Topic 1a - What is Earth Observation?
Welcome to the first topic of the course. For each new topic you should watch the main video (above), read the explanatory text (below), and you can find links to optional further reading at the external websites in the ‘See Also’ section at the bottom of the page. You can also find more information about the data, imagery, animations and satellite missions featured in each topic in the step that follows each video.
Earth observation (EO) is a collective term for the gathering of data and information about our planet’s physical, chemical and biological systems through the process of ‘remote sensing’. Earth observation provides a way of monitoring the status of these systems and checking on their health. In essence, this is done by sensing and recording reflected or emitted electromagnetic radiation which is then processed, analysed, and utilised across a wide range of applications.
In this video, Professor Martin Wooster will introduce you to some of the basic principles of Earth observation and why viewing our planet from space is so important. He will explain some of the key remote sensing methods and techniques, and will illustrate how aspects of the entire Earth system are only possible to observe using EO technology.
A brief overview of Earth Observation and Remote Sensing
Remote sensing is the measurement of electromagnetic radiation (EMR) emitted or reflected by the Earth. This includes measurements made at the wavelengths of visible light, (for example from cameras on satellites, the International Space Station or even astronaut photography), but also those made at shorter wavelengths such as in the ultraviolet, or longer wavelengths such as in the near infrared, thermal infrared and microwave regions of the electromagnetic spectrum.
Depending on the exact wavelength and nature of the electromagnetic radiation (EMR) measurements, and the type of technology employed to make them, EO can provide information related to the Earth’s atmosphere, land surface, rocks, soils, vegetation, its rivers, lakes and oceans, and on places where water is frozen into ice or snow (the cryosphere). It can even provide information related to processes happening below the surface, such as changing levels of ground-water or inside volcanoes.
A mix of mathematical equations, computer based algorithms and human interpretation is used to convert these measurements of reflected and emitted EMR into the desired information about the Earth system. For example, when using measurements of reflected sunlight over land, we can understand the distribution and health of plants in croplands or forests. Or when measuring sunlight over the ocean, we can look at the abundance of micro-algae (phytoplankton). When making measurements at much longer thermal infrared wavelengths, we can map the changing temperature of the land and ocean surface, or the distribution of water vapour and other gases in the atmosphere.
All of the measurements mentioned in the paragraph above generally rely on so-called ‘passive’ remote sensing techniques, meaning they use naturally occurring EMR signals. Engineers and scientists have also developed various types of ‘active’ remote sensing instruments, which act as EMR sources as well as receivers. The most commonly used ‘active’ technique is microwave radar, the signals of which can reach the Earth’s surface even through thick cloud cover. This technique can be used for example to measure the height of the Earth’s ocean surface when deployed as a ‘radar altimeter’.
Remote sensing from Earth observation satellites has become an increasingly dominant and extremely important aspect of monitoring the Earth, and has greatly aided our understanding of the interconnected nature of its numerous environmental and climatic systems and processes. EO satellites can be placed in orbits relatively close to the Earth, (e.g. at a height of less than 1000 km), or much further away including those in ‘geostationary orbits’ (where the satellite remains at an apparent fixed point relatively to the rotation of the Earth) 36,000 km away from the Earth. The EMR signals measured by remote sensing instruments on satellites generally have to pass through the entire depth of Earth’s atmosphere, so ‘atmospheric effects’ also have to be understood and taken into account in the design of these instruments and the data processing techniques.
The information collected by EO satellite missions is often used in computer models, with probably the most common example being that of weather forecasting, which exploits enormous amounts of satellite data alongside information from meteorological stations, aircraft and balloons. Other models that exploit EO data help us to understand key changes in the Earth system we see today, for example in the covering of Arctic sea ice, the changing size and shape of the Antarctic ozone hole, and changing rates of deforestation. This allows us to so better forecast future changes and factor these more effectively into projections of global climate and environmental change.
The imagery and data provided by EO satellites over almost half a century have been the cornerstone of work to better predict climate change, and the data they are collecting today and into the future are vital for our understanding of the Earth’s interconnected systems in ever more detail. These techniques allow us to monitor the Earth’s health and keep watch for key trends and abrupt or unexpected changes, which in turn allow for better informed policy and decision-making.
- Professor Martin Wooster
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Don’t forget you can download the video and transcript with the links on the right.