## 2 Landsat Overview  
### Landsat Legacy  
The concept of space-based land cover monitoring was developed as early as the 1960s by NASA and USGS when space-borne observation platforms became available [42]. The launch of Landsat-1, also known as the Earth Resources Technology Satellite (ESTS), on 23 July 1972 represented a new wave of opportunity for the remote sensing community [43] with global data acquisition ambitions. Currently, it is the longest-running uninterrupted Earth imaging program (Figure 1). Based upon this heritage Landsat has become a critical reference for scientific, commercial, and governmental projects, providing land use information to environmental decision-makers and international agreements at both local and global scales [23, 44]. The first near polar-orbiting EO satellite is Landsat-1 [45], which carried a Return Beam Vidicon (RBV) and Multispectral Scanner System (MSS) with ~60 m spatial resolution, a 6-bit quantization system, and 4 bands. Later operating alongside Landsat-1 and sharing the same design, Landsat-2 was implemented to achieve complete global coverage and capture periodic images. Landsat-3 carried some sensor improvements, including redesigning the RBV system to use side-by-side mounted cameras with 40 m ground resolution panchromatic spectral response and adding a thermal band, though it did not perform well at the time. Landsat-4 was the first NASA satellite with a global positioning system (GPS) instrument and carried some improvements in spacecraft engineering, such as solar arrays, greater altitude, and orbital control and adjustment. Landsat-4 launched with the Thematic Mapper (TM) sensor carrying 7 bands, 8-bit quantized data, and a 30 m spatial resolution. Landsat-4 marked the beginning of the collection of thermal infrared data with 120 m spatial resolution by the Landsat mission. As seen in Figure 1, Landsat-5, which was originally designed for only three years of operation, had the longest individual operating satellite lifetime, covering the failure of the proceeding Landsat-6, which did not achieve orbit. Next, Landsat-7 launched with the Enhanced Thematic Mapper (ETM+) sensor, an improved version of the previous TM, offering 15 m panchromatic and 60 m thermal infrared spatial resolution bands. Unfortunately, the Landsat-7 Scan Line Corrector (SLC) failure occurred in June 2003, and since then, the data delivered from this sensor contained gaps. Landsat-8 was launched on 11 February 2013. By this time, high spatial resolution sensors were developed; however, Landsat-8 data still has the 30 m spatial resolution to maintain data continuity and consistency with its predecessors. Landsat-8 carries the Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) with improvements in quantization (12-bit) and it also has some additional bands (i.e., coastal aerosol and cirrus). Despite the increasing number of EO satellites, including high-resolution commercial satellites, CubeSats, national programs, and satellite constellations, the Landsat program continues to play a key role in applications of optical imagery, acting as a reference point that is considered \"_not replaceable_\" [9]. This is because there is no other science-grade, long-term uninterrupted EO program, which provides a free data policy, global coverage, radiometric consistency between sensors with appropriate spatial and temporal resolutions for resource and environment monitoring. Today, remote sensing scientific communities, commercial programs, and governments rely on the consistency and availability of Landsat data [46]. The European Space Agency (ESA) launched the Sentinel-2 constellation with a multispectral sensor in 2014 and 2017 as a part of the European Union's Copernicus EO program, providing spatial resolution of 10 m (visible and near-infrared bands), 20 m (red-edge and shortwave infrared bands) and 60 m (atmospheric correction bands), a five-day temporal resolution from two-satellite constellation (at equator), and free access policies [47].