# An Overview of the Applications of Earth Observation Satellite Data: Impacts and Future Trends
## 3 Results
### Knowledge Base Analysis  
We analyzed articles referencing all EO satellites (45,674) as well as the Sentinel (4016), Landsat (18,366), MODIS (14,494), Gaofen (678), and WorldView (1103) satellites using the CiteSpace bibliometric software [63]. Based on a co-citation analysis of the cited papers, we explored the knowledge base constituted by the published documents that referenced EO satellite data (Table S4). Table 2 lists the top 10 application clusters found for the application of data from all EO satellites and for Sentinel, Landsat, MODIS, Gaofen, and WorldView. This list consists of clusters that indicate the knowledge base of the application of EO satellite data. The clusters for all EO satellites suggest that EO satellite data have been widely used in Earth science including in studies of the land (land-cover change and land surface temperature), the atmosphere (evapotranspiration, aerosols, and AOD (aerosol optical depth)), water (Great Lakes), phenology, and biomass (biomass burning). It can also be seen that, currently, Landsat can be considered the most important EO satellite and the random forest (RF) algorithm the most important image processing algorithm. In terms of clusters related to articles that reference Sentinel, the leaf area index, deforestation, flood mapping, soil moisture, and water quality are the main applications. There have also been many studies on the processing of Sentinel images using techniques such as deep learning, RF, and image fusion. In the Landsat clusters, the main applications are spectral mixture analysis, habitat, savanna, landscape ecology, climate change, PM 2.5, and land surface temperature. The emergence of Google Earth Engine (GEE) has provided a convenient way of downloading and processing EO data. Sentinel-2 can act as a replacement for Landsat to a certain extent and has a better spatial resolution. From the clusters formed by articles that reference MODIS, it can be seen that MODIS data have mainly been used in studies related to phenology, aerosols, land surface temperature, aerosol optical depth (AOD), evapotranspiration, the leaf area index, and biomass burning. The clusters for all EO satellites and MODIS are similar, which indicates that the applications of MODIS data are widespread. The MODIS instruments provide images with a high radiometric (12-bit) resolution in 36 spectral bands (0.4-14.4 um); these offer unprecedented Earth-observation capabilities for a vast and diverse community of users worldwide. Currently, there are few articles related to the Gaofen mission, and only six clusters are listed in the table. These clusters are primarily associated with image processing methods. The clusters of references to WorldView are mainly associated with bathymetry, indigenous forests, and urban areas. The clusters of articles referencing WorldView data are more concerned with the processing of EO data than its application and include references to object-based image analysis (OBIA), pan-sharpening, deep learning, and linear discriminant analysis. Comparing the clusters for the five individual EO missions that were analyzed separately, MODIS is currently the EO mission that provides EO data for the widest range of EO applications. This is due to the width of and particular wavelengths covered by the MODIS spectrum. The CEOS has concluded that MODIS data have the potential to be applied in 16 different measurement categories including categories related to the land, oceans, atmosphere, and ice and snow. Landsat data are at a disadvantage compared to MODIS in relation to these categories. Although the remaining four satellite missions have more comprehensive launch plans for different EO applications, these have not yet been fully completed. For example, the Sentinel missions are designed for a wide range of EO applications, but various Sentinel missions are currently underway (Sentinel-4, -5, and -6): Sentinel-4 and -5 are dedicated to air quality monitoring and Sentinel-6 is dedicated to the sea-surface height measurement.