## 7 Recent and Future Space-Borne Satellite Missions for Biomass Estimation  
Global and pan-tropical studies of AGB estimate from the current satellite observations and integrated analysis of field inventories identify uncertainties and gaps in quantitative carbon stock assessments. Therefore, many space-born remote sensing missions have been launched recently, aiming at enhanced quantifications of carbon stocks across different biomes and carbon exchange associated with deforestation, degradation and LCLU changes (Figure 7). BIOMASS, one of the Earth Explorer mission from European Space Agency, is a P-band SAR satellite remote sensing mission, which is planned to be launched by the ESA in 2022. Three primary products will be derived from the data acquired by the BIOMASS mission, including AGB, forest canopy height (upper canopy height) and forest disturbance maps. The AGB and forest height will be estimated at a 200 m spatial resolution, while forest disturbance maps will be produced at a 50 m spatial resolution [186]. A new era of forest covers dynamic monitoring will be opened with the launch of Tadem-L in 2022 by the German Aerospace Center (DLR). It is a SAR mission with two satellites operating in the L-band. The mission will provide high-resolution data to extract annual and seasonal 3-D structure, and biomass of global forests twice a year at a voxel size of \(10\times 50\times 50\) m\({}^{3}\). It will provide a tree height map at 30 m and a biomass map at a 50 m spatial resolution across the globe [187]. Figure 6: Capacities of the tropical countries for remote sensing based (**a**) change mapping and (**b**) forest inventory development, in the context of the REDD+ initiative [182]. GEDI is the first high-resolution space-borne LiDAR mission to measure the magnitude of forest biomass and vertical structure of forests between \(\pm\) 51.6\({}^{\circ}\) latitude. It was deployed on ISS in 2018 by NASA for two years [188]. It measures surface elevation, forest canopy height and the vertical structure of the forest, which is imperative for an accurate estimate of existing forest biomass and its changes due to anthropogenic and climatic factors. The significant science products developed from GEDI include, but are not limited to, Canopy Cover Fraction (CCF), LAI, and LAI profiles at 25 m, AGB at a footprint level of 25 m, and Gridded AGB at a grid interval of 1 km [108]. The precision of the products will help to reduce uncertainties in carbon stock mapping due to deforestation, degradation and regenerating tropical forests [189]. NISAR (NASA-ISRO Synthetic Aperture Radar) is a 3-year multidisciplinary SAR mission emerged through collaboration between NASA and Indian Space Research Organization (ISRO). It is planned to be launched in 2021 and it will be hosting both S-band and L-band radars, with a spatial resolution of 7 m with a revisit time of 12 days. The mission will develop an annual global biomass product at a spatial resolution of 1 ha, which will be instrumental to monitor gains and losses in forest biomass across the tropical and other parts of the world [117; 189; 190]  
NASA's ICESat-2 was launched in September 2019. The ATLAS onboard ICESat-2 is promised to provide altimetry observations of Earth's land surface. The primary objective of the ICESat is to measure ice sheets and land elevation. However, the data are used for the canopy height and biomass estimation; thus, the ICESat-2 generates two vegetation products ATL08 (Level-3A product) and ATL18 (Level-3B product). The ALT18 gridded land product will provide coarse resolution annual maps of canopy height and canopy cover, while ALT08 will generate the vegetation products at a 100 m spatial resolution [117]. Several studies have utilized ICESat-1's GLAS datasets to estimate pan-tropical biomass [32; 105] and the new observations from ICESat-2 will help determine the changes in the AGB, as well as improving the accuracy of current carbon stock assessments [118; 119].