SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data, Version 4

SMAP Level-4 (L4) surface and root zone soil moisture data are provided in three products:
* SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data (SPL4SMGP, DOI: 10.5067/KPJNN2GI1DQR)
* SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Analysis Update (SPL4SMAU, DOI: 10.5067/60HB8VIP2T8W)
* SMAP L4 Global 9 km EASE-Grid Surface and Root Zone Soil Moisture Land Model Constants (SPL4SMLM, DOI: 10.5067/KGLC3UH4TMAQ).
For each product, SMAP L-band brightness temperature data from descending and ascending half-orbit satellite passes (approximately 6:00 a.m. and 6:00 p.m. local solar time, respectively) are assimilated into a land surface model that is gridded using an Earth-fixed, global cylindrical 9 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0) projection.

SMAP四级（L4）表层与根区土壤湿度数据集共包含三类产品：
* SMAP L4全球每3小时9公里等面积可缩放地球格网（Equal-Area Scalable Earth Grid，简称EASE-Grid）表层与根区土壤湿度地球物理数据（SPL4SMGP，DOI: 10.5067/KPJNN2GI1DQR）
* SMAP L4全球每3小时9公里EASE-Grid表层与根区土壤湿度分析更新数据（SPL4SMAU，DOI: 10.5067/60HB8VIP2T8W）
* SMAP L4全球9公里EASE-Grid表层与根区土壤湿度陆面模型常数数据（SPL4SMLM，DOI: 10.5067/KGLC3UH4TMAQ）。
针对每类产品，将SMAP L波段亮温数据（源自降轨与升轨半轨道卫星过境，分别对应当地太阳时约6:00与18:00）同化至采用地球固定的全球圆柱等面积可缩放地球格网2.0版（EASE-Grid 2.0）投影进行网格化的9公里分辨率陆面模型中。