A/D
Analog to Digital. Used to refer to the conversion of analog data to
its digital equivalent.
Absolute Humidity
In a system of moist air, the ratio of the mass of water vapor
present to the volume occupied by the mixture; that is, the density of
the water vapor component. Absolute humidity is normally expressed in
grams of water vapor in a cubic meter of air (25 g/m3)
Absorption
The process in which radiant energy is retained by a substance. A
further process always results from absorption, that is, the
irreversible conversion of the absorbed radiation into some other form
of energy within and according to the nature of the absorbing medium.
The absorbing medium itself may emit radiation, but only after an energy
conversion has occurred.
Acid Rain
Acids form when certain atmospheric gases (primarily carbon dioxide,
sulfur dioxide, and nitrogen oxides) come in contact with water in the
atmosphere or on the ground and are chemically converted to acidic
substances. Oxidants play a major role in several of these acid-forming
processes. Carbon dioxide dissolved in rain is converted to a weak acid
(carbonic acid). Other gases, primarily oxides of sulfur and nitrogen,
are converted to strong acids (sulfuric and nitric acids). Although rain
is naturally slightly acidic because of carbon dioxide, natural
emissions of sulfur and nitrogen oxides, and certain organic acids,
human activities can make it much more acidic. Occasional pH readings of
well below 2.4 (the acidity of inegar) have been reported in
industrialized areas. The principal natural phenomena that contribute
acid-producing gases to the atmosphere are emissions from volcanoes and
from biological processes that occur on the land, in wetlands, and in
the oceans. The effects of acidic deposits have been detected in glacial
ice thousands of years old in remote parts of the lobe. Principal human
sources are industrial and power-generating plants and transportation
vehicles. The gases may be carried hundreds of miles in the atmosphere
before they are converted to acids and deposited. Since the industrial
revolution, emissions of sulfur and nitrogen oxides to the atmosphere
have increased. Industrial and energy-generating facilities hat burn
fossil fuels, primarily coal, are the principal sources of increased
sulfur oxides. These sources, plus the transportation sector, are the
major originators of increased nitrogen oxides. The problem of acid rain
not only has increased with population and industrial growth, it has
become more widespread. The use of tall smokestacks to reduce local
pollution has contributed to the spread of acid rain by releasing gases
into regional atmospheric circulation. The same remote glaciers that
provide evidence of natural variability in acidic deposition show, in
their more recently formed layers, the increased deposition caused by
human activity during the past half century.
Acquisition of Signal
The time you begin receiving a signal from a spacecraft. For
polar-orbiting satellites, radio reception of the APT signal can begin
only when the polar-orbiting satellite is above the horizon of a
particular location. This is determined by both the satellite and its
particular path during orbit across the reception range of a ground
station.
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 system. Compare
with passive system.
Advanced Very High Resoution 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,
and ice. TIROS Automatic Picture Transmissions (APT) are derived from
this nstrument. See TIROS.
Aerosol
Particles of liquid or solid dispersed as a suspension in
gas.
Afforestation
The act or process of establishing a forest, especially on land not
previously forested.
AI
See artificial intelligence.
Air Mass
Large body of air, often hundreds or thousands of miles across,
containing air of a similar temperature and humidity. Sometimes the
differences between air masses are hardly noticeable, but if colliding
air masses have very different temperatures and humidity values, storms
can erupt. See front.
Air Pollution
The existence in the air of substances in concentrations that are
determined unacceptable to human health and the environment.
Contaminants in the air we breathe come mainly from manufacturing
industries, electric power plants, exhaust from automobiles, buses, and
trucks.
Air Pressure
The weight of the atmosphere over a particular point, also called
barometric pressure. Average air exerts approximately 14.7 pounds (6.8
kg) of force on every square inch (or 101,325 newtons on every square
meter) at sea level.
AIR
Airborne Imaging Radar.
aka
Also known as.
Albedo
The ratio of the outgoing solar radiation reflected by an object to
the incoming solar radiation incident upon it.
Alkaline
Substance capable of neutralizing acid, with a pH greater than 7.0.
See pH.
Algorithm
A mathematical relation between an observed quantity and a variable
used in a step-by-step mathematical process to calculate a quantity. In
the context of remote sensing, algorithms generally specify how to
determine higherlevel data products from lower-level source data. For
example, algorithms prescribe how atmospheric temperature and moisture
profiles are etermined from a set of radiation observations originally
sensed by satellite sounding instruments.
Altimeter
An active instrument (see active system) used to measure the
altitude of an object above a fixed level. For example, a laser
altimeter can measure height from a spacecraft to an icesheet. That
measurement, coupled with radial orbit knowledge, will enable
determination of the topography.
Altitude
Height above the Earth's surface.
AM
See amplitude modulation.
Analog
Transmission of a continuously variable signal as opposed to a
discretely variable signal. Compare with digital. A system of
transmitting and receiving information in which one value (i.e.,
voltage, current, resistance, or, in the APT system, the volume level of
the video tone) can be compared directly to the information (in the APT
system, the white, black, and gray values) in the image.
Ancillary Data
Data other than instrument data required to perform an instrument's
data processing. Ancillary data includes such information as orbit
and/or attitude data, time information, spacecraft engineering data, and
calibration information.
Anemometer
Instrument used to measure wind speed, usually measured either from
the rotation of winddriven cups or from wind pressure through a tube
pointed into the wind.
Anomaly
1. The deviation of (usually) temperature or precipitation in a
given region over a specified period from the normal value for the same
region. 2. The angular distance of an Earth satellite (or planet) from
its perigee (or perihelion) as seen from the center of the Earth (sun).
See Keplerian elements for examples of use.
Antenna Array
An ordered assembly of elementary antennae spaced apart and fed in
such a manner that the resulting radiation is concentrated in one or
more directions.
Antenna Beam
The focused pattern of electromagnetic radiation that is either
received or transmitted by an antenna.
Antenna
A wire or set of wires used to send and receive electromagnetic
waves. Two primary features must be considered when selecting antennas:
beamwidth, or the "width" of the antenna pattern (wide beamwidth
suggests the ability to receive signals from a number of different
directions), and gain, or the increase in signal level. Generally
beamwidth or gain can be ncreased only at the expense of the other. Gain
can be increased by multiplying the number of antenna elements, although
this adds "directionality" that reduces beamwidth. Important antenna
considerations include the following:
- The physical size of antenna
components is determined by the frequency of the transmissions it will
receive--the higher the requency, the shorter the elements. At high
frequencies, use of a satellite dish will compensate for the reduced
amount of energy intercepted by shortened components.
- The antenna
design should fit the type of radio frequency (RF) signal polarization
it will receive. The orientation of radio waves in space is a function
of the orientation of the elements of the ransmitting antenna. A
circularly polarized wave rotates as it propagates through space.
Antennas can be designed for either right or left-handed circular
polarization. Earth-based communication antennas are either vertical or
horizontal in polarization, and not suited for space communication.
Police and cellular phone transmissions use vertical polarization
because simple vertical whip antenna is the easiest sort of
omnidirectional antenna to mount on a vehicle.
- The antenna needs to
produce sufficient signal gain to produce noise-free reception.
- The
antenna should be clear of conductive objects such as power lines, phone
wires, etc., so height above the ground becomes important. Basic antenna
components are:
- driven element--the parts connected to and receiving
power from the receiver/transmitter;
- Parasitic elements--the parts
dependent upon resonance rather than connection to a power source;
- A director or parasitic element that rein forces radiation on a line
pointing to it from the driven element;
- A reflector or parasitic element that rein forces radiation on a line
pointing from it to the driven element. A fundamental form of antenna is
a single wire whose length approximately equals half the transmitting
wavelength. Known as a dipole antenna, it is the unit from which many
more complex forms of antennas are constructed. One of the most common
forms of VHF antenna is the Yagi/beam, named for the Japanese scientist
ho first described the principles of combining a basic dipole (driven
element) and parasitic elements. A common TV antenna is an example of
this type. A Yagi/beam antenna is directional and therefore includes a
rotator to aim (direct) the antenna. See yagi. An omnidirectional
antenna has a wide beamwidth and consequently does not require
"tracking" (aiming the antenna toward the signal source). An example of
an omnidirectional antenna is the turnstile antenna, a variation of the
standard dipole antenna well suited for space communications. The
quadrifilar helix antenna is omnidirectional and an inherently excellent
antenna for ground station use. Quadrifilars are also used on NOAA's
polarorbiting environmental satellites. The parabolic reflector or
satellite dish antenna collects RF signals on a passive dishshaped
surface. A feedhorn antenna--a simple dipole antenna mounted in a
resonant tube structure (cylinder with one open end)--transfers the RF
energy to a transmission line. The bigger the dish, the greater the
amount of RF energy intercepted, and therefore the reater the gain from
the signal.
Anticyclone
A high pressure area where winds blow clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere. See cyclone,
wind.
AOS
See Acquisition of Signal.
Apogee (aka Apoasis or Apifocus)
On an elliptical orbit path, point a which a satellite is farthest
from the Earth. See perigee diagram
APT
See Automatic Picture Transmission.
Aquifier
Layer of water-bearing permeable rock, sand, or gravel capable of
providing significant amounts of water.
Arctic Circle
The parallel of latitude that is approximately 66.5 degrees north of
the equator and that circumscribes the northern frigid zone.
ARGOS
French random-access Doppler data collection system. Used on NOAA's
Polar-Orbiting Environmental Satellites (POES), ARGOS receives platform
and buoy transmissions on 401.65 MHz. This data collection system now
monitors more than 4,000 platforms worldwide, outputs data via VHF link,
and stores them on tape for relay to a central processing
facility.
Argument of Perigee (aka ARGP or w)
One of the six Keplerian elements, it gives the rotation of the
satellite on the orbit. The argument (argument meaning angle) of
perigee-- perigee is the point on an orbital path when the satellite is
closest to the Earth--is the angle (measured from the center of the
Earth) from the ascending node to perigee. Example: When ARGP = 0
degrees, the perigee occurs at the same place as the ascending node.
That means that the satellite would be closest to Earth just as it rises
up over the equator. When ARGP = 180 degrees, apogee would occur at the
same place as the descending node. This means that the satellite would
be farthest from Earth just as it rises over the equator. See Keplerian
elements for diagram.
Artificial Intelligence
Neural networks. The branch of computer science that attempts to
program computers to respond as if they were thinking--capable of
reasoning, adapting to new situations, and learning new skills. Examples
of artificial intelligence programs include those that can locate
minerals underground and understand human speech.
Ascending Node
The point in an orbit (longitude) at which a satellite crosses the
equatorial plane from south to north.
Aspect Ratio
The ratio of image width to image height. Weather Facsimile (WEFAX)
images have a 1:1 aspect ratio (square); a conventional TV aspect ratio
is 4:3 (rectangle).
Astronomical Unit (AU)
The distance from the Earth to the sun. On average, the sun is
149,599,000 kilometers from Earth.
ATLAS (Atmospheric Laboratory for Applications and Science)
mission
The focus of ATLAS is to study the chemistry of the Earth's upper
atmosphere (mainly the stratosphere/mesosphere) and the solar radiation
incident on the Earth system (both total solar irradiance and spectrally
resolved radiance, especially ultraviolet). Science operations onboard
ATLAS 1 (March 1992) and ATLAS 2 (March-April, 1993) began a
comprehensive and systematic collection of data that will help establish
benchmarks for atmospheric conditions and the sun's stability.
Atmosphere
The air surrounding the Earth, described as a series of shells or
layers of different characteristics. The atmosphere, composed mainly of
nitrogen and oxygen with traces of carbon dioxide, water vapor, and
other gases, acts as a buffer between Earth and the sun. The layers,
troposphere, stratosphere, mesosphere, thermosphere, and the exosphere,
vary around the globe and in response to seasonal changes. Troposphere
stems from the Greek word tropos, which means turning or mixing. The
troposphere is the lowest layer of the Earth's atmosphere, extending to
a height of 8-15 km, depending on latitude. This region, constantly in
motion, is the most dense layer of the atmosphere and the region that
essentially contains all of Earth's weather. Molecules of nitrogen and
oxygen compose the bulk of the troposphere. The tropopause marks the
limit of the troposphere and the beginning of the stratosphere. The
temperature above the tropopause increases slowly with height up to
about 50 km. The stratosphere and stratopause stretch above the
troposphere to a height of 50 km. It is a region of intense interactions
among radiative, dynamical, and chemical processes, in which horizontal
mixing of gaseous components proceeds much more rapidly than vertical
mixing. The stratosphere is warmer than the upper troposphere, primarily
because of a stratospheric ozone layer that absorbs solar ultraviolet
energy. The mesosphere, 50 to 80 km above the Earth, has diminished
ozone concentration and radiative cooling becomes relatively more
important. The temperature begins to decline again (as it does in the
troposphere) with altitude. Temperatures in the upper mesosphere fall to
-70° to -140° Celsius, depending upon latitude and season. Millions of
meteors burn up daily in the mesosphere as a result of collisions with
some of the billions of gas particles contained in that layer. The
collisions create enough heat to burn the falling objects long before
they reach the ground. The stratosphere and mesosphere are referred to
as the middle atmosphere. The mesopause, at an altitude of about 80 km,
separates the mesosphere from the thermosphere--the outermost layer of
the Earth's atmosphere. The thermosphere, from the Greek thermo for
heat, begins about 80 km above the Earth. At these high altitudes, the
residual atmospheric gases sort into strata according to molecular mass.
Thermospheric temperatures increase with altitude due to absorption of
highly energetic solar radiation by the small amount of residual oxygen
still present. Temperatures can rise to 2,000° C. Radiation causes the
scattered air particles in this layer to become charged electrically,
enabling radio waves to bounce off and be received beyond the horizon.
At the exosphere, beginning at 500 to 1,000 km above the Earth's
surface, the atmosphere blends into space. The few particles of gas here
can reach 4,500° F (2,500° C) during the day.
Atmospheric
Having to do with the atmosphere. For example, "atmospheric conditions" refer to
conditions in the atmosphere.
Atmospheric Infrared Sounder
Advanced sounding instrument selected to fly on the EOS-PM1 mission
(intermediate-sized, sun-synchronous, morning satellite) in the year
2000. It will retrieve vertical temperature and moisture profiles in the
troposphere and stratosphere. Designed to achieve temperature retrieval
accuracy of 1°C with a 1 km vertical resolution, it will fly with two
operational microwave sounders. The three instruments will constitute an
advanced operational sounding system, relative to the TIROS Operational
Vertical Sounder (TOVS) currently flying on NOAA polar-orbiting
satellites. See Earth Observing System, TIROS-N/NOAA Satellites.
Atmospheric Pressure
The amount of force exerted over a surface area, caused by the
weight of air molecules above it. As elevation increases, fewer air
molecules are present. Therefore, atmospheric pressure always decreases
with increasing height. A column of air, 1 square inch in cross section,
measured from sea level to the top of the atmosphere would weigh
approximately 14.7 b/in2. The standard value for atmospheric pressure at
sea level is: 29.92 inches or 760 mm of mercury 1013.25 millibars (mb)
or 101,325 pascals (Pa)
Atmospheric Radiation Measurements Program (ARM)
U.S. Department of Energy program for the continual, ground-based
measurements of atmospheric and meteorological parameters over
approximately a ten-year period. The program will study radiative
forcing and feedbacks, particularly the role of clouds. The general
program goal is to improve the performance of climate models,
particularly general circulation models of the atmosphere.
Atmospheric Response Variables
Variables that reflect the response of the atmosphere to external
forcing (e.g., temperature, pressure, circulation, and
precipitation).
Atmospheric Windows
The range of wavelengths at which water vapor, carbon dioxide, or
other atmospheric gases only slightly absorb radiation. Atmospheric
windows allow the Earth's radiation to escape into space unless clouds
absorb the radiation. See greenhouse effect.
Atoll
A coral island consisting of a ring of coral surrounding a central
lagoon. Atolls are common in the Indian and Pacific Oceans.
Attenuation
The decrease in the magnitude of current, voltage, or power of a
signal in transmission between points. Attenuation may be expressed in
decibels, and can be caused by interferences such as rain, clouds, or
radio frequency signals.
Audio Frequencies
Frequencies that the human ear can hear (usually 30 to 20,000 cycles
per second).
Auroras
See solar wind.
Automatic Picture Transmission (APT)
System developed to make real-time reception of satellite images
possible whenever an APTequipped satellite passes within range of an
environmental satellite ground station. Transmission (analog video
format) consists of an amplitude-modulated audible tone that can be
displayed as an image on a computer monitor when received by an
appropriate ground station. APT images are transmitted by polar-orbiting
satellites such as the TIROS-N/NOAA satellites, Russia's METEOR, and the
Chinese Feng Yun, which orbit 500-900 miles above the Earth, and offer
both visible and infrared images. An APT image has thousands of squares
called picture elements or pixels. Each pixel represents a four-km
square.
AVHRR
See Advanced Very High Resolution Radiometer.
Azimuth
The direction, in degrees referenced to true north, that an antenna
must be pointed to receive a satellite signal (compass direction). The
angular distance is measured in a clockwise direction.
Band
1. In radio, a continuous sequence of broadcasting frequencies
within given limits. 2. 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.
3. 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.
Bandwidth
The total range of frequency required to pass a specific modulated
signal without distortion or loss of data. The ideal bandwidth allows
the signal to pass under conditions of maximum AM or FM adjustment. (Too
narrow a bandwidth will result in loss of data during modulation peaks.
Too wide a bandwidth will pass excessive noise along with the signal.)
In FM, radio frequency signal bandwidth is determined by the frequency
deviation of the signal.
Barometer
An instrument used by weather forecasters and scientists to measure air pressure
at any given moment for a particular place. A standard
mercury barometer has a glass column about 30 inches long, closed at one
end, with a mercury-filled reservoir. Air pressure is usually measured in
millibars or in inches of mercury. Mercury in the tube adjusts until
the weight of the mercury column balances the atmospheric force exerted
on the reservoir. High atmospheric pressure forces the mercury higher in
the olumn. Low pressure allows the mercury to drop to a lower level in
the column. An aneroid barometer uses a small, flexible metal box called
an aneroid cell. The box is tightly sealed after some of the air is
removed, so that small changes in external air pressure cause the cell
to expand or contract.
Base
A substance that forms a salt when it reacts with acid. A base is a
substance that removes hydrogen ions (protons) from an acid and combines
with them in a chemical reaction.
BASIC
See Beginners All-purpose Symbolic Instruction Code.
Baud
Unit of signaling speed. The speed in bauds is the number of
discrete conditions or signal events per second. If each signal event
represents only one bit condition, baud is the same as bits per
second.
Bay
A wide area of water extending into land from a sea or lake.
Beamwidth
The measure of the "width" of an antenna pattern, measured in
degrees of arc. Generally an antenna with low gain has a wide pattern,
receiving signals well from a number of different directions. See
antenna.
Bearing
The combination of antenna azimuth and elevation required to point
(aim) an antenna at a spacecraft. The bearing for geostationary (i.e.,
GOES) satellites is constant. The bearing for polar-orbiting satellites
varies continuously.
Beginners All-Purpose Symbolic Instruction Code (BASIC)
A most popular and widespread "high level" language for
microcomputers. BASIC uses a sequence of English-like commands and
statements.
Binary
A numbering system that uses only 1 and 0 (e.g., 1 is one, 10 is
two, 11 is three). In digital integrated circuits, a 0 is indicated by a
logic low and a 1 by a logic high.
Bioassay
A measurement of the effects of a substance on living
organisms.
Biodegradation
Decomposition of material by microorganisms.
Biogeochemical Cycles
Movements through the Earth system of key chemical constituents
essential to life, such as carbon, nitrogen, oxygen, and
phosphorus.
Biomass
The amount of living material in unit area or volume, usually
expressed as mass or weight.
Biome
Well-defined terrestrial environment (e.g., desert, tundra, or
tropical forest). The complex of living organisms found in an ecological
region.
Biosphere
Part of the Earth system in which life can exist, between the outer
portion of the geosphere and the inner portion of the
atmosphere.
Biota
The plant and animal life of a region or area.
Bit Rate
The speed at which bits are transmitted, usually expressed in bits
per second. See baud.
Bit
A contraction of "binary digit." The basic element of a two-element
(binary) computer language.
Blizzard
A severe weather condition characterized by low temperatures and
strong winds (greater than 35 mph) bearing a great amount of snow,
either falling or blowing. When these conditions persist after snow has
stopped falling, it is called a ground blizzard.
Boundaries
Lines indicating the limits of countries, states, or other political
jurisdictions, or different air masses.
British Thermal Unit (BTU)
The amount of heat needed to raise the temperature of one pound of
water by one degree Fahrenheit. Compare with calorie.
Bus
The basic frame of a satellite system that includes the propulsion
and stabilization systems, but not the instruments or data
systems.
Byte
A unit of eight bits of data or memory in microcomputer
systems.
Calibration
Act of comparing an instrument's measuring accuracy to a known
standard.
Calorie
The amount of heat needed to raise the temperature of one gram of
water at 15° centigrade one degree centigrade. Compare with British
Thermal Unit.
Canal
A man-made watercourse designed to carry goods or water.
Canopy
The layer formed naturally by the leaves and branches of trees and
plants.
Canyon
A large but narrow gorge with deep sides.
Cape (or Point)
A piece of land extending into water.
Carbon Cycle
All parts (reservoirs) and fluxes of carbon. The cycle is usually
thought of as four main reservoirs of carbon interconnected by pathways
of exchange. The reservoirs are the atmosphere, terrestrial biosphere
(usually includes freshwater systems), oceans, and sediments (includes
fossil fuels). The annual movements of carbon, the carbon exchanges
between eservoirs, occur because of various chemical, physical,
geological, and biological processes. The ocean contains the largest
pool of carbon near the surface of the Earth, but most of that pool is
not involved with rapid exchange with the atmosphere. See appendix for
diagram.
Carbon Dioxide (CO2)
A minor but very important component of the atmosphere, carbon
dioxide traps infrared radiation. Atmospheric CO2 has increased about 25
percent since the early 1800s, with an estimated increase of 10 percent
since 1958 (burning fossil fuels is the leading cause of increased CO2,
deforestation the second major cause). The increased amounts of CO2 in
the tmosphere enhance the greenhouse effect, blocking heat from escaping
into space and contributing to the warming of Earth's lower
atmosphere.
Carrier
Radio frequency capable of being modulated with some type of
information. See modulation.
Carrying Capacity
The steady-state density of a given species that a particular
habitat can support.
Catalog Number
A five-digit number assigned to a cataloged orbiting object. This
number may be found in the NASA Satellite Situation Report and on the
NASA Prediction Bulletins.
Cathode Ray Tube (CRT)
A television picture tube for image display.
CD_ROM
See Compact Disk–Read Only Memory.
Centigrade
Temperature scale proposed by Swedish astronomer Anders Celsius in
1742. A mixture of ice and water is zero on the scale; boiling water is
designated as 100 degrees. A degree is defined as one hundredth of the
difference between the two reference points, resulting in the term
centigrade (100th part). To convert centigrade to Fahrenheit: multiply
the centigrade emperature by 1.8 and add 32°. (F = 9/5 C + 32) To
convert Fahrenheit to centigrade: subtract 32° from the Fahrenheit
temperature and divide the quantity by 1.8. (C = (F -32) / 1.8).
Central Processing Unit (CPU)
Main part of a computer consisting of an arithmetic logic unit and a
control unit. See microprocessor.
CFC
See chlorofluorocarbon.
Chloroflourocarbon (CFC)
A family of compounds of chlorine, fluorine, and carbon, entirely of
industrial origin. CFCs include refrigerants, propellants for spray cans
(this usage is banned in the U.S., although some other countries permit
it) and for blowing plastic-foam insulation, styrofoam packaging, and
solvents for cleaning electronic circuit boards. The compounds’
lifetimes vary over a wide ange, exceeding 100 years in some cases.
CFCs’ ability to destroy stratospheric ozone through catalytic cycles is
contributing to the depletion of ozone worldwide. Because CFCs are such
stable molecules, they do not react easily with other chemicals in the
lower atmosphere. One of the few forces that can break up CFC molecules
is ultraviolet radiation, however the ozone layer protects the CFCs from
ultraviolet radiation in the lower atmosphere. CFC molecules are then
able to migrate intact into the stratosphere, where the molecules are
bombarded by ultraviolet rays, causing the CFCs to break up and release
their chlorine atoms. The released chlorine atoms participate in ozone
destruction, with a single atom of chlorine able to destroy ozone
molecules over and over again. International attention to CFCs resulted
in a meeting of diplomats from around the world in Montreal in 1987.
They forged a treaty that called for drastic reductions in the
production of CFCs. In 1990, diplomats met in London and voted to
significantly strengthen the Montreal Protocol by calling for a complete
elimination of CFCs by the year 2000. See Montreal Protocal.
Circadian Rhythm
The cyclical changes in physiological processes and functions that
are related to the 24-hour diurnal cycle.
Circuit
The complete path of an electric current; an assemblage of
electronic elements; a means of two-way communication between two
points--comprised of associated "go" and "return" channels.
Circularly Polarized RF
Radio frequency transmissions where the wave energy is divided
equally between a vertically and a horizontally-polarized
component.
Clarke Belt
A belt 22,245 miles (35,800 kilometers) directly above the equator
where a satellite orbits the Earth at the same speed the Earth is
rotating. Science fiction writer and scientist Arthur C. Clarke wrote
about this belt in 1945, hence the name.
Climate
The average weather conditions in an area determined over a period
of years.
Climatology
Science dealing with climate and climate phenomena.
Clone
A person or thing very much like another, e.g., a copy of another
manufacturer's computer.
Cloud Streets
Lines or rows of cumuliform clouds.
Cloudburst
Any sudden, heavy rain shower.
Clouds
A visible mass of water vapor suspended in the atmosphere above
Earth's surface. Clouds form in areas where air rises and cools. The
condensing water vapor forms small droplets of water (0.012 mm) that,
when combined with billions of other droplets, form clouds. Clouds can
form along warm and cold fronts, where air flows up the side of the
mountain and cools as it rises higher into the atmosphere, and when warm
air blows over a colder surface, such as a cool body of water. Clouds
fall into two general categories: sheetlike or layer-looking stratus
clouds (stratus means layer) and cumulus clouds (cumulus means piled
up). These two cloud types are divided into four more groups that
describe the cloud's altitude. High clouds form above 20,000 feet in the
cold region of the troposphere, and are denoted by the prefix CIRRO or
CIRRUS. At this altitude water almost always freezes so clouds are
composed of ice crystals. The clouds tend to be wispy, are often
transparent, and include cirrus, cirrocumulus, and cirrostratus. Middle
clouds form between 6,500 and 20,000 feet and are denoted by the prefix
ALTO. They are made of water droplets and include altostratus and
altocumulus. Low clouds are found up to 6,500 feet and include the
stratocumulus and nimbostratus clouds. When stratus clouds contact the
ground they are called fog. Vertical clouds, such as cumulus, rise far
above their bases and can form at many heights. Cumulonimbus clouds, or
thunderheads, can start near the ground and soar up to 75,000
feet.
Coastal Zone Color Scanner (CZCS)
The first spacecraft instrument devoted to measurement of ocean
color. Although instruments on other satellites have sensed ocean color,
their spectral bands, spatial resolution, and dynamic range were
optimized for geographical or meteorological use. In the CZCS, every
parameter is optimized for use over water to the exclusion of any other
type of sensing. The CZCS flew on the Nimbus-7spacecraft.
Coaxial Cable
A hollow copper cylinder, or other cylindrical conductor,
surrounding a single-wire conductor having a common axis (hence
coaxial). The space between the cylindrical shell and the inner
conductor is filled with an insulator which may be plastic or mostly
air, with supports separating the shell and the inner conductor every
inch or so. The cable is used to carry radio frequency signals to or
from antennas, etc.
COBOL
See Common Business Oriented Language.
Comma Cloud
Band of organized cumuliform clouds that look like a comma from a
satellite's perspective. Comma clouds are indicators of heavy
storms.
Common Business Oriented Language (COBOL)
A computer programming language written for business
application.
Compact Disk–Read Only Memory (CD-ROM)
Type of computer memory that reads and uses information, but does
not allow information to be added, changed, or erased. Digital
information is read by laser. CD-ROM does not depend upon any
proprietary hardware or software, making it an accessible vehicle for
electronic publishing.
Computer
Electronic machine capable of performing calculations and other
manipulations of various types of data, under the control of a stored
set of instructions. The machine itself is the hardware; the
instructions are the program or software. Depending upon size, computers
are called mainframes, minicomputers, and microcomputers. Microcomputers
include desk-top and portable personal computers.
Condensation
Change of a substance to a denser form, such as gas to a liquid. The
opposite of evaporation.
Conduction
The transfer of heat from one substance to another by direct
contact. Denser substances are better conductors; the transfer is always
from warmer to colder substances.
Continent
One of the large, continuous areas of the Earth into which the land
surface is divided. The six geographically defined continents are
politically defined as seven; Africa, Asia, Australia, Europe, North
America, South America, and Antarctica.
Continental Drift
See plate tectonics.
Contrails
Condensation trails. Artificial clouds made by the exhaust of jet
aircraft.
Convection
The rising of warm air and the sinking of cool air. Heat mixes and
moves air. When a layer of air receives enough heat from the Earth's
surface, it expands and moves upward. Colder, heavier air flows under it
which is then warmed, expands, and rises. The warm rising air cools as
it reaches higher, cooler regions of the atmosphere and begins to sink.
Convection causes local breezes, winds, and thunderstorms.
Coordinated Universal Timing (UTC)
(aka Greenwich Mean Time [GMT]) Local time at zero degrees longitude
at the Greenwich Observatory, England. UTC uses a 24-hour clock, i.e.,
2:00 a.m. is 0200 hours, 2:00 p.m. is 1400 hours, midnight is 2400 or
0000 hundred hours.
Coriolis Force
The apparent tendency of a freely moving particle to swing to one
side when its motion is referred to a set of axes that is itself
rotating in space, such as Earth. The acceleration is perpendicular to
the direction of the speed of the article relative to the Earth's
surface and is directed to the right in the northern hemisphere. Winds
are affected by rotation of the Earth so that instead of a wind blowing
in the direction it starts, it turns to the right of that direction in
the northern hemisphere; left in the southern hemisphere.
Coupled System
Two or more processes that affect one another.
CPU
See central processing unit.
Crop Calendar
The schedule of the maturing and harvesting of seasonal
crops.
CRT
See cathode ray tube.
Cryosphere
One of the interrelated components of the Earth's system, the
cryosphere is frozen water in the form of snow, permanently frozen
ground (permafrost), floating ice, and glaciers. Fluctuations in the
volume of the cryosphere cause changes in ocean sea-level, which
directly impact the atmosphere and biosphere.
Culmination
The point at which a satellite reaches its highest position or
elevation in the sky, relative to an observer (aka the closest point of
approach).
Cyclone
An area of low pressure where winds blow counterclockwise in the
Northern Hemisphere and clockwise in the Southern Hemisphere. See
anticyclone, wind.
CZCS
See Coastal Zone Color Scanner.
Data Collection System (DCS)
DCS units are flown on both GOES and NOAA polar-orbiting spacecraft.
They gather and relay data from both mobile and stationary platforms at
various locations. DCS units on NOAA satellites can also determine the
precise location of moving platforms at the time the data were acquired.
See TIROS-N/NOAA satellites.
Data Rate
The amount of information transmitted per unit time.
Decay
See period decay.
Decibel (dB)
A tenth of a bel. A unit used to measure the volume of a sound,
equal to ten times the common logarithm of the ratio of the intensity of
the sound to the intensity of an arbitrarily chosen standard sound. The
decibel also is used to measure relative strengths of antenna and
amplified signals and always refers to a ratio or difference between two
values.
Declination
The angular distance from the equator to the satellite, measured as
positive north and negative south.
Defense Meteorological Satellite Program (DMSP)
A U.S. Air Force-managed meteorological satellite program with
satellites circling in sun-synchronous orbit. Imagery is collected in
the visible- to near-infrared band (0.4 to 1.1 micrometers) and in the
thermal-infrared band (about 8 to 13 micrometers) at a resolution of
about three kilometers. DMSP data is available directly from the
satellite for local use aboard ships and at military deployment
locations, but is also usually available to civilian users.
Degree
A unit of angular measure represented by the symbol °. The
circumference of a circle contains 360 degrees. When applied to the
roughly spherical shape of the Earth for geographic and cartographic
purposes, degrees are each divided into 60 minutes.
Delta
The fan-shaped area at the mouth or lower end of a river, formed by
eroded material that has been carried downstream and dropped in
quantities larger than can be carried off by tides or currents.
Demodulation
The process of retrieving information (data) from a modulated
carrier wave, the reverse of modulation.
Department of the Interior (DOI)
Responsible for our nationally-owned public lands and natural
resources, the DOI is chartered to foster the wisest use of our land and
water resources, protect fish and wildlife, preserve the environmental
and cultural values of national parks and historical places, and provide
for the enjoyment of life through outdoor recreation. The department
assesses energy and mineral resources and is responsible for assuring
that their development is in the best interest of all citizens. The U.S.
Geological Survey (USGS) is part of the DOI.
Descending Node
The point in a satellite's orbit at which it crosses the equatorial
plane from north to south. See diagram, Keplerian elements.
Desert
A land area so dry that little or no plant or animal life can
survive.
Desertification
The man-made or natural formation of desert from usable
land.
Detector
A device in a radiometer that senses the presence and intensity of
radiation. The incoming radiation is usually modified by filters or
other optical components that restrict the radiation to a specific
spectral band. The information can either be transmitted immediately or
recorded for transmittal at a later time.
Dew Point
The temperature to which air must be cooled for saturation to occur,
exclusive of air pressure or moisture content change. At that
temperature dew begins to form, and water vapor condenses into
liquid.
Dew
Atmospheric moisture that condenses after a warm day and appears
during the night on cool surfaces as small drops. The cool surfaces
cause the water vapor in the air to cool to the point where the water
vapor condenses.
Digital Image
An analog image converted to numerical form so that it can be stored
and used in a computer. The image is divided into a matrix of small
regions called picture elements or pixels. At sub-satellite point each
pixel represents a specific amount of area. For example, in APT each
pixel represents 4.1 kilometers. Each pixel has a numerical value or
data number value, uantifying the radiance of the image at that spot.
The data number value of each pixel usually represents a value between
black and white, i.e., shades of gray. False color can be applied to the
image by assigning a graduated color palette to represent the gray
shades. The color is "false" because it represents an assigned, not
actual, color.
Digital System
A system in which information is transmitted in a series of pulses.
The source is periodically sampled, analyzed, and converted or coded
into numerical values and transmitted. Digital transmissions typically
use the binary coding used by computers so most data is in appropriate
form, but verbal and visual communication must be converted. Many
satellite transmissions use digital formats because noise will not
interfere with the quality of the end product, producing clear and
higher-resolution imagery.
Direct Readout
The capability to acquire data directly from environmental
satellites via an Earth station. Data can be acquired from NOAA and
other nations’ environmental satellites, which offer weather information
from geostationary and polar-orbiting satellites.
Director
Parasitic element(s) of a VHF antenna located forward of the driven
element. See antenna.
DIS
Data and Information System.
Diurnal Arc
The apparent arc described by heavenly bodies from their rising to
their setting.
Diurnal
Performed in twenty-four hours, such as the diurnal revolution of
the Earth.
Dobson Unit (DU)
The standard way to express ozone amounts in the atmosphere. One DU
is 2.7 x 1016 ozone molecules per square centimeter. One Dobson unit
refers to a layer of ozone that would be 0.001 cm thick under conditions
of standard temperature (0° C) and pressure (the average pressure at the
surface of the Earth). For example, 300 Dobson units of ozone brought
down to the surface of the Earth at 0° C would occupy a layer only 0.3
cm thick in a column. Dobson was a researcher at Oxford University who,
in the 1920s, built the first instrument (now called the Dobson meter)
to measure total ozone from the ground.
Doldrums
Region near the equator characterized by low pressure and light
shifting winds. See wind.
Doppler Effect (aka Doppler Shift)
The apparent change in frequency of sound or light waves, varying
with the relative velocity of the source and the observer. If the source
and observer draw closer together, the frequency is increased. Named for
Christian Doppler, Austrian mathematician and physicist
(1803–1853).
Doppler Radar
The weather radar system that uses the Doppler shift of radio waves
to detect air motion that can result in tornadoes and precipitation, as
previously-developed weather radar systems do. It can also measure the
speed and direction of rain and ice, as well as detect the formation of
tornadoes sooner than older radars.
Downconverter
Any radio frequency circuit that converts a higher frequency to a
lower frequency. This enables signal processing by a receiver. A typical
downconverter will feature one or more states of RF preamplification, a
mixer where the frequency conversion occurs, a local oscillator chain,
and often one or more intermediate frequency preamplifiers to minimize
the effect of line losses between the converter and the
receiver.
Drag (aka N1)
A retarding force caused by the Earth's atmosphere. Thus by
definition, drag will act opposite to the vehicle's instantaneous
velocity vector with respect to the atmosphere. The magnitude of the
drag force is directly proportional to the product of the vehicle's
crosssectional area, its drag coefficient, its velocity, and the
atmospheric density, and inversely proportional to its mass. The effect
of drag is to cause the orbit to decay, or spiral downward. A satellite
of very high mass and very low cross-sectional area, and in a very high
orbit, may be very little affected by drag, whereas a large satellite of
low mass, in a low altitude orbit may be affected very strongly by drag.
Drag is the predominant force affecting satellite lifetime.
Driven Element
See antenna.
Dynamics
The study of the action of forces on bodies and the changes in
motion they produce.
Earth Observing System (EOS)
A series of small- to intermediate-sized spacecraft that is the
centerpiece of NASA's Mission to Planet Earth (MTPE). Planned for launch
beginning in 1998, each of the EOS spacecraft will carry a suite of
instruments designed to study global climate change. MTPE will use
space-, aircraft-, and ground-based measurements to study our
environment as an integrated system. Designing and implementing the MTPE
is, of necessity, an international effort. The MTPE program involves the
cooperation of the U.S., the European Space Agency (ESA), and the
Japanese National Space Development Agency (NASDA). The MTPE program is
part of the U.S. interagency effort, the Global Change Research
Program.
Earth Observing System Data & Information System (EOSDIS)
The system that will manage a dataset of Earth science observations
to be collected over a 15-year period. Existing data indicates that the
Earth is changing, and that human activity increasingly contributes to
this change. To monitor these changes, a baseline of "normal"
performance characteristics must be obtained. For the Earth, these
baseline characteristics must cover a global scale and a long enough
period that the variation caused by seasonal changes and other cyclical
or periodic events (e.g., El Niño and the solar cycle) may be included
in the analyses. The baseline characteristics also must enable
scientists to quantify processes that govern the Earth's system.
Functionally, EOSDIS will provide computing and networking facilities
supporting EOS research activities, including data interpretation and
modeling; processing, distribution, and archiving of EOS data; and
command and control of EOS observatories.
Earth Probes
Discipline-specific satellites and instruments that will be used by
NASA to obtain observations before the launch of EOS spacecraft.
Generally smaller than the EOS satellites and instruments, Earth Probes
are planned to complement the broad environmental measurements from EOS
with highly focused studies in areas such as tropical rainfall (TRMM),
ocean productivity (SeaWiFS), atmospheric ozone (TOMS), and ocean
surface winds (NSCAT).
Earth Radiation Budget Experiment (ERBE)
An experiment to obtain data to study the average radiation budget
of the Earth and determine the energy transport gradient from the
equator to the poles. Three satellites were flown in different orbits to
obtain the data: the Earth Radiation Budget Satellite, ERBS (launched in
October 1984), NOAA-9 (launched in December 1984), and NOAA-10 (launched
in September 1986). See Television and Infrared Observation Satellite
(TIROS).
Earth Station (aka Ground Station)
Hardware necessary to acquire data directly from environmental
satellites. The WEFAX Earth station diagram illustrates a basic ground
station configuration for obtaining direct readout data from
geostationary environmental (weather) satellites.
Earth System Science
An integrated approach to the study of the Earth that stresses
investigations of the interactions among the Earth's components in order
to explain Earth dynamics, evolution, and global change.
Earth System
The Earth regarded as a unified system of interacting components,
including geosphere (land), atmosphere (air), hydrosphere (water and
ice), and biosphere (life).
Eccentricity
(aka ecce or E0 or e) One of six Keplerian elements, it describes
the shape of an orbit. In the Keplerian orbit model, the satellite orbit
is an ellipse, with eccentricity defining the "shape" of the ellipse.
When e=0, the ellipse is a circle. When e is very near 1, the ellipse is
very long and skinny.
Eclipse Blindness
Focus-point type of vision loss caused by looking at the sun for too
long a time, which can burn a hole in the retina of the eye.
Eclipse
The partial or total apparent darkening of the sun when the moon
comes between the sun and the Earth (solar eclipse), or the darkening of
the moon when the full moon is in the Earth's shadow (lunar
eclipse).
Ecology
Science dealing with the interrelationships between living organisms
and their environments.
Ecosystem
Any natural unit or entity including living and non-living parts
that interact to produce a stable system through cyclic exchange of
materials.
El Nino
A warming of the surface waters of the eastern equatorial Pacific
that occurs at irregular intervals of 2–7 years, usually lasting 1–2
years. Along the west coast of South America, southerly winds promote
the upwelling of cold, nutrient-rich water that sustains large fish
populations, that sustain abundant sea birds, whose droppings support
the fertilizer industry. Near the end of each calendar year, a warm
current of nutrient-poor tropical water replaces the cold, nutrient-rich
surface water. Because this condition often occurs around Christmas, it
was named El Niño (Spanish for boy child, referring to the Christ
child). In most years the warming lasts only a few weeks or a month,
after which the weather patterns return to normal and fishing improves.
However, when El Niño conditions last for many months, more extensive
ocean warming occurs and economic results can be disastrous. El Niño has
been linked to wetter, colder winters in the United States; drier,
hotter summers in South America and Europe; and drought in Africa. See
ENSO.
Electromagnetic Radiation
Energy propagated as time-varying electric and magnetic fields.
These two fields are inextricably linked as a single entity since
timevarying electric fields produce time-varying magnetic fields and
vice versa. Light and radar are examples of electromagnetic radiation
differing only in their wavelengths (or frequency). Electric and
magnetic fields propagate through space at the speed of light.
Electromagnetic Spectrum
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.
Electromotive Force
The force that can alter the motion of electricity, measured in
terms of the energy per unit charge imparted to electricity passing
through the source of this force. Electromotive force causes current
flow in a circuit.
Electromotive
Producing an electric current through differences in
potential.
Element Set (aka Keplerian elements, classical elements,
satellite elements)
Specific information used to define and locate a particular
satellite. The set includes the catalog number; epoch year, day, and
fraction of day; period decay rate; argument of perigee; inclination;
eccentricity; right ascension of the ascending node; mean anomaly; mean
motion; revolution number at epoch; and element set number. This data is
contained in the two-line orbital elements provided by NASA in the NASA
Prediction Bulletin. See Keplerian elements.
Elevation
The angle at which an antenna must be pointed above the horizon for
optimal reception from a spacecraft.
Elliptical Orbits
Bodies in space orbit in elliptical rather than circular orbits
because of factors such as gravity and drag. The point where the
orbiting satellite is closest to Earth is the perigee, sometimes called
peri-apsis or perifocus. The point where the satellite is farthest from
Earth is called apogee, apoapsis, or apifocus. A line drawn from perigee
to apogee is the line-of-apsides, sometimes called the major-axis of the
ellipse. It's simply a line drawn through the ellipse the long
way.
ELT
Emergency Locator Transmitter. See Search and Rescue.
Energy Budget
A quantitative description of the energy exchange for a physical or
ecological system. The budget includes terms for radiation, conduction,
convection, latent heat, and for sources and sinks of energy.
ENSO (El Nino-Southern Oscillation)
Interacting parts of a single global system of climate fluctuations.
ENSO is the most prominent known source of interannual variability in
weather and climate around the world, though not all areas are affected.
The Southern Oscillation (SO) is a global-scale seesaw in atmospheric
pressure between Indonesia /North Australia, and the southeast Pacific.
In major warm events El Niño warming extends over much of the tropical
Pacific and becomes clearly linked to the SO pattern. Many of the
countries most affected by ENSO events are developing countries with
economies that are largely dependent upon their agricultural and fishery
sectors as a major source of food supply, employment, and foreign
exchange. New capabilities to predict the onset of ENSO events can have
a global impact. While ENSO is a natural part of the Earth's climate,
whether its intensity or frequency may change as a result of global
warming is an important concern.
Environment
The complex of physical, chemical, and biological factors in which a
living organism or community exists.
EOS
See Earth Observing System.
EOSDIS
See Earth Observing System Data and Information System.
EPA (Environmental Protection Agency)
U.S. agency that ensures: Federal environmental laws are implemented
and enforced effectively; U.S. policy--both foreign and domestic--fosters
the integration of economic development and environmental protection so
that economic growth can be sustained over the long term; public and
private decisions affecting energy, transportation, agriculture,
industry, international trade, and natural resources fully integrate
considerations of environmental quality; national efforts to reduce
environmental risk are based on the best available scientific
information communicated clearly to the public; everyone in our society
recognizes the value of preventing pollution before it is created;
people have the information and incentives they need to make
environmentally-responsible choices in their daily lives; and schools
and community institutions promote environmental stewardship as a
national ethic.
Ephemeris
A tabulation of a series of points that define the position and
motion of a satellite. See Keplerian elements.
EPIRB
Emergency Position Indicating Radio Beacon. See Search and
Rescue.
Epoch (aka Epoch Time or T0)
Epoch specifies the time of a particular description of a satellite
orbit. See Keplerian elements.
Equator
An imaginary circle around the Earth that is everywhere equally
distant (90°) from the North Pole and the South Pole. The equator is a
great circle and defines latitude 0°.
ERBE
See Earth Radiation Budget Experiment.
ESA
European Space Agency.
Eutrophication
The process whereby a body of water becomes rich in dissolved
nutrients through natural or man-made processes. This often results in a
deficiency of dissolved oxygen, producing an environment that favors
plant over animal life.
Evaporation
Change from a liquid (more dense) to a vapor or gas (less dense)
form. When water is heated it becomes a vapor that increases humidity.
Evaporation is the opposite of condensation.
Exosphere
The uppermost layer of the atmosphere, its lower boundary is
estimated at 500 km to 1000 km above the Earth's surface. It is only
from the exosphere that atmospheric gases can, to any appreciable
extent, escape into outer space.
External Forcing
Influence on the Earth system (or one of its components) by an
external agent such as solar radiation or the impact of extraterrestrial
bodies such as meteorites.
FAA
Federal Aviation Administration.
Facsmile (FAX)
A process by which graphic or photographic information is
transmitted or recorded by electronic means.
Fahrenheight
Temperature scale designed by the German scientist Gabriel
Fahrenheit in 1709, based upon water freezing at 32 °F and water boiling
at 212 °F under standard atmospheric pressure. Compare with
centrigrade.
False Color
See digital image.
Far Infrared
Electromagnetic radiation, longer than the thermal infrared, with
wavelengths between about 25 and 1000 micrometers. See electromagnetic
spectrum.
Feedhorn
A metallic cylinder closed at one end, used to obtain and direct
radio frequency (RF) energy reflected from a satellite dish. It acts as
a wave guide at microwave frequencies. RF energy inside the horn is
picked up by a small probe; once inside the horn, the wavelength
(energy) of the microwave radiation changes to a guided wave.
FEMA
U.S. Federal Emergency Management Agency.
Feng Yun
Chinese geostationary environmental satellite that was destroyed by
an explosion before launch in April 1994. The name Feng Yun, meaning
Wind and Cloud, was originally applied to the Chinese polar-orbiting
environmental satellite launched in September 1991 (Feng Yun 1-2), which
offered direct readout services. The Chinese polar-orbiter program has
since been abandoned.
Field of View
The range of angles that are scanned or sensed by a system or
instrument, measured in degrees of arc.
Field
The set of influences (electricity, magnetism, gravity) that extend
throughout space.
Filter
Device that while selectively passing desired frequencies removes
undesired ones.
FM
See frequency modulation.
Focal Length
1) In optics, the distance--usually expressed in millimeters--from the
principal point of a lens or concave mirror to its focal point. 2) The
distance, measured from the center of the surface of a parabolic or
spherical reflector (e.g., satellite dish) where RF energy is brought to
essential point focus.
Focal Point
The area where weak signals collected by a satellite dish,
concentrated into a smaller receiving area, converge.
Fog
A cloud on the ground.
Fossil Fuel
Any hydrocarbon deposit that can be burned for heat or power, such
as petroleum, coal, and natural gas.
Fossil
Hardened remains or traces of plant or animal life from a previous
geological period preserved in the Earth's crust.
Frame
A single image or picture. A single complete vertical scan of the
cathode ray tube (CRT).
Free Radicals
Atomic or molecular species with unpaired electrons or an otherwise
open shell configuration, usually very reactive. Specific to atmospheric
chemistry, free radicals are: short-lived, highly reactive, intermediate
species produced by dissociation of the source molecules by solar
ultraviolet radiation or by reactions with other stratospheric
constituents. Free radicals are the key to intermediate species in many
important stratospheric chain reactions in which an ozone molecule is
destroyed and the radical is regenerated. See ozone.
Frequency (F)
Number of cycles and parts of cycles completed per second. F=1/T ,
where T is the length of one cycle in seconds.
Frequency Division Multiplexing
The combining of a number of signals to share a medium by dividing
it into different frequency bands for each signal. See signal.
Frequency Modulation (FM)
The instantaneous variation of the frequency of a carrier wave in
response to changes in the amplitude of a modulating signal. As applied
to APT, the radio signal from the satellite is broadcast on an FM
transmitter and received on the ground on an FM radio receiver. See
frequency division multiplexing, signal.
Front
A boundary between two different air masses. The difference between
two air masses sometimes is unnoticeable. But when the colliding air
masses have very different temperatures and amounts of water in them,
turbulent weather can erupt. A cold front occurs when a cold air mass
moves into an area occupied by a warmer air mass. Moving at an average
speed of about 20 mph, the heavier cold air moves in a wedge shape along
the ground. Cold fronts bring lower temperatures and can create narrow
bands of violent thunderstorms. In North America, cold fronts form on
the eastern edges of high pressure systems. A warm front occurs when a
warm air mass moves into an area occupied by a colder air mass. The warm
air is lighter, so it flows up the slope of the cold air below it. Warm
fronts usually form on the eastern sides of low pressure systems, create
wide areas of clouds and rain, and move at an average speed of 15 mph.
When a cold front follows and then overtakes a warm front (warm fronts
move more slowly than cold fronts) lifting the warm air off the ground,
an occluded front forms.
Frost
Water condensation occurring on surfaces below freezing. Condensing
water turns to ice.
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.
Gain
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.
Geodesy
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.
Geoid
A surface of constant gravitational potential around the Earth--an
averaged surface perpendicular to the force of gravity.
Geosphere
The physical elements of the Earth's surface, crust, and
interior.
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 Earth's 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 oints 35,
790 km (22,240 miles) above the equator at 75° west and 135°
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 geostationary. 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:
- 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 imagery once every thirty
minutes.
- 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.
- The Space Environment Monitor (SEM) measures the condition of
the Earth's magnetic
field, the solar activity and radiation around the spacecraft, and
transmits these data to a central processing facility.
- The Data Collection System (DCS) receives transmitted
meteorological data from
remotelylocated platforms and relays the data to the end users.
- 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.
Geostationary
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. GOES, METEOSAT (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.
Geosynchronous (aka GEO)
Synchronous with respect to the rotation of the Earth. See
geostationary.
Glacier
A multi-year surplus accumulation of snowfall in excess of snowmelt
on land and resulting in a mass of ice at least 0.1 km2 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 NASA's central contribution to the U.S.
Global Change Research Program. 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 system's geosphere,
hydrosphere, atmosphere, and biosphere and their components are, or
potentially are, global variables.
GMS
See Geostationary Meteorological Satellite.
GOES I/GOES 8
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.
GOES NEXT
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
GOES
See Geostationary Operational Environmental Satellite.
Grayscale
Environmental satellite scanners, rather than photographing a scene,
scan a scene line-byline measuring light or heat levels and transmitting
this information as a video image via an amplitude modulated (AM)
subcarrier contained in the satellite's 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 Earth's
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
sun's radiant energy, allowing them to penetrate deep into the
atmosphere or all the way into the Earth's surface. Greenhouse gases and
clouds prevent some of infrared radiation from escaping, trapping the
heat near the Earth's 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 global warming.
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 Earth's center. The intersection
on the line with the Earth's 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 Earth's rotation, its ground track is reduced to an apparent
stationary point on the equator.
GSFC
NASA Goddard Space Flight Center, located in Greenbelt, Maryland.
See NASA Centers.
Guided Wave
Electromagnetic or acoustic wave that is constrained within certain
boundaries, as in a wave guide (transmission line).
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.
Gulf
A large arm of an ocean or sea extending into a land mass.
Habitat
The area or region where a particular type of plant or animal lives
and grows.
Hail
Precipitation composed of balls or irregular lumps of ice. Hail is
produced when large frozen raindrops, or almost any particles, in
cumulonimbus clouds act as embryos that grow by accumulating supercooled
liquid droplets. Violent updrafts in the cloud carry the particles in
freezing air, allowing the frozen core to accumulate more ice. When the
piece of hail becomes too heavy to be carried by upsurging air currents
it falls to the ground.
Hardware
The electrical and mechanical components of a system, as opposed to
software.
Haze
Fine dry or wet particles of dust, salt, or other impurities that
can concentrate in a layer next to the Earth when air is stable.
Heat Balance
The equilibrium existing between the radiation received and emitted
by a planetary system.
Heat Capacity Mapping Mission (HCMM)
A two-channel radiometer launched by NASA to measure the thermal
properties of the terrestrial surface. It had an application to identify
and locate rocks and minerals. One radiometer channel was in the visible
to near infrared (0.5–1.1 micrometers), and the other in the thermal
infrared (10.5–12.5 micrometers). The instantaneous field of view (IFOV)
was about 600 meters.
Hemisphere
Half of the Earth, usually conceived as resulting from the division
of the globe into two equal parts, north and south or east and
west.
Hertz (Hz)
The international unit of frequency equal to one cycle per second.
Radio frequencies are usually expressed in kilohertz/kHz (1,000 cycles
per second) or megahertz/MHz (1,000,000 cycles per second).
Hertzian Waves
Radio waves or other electromagnetic radiation resulting from the
oscillations of electricity in a conductor.
High Resolution Doppler Imager (HRDI)
Carried on UARS, it measures stratospheric winds.
High-Resolution Infrared Radiation Sounder (HIRS)
Instrument carried by NOAA polar-orbiting satellites that detects
and measures energy emitted by the atmosphere to construct a vertical
temperature profile from the Earth's surface to an altitude of about 40
km. Measurements are made in 20 spectral regions in the infrared
band.
High-Resolution Picture Transmission (HRPT)
Real-time, 1.1-kilometer resolution, digital images provided by
NOAA's polar-orbiting environmental satellites, containing all five
spectral channels and telemetry data transmitted as high-speed digital
transmissions. The Advanced Very High Resolution Radiometer (AVHRR)
provides the primary imaging system for APT and HRPT. See TIROS.
High
A digital logic state corresponding to a binary "1." See
low.
Horse Latitudes
The subtropical latitudes (30–35 degrees), where winds are light and
weather is hot and dry. According to legend, ships traveling to the New
World often stagnated in this region and had to throw dead horses
overboard or eat them to survive, hence the name horse latitudes. See
wind.
HRDI
See High Resolution Doppler Imager.
HRPT
See High Resolution Picture Transmission.
Humidity
The amount of water vapor in the air. The higher the temperature,
the greater the number of water molecules the air can hold. For example:
at 60 °F (15 °C), a cube of air one yard on each side can hold up to
4.48 ounces of water. At 104 °F (40 °C), the same cube of air can hold
up to 17.9 ounces of water. Relative humidity describes the amount of
water in the air compared with how much the air can hold at the current
temperature. Example: 50
relative humidity means the air holds half the
water vapor that it is capable of holding; 100
relative humidity means
the air holds all the water vapor it can. At 100
humidity, no more
evaporation can occur until the temperature rises, or until the water
vapor leaves the air through condensation. Absolute humidity is the
ratio of the mass of water vapor present in a system of moist air to the
volume occupied by the mixture, that is, the density of water
vapor.
Hurricanes
Severe tropical storms whose winds exceed 74 mph. Hurricanes
originate over the tropical and subtropical North Atlantic and North
Pacific oceans, where there is high humidity and light wind. These
conditions prevail mostly in the summer and early fall. Since hurricanes
can take days or even weeks to form, time is usually available for
preventive or protective measures. From space, hurricanes look like
giant pinwheels, their winds circulating around an eye that is between 5
and 25 miles in diameter. The eye remains calm with light winds and
often a clear sky. Hurricanes may move as fast as 50 mph, and can become
incredibly destructive when they hit land. Although hurricanes lose
power rapidly as soon as they leave the ocean, they can cause high waves
and tides up to 25 feet above normal. Waves and heavy flooding cause the
most deaths during a hurricane. The strongest hurricanes can cause
tornadoes.
Hydrochlorofluorocarbon (HCFC)
One of a class of compounds used primarily as a CFC substitute. Work
on CFC alternatives began in the late 1970s after the first warnings of
CFC damage to stratospheric ozone. By adding hydrogen to the chemical
formulation, chemists made CFCs less stable in the lower atmosphere
enabling them to break down before reaching the ozone layer. However,
HCFCs do release chlorine and have contributed more to atmospheric
chlorine buildup than originally predicted. Development of non-chlorine
based chemical compounds as a substitute for CFCs and HCFCs
continues.
Hydrologic Cycle
The pathways through which water is cycled in the terrestrial
biosphere.
Hydrosphere
The totality of water encompassing the Earth, comprising all the
bodies of water, ice, and water vapor in the atmosphere.
Hygrometer
Instrument that measures water vapor content in the air and
communicates changes in humidity visibly and immediately through a graph
or a dial. There are three types of hygrometers:
- The hair hygrometer
uses a human hair as the sensing instrument. The hair lengthens when the
air is moist and contracts when the air is dry, but remains unaffected
by air temperature. However, the hair hygrometer cannot respond to rapid
fluctuations in humidity.
- An electric hygrometer uses a plate coated
with carbon. Electrical resistance of the carbon coating changes as the
moisture content of the air changes--changes that translate into relative
humidity. This type of hygrometer is used frequently in the
radiosonde.
- An infrared hygrometer uses a beam of light containing two
separate wave lengths to gauge atmospheric humidity. One of the
wavelengths is absorbed by water vapor, the other is unaffected,
providing an extremely accurate index of water vapor for paths of a few
inches or thousands of feet. See psychrometer.
Hz
See Hertz.
Ice Shelf
A thick mass of ice extending from a polar shore. The seaward edge
is afloat and sometimes extends hundreds of miles into the sea.
IFOV
Instantaneous Field of View. See Multispectral Scanner for sample
usage.
IGY
See International Geophysical Year.
Image Resolution
The area represented by each pixel of a satellite image. The smaller
the area represented by a pixel, the more accurate and detailed the
image. For example, if a U.S. map and a world map are printed on
identically sized sheets of paper, one square inch on the U.S. map will
represent far less area and provide for more detail than one square inch
on the world map. In this example the U.S. map has higher resolution.
APT has a resolution of 4 km, HRPT has a resolution of 1.1 km and WEFAX
resolution is 8 km.
Image
Pictorial representation of data acquired by satellite systems, such
as direct readout images from environmental satellites. An image is not
a photograph. An image is composed of two-dimensional grids of
individual picture elements (pixels). Each pixel has a numeric value
that corresponds to the radiance or temperature of the specific ground
area it depicts. See grayscale.
Imager
A satellite instrument that measures and maps the Earth and its
atmosphere. Imager data are converted by computer into pictures.
In Situ
Latin for "in original place." Refers to measurements made at the
actual location of the object or material measured. Compare with remote
sensing.
Inclination (aka i)
One of the six Keplerian elements, it indicates the angle of the
orbit plane to the central body's equator. See Keplerian elements for
diagram. The elliptical path of a satellite orbit lies in a plane known
as the orbital plane. The orbital plane always goes through the center
of the Earth but may be tilted at any angle relative to the equator.
Inclination is the angle between the equatorial plane and the orbital
plane measured counter-clockwise at the ascending node. A satellite in
an orbit that exactly matches the equator has an inclination of 0°,
whereas one whose orbit crosses the Earth's poles has an inclination of
90°. Because the angle is measured in a counterclockwise direction, it
is quite possible for a satellite to have an inclination of more than
90°. An inclination of 180° would mean the satellite is orbiting the
equator, but in the opposite direction of the Earth's rotation. Some
sun-synchronous satellites that maintain the same ground track
throughout the year have inclinations of as much as 98°. U.S. scientific
satellites that study the sun are placed in orbits closer to the
equator, frequently at 28° inclination. Most weather satellites are
placed in high-inclination orbits so they can oversee weather conditions
worldwide. See orbital inclination.
Information System
All of the means and mechanisms for data receipt, processing,
storage, retrieval, and analysis. Information systems can be designed
for storage and dissemination of a variety of data products--including
primary data sets and both intermediate and final analyses-- and for an
interface providing connections to external computers, external data
banks, and system users. To be effective, the design and operation of an
information system must be carried out in close association with the
primary producers of the data sets, as well as other groups producing
integrated analyses or intermediate products.
Infrared Radiation (IR)
Infrared is electromagnetic radiation whose wavelength spans the
region from about 0.7 to 1000 micrometers (longer than visible
radiation, shorter than microwave radiation). Remote-sensing instruments
work by sensing radiation that is naturally emitted or reflected by the
Earth's surface or from the atmosphere, or by sensing signals
transmitted from a satellite and reflected back to it. In the visible
and near-infrared regions, surface chemical composition, vegetation
cover, and biological properties of surface matter can be measured. In
the mid-infrared region, geological formations can be detected due to
the absorption properties related to the structure of silicates. In the
far infrared, emissions from the Earth's atmosphere and surface offer
information about atmospheric and surface temperatures and water vapor
and other trace constituents in the atmosphere. Since IR data are based
on temperatures rather than visible radiation, the data may be obtained
day or night.
INSAT
Indian National Satellite.
Insolation
Solar radiation incident upon a unit horizontal surface on or above
the Earth's surface.
Instantaneous Field of View (IFOV)
The field of view of a scanning detector system at a given instant.
The range of angles scanned by the system is then called the field of
view, or swath width.
Integrated Circuit (IC)
A solid state electronic circuit that consists of several
micro-components constructed to perform a special function.
International Date Line
An imaginary line of longitude 180° east or west of the prime
meridian.
International Designator
An internationally agreed-upon naming convention for satellites. The
designator contains the last two digits of the launch year, the launch
number of the year, and the part of the launch, i.e., "A" indicates
payload, "B" the rocket booster, or second payload, etc.
International Geophysical Year (IGY)
(1957–58) The IGY was organized by the scientific community through
the International Council of Scientific Unions (ICSU) . It was
highlighted by international cooperation in the exploration of
world-wide geophysical phenomena and by the inauguration of the space
age through the launching of the first satellites (USSR's Sputnik I and
US Explorer 1) to study the upper atmosphere and Earth's nearby
environment.
International Space Year (ISY)
(1992) Designated the first international celebration of humanity's
future in the space age. Themes included the global perspective of the
space age, discovery, exploration, and scientific inquiry. An important
ISY scientific focus was Mission to Planet Earth. A wide range of
educational programs and public events emphasized ISY's global
perspective. 1992 also commemorated the 500th anniversary of Columbus’
voyage to the New World and the 35th anniversary of the International
Geophysical Year.
International System of units (SI)
The International System of Units prescribes the symbols and
prefixes shown in the table to form decimal multiples and submultiples
of SI units. The following examples illustrate the use of these
prefixes. 0.000,001 meters = 10-6 meters =1 micrometer = 1µm; 1000
meters = 103 meters = 1 kilometer = 1 km; 1,000,000 cycles per second =
106 hertz = 1 megahertz = 1 MHz
Ion
Atom or molecule that has acquired an electric charge by the
loss/gain of one or more electrons.
IPS
Inches per second.
IR
See infrared.
Isobars
Lines drawn on a weather map joining places of equal barometric
pressure.
Isothermal
Of or indicating equality of temperature.
Isotherms
Lines connecting points of equal temperature on a weather
map.
Isthmus
Narrow strip of land located between two bodies of water, connecting
two larger land areas.
ITOS (Improved TIROS Operational Satellite)
Second generation, polar-orbiting, environmental satellites utilized
to augment NOAA's world-wide weather observation capabilities. ITOS were
launched from 1970–1976, but eventually replaced by the third generation
of polar-orbiting, environmental satellites TIROS-N (first launched in
1978). See TIROS.
Japanese National Space Development Agency (NASDA)
The agency reports to the Japanese Ministry of Science and
Technology.
Jet Stream
Rivers of high-speed air in the atmosphere. Jet streams form along
the boundaries of global air masses where there is a significant
difference in atmospheric temperature. The jet streams may be several
hundred miles across and 1–2 miles deep at an altitude of 8–12 miles.
They generally move west to east, and are strongest in the winter with
core wind speeds as high as 250 mph. Changes in the jet stream indicate
changes in the motion of the atmosphere and weather.
Joint Education Initiative (JEI)
The JEI project was developed by USGS, NOAA, NASA, industry, and
teachers to enable teachers and students to explore the massive
quantities of Earth science data published by the U.S. Government on
CD-ROM. JEI encourages a research and analysis approach to science
education.
JPL (Jet Propulsion Laboratory)
See NASA Centers.
JSC (Johnson Space Center)
See NASA Centers.
Kepler's Three Laws of Motion
Any spacecraft launched into orbit obeys the same laws that govern
the motions of the planets around our sun, and the moon around the
Earth. Johannes Kepler formulated three laws that describe these
motions: 1) Each planet revolves around the sun in an orbit that is an
ellipse with the sun as its focus or primary body. Kepler postulated the
lack of circular 2) The radius vector--such as the line from the center
of the sun to the center of a planet, from the center of Earth to the
center of the moon, or from the center of Earth to the center of gravity
of a satellite--sweeps out equal areas in equal periods of time. 3) The
square of a planet's orbital period is equal to the cube of its mean
distance from the sun times a constant. As extended and generalized,
this means that a satellite's orbital period increases with its mean
distance from the planet. See Newton's law of universal gravitation and
laws of motion. orbits--only elliptical ones--determined by gravitational
perturbations and other factors. Gravitational pulls, according to
Newton, extend to infinity, although their forces weaken with istance
and eventually become impossible to detect. (See Newton's law of
universal gravitation.) Spacecraft orbiting the Earth are primarily
influenced by the Earth's gravity and anomalies in its composition, but
they also are influenced by the moon and sun and possibly other
planets.
Keplerian Elements (aka Satellite Orbital Elements)
The set of six independent constants which define an orbit--named for
Johannes Kepler [1571–1630]. The constants define the shape of an
ellipse or hyperbola, orient it around its central body, and define the
position of a satellite on the orbit. The classical orbital elements
are: a: semi-major axis, gives the size of the orbit, e: eccentricity,
gives the shape of the orbit, i: inclination angle, gives the angle of
the orbit plane to the central body's equator W: right ascension of the
ascending node, which gives the rotation of the orbit plane from
reference axis, w: argument of perigee is the angle from the ascending
nodes to perigee point, measured along the orbit in the direction of the
satellite's motion, q: true anomaly gives the location of the satellite
on the orbit.
Kilohertz (kHz)
One thousand hertz, i.e., one thousand cycles per second.
Kilometer (km)
Metric unit of distance equal to 3,280.8 feet or .621 statute
miles.
Knot
Unit of speed of one nautical mile (6,076.1 feet) an hour.
KSC (Kennedy Space Center)
See NASA Centers.
Lake
A body of fresh or salt water entirely surrounded by land.
Land Breeze
A nocturnal coastal breeze that blows from land to sea. In the
evening the water may be warmer than the land, causing pressure
differences. The land breeze is the flow of air from land to sea
equalizing these pressure differences. See sea breeze.
Landsat
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. Landsat has a spatial resolution
of 28.5 meters. Restructured Federal agency responsibilities for the
Landsat program are effective for the acquisition and operation of
Landsat 7. New operating policy specifies that NOAA will be responsible
for satellites after they are placed in orbit, NASA will be responsible
for the development and launch of Landsat 7, and that the U.S.
government will provide unenhanced data to users at no cost beyond the
cost of fulfilling their data request.
Landsats (aka Earth Resources Satellites)
Any land remote-sensing satellites. Includes the U.S. Landsat system
and the French SPOT.
LaRC (Langley Research Center)
See NASA Centers.
Laser (Light Amplification by Stimulated Emission of
Radiation)
Active instrument that produces discretely coherent pulses of light
(light waves with no phase differences, or with predictable phases
differences, are said to be coherent).
Laser Ranging
The use of lasers to measure distances.
Latitude (aka geodetic latitude)
The angle between a perpendicular at a location, and the equatorial
plane of the Earth.
Legend
A listing that contains symbols and other information about a
map.
LeRC (Lewis Research Center)
See NASA Centers.
Light
1) Form of radiant energy that acts upon the retina of the eye,
optic nerve, etc., making sight possible. This energy is transmitted at
a velocity of about 186,000 miles per second by wavelike or vibrational
motion.
2) A form of radiant energy similar to this, but not acting on
the normal retina, such as ultraviolet and infrared radiation. Interplay
between light rays and the atmosphere cause us to see the sky as blue,
and can result in such phenomena as glows, halos, arcs, flashes, and
streamers.
Lightening
A discharge of atmospheric electricity accompanied by a vivid flash
of light. During thunderstorms, static electricity builds up within the
clouds. A positive charge builds in the upper part of the cloud, while a
large negative charge builds in the lower portion. When the difference
between the positive and negative charges becomes great, the electrical
charge jumps from one area to another, creating a lightning bolt. Most
lightning bolts strike from one cloud to another, but they also can
strike the ground. These bolts occur when positive charges build up on
the ground. A negative charge called the "faintly luminous streamer" or
"leader" flows from the cloud toward the ground. Then a positively
charged leader, called the return stroke, leaves the ground and runs
into the cloud. What is seen as a lightning bolt is actually a series of
downward-striking leaders and upward-striking return strokes, all taking
place in less than a second. Lightning bolts can heat the air to
temperatures hotter than the surface of the sun. This burst of heat
makes the air around the bolt expand explosively, producing the sound we
hear as thunder. Since light travels a million times faster than sound,
we see lightning bolts before we hear their thunderclaps. By counting
the seconds between a flash of lightning and the thunderclap and
dividing by five, we can determine the approximate number of miles to
the lightning stroke. See thunderstorm.
Limb Viewing (Occultation)
The process of viewing the atmosphere at a tangent to the Earth's
surface. The viewing signal, from a star or another satellite, is
occulted or obscured by the intervening atmosphere. The absorption of
light from the sun or star provides information on the properties of the
atmosphere at different heights. Limb viewing instruments can also sense
infrared or microwave-emitted radiation from the atmosphere.
Line-of-Apsides (aka Major-Axis of the Ellipse)
The straight line drawn from the perigee (point of orbit closest to
Earth) to the apogee (point of orbit farthest from Earth) is the
lineof-apsides.
Line-of-Nodes
The line created by the intersection of the equatorial plane and the
orbital plane.
Line-of-Site
Area within which visible contact can be made. For example, NOAA
polar-orbiting satellites continuously transmit the APT signal. Radio
reception of the APT signal is possible only when the satellite is above
the horizon of a particular location (not obstructed by the Earth's
surface), with a line-of-sight contact with the satellite.
Logarithm
Exponent of the power to which it is necessary to raise a fixed
number (the base) to produce the given number. For example, the
logarithm of 100 (base 10) is 2 because 102 = 100.
Longitude
The angular distance from the Greenwich meridian (0°), along the
equator. This can be measured either east or west to the 180th meridian
(180°) or 0° to 360° W.
Loss of Signal (LoS)
The inability to receive a satellite signal because the satellite's
orbital path has taken it below the antenna's horizon. This term is
relevant to all satellites except geostationary.
Low or Low-Pressure System
A horizontal area where the atmospheric pressure is less than it is
in adjacent areas. Since air always moves from areas of high pressure to
areas of low pressure, air from these adjacent areas of higher pressure
will move toward the low pressure area to equalize the pressure. This
inflow of air toward the low will be affected by the Earth's rotation
(see Coriolis force) and will cause the air to spiral inward in a
counterclockwise direction in the northern hemisphere. The air
eventually rises near the center of the low, causing cloudiness and
precipitation. The air in a low rotates in a counterclockwise direction
in the Northern Hemisphere, and in a clockwise direction in the Southern
Hemisphere. Low-pressure cells are called cyclones.
Low
A logic state corresponding to a binary "0". Satellite imagery is
displayed on a computer monitor by a combination of highs and lows. See
high.
Magnetosphere
Region surrounding a celestial body where its magnetic field
controls the motions of charged particles. The Earth's magnetic field is
dipolar in nature. That is, it behaves as if produced by a giant bar
magnet located near the center of the planet with its north pole tilted
several degrees from Earth's geographic north pole. The Earth's magnetic
field presents an obstacle to the solar wind, as a rock in a running
stream of water. This obstacle is called a bow shock. The bow shock
slows down, heats, and compresses the solar wind, which then flows
around the rest of Earth's magnetic field. See Van Allen belts.
MAPS-NET
MAryland Pilot Earth Science and Technology Education NETwork.
NASAsponsored education project designed to complement NASA's Mission to
Planet Earth. MAPS-NET has been developed to enrich math and science
curricula and enhance teacher preparation in Earth system science.
Middle and high school teachers learn about Earth sciences and satellite
direct readout at graduate-level summer workshops; academia, federal
agencies, and the private sector form the support network.
Mean Anomaly (aka M0 or MA or phase)
Specifies the mean location (true anomaly specifies the exact
location) of a satellite on an orbit ellipse at a particular time,
assuming a constant mean motion throughout the orbit . Epoch specifies
the particular time at which the satellites position is defined, while
mean anomaly specifies the location of the satellite at epoch. Mean
anomaly is measured from 0° to 360° during one revolution.
It is defined
as 0° at perigee, and hence is 180° at apogee. See Keplerian
elements.
Mean Motion (aka N0)
Averaged speed of a satellite in a non-circular orbit (i.e.,
eccentricity>0). Diagram, page 19. Satellites in circular orbits travel
at a constant speed. Satellites in non-circular orbits move faster when
closer to the Earth, and slower when farther away. Common practice is to
compute the mean motion (average the speed), which is measured in
revolutions per day.
Measurement System Integrity
The tracking and documentation over the long term of all causes of
error or uncertainty in a final data-analysis product. These include
instrument calibration, adequacy of measurement validation, data
coverage and sampling density, availability and quality of ancillary
data, procedures for data analysis and reduction, the results of checks
against independent measurement, and quantitative error
analysis.
Measurement Validation
The establishment of confidence in the numerical relationship
between the calibrated sensor output and the actual variable being
measured.
Mercator Projection
A method of making maps in which the Earth's surface is shown as a
rectangle with the meridians as parallel straight lines spaced at equal
intervals and the parallels of latitude as parallel straight lines
intersecting the meridians at right angles. Areas away from the equator
appear larger than they are, with the greatest distortion near the
poles.
Mesopause
The upper boundary of the mesosphere where the temperature of the
atmosphere reaches its lowest point.
Mesosphere
The atmospheric layer above the stratosphere, extending from about
50 to 85 kilometers altitude. The temperature generally decreases with
altitude.
Metadata
Information describing the content or utility of a data set. For
example, the dates on which data were procured are metadata.
Meteor
The former Soviet Union's series of polar-orbiting weather
satellites. The Meteor satellites transmit images in a system compatible
with the NOAA polar-orbiting satellites.
Meteorology
Study of the atmosphere and its phenomena.
METEOSAT
METEOrological SATellite. Europe's geostationary weather satellite,
launched by the European Space Agency and now operated by an
organization called Eumetsat. METEOSAT transmits at 1691 and 1694.5
MHz.
Metsat
Generic term for meteorological (weather) satellites.
MHz (megahertz)
10^6 hertz.
Micrometer (aka micron)
One millionth of a meter, used to measure wavelengths in the
electromagnetic spectrum.
Microprocessor
Controlling unit of a microcomputer, laid out on a tiny silicon chip
and containing the logical elements for handling data, performing
calculations, carrying out stored instructions, etc.
Microwave
Electromagnetic radiation with wavelengths between about 1000
micrometers and one meter.
Middle Infrared
Electromagnetic radiation between the near infrared and the thermal
infrared, about 2–5 micrometers.
Millibar (mb)
One thousandth of a bar, a unit of atmospheric pressure. The average
atmospheric pressure at sea level is 1.01325 bars or 1013.25 mb. See
pascal (Pa), atmospheric pressure.
Mission to Planet Earth (MTPE)
International research program to understand our planet's
environment as a system. A major challenge of MTPE is to observe,
understand, model, assess, and eventually predict global change. Meeting
this challenge will help to evaluate the impact that human activity
(e.g., clearing forests and burning fossil fuels) has on our
environment, and to distinguish human-induced changes from the effects
of natural events (e.g. volcanic eruptions, erosion). NASA's MTPE uses
space-, aircraft-, and ground-based measurements to provide the
scientific basis for understanding global change. The program will
produce longterm global maps of clouds, land and ocean vegetation,
atmospheric ozone, sea-surface temperature, and other global processes
necessary to understand the state of the Earth and to detect any
patterns of change. This information will be available to scientists and
policy makers through the Earth Observing System Data and Information
System (EOSDIS). The centerpiece of NASA's MTPE will be the Earth
Observing System (EOS), a series of satellites planned for launch
beginning in 1998. Measurements from EOS will be complemented by the
Earth Probes, a series of discipline-specific satellites and instruments
designed to observe Earth processes where smaller platforms and/or
different orbits from EOS are required. Planned Earth Probes will
measure tropical rainfall, ocean productivity, ozone, and ocean surface
winds. In addition, MTPE includes current NASA Earth science missions
collecting important data on the global environment, such as the Upper
Atmosphere Research Satellite (UARS) and the Ocean Topography Experiment
(TOPEX/POSEIDON), Space Shuttle experiments such as ATLAS, and aircraft
campaigns.
Model (noun)
A mathematical representation of a process, system, or object
developed to understand its behavior or to make predictions. The
representation always involves certain simplifications and
assumptions.
Modem (Modulator/demodulator)
Device that allows two computers (which use binary data in the form
of bits) to communicate using a telephone line (which uses tones). When
the computer is transmitting data, the modem is needed to modulate
binary data into tones. When receiving data, the device is needed to
demodulate the tones to obtain the binary data required by the computer.
Since the computer must be both a transmitter and receiver of data, the
modem must be able to modulate and demodulate data.
Modulation
Variation in the frequency of a radio wave in accordance with some
other impulse. Modulation is essential to communication systems in which
a number of different signals must all share the same medium. One way
this sharing can be accomplished is to place each signal in its own band
of frequencies in the medium. Amplitude modulation and frequency
modulation are two ways in which signals The combining of a number of
signals to share a communication medium by dividing it into different
frequency bands for each signal is called frequency-division
multiplexing. Amplitude modulation (AM) is technologically quite simple,
and the bandwidth of the amplitude-modulated carrier is at most twice
the bandwidth of the modulating signal. However, an amplitude-modulated
carrier is very prone to the effects of additive noise. Frequency
modulation (FM) is more complicated than amplitude modulation, and the
bandwidth of the frequency-modulated carrier can be many times that of
the modulating signal. However, the process of demodulating a
frequency-modulated carrier eliminates much of the deleterious effects
of additive noise. This trade-off between bandwidth and noise reduction
characterizes most communication situations.
Monsoon
Heavy winds characterized by a pronounced seasonal change in
direction. Winds usually blow from land to sea in the winter, while in
the summer, the flow reverses and precipitation is more common. Monsoons
are most typical in India and southern Asia.
Montreal Protocol
An international agreement to drastically reduce CFC production, the
Protocol was adopted in Montreal in 1987. It was significantly
strengthened at a subsequent meeting in London in 1990 that called for a
complete elimination of CFCs by the year 2000. The agreement was again
amended |