Dear Colleagues,

This is the 2nd volume of the Special Issue “Microwave Remote Sensing of Soil Moisture”, which achieved great success. Based on the previous research results, this volume is aimed at the presentation of recent advances in microwave remote sensing of soil moisture.

Soil moisture is well-recognized as a pivotal parameter to link the water, energy, and carbon cycles. Active and passive microwave remote sensing has been well-recognized as the most promising means to infer soil moisture spatially and temporally. Active microwave remote sensing, particularly the synthetic aperture radar (SAR), has a much finer spatial resolution than passive sensors but suffers more from the geometrical features of the scene (e.g., surface roughness, vegetation, and topography). Passive microwave remote sensing has higher sensitivity to soil moisture than active radar but is limited by its coarse spatial resolution. Moreover, active and passive microwave signals respond differently to soil and vegetation parameters and can thus provide complementary information for each other.

Over the past several decades, great progress has been made in microwave remote sensing of soil moisture. Several field or aircraft experiments (e.g., SGP, SMEX, HiWATER, SMAPEx1-5, and SMAPVEX) have been organized to support the assessment and refinement of active and passive microwave soil moisture retrieval algorithms. At the same time, a number of microwave spaceborne satellites/sensors have been successfully launched to provide valuable opportunities to obtain soil moisture with various spatial scales from meters to tens of kilometers. These include the passive microwave instruments, such as the multi-frequency AMSR-E/2 (2002-), FY-3 MWRI (2008-), L-band SMOS (2009-), and SMAP (2015-), as well as the active microwave instruments, such as the scatterometer-based Metop/ASCAT series (2006-), monostatic ALOS-2 (2014-), Sentinel-1 (2014-), and Gaofen-3 (2016-), bistatic CYGNSS (2016-), and the P-band Biomass (planned launch in 2023). All of these open a wide range of possibilities to estimate soil moisture at regional and global scales. In this context, this Special Issue aims to present the most advanced theories, models, algorithms, and products related to microwave remote sensing of soil moisture.

The topics of the Special Issue include, but are not limited to, the following:

• Review on microwave remote sensing of soil moisture;
• Introduction to field or aircraft experiments and future satellite missions for soil moisture;
• Evaluation or comparison of remotely sensed soil moisture products using in situ measurements, model simulations, or other mathematical approaches (e.g., TCA, TCH, IVd, etc.);
• Development, calibration, or validation of the theoretical or semi-empirical forward models (e.g., microwave scattering model and radiative transfer model) used for soil moisture retrieval;
• Development, improvement, or comparison of remotely sensed soil moisture retrieval algorithms;
• Development, improvement, or comparison of spatial downscaling/upscaling methods and spatiotemporal fusion techniques of remotely sensed soil moisture;
• Application of remotely sensed soil moisture products in data assimilation, agriculture, ecology, hydrology, and other fields.

Dr. Jiangyuan Zeng
Prof. Dr. Jian Peng
Dr. Wei Zhao
Dr. Chunfeng Ma
Dr. Hongliang Ma
Guest Editors

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• soil moisture
• active microwave remote sensing
• passive microwave remote sensing
• product validation and error analysis
• retrieval algorithms
• downscaling/upscaling methods
• spatiotemporal fusion techniques
• GNSS-R
• data assimilation
• eco-hydrological applications

• Microwave Remote Sensing of Soil Moisture in Remote Sensing (11 articles)