront of deploying PMWI technology [12]. The journey of PMWIs began in 1968 with the launch of a PMWI sensor by the Soviet Union. A significant leap in this technology came with NASA's Nimbus-5 and Nimbus-6 satellites, which carried the Electrically Scanning Microwave Radiometer (ESMR) in 1972 and 1975 [1; 8]. This was followed by the deployment of the Scanning Multichannel Microwave Radiometer (SMMR) on Nimbus-7 and SeaSat-1 in 1978 and the Special Sensor Microwave Imager (SSM/I) in 1987 as part of the DMSP. These early sensors laid the foundation for current PMWIs, providing valuable data on Sea Surface Temperatures (SSTs), wind stress, snow cover, and rainfall rates. The subsequent Special Sensor Microwave Imager/Sounder (SSMIS) series, starting with DMSP-F16 in 2004, expanded the frequency range and applications, covering atmospheric temperature, moisture, and surface parameters [13]. More detailed information about all existing PMWIs is provided in Table 1.  
PMWIs have found a wide array of applications in monitoring and understanding various environmental phenomena [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. For instance, the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) aboard NASA's Aqua satellite has provided crucial data on precipitation intensity, SST, sea ice cover, soil moisture, and wind speed since 2002. Similarly, the Tropical Rainfall Measuring Mission (TRMM) launched by NASA and JAXA in 1998 and its successor, the GPM mission in 2014, have been instrumental in enhancing precipitation estimates [26; 27; 28]. In parallel, EUMETSAT's Meteorological Operational Satellite-Second Generation B series (Metop-SG-B) will carry the Ice Cloud Imager (ICI) and Microwave Imager (MWI) by 2026. The MWI will provide data on precipitation intensity at the surface, sea ice cover, and near-surface wind speed, while the ICI will focus on cloud characteristics and parameters. Meanwhile, JAXA's GCM-W1 satellite, launched in 2012, carries the Advanced Microwave Scanning Radiometer 2 (AMSR2), operating at frequencies from 6.925 GHz to 89.0 GHz, providing precipitation, SST, soil moisture, and wind speed products [29; 30]. Additionally, the JPSS serves NOAA's requirements by supporting the delivery of ocean products, including normalized water-leaving radiances from the AMSR2 instrument on GCM-W, to provide global environmental data and enhance the understanding and prediction of changes in weather, climate, oceans, and coasts [10].