# Satellite Remote Sensing of Surface Urban Heat Islands: Progress, Challenges, and Perspectives
## 2 Historical Review
### Milestones in Remote Sensing of SUHI  
A series of milestones can be identified in the history of SUHI research in terms of data availability. [PERSON] who is a research physical scientist of the National Environmental Satellite Service (NESS) of the National Oceanic and Atmospheric Administration (NOAA) was the forerunner to study SUHI using satellite thermal data. He found evident urban signatures on ITOS-1 thermal images in the Eastern United States, and highlighted the potential of thermal remote sensing in SUHI research [33]. This led to further SUHI research, but at a relatively coarse spatial resolution (\(\sim\)1.1 km) and low accuracy through meteorological satellite imagery [55, 56, 57]. To facilitate the comparison analysis, high spatial (temporal) resolution data refer to the data with a resolution of \(<\)120 m (\(<\)1 day), while others are considered as coarse resolution data in this study. In 1990, Landsat data were first applied in SUHI research [58]. Even though only available in the daytime, the Landsat archive soon  
Figure 3: The study time of day (**a**), season (**b**), and foci (**c**) of the selected SUHI studies.  
became the major satellite record for the SUHI analyses because of its comparatively high spatial resolution (60-120 m), long time series, and high data quality, especially after the free release of the entire Landsat archive in 2008. The Moderate Resolution Imaging Spectroradiometer (MODIS) LST product [59] constitutes another important data source for SUHI research due to its high temporal resolution, medium spatial resolution (1 km), global coverage, and free availability. MODIS enables the study of diurnal, inter-annual, and intra-annual variability of SUHI at various spatial scales. The standard MODIS LST data were initially released in 2000, while MODIS-based SUHI research was first published in English in 2004 [60]. [PERSON] [61] provided the first high-resolution (\(\sim\)90 m) study of SUHI during the day and night using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data in 2005, even though the standard ASTER LST products has been available since 2001. The entire archive of ASTER data became available free of charge in 2016. Details of these data are reviewed in Section 3.  
SUHI research has also been greatly advanced by a suite of breakthroughs in scientific knowledge. For example, [PERSON] et al. [62] provided the first full discussion of the applications and limitations of thermal remote sensing in urban climate research. The metrological questions that they raised were then systematically reviewed by [PERSON] and [PERSON] [29]. These two efforts provided critical overviews of the theoretic basis for studying SUHI. [PERSON] et al. [63] explored the relationship between SUHI and NDVI for the first time, which still constitutes one of the major indicators to measure the LST-vegetation relationship. [PERSON] et al. [64] introduced the vegetation fraction, which is an alternative indicator of vegetation to SUHI research. They found that the vegetation fraction possessed a slightly stronger relationship with LST than NDVI in urban areas. [PERSON] et al. [65] published the first study of the impacts of global urban areas on climate. [PERSON] and [PERSON] [66] explored the role of an impervious surface area (ISA) in influencing SUHI patterns for the first time, and this soon became one of the most widely studied land surface properties in SUHI research. Two years later, the landscape ecology methods were first applied in SUHI research [67]. [PERSON] et al. [68] provided the first comprehensive comparison analysis of indicators for quantifying the SUHI intensity and emphasized the large uncertainties associated with the quantification methods. Recently, [PERSON] et al. [69] made the first effort to evaluate the impact of directional anisotropy on the derived SUHI. They demonstrated that anisotropic effects can modify the SUHI by 25% to 50%. All of these works have promoted rapid progress in SUHI research.