Planet Earth Online

Eyes in the sky

18 May 2012

But for every dollar spent, satellites provide invaluable information we could never gather from the ground. Bristling with sophisticated sensors, they monitor vast swathes of Earth and its atmosphere, repeating their measurements every orbit. This gives scientists long-term and almost real-time information about changes to our planet, which they once had to estimate from a handful of terrestrial instrument stations.

Here are just a few examples of satellite applications from the last few years:

• The same Global Positional System (GPS) signals used in your sat nav can measure movements of the Earth's crust as slow as a millimetre a year. This reveals tiny deformations and patterns of strain which can indicate where disaster may strike next - information that could save thousands of lives around the world.

• The GOCE satellite - Gravity field and Ocean Circulation Explorer - measures variations in Earth's gravity which, until now, we've only known about in general terms. GOCE has already enabled scientists to map gravity more precisely than was ever possible before, and will carry on collecting data until 2013.

• The Soil Moisture and Ocean Salinity satellite, SMOS, is transforming our understanding of how seawater's saltiness varies in different parts of the oceans, and of the varying degrees of moisture in soils all over the world.

• GPS has a range of other environmental science applications too - everything from tracking migrating animals to monitoring long-term changes in the topography of Antarctic ice.

• Infra-red imaging has revealed pyramids and settlements that have been buried for millennia under Egyptian sands. Because the buildings are denser than the overlying sediments they absorb heat differently from their surroundings and can be detected from space even if there's no sign of them on the ground.

• Scientists are using satellite-based LIDAR (light detection and ranging) to investigate everything from weather processes to the intricacies of rainforest canopies. Just as RADAR uses radio waves, LIDAR senses its target's distance or speed from how long it takes for a laser to be reflected back to its receiver.

• Cryosat 2 uses pulses of microwave energy to obtain detailed measurements of Arctic and Antarctic ice - vital to our understanding of the relationship between climate change and the poles. The satellite is also shedding new light on patterns of ocean circulation and sea-level rise.

• When the sun's energy reflects off the Earth, some of it is absorbed by atmospheric gases like carbon dioxide and methane. Different gases absorb energy at different wavelengths, so researchers can monitor their levels using satellites to detect the different wavelengths of radiation that make it back out to space. This is giving us a more sophisticated understanding of the carbon cycle and how human activity is affecting it.

Keywords: Climate system, Earth observation, Environmental change, Pollution, Satellites, Space, Technology,

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