## 1 Introduction  
Passive microwave observations from space provide unique information content about atmospheric temperature, water vapor, cloud liquid water, ice water content, precipitation (rain and snow fall rate), ocean water salinity, ocean surface wind, and soil moisture, as well as aid in the characterization of sea ice, snow, and land surfaces [1, 2, 3]. There are numerous satellites carrying passive microwave instruments observing Earth's surfaces and atmosphere. Satellite and conventional observations are assimilated at numerical weather prediction centers for daily weather forecasting [4, 5, 6]. Passive microwave observations contribute around 30\(-\)40% of the overall improvement of short-range forecast skill and provide around 15\(-\)30% of the total number observations assimilated [7, 8, 9]. English [8] studied the requirement for 23.6\(-\)24.0-GHz observations for weather forecasting and demonstrated the importance for acquiring water vapor information. Passive microwave observations from space began with measurements of Venus' surface temperature during the Mariner-2 mission in 1962. Cosmos-243 in 1968 carried the first microwave radiometer to measure Earth's atmosphere and surface from space. It was followed by three passive microwave remote sensing research missions: the Electrically Scanning Microwave Radiometer (ESMR) with a single frequency at 19.35 GHz and at single polarization on Nimbus-5 in 1972; ESMR operating with a single frequency at 37 GHz in dual polarizations on Nimbus-6 in 1975; and five dual-polarization channels of the Scanning Multichannel Microwave Radiometer (SMMR) on Nimbus-7 in 1978. The TIROS-N satellite was the NOAA's (National Oceanic and Atmospheric Administration's) first operational satellite equipped with the Microwave Sounding Unit (MSU) designed for acquiring atmospheric temperature profiles. The U.S. Advanced Microwave Sounding Unit-A (AMSU-A) and B on NOAA-15 (since 2005, AMSU-B on follow-on satellites was replaced by the microwave humidity sounder (MHS)) was launched on 13 th May 1998 and has been measuring atmosphere/surface for more than 21 years. Today, we have NOAA-15 AMSU-A/B, NOAA-18 AMSU-A+MHS, NOAA-19 AMSU-A+MHS, NASA Aqua AMSU-A, Suomi NPP and NOAA-20 Advanced Technology Microwave Sounder (ATMS), Advanced Microwave Scanning Radiometer-2 (AMSR-2), and AMSU-A+MHS on the European satellites Metop-A, B, and C. All these recent satellites contain radiometers with a channel situated at 23.8 GHz, whose main purpose is the estimation of atmospheric water vapor. Currently, there are ongoing discussions within the government, academia, and industry about future 5G wireless technology that could potentially impact passive microwave remote sensing due to radio frequency interference (RFI). In September 2018, the European Center for Medium-Range Weather Forecasts (ECMWF) hosted a workshop on the role and value of microwave observations from space in numerical weather prediction (NWP) with the specific goal of providing detailed guidance on the potential socioeconomic value of microwave bands allocated to the Earth Exploration Satellite Service (EESS) [7]. Weather forecasting is important to daily life as well as to the economy and society [10]. The radio frequency interference workshop reported that the value of microwave humidity observations has increased from just a few percent of the total to over 20% of the total over the last decade; microwave temperature sounders also provide around 20% of all observation impact. NWP relies on accurate observational information when potentially high-impact weather systems are in an early stage of development, providing advanced warning and time to take necessary action [9]. As noted, most passive microwave sensors in space have a water vapor channel at 23.8 GHz. The proposed 5G wireless technology may use a frequency close to this, thus potentially contaminating the water vapor channel measurements at 23.8 GHz. There are significant concerns about the impact without the water vapor channel.