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Words begining with "A"
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).
absolute humidity = mass of water vapor/volume of air
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 stronq 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 vinegar)
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 globe. 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 that 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 (AOS)
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.
A/D
Analog to Digital. Used to refer to the conversion of analoq data
to its diqital equivalent.
Advanced Very High Resolution Radiometer (AVHRR)
A five-channel scanning instrument that quantitatively measures
electromagnetic radiation, flown on NOAA environmental satellites.
AVHRR remotely determines cloud cover and surface temperature.
Visible and infrared detectors observe vegetation, clouds, lakes,
shorelines, snow, and ice. TIROS Automatic Picture Transmissions
(APT) are derived from this instrument. 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.
AIR
Airborne Imaging Radar.
Air Mass
Large body of air, often hundreds or thou- sands of miles across,
containing air of a simi- lar 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.
Primary Air Pollutants
- sulfur dioxide
- carbon monoxide
- nitrogen dioxide
- ground-level ozone
- lead
- carbon particles
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.
AKA
Also Known As.
Albedo
The ratio of the outgoing solar radiation reflected by an object
to the incoming solar radiation incident upon it.
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 determined from a set of radiation observations
originally sensed by satellite sounding instruments.
Alkaline
Substance capable of neutralizing acid, with a pH greater than
7.0. See pH.
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 topoqraphy.
Altitude
Height above the Earth's surface.
Ampere (Amp)
Standard unit to measure the strength of an electric current. One
amp is the amount of current produced by an electromotive force of
one volt acting through the resistance of one ohm. The ampere is .1
of the theoretical electromagnetic unit of current. Named for the
French physicist Andre Marie Ampere. See ohm.
Amplitude
The magnitude of the displacement of a wave from a mean value. For
a simple harmonic wave, it is the maximum displacement from the mean.
For more complex wave motion, amplitude is usually taken as one-half
of the mean distance (or difference) between maxima and minima.
Amplitude Modulation (AM)
One of three ways to modify a sine wave signal in order to make it
"carry" information. The strength (amplitude) of a signal varies
(modulates) to correspond to the transmitted information. As applied
to APT, an audible tone of 2400 Hz is amplitude
modulated, with the maximum signal corresponding to light areas of a
photograph, the minimum levels black, and the intermediate strengths
various shades of qray. See grayscale.
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 wind-driven cups or from wind pressure through a
tube pointed into the wind.
Anomaly
- The deviation of (usually) temperature or precipitation in a
given region over a specified period from the normal value for the
same region.
- 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
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 increased 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
- The physical size of antenna components is determined by the
frequency of the transmissions it will receive - the higher the
frequency, 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 transmitting 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 a
simple vertical whip antenna is the easiest sort of omnidirectinal
antenna to mount on a vehicle.
- The antenna needs to produce sufficient signal qain 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/ trsnsmitter
- 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 who 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 NOAAs 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 greater the gain from the signal.
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.
Anticyclone
A high pressure area where winds blow clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere. See
cyclone, wind.
Apogee (aka Apoapsis or Apifocus)
On an elliptical orbit path, point a which a satellite is farthest
from the Earth. See Perigee
Diagram.
Aquifer
Layer of water-bearing permeable rock, sand, or gravel capable of
providing significant amounts af water
ARGOS
French random-access Doppler data collection system. Used on NOAAs
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.
Arctic Circle
The parallel of latitude that is approximately 66.5 degrees north
of the equator and that circumscribes the northern friqid zone.
Artificial Intelligence (AI)
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, or 93,440,974 miles 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 1 992) and ATLAS 2 (March-April,
1993) began a comprehensive and systematic collection of data that
will help establish benchmarks for atmospheric conditions and the
suns 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 qlobe and in response to seasonal chanqes.
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 (5 - 9 mi), 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 (31 mi). 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 degrees 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 degrees Celsius.
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 Earths surface, the atmosphere blends into space.
The few particles of gas here can reach 4,500 degrees F (2,500
degrees C) during the day.
Atmospheric Infrared Sounder
Advanced sounding instrument selected to fly on the EOS-PM I
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 degree 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 Ib/in2. The standard value for
atmospheric pressure at sea level is:
- 29.92 inches of mercury
- 760 mm of mercury
- 1013.25 millibars (mb)
- 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 Window
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).
Automatic Picture Ttansmission (APT)
System developed to make real-time reception of satellite images
possible whenever an APT- equipped 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.
Azimuth
The direction, in degrees referenced to true north, that an
antenna must be pointed to receive a satellite signal (compass W
direction). The angular distance is measured in a Clockwise
direction.
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