Land-cover mapping is one of the foundations of Earth science. As a result of the combined efforts of many scientists, numerous global land-cover (GLC) products with a resolution of 30&#x2009;m have so far been generated. However, the increasing number of fine-resolution GLC datasets is imposing additional workloads as it is necessary to confirm the quality of these datasets and check their suitability for user applications. To provide guidelines for users, in this study, the recent developments in currently available 30&#x2009;m GLC products (including three GLC products and thematic products for four different land-cover types, i.e., impervious surface, forest, cropland, and inland water) were first reviewed. Despite the great efforts toward improving mapping accuracy that there have been in recent decades, the current 30&#x2009;m GLC products still suffer from having relatively low accuracies of between 46.0&#x0025; and 88.9&#x0025; for GlobeLand30-2010, 57.71&#x0025; and 80.36&#x0025; for FROM_GLC-2015, and 65.59&#x0025; and 84.33&#x0025; for GLC_FCS30-2015. The reported accuracies for the global 30&#x2009;m thematic maps vary from 67.86&#x0025; to 95.1&#x0025; for the eight impervious surface products that were reviewed, 56.72&#x0025; to 97.36&#x0025; for the seven forest products, 32.73&#x0025; to 98.3&#x0025; for the six cropland products, and 15.67&#x0025; to 99.7&#x0025; for the six inland water products. The consistency between the current GLC products was then examined. The GLC maps showed a good overall agreement in terms of spatial patterns but a limited agreement for some vegetation classes (such as shrub, tree, and grassland) in specific areas such as transition zones. Finally, the prospects for fine-resolution GLC mapping were also considered. With the rapid development of cloud computing platforms and big data, the Google Earth Engine (GEE) greatly facilitates the production of global fine-resolution land-cover maps by integrating multisource remote sensing datasets with advanced image processing and classification algorithms and powerful computing capability. The synergy between the spectral, spatial, and temporal features derived from multisource satellite datasets and stored in cloud computing platforms will definitely improve the classification accuracy and spatiotemporal resolution of fine-resolution GLC products. In general, up to now, most land-cover maps have not been able to achieve the maximum (per class or overall) error of 5&#x0025;&#x2013;15&#x0025; required by many applications. Therefore, more efforts are needed toward improving the accuracy of these GLC products, especially for classes for which the accuracy has so far been low (such as shrub, wetland, tundra, and grassland) and in terms of the overall quality of the maps.

Liangyun Liu

Xiao Zhang

Yuan Gao

Xidong Chen

Xie Shuai

Jun Mi

Finer-Resolution Mapping of Global Land Cover: Recent Developments, Consistency Analysis, and Prospects

Topsoil structures and inhomogeneous distribution of moisture in the soil volume will induce dielectric discontinuities from air to bulk soil, which in turn may induce multiple and volume scattering and affect the microwave surface emission. <i>In situ</i> ELBARA-III L-band radiometer observations of brightness temperature <svg xmlns="http://www.w3.org/2000/svg" id="M1" style="vertical-align:-4.6325pt" height="15.0984pt" version="1.1" viewBox="-0.0498162 -10.4659 15.5446 15.0984" width="15.5446pt">


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</svg> =H or V polarization) at the Maqu site on the Eastern Tibetan Plateau are exploited to understand the effect of surface roughness on coherent and incoherent emission processes. Assisted with <i>in situ</i> soil moisture (SM) and temperature profile measurements, this study develops an air-to-soil transition (ATS) model that incorporates the dielectric roughness (i.e., resulted from fine-scale topsoil structures and the soil volume) characterized by SM and geometric roughness effects, and demonstrates the necessity of the ATS model for modelling L-band <svg xmlns="http://www.w3.org/2000/svg" id="M3" style="vertical-align:-4.6325pt" height="15.0984pt" version="1.1" viewBox="-0.0498162 -10.4659 15.5446 15.0984" width="15.5446pt">


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</svg>. The Wilheit (1978) coherent and Lv et al. (2014) incoherent models are compared for determining the dielectric constant of bulk soil in the ATS zone and for calculating soil effective temperature <svg xmlns="http://www.w3.org/2000/svg" id="M4" style="vertical-align:-3.25759pt" height="11.5729pt" version="1.1" viewBox="-0.0498162 -8.31531 17.4207 11.5729" width="17.4207pt">


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</svg>. The Tor Vergata discrete scattering model (TVG) integrated with the advanced integral equation model (AIEM) is used as the baseline model configuration for simulating L-band <svg xmlns="http://www.w3.org/2000/svg" id="M5" style="vertical-align:-4.6325pt" height="15.0984pt" version="1.1" viewBox="-0.0498162 -10.4659 15.5446 15.0984" width="15.5446pt">


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</svg>. Whereafter, the ATS model is integrated with the foregoing model for assessing its performance. Results show the ATS-based models reduce the underestimation of <svg xmlns="http://www.w3.org/2000/svg" id="M6" style="vertical-align:-4.6325pt" height="15.0984pt" version="1.1" viewBox="-0.0498162 -10.4659 15.5446 15.0984" width="15.5446pt">


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</svg> (≈20-50 K) by the baseline simulations. Being dynamic in nature, the proposed dielectric roughness parameterization in the ATS model significantly improves the ability in interpreting <svg xmlns="http://www.w3.org/2000/svg" id="M7" style="vertical-align:-4.6325pt" height="15.0984pt" version="1.1" viewBox="-0.0498162 -10.4659 15.5446 15.0984" width="15.5446pt">


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</svg> dynamics, which is important for improving SM retrieval at the global scale.

Hong Zhao

Yijian Zeng

Jun Wen

Xin Wang

Zuoliang Wang

Xianhong Meng

Zhongbo Su

An Air-to-Soil Transition Model for Discrete Scattering-Emission Modelling at L-Band

Crop phenology is critical for agricultural management, crop yield estimation, and agroecosystem assessment. Traditionally, crop growth stages are observed from the ground, which is time-consuming and lacks spatial variability. Remote sensing Vegetation Index (VI) time series has been used to map land surface phenology (LSP) and relate to crop growth stages mostly after the growing season. In recent years, high temporal and spatial resolution remote sensing data have allowed near-real-time mapping of crop phenology within the growing season. This paper summarizes two classes of near-real-time mapping methods, i.e., curve-based and trend-based approaches. The curve-based approaches combine the time series VIs and crop growth stages from historical years with the current observations to estimate crop growth stages. The curve-based approaches are capable of a short-term prediction. The trend-based approaches detect upward or downward trends from time series and confirm the trends using the increasing or decreasing momentum and VI thresholds. The trend-based approaches only use current observations. Both curve-based and trend-based approaches are promising in mapping crop growth stages timely. Nevertheless, mapping crop phenology near real-time is challenging since remote sensing observations are not always sensitive to crop growth stages. The accuracy of crop phenology detection depends on the frequency and availability of cloud-free observations within the growing season. Recent satellite datasets such as the harmonized Landsat and Sentinel-2 (HLS) are promising for mapping crop phenology within the season over large areas. Operational applications in the near future are feasible.

Feng Gao

Xiaoyang Zhang

Mapping Crop Phenology in Near Real-Time Using Satellite Remote Sensing: Challenges and Opportunities

The MODIS LAI/FPAR products have been widely used in various fields since their first public release in 2000. This review intends to summarize the history, development trends, scientific collaborations, disciplines involved, and research hotspots of these products. Its aim is to intrigue researchers and stimulate new research direction. Based on literature data from the Web of Science (WOS) and associated funding information, we conducted a bibliometric visualization review of the MODIS LAI/FPAR products from 1995 to 2020 using bibliometric and social network analysis (SNA) methods. We drew the following conclusions: (1) research based on the MODIS LAI/FPAR shows an upward trend with a multiyear average growth rate of 24.9&#x0025; in the number of publications. (2) Researchers from China and the USA are the backbone of this research area, among which the Chinese Academy of Sciences (CAS) is the core research institution. (3) Research based on the MODIS LAI/FPAR covers a wide range of disciplines but mainly focus on environmental science and ecology. (4) Ecology, crop production estimation, algorithm improvement, and validation are the hotspots of these studies. (5) Broadening the research field, improving the algorithms, and overcoming existing difficulties in heterogeneous surface, scale effects, and complex terrains will be the trend of future research. Our work provides a clear view of the development of the MODIS LAI/FPAR products and valuable information for scholars to broaden their research fields.

Kai Yan

Dongxiao Zou

Guangjian Yan

Hongliang Fang

Marie Weiss

Miina Rautiainen

Yuri Knyazikhin

Ranga B. Myneni

A Bibliometric Visualization Review of the MODIS LAI/FPAR Products from 1995 to 2020

Optical remote sensing (ORS) of reflected sun light has been used to assess oil spills in the ocean for several decades. While most applications are toward simple presence/absence detections based on the spatial contrast between oiled water and oil-free water, recent advances indicate the possibility of classifying oil types and quantifying oil volumes based on their spectral contrasts with oil-free water. However, a review of the current literature suggests that there is still confusion on whether this is possible and, if so, how. Here, based on the recent findings from numerical models, laboratory measurements, and applications to satellite or airborne imagery, we attempt to clarify this situation by summarizing (1) the optics behind oil spill remote sensing, and in turn, (2) how to interpret optical remote sensing imagery based on optical principles. In the end, we discuss the existing limitations and challenges as well as pathways forward to advance ORS of oil spills.

Chuanmin Hu

Yingcheng Lu

Shaojie Sun

Yongxue Liu

Optical Remote Sensing of Oil Spills in the Ocean: What Is Really Possible?

Yirong Wu

Xiaofeng Li

Jean-Philippe Gastellu-Etchegorry

Jun Yan

Journal of Remote Sensing: A Platform to Promote International High-Quality Research and Innovation

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<p>The <em>Journal of Remote Sensing</em>, an Open Access journal published in association with AIR-CAS, promotes the theory, science, and technology of remote sensing, as well as interdisciplinary research within earth and information science.</p>


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<p>The <em>Journal of Remote Sensing’s</em> editorial board is led by Yirong Wu (Aerospace Information Research Institute, Chinese Academy of Sciences)&nbsp;and is comprised of experts who have made significant and well recognized contributions to the field.</p>


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