# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 3 Satellite Measurement of Precipitation
### Science and Technology Advances  
#### 3.1.1 Synergy of Sensors for Precipitation Estimates  
\"Measuring\" precipitation from space is an indirect process that aims at retrieving precipitation from the passive or active remote sensing of radiation in a cloudy atmosphere. The scientific community has come a long way since the early estimates only from cloud top temperature thermal IR measurements linked to precipitation through cold cloud duration algorithms, or simply by linking temperature/height and precipitation potential. The launch of passive microwave (PMW) sensors in the 1980s brought the necessary information from within the cloud, as opposed to merely the cloud top. However, it was only in 1997, with the launch of the Tropical Rainfall Measuring Mission (TRMM) [109] with the first Precipitation Radar (PR), that active measurements introduced more direct information on the precipitation formation in clouds, providing a 3D description of the precipitation field. The PR demonstrated that cloud horizontal and vertical structure sensing is necessary to improve our ability to measure precipitation at the local and global levels through the use of passive and active microwave (MW) sensors. TRMM was followed by CloudSat in 2006 [110], and the by the Global Precipitation Measurement (GPM) mission in 2014, with the new Dual-frequency Precipitation Radar (DPR) [111].  
The introduction of cloud and precipitation radars in space has opened the way to correcting errors deriving from the misidentification of cloudy and precipitating scenes on the one hand, and from the sensitivity of the radiative transfer model to cloud parameters on the other. Validation studies conducted over several decades have demonstrated that problems still arise in comparing satellite-derived precipitation products against those of ground radars when assessing their ability to represent the spatial variability of rainfall and precipitation intensity (e.g., [112]); differences in precipitation schemes and observation geometry are the basic reasons for this, and the advent of cloud radars and precipitation radars in space is narrowing the gap.  
Nevertheless, significant problems are inherent to maintaining the actual constellation of PMW sensors in orbit [113], which is expensive and is affected by calibration problems between different instruments, or even between sensors of the same category. The radar constellation (two radars are in orbit at the time of writing) is rapidly evolving, and future missions hosting clouds and precipitation radars such as the Earth Clouds, Aerosol, and Radiation Explorer (EarthCARE, [114]) are planned. Doppler radar technology is not yet available in space, and this is the gap that the proposed Wind Velocity Radar Nephoscope (WIVERN, [115]) mission wants to fill, in order to provide global measurements of in-cloud winds using the Doppler-shifted radar returns from hydrometeors. At the same time, the continuous development of an observing strategy for precipitation is part of a wider strategy for maintaining the global constellation of \"conventional\" operational meteorological satellites through the combined efforts of governments, research institutions, and industry [116].