National Aeronautics and Space Administration

Earth Observing System 

The Earth Observing System (EOS) is a program of space-based instruments that observe various land, ocean, and atmospheric conditions, usually over a number of years. EOS includes eleven missions - Terra, Aqua, Aura, Landsat-7, Jason-1, ICEsat, ACRIMSAT, Seawinds/ADEOS II, SORCE, SAGE III, QuikSCAT - and uses interdisciplinary science investigations to develop a comprehensive understanding of how the Earth functions as a unified system. 

"The overall goal of the Earth Observing System is to advance the understanding of the entire Earth system on a global scale by improving our knowledge of the components of the system, the interactions between them, and how the Earth system is changing. The EOS data will be used to study the atmosphere, oceans, cryosphere, biosphere, land surface and solid earth, particularly as their relationships are manifested in the flow of energy and in the cycling of water and other chemicals through the Earth system." 

Terra, Aqua, and Aura are the three largest elements of EOS: 

Terra, launched December 1999, is one of the three major EOS spacecraft and the most important launched to date. Terra carries five instruments: 

• Clouds and the Earth's Radiant Energy System (CERES), which "measures the Earth's total radiation budget and provides cloud property estimates that enable scientists to assess clouds' roles in radiative fluxes from the surface to the top of the atmosphere."  
• Moderate-resolution Imaging Spectroradiometer (MODIS), which views "every point on our world every 1-2 days in 36 discrete spectral bands." MODIS, together with MISR and CERES, will study the impact of clouds and aerosols on the Earth's energy budget.  MODIS can also "measure the photosynthetic activity of land and marine plants (phytoplankton) to yield better estimates of how much of the greenhouse gas is being absorbed and used in plant productivity. Coupled with the sensor's surface temperature measurements, MODIS' measurements of the biosphere are helping scientists track the sources and sinks of carbon dioxide in response to climate changes."  
• Multi-angle Imaging Spectro-Radiometer (MISR), which observes the Earth through cameras pointed at nine different angles and using each of four wavelengths (blue, green, red, and near-infrared). MISR studies scattering of sunlight, atmospheric aerosol particles, the amount, types, and heights of clouds; and the distribution of land surface cover, including vegetation canopy structure.  
• Japanese Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), which "obtains high-resolution images of the Earth in 14 different wavelengths of the electromagnetic spectrum, ranging from visible to thermal infrared light. Scientists use ASTER data to create detailed maps of land surface temperature, emissivity, reflectance, and elevation."  
• Canadian Measurements of Pollution in the Troposphere (MOPITT), which is "designed to enhance our knowledge of the lower atmosphere and to particularly observe how it interacts with the land and ocean biospheres. Its specific focus is on the distribution, transport, sources, and sinks of carbon monoxide and methane in the troposphere."

Aqua has a mission that is related to Terra and carries two instruments also carried by Terra.  Aqua studies atmospheric temperatures and humidity profiles, clouds, precipitation, and radiative balance; terrestrial snow and sea ice; sea-surface temperature and ocean productivity; soil moisture; and the improvement of numerical weather prediction.  Aqua's instruments focus on cloud formation, precipitation, and radiative properties.  Aqua carries six major instruments:

• Clouds and the Earth's Radiant Energy System (CERES) - see Terra above.  
• Moderate Resolution Imaging Spectroradiometer (MODIS) - see Terra above  
• Atmospheric Infrared Sounder (AIRS), which makes highly accurate measurements of air temperature, humidity, clouds, and surface temperature  
• Advanced Microwave Scanning Radiometer-EOS (AMSR-E), a Japanese instrument which measures precipitation rate, water vapor content, and surface moisture content.  
• Humidity Sounder for Brazil (HSB), a Brazilian instrument which, used in conjunction with the AIRS data, provides humidity profile corrections in the presence of clouds.  
• Advanced Microwave Sounding Unit (AMSU), a NOAA instrument, operates in conjunction with the AIRS and HSB instruments to provide atmospheric temperature and water vapor data both in cloudy and cloud-free areas.

Aura, with a planned launch in June 2003, will carry a "suite of scientific instruments designed to make the most comprehensive measurements ever undertaken of atmospheric trace gases. Its objective is to study the chemistry and dynamics of the Earth's atmosphere with emphasis on the upper troposphere and lower stratosphere (0-30km). The mission will measure ozone, aerosols, and several key atmospheric constituents that play an important role in atmospheric chemistry, air quality, and climate." Aura will carry four instruments:

• Tropospheric Emission Spectrometer  
• Microwave Limb Sounder, which will measure the stratospheric temperature and various chemical species  
• High-Resolution Dynamics Limb Sounder, a joint US/UK instrument that will "sound the upper troposphere, stratosphere and mesosphere"  
• Ozone Monitoring Instrument, a joint Dutch/Finnish instrument that will provide global mapping of ozone, trace gasses and aerosols

Special Spacecraft "will be designed to study atmospheric aerosols, ocean circulation, ice-sheet mass balance, cloud physics, atmospheric radiation properties, and solar irradiance." 

The Stratospheric Aerosol and Gas Experiment (SAGE III) measures aerosol and gaseous constituents of the atmosphere.  Three copies of the instrument were built:  one SAGE III instrument is mounted on the Meteor-3M spacecraft, a second will be placed on the International Space Station in 2005, and a third instrument "has been built and is awaiting identification of an appropriate flight opportunity."

The Japanese will provide the Advanced Earth Observing System II (ADEOS II) spacecraft for the Seawinds instrument to measure ocean surface wind velocity as a follow-up to the NASA Scatterometer instrument on ADEOS-I and the Seawinds instrument on QuikScat. 

The Radar Altimetry mission, Jason-I , was launched in December 2001 as a follow-on to the TOPEX/Poseidon.  The Jason-I mission is a collaboration between NASA and the French Space Agency (CNES), with data provided to NOAA.  Jason-I "monitors global climate interactions between the sea and the atmosphere. Initial maps of sea-level anomalies, significant wave height and ocean wind speed confirm the health of the spacecraft science instruments and the mission's ability to meet its data turnaround requirements." 

Ice, Cloud, and Land Elevation Satellite (ICESat) provides cloud property information not otherwise available from passive sensors, especially the high ice clouds common over polar areas. It provides a land-topography data set by processing the altimeter data throughout its orbit, in addition to the measuring ice sheet height and volume for long-term climate variability studies. 

Active Cavity Radiometer Irradiance Monitors (ACRIMSAT), which was launched in December 1999, measures total solar irradiance, continuing the measurement of Total Solar Irradiance (TSI) begun by the ACRIM instruments on the Solar Maximum Mission and UARS. 

The Total Solar Irradiance Mission (TSIM) was combined with the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) Mission to form the Solar Radiation and Climate Experiment (SORCE) Mission. SORCE will accomplish all the original science objectives of both TSIM and SOLSTICE, including those requirements defined by the National Polar-orbiting Operational Environment Satellite (NPOESS). The SORCE spacecraft was launched in January 2003.

QuikScat -Quick  Scatterometer was launched June 1999 to acquire all-weather, high-resolution measurements of near-surface winds over global oceans, determine atmospheric forcing, ocean response, and air-sea interaction mechanisms on various spatial and temporal scales, combine wind data with measurements from scientific instruments in other disciplines to help us better understand the mechanisms of global climate change and weather patterns, study both annual and semi-annual rain forest vegetation changes, and study daily/seasonal sea ice edge movement and Arctic/Antarctic ice pack changes. 

Landsat-7 was launched April 1999 to "acquire remotely sensed images of the Earth's land surface and surrounding coastal regions." This satellite provides high spatial resolution images and measurements of land surface and surrounding coastal regions with data to be used for global change