Earth Observation (for review)

Anything that relates to measuring properties of the surface of Earth. Whether the measurements are taken on the ground, by satellites, or other forms of remote sensing, all are considered Earth Observation. 

A satellite is a "moon, planet or machine that orbits a planet or sun", so technically Earth is a satellite. But when we talk about satellites in Earth observation, we mean a machine that orbits Earth and communicates information from it's sensors back to Earth. 

How EO satellites work depends on the wavelength of the property being measured and therefore the types of sensors used.

Radar is an active sensor in that it utilises a human or machine created signal. They send a microwave to earth and measure how much and in what way it is reflected back at each point of the earth, creating a greyscale image regardless of weather or darkness. The reflection is affected by the roughness of a surface, the kind of material, and its moisture content, allowing us to detect oil spills, observe currents in shallow waters, measure ocean waves, map land and ocean floor topography, monitor ice caps and flooding, and examine deforestation and crop growth. 

Optical sensors don't rely on the reflection of a signal they've sent, instead they rely on the many waves already present in our atmosphere. Some optical satellites measure waves of visual light from the sun, operating just like our eyes- by measuring the amount of waves detected from the wavelength each colour transmits. They provide true colour images just as we would see things if we had that vantage point, but also rely on sunlight and low-cloud coverage to see things. 

Other optical sensors can see light that is invisible to us, for example infrared which represents heat. Sensors that detect infrared waves can detect heat in any circumstance, independent of sunlight and low cloud cover, as they measure light with a different wavelength of the electromagnetic spectrum that sunlight does not affect. Infrared data can produce images with heat mapped using a colour scale.  

The information gained from satellite technology is often spot-checked with information from earth-based sensors and imagery to verify accuracy. Satellites are also assigned an accuracy level based on how they calibrate information and assign the data they receive to specific points on earth, so we can investigate the data with full awareness of its limitations and an understanding of potential causes of data anomalies. 

Satellites provide valuable information about the sea floor, shallow waters and the sea's surface. Satellites using the visible spectrum can display algal blooms, infrared satellites can measure sea surface temperature, and radars can detect large oil spills, measure the height and shape of points on the sea floor, the height, shape, and direction of waves, and can display changes in the height and spread of water bodies and ice. 

While satellites can't take an image below the surface of the ground, they can be used to build a picture of what's there by measuring phenomena above ground. By using infrared sensors and focussing on a mineral's signature wavelength,  the presence of that mineral can be detected by measuring which wavelengths are reflected and those that are not as an indication of the presence of those minerals.

Astrophysicists can study the effect a location has on a satellite's measurements to provide information on what's under the ground at that location by using data models. 

Satellite data can be provided in numerous forms. Images can be produced from information on a single wavelength or from multiple wavelengths layered to create a fuller picture. These images can then be presented as stand alone images or they can be added to a Geographical Information System (GIS) where they are associated with points on Earth and viewed in context, often with similar images of other areas composited together to create a global view. 

How much detail a satellite can see is measured in land area per pixel and depends on a few factors: which wavelengths it's measuring, the technology of the satellite and it's producers, and any image processing techniques used to improve clarity. 

The freely available satellite with the best resolution is the European Space Agency's Sentinel-2, which currently has the best resolution for free satellite imagery, offers 10-meter resolution in red, green, blue and near-infrared, but has other 20m and 60m resolutions for other bands.

Commercial satellites can offer higher resolutions of up to 30cm, and more detailed images can be displayed via image processing techniques.