## 3 Potential of Earth Observation (EO) for Supporting SDG Indicators  
The era of observing the Earth through satellites began in 1957 with the launch of the first satellites (Sputnik-1 and Sputnik-2) by the Soviet Union followed soon after (1958 to 1959) by NASA launching the Explorer satellite series. While images of the Earth had been captured by sub-orbital craft in the 1940s, the first orbital satellite image of the Earth was captured by Explorer 6 in 1959 [37]. Technology progressed rapidly and, in 1972, Landsat 1 provided the first images with a resolution of 80 m, followed by the European Space Agency (ESA) that has observed the Earth from space since the launch of its first meteorological mission in 1977 and later by European Remote Sensing (ERS) and Envisatmissions satellites [38]. Today the main space agencies provide high-resolution satellite data at no cost to the user, including Landsat from NASA with 30 m and ESA Sentinel 2 with up to 10 m resolutions. There are currently 260 operational EO satellites (having full functionality), 333 non-operational (usually historical satellites that have stopped sending data for several reasons) and approximately 200 satellites are in development (this includes satellites that have been approved and planned to be launched). However, it needs to be noted here that a single satellite may carry several instruments (sensors) that provide EO data. A list of important current satellites, non-operational satellites and satellites in development (along with their sensors) is given in Supplementary Material Table S1 (which contains a list of 300 satellites, where 162 are operational, 116 non-operational, and 22 in development), based on data provided in three sources: WMO-Oscar [39], EO satellite portal [40] and CEOS [41]. The list covers the satellites' characteristics (e.g., date and lifetime, type of instrument, spectral coverage), capabilities (temporal and spatial resolution, primary mission, imaging capability, data accessibility), and main applications. The satellites and instruments in Table S1 have been selected based on three criteria:  
* they have been or are used in a wide range of programmes in sustainable development by organisations such as GEO, ESA, NASA, JAXA, etc. * the instruments on board the satellites have provided data for more than 6 consecutive months
* the instruments on the satellites can offer time-series data from a series of similar satellites (e.g., Landsat, Sentinel, NOAA, GOES, METEOSAT, HY). EO satellites can be passive (measuring the light reflected from or the thermal energy emitted by the Earth) or active (measuring the interaction of energy emitted by the satellite with the Earth, e.g., through radar). Passive EO satellite sensors provide data at various spectral, spatial, and temporal resolutions. The spectral resolution refers to the wavelengths of radiation that can be detected by the instrument and, for example, in the visible and shortwave infrared regions, the information in different spectral bands can provide a spectral'signature' for different land cover types (e.g., vegetation, soil, water, buildings). Spatial resolution (typically reported as the number of metres covered by one dimension of a single pixel) is a measure of the observable detail in an image, and modern optical sensors provide multispectral and panchromatic (PAN) imagery at much finer spatial resolutions than seen in previous decades [42] (although the chosen resolution for a modern mission is a compromise between resolution and coverage.) Different space agencies, companies, and other organizations categorise the spatial resolution of satellite images differently; thus, 'high resolution' can have a variety of meanings. In general, the categories used for images are classified as shown in Table 3. Satellite data can also be categorised by the type of orbit the satellite is in, whether it is in a high geostationary orbit (so it always observes the same locations on Earth) or in a low Earth orbit (so it tracks the surface as it orbits).