Earth observation satellites are by far the most impactful space technology for climate action. Earth observation satellites use remote sensing capabilities (long-distance detection of the physical characteristics of a system through radiation emission and reflection) to capture images of the Earth or gather other types of data about the atmosphere or about Earth’s surface. As of 2022, there are about 172 satellites currently being used by the Committee on Earth Observation Satellites (CEOS) as part of Earth observation programs.
Most Earth observation satellites operate from low Earth orbit (LEO), at about 500 km of altitude, where they are close to Earth’s surface relative to other orbits. This is beneficial in terms of distance between the sensors and their target, and in terms of mission cost and complexity, at the expense of global coverage and lifetime. They are sometimes operated as part of satellite constellations and often positioned in polar orbits to iteratively provide global imagery as Earth rotates below them, despite their narrow instantaneous coverage. Many Earth observation satellites – weather satellites especially – are also launched to geostationary orbit (GEO), at about 36,000 km of altitude, for their constant positioning relative to Earth’s surface and their very large coverage.
The performance of these sensing capabilities is typically characterized by the spatial resolution (“pixel size”) obtained, the spectral resolution (the specific wavelengths of electromagnetic radiations captured) and the temporal resolution (how often a location is imaged). Spatial resolutions are typically from below a meter to a few dozens of meters. Spectral resolution can be very diverse depending on the objective, but many satellites cover the visible spectrum (between 400 and 800 nm). Temporal resolutions are also very diverse, but often in the order of magnitude of a few days.
Through their sensors, Earth observation satellites can capture a lot of valuable data: temperatures, atmospheric pressure, ocean and land color changes, greenhouse gases and aerosol concentrations, ocean currents, fires, hurricanes, storms, etc. Applications of these measurements contributing to climate action include: climate monitoring and modeling, tracking sources of pollution, monitoring biodiversity levels, monitoring oceans, fires, floods, agriculture, land-use, etc.
Example of a wildfire near Salamanca, Spain, captured by Sentinel-2. Credits: ESA