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| Satellites orbited on the atmosphere of the Earth |
Atmosphere. NASA launched eight Nimbus Earth observation satellites between 1964 and 1978. They pioneered the use of ‘sounders’ that measure the humidity and temperature of the atmosphere. They obtain temperature measurements by analysing infrared radiation (IR) on wavelengths linked with oxygen and carbon dioxide. IR or microwave sounders identify water vapour in the atmosphere to measure the humidity. Microwave sounders have a lower resolution, but those can be used in all weather conditions as they can sound through clouds.
Oceans. In the 1970s, the USA and Soviet Union issued ocean observation satellite programmes, which carried synthetic aperture radar (SAR) equipment. A number of radar images are taken by SARs and combined to produce a single detailed image. This is able to determine the height of sea levels, waves, currents, and their distribution and it can detect oil slicks and shipping movements. The Jason 1 and 2 spacecraft currently use these techniques to study the topography and characteristics of the oceans, to give a better warning of floods or climate changes.
Radiation. Visible red, green and blue light only provides a limited amount of information about the Earth’s surface, so satellites use spectrometers to study the invisible near-infrared and infrared parts of the electromagnetic spectrum.
They can identify and track the growth of plant species, as they all reflect infrared light. The infrared ‘fingerprint’ of plants can also indicate the amount of water present and can warn of potential droughts. Likewise, exposed rocks radiate their own infrared fingerprint that allows geologists to identify valuable mineral/ oil deposits.
Infrared data from satellites is ‘false coloured’, so invisible light from up to three wavelengths is rendered into a combination of visible red, green and blue.
Land. The Shuttle Radar Topography Mission (SRTM) by the Endeavour space shuttle in February 2000 used two radar antennas to produce the most comprehensive high-resolution digital topographical map of the Earth’s terrain. This data is used by Google Earth to create maps that can be viewed in 2D or 3D.
The Earth observation satellites are important in monitoring the seasonal variation of vegetation. Besides studying long-term changes, they are also used to observe and issue warnings of natural disasters such as volcanic eruptions, forest fires and earthquakes.
Ice. Carrying of from the work of Envisat, which discovered that every decade since 1978; the Arctic ice fields have shrunk by 2.7%, the European Space Agency (ESA) launched CryoSat-2 on 8 April 2010. It uses radar altimeters with SAR technology, specifically designed for its mission to study the thickness and distribution of ice in the polar oceans. NASA’s ICESat (2004) carried a Geoscience Laser Altimeter System (GLAS), which used pulses of laser light to measure the height and characteristics of Greenland and Antarctic ice fields. These satellites have indicated the role of greenhouse gases in the polar atmosphere and that the ozone layer has shown signs of recovery.
Gravity. The European gravity field and steady-state ocean circulation explorer (GOCE) launched in March 2009, carries an Electrostatic Gravity Gradiometer (EGG) to measure the gravity field of Earth. By measuring the minute variations in the tug of gravity, it enables the production of Geoid maps of the globe that can indicate ocean circulation and changes, the movement and composition of polar ice sheets and the physics of the Earth’s interior.
In March 2002, NASA launched two Gravity Recovery And Climate Experiment (GRACE) spacecraft. They use a microwave system that accurately measures any minute changes between their speed and distance, indicating the influence of the Earth’s gravitational pull.