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Words begining with "G"

Gaia Hypothesis

The hypothesis that the Earth's atmosphere, biosphere, and its living organisms behave as a single system striving to maintain a stability that is conducive to the existence of life.


The increase in signal power produced by an amplifier, usually expressed in decibels as the ratio of the output to the input. A measure of the effectiveness of a directional antenna as compared to a non-directional antenna. See antenna.


A branch of applied mathematics concerned with measuring the shape of the Earth and describing variations in the Earth's gravity field.

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, tables, etc.


A surface of constant gravitational potential around the Earth - an averaged surface perpendicular to the force of gravity.


The physical elements of the Earth's surface, crust, and interior.


Describes an orbit in which a satellite is always in the same position (appears stationary) with respect to the rotating Earth. The satellite travels around the Earth in the same direction, at an altitude of approximately 35,790 km (22,240 statute miles) because that produces an orbital period equal to the period of rotation of the Earth (actually 23 hours, 56 minutes, 04.09 seconds). A worldwide network of operational geostationary meteorological satellites provides visible and infrared images of Earth's surface and atmosphere. The satellite systems include the U.S. GEOS, METOSAT (launched by the European Space Agency and operated by the European Weather Satellite Organization-EUMETSAT), the Japanese GMS, and most commercial, telecommunications satellites. See Clarke Belt.

Geostationary Meteorological Satellite (GMS)

Japan's geostationary weather satellite.

Geostationary Operational Environmental Satellite (GOES)

NASA-developed, NOAA-operated series of satellites that:

  • provide continuous day and night weather observations;
  • monitor severe weather events such as hurricanes. thunderstorms. and flash floods;
  • relay environmental data from surface collection platforms to a processing center;
  • perform facsimile transmissions of processed weather data to low-cost receiving stations;
  • monitor the Earths magnetic field, the energetic particle flux in the satellite's vicinity, and x-ray emissions from the sun;
  • detect distress signals from downed aircraft and ships.

GOES observes the U.S. and adjacent ocean areas from vantage points 35, 790 km (22,240 miles) above the equator at 75 degrees west and 135 degrees west. GOES satellites have an equatorial, Earth-synchronous orbit with a 24-hour period, a resolution of 8 km, an IR resolution of 4 km, and a scan rate of 1864 statute miles in about three minutes. See geostatinary. The transmission of processed weather data (both visible and infrared) by GOES is called weather facsimile (WEFAX). GOES WEFAX transmits at 1691 + MHz and is accessible via a ground station with a satellite dish antenna. GOES carries the following five major sensor systems:

  1. The imager is a multispectral instrument capable of sweeping simultaneously one visible and four infrared channels in a north-to south swath across an east-to-west path, providing full disk imaqery once every thirty minutes.
  2. The sounder has more spectral bands than the imager for producing high-quality atmospheric profiles of temperature and moisture. It is capable of stepping one visible and eighteen infrared channels in a north-to-south swath across an east-to-west path.
  3. The Space Environment Monitor (SEM) measures the condition of the Earths magnetic field, the solar activity and radiation around the spacecraft, and transmits these data to a central processing facility.
  4. The Data Collection System (DCS) receives transmitted meteorological data from remotely - located platforms and relays the data to the end users.
  5. The Search and Rescue Transponder can relay distress signals at all times, but cannot locate them. While only the polar-orbiting satellite can locate distress signals, the two types of satellites work together to create a comprehensive search and rescue system.

Geosynchronus (aka GEO)

Synchronous with respect to the rotation of the Earth. See geostationary.


A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.1 square kilometers in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.

Global Change Research Program (GCRP)

The USGCRP is a government-wide program whose goal is "to establish a scientific basis for national and international policy-making relating to natural and human-induced changes in the global Earth system." Mission to Planet Earth is NASAs central contribution to the U.S.

Global Change Research Proqram. The Global Change Research Program coordinates and guides the efforts of federal agencies. The program examines such questions as, is the Earth experiencing global warming? Is the depletion of the ozone layer expanding? How do we determine and understand the causes of global climate changes? Are they reversible? What are the implications for human needs and activities?

Global Measurement

All of the activities required to specify a global variable, such as ozone. These activities range from data acquisition to the generation of a data-analysis product, and include estimates of the uncertainties in that product. A global measurement often will consist of a combination of observations from a spacecraft instrument (required for global coverage) and measurements in situ (needed to provide reference points for long-term accuracy).

Global Variables

Functions of space and time that describe the large scale state and evolution of the Earth system. The Earth systems geosphere, hydrosphere, atmosphere, and biosphere and their components are, or potentially are, global variables.


See Geostationary Meteorological Satellite.


See Geostationary Operational Environmental Satellite.


NOAA geostationary satellite launched in April 1994 (alphabetical designators are used while on the ground and before geostationary orbit, after it achieves geostationary orbit it became GOES 8). GOES 8 is the first in a series of five new geostationary satellites that will ensure dual-satellite coverage of the U.S. into the next century, and will provide better advanced warnings of thunderstorms, flash floods, hurricanes, and other severe weather. GOES 8 will also contribute important information to a new flood and water management system which will assist decision-makers with the allocation of precious western water resources.


The next generation of NOAA geostationary satellites, scheduled for launch beginning sometime after 2003. Currently in the planning phase, these satellites will follow the series of five geostationary satellites which are being launched beginning in 1994. See GOES I.


Environmental satellite scanners, rather than photographing a scene, scan a scene line-by-line measuring light or heat levels and transmitting this information as a video image via an amplitude modulated (AM) subcarrier contained in the satellites FM signal. The video image a 2400 Hz tone - is amplitude modulated to correspond to the light and dark areas sensed, with the louder portion of the tone representing the lighter areas of the image and the lower portion of the tone representing the darker areas of the image. Intermediate volumes form the shades of the grayscale (up to 256 shades) needed to complete the image. This is an analog type of data transmission, and enables the assessment of such features as heat, light, temperature, and cloud heights.

Greenhouse Effect

Process by which significant changes in the chemistry of Earths atmosphere may enhance the natural process that warms our planet and elevates temperatures. If the effect is intensified and Earth's average temperatures change, a number of plant and animal species could be threatened with extinction.

Certain gaseous components of the atmosphere, called greenhouse gases, transmit the visible portion of solar radiation but absorb specific spectral bands of thermal radiation emitted by the Earth. The theory is that terrain absorbs radiation, heats up, and emits longer wavelength thermal radiation that is prevented from escaping into space by the blanket of carbon dioxide and other greenhouse gases in the atmosphere. As a result, the climate warms.

Because atmospheric and oceanic circulations play a central role in the climate of the Earth, improving our knowledge about their interaction becomes essential.

Greenhouse Gas

A gaseous component of the atmosphere contributing to the greenhouse effect. Greenhouse gases are transparent to certain wavelengths of the suns radiant energy, allowing them to penetrate deep into the atmosphere or all the way into the Earths surface. Greenhouse gases and clouds prevent some of infrared radiation from escaping, trapping the heat near the Earths surface where it warms the lower atmosphere. Alteration of this natural barrier of atmospheric gases can raise or lower the mean global temperature of the Earth.

Greenhouse gases include carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and water vapor. Carbon dioxide, methane, and nitrous oxide have significant natural and human sources while only industries produce chlorofluorocarbons. Water vapor has the largest greenhouse effect, but its concentration in the troposphere is determined within the climate system. Water vapor will increase in response to global warming, which in turn may further enhance qlobal warminq.

Greenwich Mean Time (GMT)

See Coordinated Universal Time.

Gross Feature Map

Map that displays geographic characteristics rather than political boundaries.

Ground Control (points)

Identifiable points on the ground whose locations on the surface of the Earth are accurately known for use as geodetic references in mapping, charting, and other related mensuration applications.

Ground Station

See Earth station.

Ground Track

The inclination of a satellite, together with its orbital altitude and the period of its orbit, creates a track defined by an imaginary line connecting the satellite and the Earths center. The intersection on the line with the Earths surface is the subsatellite point. As the Earth turns on its axis and the satellite orbits overhead, a line is created by the satellite's apparent path over the ground (the series of subsatellite points connected). A geostationary satellite has an inclination of essentially zero, and, because its orbital period exactly matches the Earths rotation, its ground track is reduced to an apparent stationary point on the equator.


NASA Goddard Space Flight Center, located in Greenbelt, Maryland. See NASA Centers

Guided Wave

Electromagnetc or acoustic wave that is constrained within certain boundaries, as in a wave guide (transmission line).


A large arm of an ocean or sea extending into a land mass.

Gulf Stream

A warm, swift ocean current that flows along the coast of the Eastern United States and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 13 2021

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