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Remote sensing

View from space. Image credit: NASA

Remote Sensing (RS) allows us to collect information about the environment without making physical contact with it. This is usually by using sensors placed on satellites in space. These sensors are pointed down at the earth and are set to record reflected light and electromagnetic energy given off by the earth's surface and by vegetation. Once beamed back to satellite receiving stations, this information can be recorded, measured and interpreted, often by being placed alongside other datasets in a geographical information system (GIS). Remote sensing offers a range of important tools for land managers trying to understand changes on the earth and for decision makers planning the best way to manage natural resources.

In Space

Images taken of the earth from space can help soil and other environmental scientists understand the world and how it is changing There are many satellites put into space every year. Many of these have special sensors for gathering reflected light or electromagnetic energy. These sensors work a bit like a digital camera, in that a series of lenses direct the light or energy onto a sensor which makes the conversion into a digital dataset. This dataset can then be transmitted by radio down to a receiving station on earth. At the receiving station, the dataset can be converted into an image for use in software packages like Geographical Information Systems (GIS). Many of the images require processing to make them understandable, as well as comparable with other datasets. Images have a resolution, again similar to digital cameras, that determine what size a pixel relates to on the earth's surface (for instance a 30m pixel represents a block 30m by 30m on the ground). Sensors can detect light at a range of different wavelengths (such as green, red, infra-red etc.) Some sensors can detect multiple wavelengths of light - these are termed 'multi-spectral' sensors.

Landsat MSS imagery processed to produce the Soil Brightness Index (SBI). SBI is used to show variations in the soil Satellites are placed into space by rockets or the space shuttle. Once in space they take up their position. There are two typical ways satellites travel once in space:

  • Geostationary Here the satellite travels above a particular position on the earth's surface, travelling through space to keep to this position as the earth rotates. The satellite always looks down on the same position.
  • Orbiting Here the satellite travels around the axis of the earth, passing near to the northern and southern polar regions. With the earth constantly rotating below, this means the satellite can view the entire earth's surface over a given period of time.

There are many types of remote sensing satellites in space. Here are some of the more important ones used by soil and land-resource scientists.

MeteosatUsed for collecting meteorological data, analysing weather patterns and measuring atmospheric conditions. More informationGeostationary orbiting 35,900km above the equator
NOAAUsed for collecting meteorological data such as water vapour, cloud patterns and temperature. More informationNear-polar orbit 850km high
LandsatAThe frequently used Landsat series of satellites, the first being launched in 1972. Landsat's 'Thematic Mapper' (TM) measures light at seven wavelengths at up to a 30m resolution. Used for imaging the environment. More informationNear-polar orbit 705km high
SPOTA series of French satellites, capable of high resolution images up to 10m pixel size, measuring light over several wavelengths. More informationNear-polar orbit 832km high
IRSIndian Remote Sensing satellites, first launched in 1995, with a range of high resolution sensors at multiple wavelengths. More informationNear-polar orbit 817km high
TERRA / ASTERA NASA satellite, launched in 1999, designed to explore climate change. Measuring fourteen wavelengths ASTER can provide stereo imagery. More informationNear-polar orbit 705km high
TERRA / MODISMeasuring 36 wavelength bands, MODIS (Moderate Resolution Imaging Spectrophotometer) was designed to understand climate changes, for instance through daily cloud cover variations. Covering the entire earth every 1-2 days with a resolution of 250m to 1km depending on the sensor used. More informationNear-polar orbit 705km high
IKONOSA very high resolution French satellite, launched in 1999 capturing four channels of multi-spectral data at between 4 and 1m pixel resolution. 1m imagery from space! More informationNear-polar orbit 681km high
QuickBirdA very high resolution satellite launched in 2001 offering up to 61cm pixel resolution images. More informationNear-polar orbit 450km high