Terra (EOS AM-1) is a multi-national NASA scientific research satellite in a sun-synchronous orbit around the Earth. It is the flagship of the Earth Observing System (EOS).   This is a multi-national, multi-disciplinary mission involving partnerships with the aerospace agencies of Canada and Japan. Managed by NASA’s Goddard Space Flight Center, the mission also receives key contributions from the Jet Propulsion Laboratory and Langley Research Center. Terra is an important part of NASA’s Science Mission, helping in a better understand and protection of earth’s environments.

TERRA was launched from Vandenberg Air Force Base on December 18, 1999 and began collecting data on February 24, 2000.  It operates in a polar sun-synchronous orbit at 705 km above the Earth's surface, crossing the equator on descending passes at 10:30 AM, when daily cloud cover is typically at a minimum over land. Because of this morning equatorial crossing time, "Terra" (a mythical name for "Mother Earth") was originally named EOS-AM-1. Terra has a repeat cycle of 16 days. It is roughly the size of a small school bus.

Terra carries a payload of five remote sensors designed to monitor the state of Earth's environment and ongoing changes in its climate system:

·     ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer)

·     CERES (Clouds and the Earth's Radiant Energy System)

·     MISR (Multi-angle Imaging SpectroRadiometer)

·     MODIS (Moderate-resolution Imaging Spectroradiometer)

·     MOPITT (Measurements of Pollution in the Troposphere)

In this article, the ASTER instrument is discussed in some detail.

ASTER  The Advanced Spaceborne Thermal Emission and Reflection Radiometer, is designed for multispectral remote sensing of the earth’s surface and obtains high-resolution (15 to 90 square meters per pixel) 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. The instrument was provided for Terra by the Japanese Ministry of International Trade and Industry.   It is designed to bridge the gap between field observations and data from the MODIS and MISR instruments.

As its name implies, ASTER operates in the visible through thermal infrared portions of the electromagnetic spectrum. Of its 14 bands, three are in the visible and near-infrared (VNIR) between 05.-0.9 µm, six are in the short-wave infrared (SWIR) between 1.6-2.43 µm, and five are in the thermal infrared (TIR) between 8-12 µm. VNIR channels have 15-m resolution, SWIR have 30-m resolution, and TIR channels have 90-m resolution. ASTER has a 60-km swath width, with a cross-track adjustable swath center.

A special feature of ASTER is an aft pointing additional VNIR telescope for creating stereo views: these images have a base-to-height ratio of 0.6.

ASTER has the following existing and planned data products: (1) unprocessed instrument data, (2) registered radiance at sensor, (3) decorrelation stretch, (4) brightness temperature, (5) surface reflectance, (6) surface radiance, (7) surface emissivity, (8) surface kinetic temperature, (9) polar surface and cloud classification, and (10) digital elevation models (DEMs). The start date of data acquisition for input to these products is either March 8 or 13 of 2000 depending on the product. All products are already available; products 1-4 and 10 are validated, products 5-6 are provisional, and products 7-8 are only beta versions. The Eros Data Center (EDC) in Sioux Falls, SD is the DAAC responsible for archiving these products.

These ASTER data products use combinations of VNIR, SWIR, & TIR and are important for cloud studies, surface mapping, soil and geologic studies, volcano monitoring, and investigations of land-use and vegetation patterns. TIR data—land and ocean brightness temperatures and emissivities—are also important in determining radiative heat balances as well as thermal intertia, vegetation health, soil moisture, and evaporation. Also, the ability to change viewing angles provides stereoscopic capability for generating DEMs and for making useful observations of cloud structure, volcanic plumes, and glacial changes.

CERES stands for Clouds and the Earth's Radiant Energy System. Its purpose is to measure Earth's radiation budget and atmospheric radiation from the top of the atmosphere to the surface and to provide cloud property estimates. It consists of two broadband scanning radiometers: one cross-track mode and one rotating plane (biaxial scanning). The cross-track scanning instrument will allow continuation of measurements made by the Earth Radiation Budget Experiment (ERBE) satellite while the biaxial scanner was created to improve/double the accuracy of those measurements.

MISR stands for Multi-angle Imaging SpectroRadiometer. Whereas most satellite instruments only look straight down, MISR was created to take multiple-angle observations to help assess the amount of sunlight scattered in different directions. These multidirectional measurements will help provide top-of-atmosphere, cloud, and surface reflectance measurements as well as global maps of surface albedo, aerosol and vegetation properties.

MODIS stands for Moderate-Resolution Imaging Spectroradiometer. It is a multispectral cross-track scanning radiometer that operates in the visible through the thermal infrared. A multidisciplinary instrument, MODIS was designed to measure high-priority atmospheric, oceanic, land surface, and cryospheric features on a global basis every 1-to-2 days, measuring a wider array of parameters than any other Terra instrument. MODIS was thus designed to make a major contribution to understanding the global Earth system as a whole and the interactions among its various processes. It takes heritage from the Advanced Vergy High Resolution Radiometer (AVHRR), Landsat Thematic Mapper (TM), and the Coastal Zone Color Scanner (CZCS).

MOPITT stands for Measurements Of Pollution In The Troposphere. It was provided for Terra by the Canadian Space Agency (CSA). The purpose of the sensor is to measure methane (CH4) and carbon monoxide (CO) in the troposphere for determining sinks and sources of these harmful pollutions as well as their abundancy and distribution over time.

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