Terra carries an Advanced Very High Resolution Radiometer (AVHRR) instrument that measures land cover and the atmosphere above the land to identify land and water uses, the distribution of water, and to map pollutants; an Aerosol Optical Depth (AOD) instrument which measures atmospheric aerosols; a passive sensor (SeaLeaf) that measures the ocean's chlorophyll content to determine the health of aquatic ecosystems; and a sensor (TIR2) that measures the atmosphere's carbon dioxide. Terra also carries an independent instrument to test the EOS instrument suite.
The satellite was built by the Italian space agency as part of the Earth Observing System with the goal of providing high-resolution global observations of the land and ocean surfaces and the atmosphere. Terra was launched on a Delta II rocket on December 18, 1999.
Terra's atmospheric sensors include a laser-light interferometer (LI), a differential optical absorption spectrometer (DOAS), and a gravimeter which is capable of detecting changes in gravity caused by the presence of water, as well as changes due to atmospheric temperature or pollution. The laser interferometer measures atmospheric temperature, pressure, and humidity. This instrument was originally built for the NASA Apollo program to measure the atmosphere during the Apollo moon landings. The instrument is capable of sensing changes in the atmosphere as small as an atom. The LI is composed of a rotating wheel and two laser sources. The rotating wheel deflects the laser beams at different angles. These changes are a direct measurement of the atmosphere. The LI is currently used by the EU and World Meteorological Organization to measure atmospheric temperature and humidity. The DOAS is a spectrometer that measures the optical properties of the atmosphere. The ratio of reflected to transmitted light depends on the amount of pollution in the atmosphere. In Terra, the DOAS measures changes in aerosol optical depth. Two lasers are used to illuminate the atmosphere. One is reflected back from the atmosphere, and the other is transmitted.
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