Envisat carries an array of ten earth-observation instruments that gather information about the earth (land, water, ice, and atmosphere) using a variety of measurement principles.
Several of the instruments are advanced versions of instruments that were flown on the earlier ERS-1 and ERS-2 missions. Examples are MERIS, that produces 2-dimensional images using a 12-band spectrometer, and ASAR, a radar-based system that can detect changes in surface heights with sub-millimeter precision.
Other instruments are using novel ways to observe the Earth's atmosphere. For example, SCIAMACHY compares light coming from the sun to light reflected by the Earth, which provides information on the atmosphere through which the earth-reflected light has passed.
The GOMOS instrument looks to stars as they descend through the Earth's atmosphere and change color, which also tells a lot about the presence of gases such as O3 (ozone), and allows for the first time a space-based measurement of the vertical distribution of these trace gases.