Earth Observation Glossary
Active System (Active Sensor)
A remote-sensing system that transmits its own radiation to detect an object or area for
observation and receives the reflected or transmitted radiation. Radar is an example of an active
Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)
ASTER is an imaging instrument Terra as part of NASA's Earth Observing System (EOS).
ASTER is used to obtain detailed maps of land surface temperature, emissivity, reflectance and
Advanced Very High Resolution Radiometer (AVHRR)
A five-channel scanning instrument that quantitatively measures electromagnetic radiation,
flown on NOAA environmental satellites. AVHRR remotely determines cloud cover and surface
temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow,
In radiometry, a relatively narrow region of the electromagnetic spectrum to which a remote
sensor responds; a multispectral sensor makes measurements in a number of spectral bands.
In spectroscopy, spectral regions where atmospheric gases absorb (and emit) radiation, e.g.,
the 15 µm carbon dioxide absorption band, the 6.3 µm water vapor absorption band, and the 9.6
µm ozone absorption band.
The layer formed naturally by the leaves and branches of trees and plants, especially in a forest.
Reflectivity that varies from less than 10% to more than 90% of the insolation and depends on
drop sizes, liquid water content, water vapor content, thickness of the cloud, and the sun's
zenith angle. The smaller the drops and the greater the liquid water content, the greater the
cloud albedo, if all other factors are the same.
Digital Elevation Model (DEM)
A representation of the topography of the Earth in digital format, that is, by coordinates and
numerical descriptions of altitude.
Distributed Active Archive Center (DAAC)
There are eight DAACs located around the United States that are tasked with processing,
storing, and distributing satellite remote sensing data for NASA and other agencies.
Performed in twenty-four hours, such as the diurnal rotation of the Earth
Earth Observing System (EOS)
A series of satellites, part of NASA's Earth Science Enterprise, designed for launch at the end of
the 1990's to gather data on global change.
The entire range of radiant energies or wave frequencies from the longest to the shortest
wavelengths-the categorization of solar radiation. Satellite sensors collect this energy, but what
the detectors capture is only a small portion of the entire electromagnetic spectrum. The
spectrum usually is divided into seven sections: radio, microwave, infrared, visible, ultraviolet, x-
ray, and gamma-ray radiation.
Enhanced Thematic Mapper-Plus (ETM+)
An eight-band multispectral scanning radiometer onboard the Landsat 7 satellite that is capable
of providing high-resolution imaging information of the Earth's surface.
A color imaging process which produces an image of a color that does not correspond to the
true or natural color of the scene (as seen by human eyes).
Traveling around the Earth's equator at an altitude of at least 35,000 kilometers and at a speed
matching that of the Earth's rotation, thereby maintaining a constant relation to points on the
Geographic Information System (GIS)
A system for archiving, retrieving, and manipulating data that has been stored and indexed
according to the geographic coordinates of its elements. The system generally can utilize a
variety of data types, such as imagery, maps. table, etc.
Global Positioning System (GPS)
A system consisting of 25 satellites in 6 orbital planes at 20,000 km altitude with 12 hr periods,
used to provide highly precise position, velocity and time information to users anywhere on
Earth or in its neighborhood at any time.
An instrument capability using many very narrow spectral frequency bands (300 or more),
enabling a satellite-based passive sensor to discriminate specific features or phenomena on the
body being observed (such as Earth).
Jet Propulsion Laboratory (JPL)
Located in Pasadena, California, JPL is operated under contract to NASA by the California
Institute of Technology. Its primary focus is the scientific study of the solar system, including
exploration of the planets with automated probes. Most of the lunar and planetary spacecraft of
the 1960s and 1970s were developed at JPL.
The characteristics of a land surface as determined by its spectral signature (the unique way in
which a given type of land cover reflects and absorbs light).
Land Remote-Sensing Satellite, operated by the U.S. Earth Observation Satellite Company
(EOSAT). Commercialized under the Land Remote-Sensing Commercialization Act of 1984,
Landsat is a series of satellites (formerly called ERTS) designed to gather data on the Earth's
resources in a regular and systematic manner. Objectives of the mission are: land use
inventory, geological/mineralogical exploration, crop and forestry assessment, and cartography.
The angle between a perpendicular at a location, and the equatorial plane of the Earth
Light amplified by simulated emission of radiation—a device that produces an intense beam of
light that may be strong enough to vaporize the hardest and most heat-resistant materials, first
constructed in 1960.
Leaf Area Index (LAI)
The area of foliage per unit area of ground. Conventionally this refers to the ration of the area of
the upper side of the leaves in a canopy projected onto a lfat surface to the area of the surface
under the canopy.
The angular distance from the Greenwich meridian (0 degree) along the equator. Can be
measured either east or west to the 180th meridian or 0-360 degrees W.
The radiation emitted in the spectral wavelength greater than 4 µm corresponding to the
radiation emitted from the earth.
Light Intersection Direction and Ranging (LIDAR)
An optical remote sensing technology that can measure the distance to, or other properties of a
target by illuminating the target with light, often using pulses from a laser.
Moderate-resolution Imaging Spectroradiometer (MODIS)
Launched into Earth orbit by NASA in 1999 on board the Terra (EOS AM) Satellite, and in 2002
on board the Aqua (EOS PM) satellite. The instruments capture data in 36 spectral bands
ranging in wavelength from 0.4 µm to 14.4 µm and at varying spatial resolutions (2 bands at 250
m, 5 bands at 500 m and 29 bands at 1 km). Together the instruments image the entire Earth
every 1 to 2 days. They are designed to provide measurements in large-scale global dynamics
including changes in Earth's cloud cover, radiation budget and processes occurring in the
oceans, on land, and in the lower atmosphere.
National Aeronautics and Space Administration (NASA)
U.S. Civilian Space Agency created by Congress. Founded in 1958, NASA belongs to the
executive branch of the Federal Government. NASA's mission to plan, direct, and conduct
aeronautical and space activities is implemented by NASA Headquarters in Washington, D.C.,
and by nine major centers spread throughout the United States. Dozens of smaller facilities,
from tracking antennas to Space Shuttle landing strips to telescopes are located around the
National Oceanic and Atmospheric Administration (NOAA)
NOAA was established in 1970 within the U.S. Department of Commerce and includes: the
National Ocean Service, the National Marine Fisheries Service, the NOAA Corps and the Office
of Oceanic and Atmospheric Research. NOAA has two main components: the National Weather
Service (NWS), and the National Environmental Satellite, Data, and Information Service
Normalized Difference Vegetation Index (NDVI)
An index, or scale, of vegetation conditions based on differences in the amount of visible and
near-infrared light reflected from plants on Earth's surface. A vegetation index is a gauge of
plant health, productivity, and density.
The path described by a heavenly body in its periodic revolution. Earth satellite orbits with
inclinations near 0 degree are called equatorial orbits because the satellite stays nearly over the
equator. Orbits with inclinations near 90 degrees are called polar orbits because the satellite
crosses over (or nearly over) the north and south poles.
Sensitive to all or most of the visible spectrum.
A system sensing only radiation emitted by the object being viewed or reflected by the object
from a source other than the system.
Photosynthetically Active Radiation (PAR)
Electromagnetic radiation in the part of the spectrum used by plants for photosynthesis.
Calibrated container that measures the amount of rainfall during a specific period of time.
The return of light or sound waves from a surface. If a reflecting surface is plane, the angle of
reflection of a light ray is the same as the angle of incidence.
The technology of acquiring data and information about an object or phenomena by a device
that is not in physical contact with it. In other words, remote sensing refers to gathering
information about the Earth and its environment from a distance.
A measure of the ability to separate observable quantities. In the case of imagery, it describes
the area represented by each pixel of an image. The smaller the area represented by a pixel,
the more accurate and detailed the image.
A nominal frequency range from 4 to 2 GHz (7 to 20 cm wavelength) within the microwave
(radar) portion of the electromagnetic spectrum. S-band radars are used for medium-range
meteorological applications, for example rainfall measurements.
The process by which electromagnetic radiation interacts with and is redirected by the
molecules of the atmosphere, ocean, or land surface.
Sea Surface Temperature (SST)
The temperature of the layer of seawater (approximately 0.5 m deep) nearest the atmosphere.
Sea-viewing Wide Field-of-view Sensor (SeaWiFS)
SeaWiFS is an ocean color sensor to study ocean productivity and interactions between the
ocean ecosystems and the atmosphere.
Satellite Pour l'Observation de la Terre (SPOT)
French, polar-orbiting Earth observation satellite(s) with ground resolution of 10 meters. SPOT
images are available commercially and are intended for such purposes as environmental
research and monitoring, ecology management, and for use by the media, environmentalists,
The radiation received from the sun and emitted in the spectral wavelengths less than 4
microns. It is also called 'solar radiation'.
Shuttle Radar Topography Mission (SRTM)
A NASA Space Shuttle mission that used C-band and X-band interferometric synthetic aperture
radars (IFSARs) to acquire topographic data over 80% of Earth's land mass (between 60degN
and 56degS) between February 11-22, 2000.
Energy received from the sun is solar radiation. The energy comes in many forms, such as
visible light (that which we can see with our eyes). Other forms of radiation include radio waves,
heat (infrared), ultraviolet waves, and x-rays. These forms are categorized within the
A characteristic that refers to a location (which may be a specific location on the Earth's surface,
or relative to an arbitrary point).
A finite segment of wavelengths in the electromagnetic spectrum.
This refers to the particular form or shape evinced by the power spectrum calculated from the
data comprising the time series of a process.
The area observed by a satellite as it orbits the Earth.
The technique of graphically representing the exact physical features of a place or region on a
map. The physical features of a place or region.
Tropical Rainfall Measuring Mission (TRMM)
TRMM is a joint space mission between NASA and the Japan Aerospace Exploration Agency
(JAXA) designed to monitor and study tropical rainfall. The term refers to both the mission itself
and the satellite that the mission uses to collect data. The satellite was launched on November
United States Geological Survey (USGS)
A bureau of the Department of the Interior. The USGS monitors resources such as energy,
minerals, water, land, agriculture, and irrigation. The resulting scientific information contributes
to environmental-policy decision making and public safety. For example, USGS identifies flood-
and landslide-prone areas and maintains maps of the United States.
A nominal frequency range from 12.5 to 8 GHz (2.4 to 3.75 cm wavelength) within the
microwave (radar) portion of the electromagnetic spectrum. X-band is a suitable frequency for
several high-resolution radar applications and has often been used for both experimental and
operational airborne systems.
NASA History Glossary
Earth Observatory Glossary