S E N T I N E L S I N T H E S K Y :
W E A T H E R S A T E L L I T E S
(PART I)
By Robert Haynes
When we talk about the weather, we often find ourselves fascinated
with its complexity and capriciousness. We are as curious as were
our ancient ancestors, who were acutely aware of how weather affected
their food supply and survival. Some tribes and cultures erected
permanent structures, with which they could continually chart the
course of the Sun and monitor seasonal changes.
Today we are no less relenting in our interest in the weather. We
have built and launched into space a family of weather satellites
that watch the weather from high above us. These "sentinels in the
Sky" are constantly on guard for those inevitable moments when
friendly weather turns to foe. They are tireless observers, telling
us when a distant low-pressure system over the Atlantic may develop
into a hurricane and threaten our lives with damaging winds. They
monitor the warming ocean currents of an El Nino that may affect
crops in the United States or Europe by changing expected
precipitation patterns. They allow us to understand the weather with
some degree of certainty, and not be victims of nature's whims.
Benjamin Franklin was the first American to suggest weather could be
predicted. From newspaper articles, Franklin deduced that severe
storms generally move across the nation from west to east. He further
deduced that if this were so, observers could follow a storm and
notify those ahead of its path that it was coming.
Franklin's ideas were finally put to practical use shortly after the
telegraph was invented in 1837. This revolutionary form of
communication soon spanned the country. It wasn't long until it was
used to link a network of weather watchers, who clicked their
observations along telegraph lines to a central office where a
national weather map was created.
Today, satellites are those observers, beeping messages to a
receiving antenna connected to a computer. Meteorologists analyze
the messages and use the data to predict how the weather will behave
and how it will affect us.
A V I E W F R O M A B O V E
Weather watchers along the telegraph lines had to base their
understanding about the weather solely on what they could see from
the ground. Well into the 20th century, meteorologists still based
most of their knowledge on ground observations. Having no way to
observe or study weather patterns over long periods, and no way to
monitor cloudtops, meteorologists had little notion of large-scale
weather behavior. If aerial views were made, they were taken from
airplanes or weather balloons, but were of too short a duration to
provide the kind of information needed. Some progress was made in
1959 when the U.S. Army Signal Corps launched VANGUARD II, but it was
also short lived.
Then, in 1960, the National Aeronautics and Space Administration
(NASA) placed in orbit the first TIROS (Television Infrared
Observational Satellite). With its tiny TV cameras, TIROS flew over
more than two-thirds of the Earth's surface. Its pictures revealed
global weather patterns of clouds, and provided meteorologists with a
new tool--a nephanalysis, or cloud chart. These high-altitude views
sharpened meteorologists' scrutiny of weather and of the environment,
and promised even greater benefits to come.
NASA built and launched bigger and better TIROS satellites. By 1965
nine more TIROS satellites had been launched. They had progressively
longer operational times and carried infrared radiometers to study
Earth's heat distribution. Several satellites were placed in polar
orbits rather than near-equatorial ones so they could take pictures
over more of the Earth's surface.
TIROS 8 had the first Automatic Picture Transmission (APT) equipment.
This instrument allowed pictures to be sent back to Earth right after
they had been taken instead of stored for later transmission.
Eventually APT pictures could be received on fairly simple ground
stations.
TIROS 9 and 10 were test satellites of improved configurations for
the TIROS Operational Satellite (TOS) system. TOS satellites were
often called ESSA after the government agency that financed and
operated them, the Environmental Sciences Services Administration.
ESSA satellites were placed in Sun-synchronous orbits, so they would
pass over the same positions on the Earth at the same time every
day. This would allow meteorologists to view local cloud cover
changes on a 24-hour basis.
With the success of TIROS, NASA created a second-generation research
satellite called NIBUS. More complex than TIROS, NIMBUS satellites
carried an APT system, an advanced TV cloud-mapping camera system,
and an infrared radiometer that allowed pictures at night for the
first time.
Seven NIMBUS satellites were placed in orbit between 1964 and 1978.
NIMBUS satellites tested space-borne meteorological equipment that
led to a fully operational weather observing service with
24-hour-a-day coverage.
Today, weather satellites scan the Earth. With their vigilance, not a
single tropical storm anywhere on Earth goes undetected. The early
detection and warnings they provide have saved thousands of lives.
When Mt. St. Helens erupted on May 18, 1980, weather satellites
looked on as tons of volcanic ash were spewed into the atmosphere.
They photographed hourly the eastwardly spread of the massive dust
clouds, allowing meteorologists to warn aircraft pilots of the danger
and to study the effects such an explosion might have on the world's
climate. Fortunately, those effects in this instance were slight.
Satellites are the workhorses of a complete weather monitoring
system. They scan the globe day and night, transmitting back weather
information such as temperatures, cloud formations, wind patterns,
sea currents, etc. For years, the swirling cloud patterns have been
standard props for TV weathercasters. Hardly anyone can tune in a
weathercast without seeing a "satellite view."
A C R O S S G O V E R N M E N T L I N E S
Our understanding of the weather has multiplied during the years
weather satellites have operated. The service these weather watchers
provide span many governmental lines both here and overseas. NASA
contributes the research and development, and over sees procurement
of these spacecraft, and evaluates their performance in flight. Once
NASA launches the satellites into their appropriate orbits, the
responsibility for operating them falls to another government agency;
however, NASA continues its research role even after the spacecraft
become operational. NASA's broad space program generates advanced
technologies that are tested and applied to produce new generations
of satellites.
The Department of Commerce's National Oceanic and Atmospheric
Administration (NOAA) is the government agency that directs the
nation's system of weather satellites. NOAA manages the processing
and distribution of the millions of bits of data and images these
satellites produce daily. The prime consumer is NOAA's Weather
Service, which employs satellite data to create forecasts for
television, radio, and weather advisory services. Satellite
information is also shared by government departments such as
Agriculture, Interior, Defense, Transportation, and Energy.
Weather satellites create an international network. Information is
routinely shared among the member nations of the World Meteorological
Organization as well as with nations that operate their own weather
satellites such as Japan, India, the Soviet Union, and members of the
European Space Agency.
N O A A S A T E L L I T E S Y S T E M
NOAA was established in 1970 inside the U.S. Department of Commerce
with the mission to ensure the safety of the general public from
atmospheric phenomena and to provide the public with an understanding
of the Earth's environment and resources. NOAA was also given the
responsibility to chart the airways, oceans, and waters of the United
States, and to guide the development of marine fisheries. One of the
ways NOAA does this is by operating its system of weather satellites.
Command and Data Acquisition Stations in Wallops, VA and Fairbanks,
AK send commands to the orbiting satellites and receive data
transmissions. A fully staffed weather satellite processing and
distribution facility is located in Suitland, MD just outside
Washington DC.
Besides looking down on weather conditions around the world,
satellites perform a host of other services. They assess crop growth
and other agricultural conditions, sense shifting ocean currents, and
measure surface temperatures of oceans and land. They relay data from
surface instruments that sense tide conditions, Earth tremors, river
levels, and precipitation.
Weather satellites also broadcast the correct time as it is precisely
measured by the Department of Commerce's National Bureau of
Standards. They receive weather data from ocean buoys, weather
balloons, and aircraft in flight, relaying this data to the Data
Acquisition Stations. Using a facsimile technique, they also
broadcast cloud cover pictures and charts. [See "The Value of Weather
Satellites"]
The TIROS-N series of satellites broadcast pictures and data so that
any properly equipped receiving station on the ground can receive the
pictures without needing to link with an expensive computer. This
capability has prompted many schools across the country to install
antennas and to build their own weather receiving systems. [Secondary
school teachers may obtain a copy of TEACHER'S GUIDE FOR BUILDING AND
OPERATING WEATHER SATELLITE GROUND STATIONS from the Educational
Programs Officer, NASA Goddard Space Flight Center, Greenbelt, MD
20771]
NOAA's operational weather satellite system comprises two types of
satellites: Polar Orbiters and Geostationary Satellites (GOES). The
polar orbiters constantly circle the globe, providing coverage of the
globe every day. They circle in low orbits (850 km) and support
large-scale, multiday forecasts. The GOES satellites circle in a much
higher orbit (35,000 km) so that their orbital rate exactly matches
the rotation of the Earth's surface. This keeps them above a fixed
spot on the surface, providing a constant vigil for severe weather
conditions that may spawn tornadoes, flash floods, hail storms, and
hurricanes. Both kinds of satellites are necessary to provide a
complete global weather monitoring system.
T H E V A L U E O F W E A T H E R S A T E L L I T E S
The value of weather satellites to save lives has been known from the
beginning. Their ability to track storms and permit early warnings
has been their greatest contribution. However, their benefits do not
end with observing the tops of clouds and storm systems.
Satellites can pinpoint different temperature boundaries in ocean
surface areas, and give commercial fishermen vital clues to the
whereabouts of commercial fish such as tuna, herring and swordfish.
They can provide early frost warnings, which can save millions of
dollars a day for citrus growers who must then heat their groves. In
Hawaii, rain warnings are provided, giving crucial information for
sugarcane harvesting. Satellites also play a role in forest
management and fire control.
NOAA's polar-orbiting satellites observe snow and ice melting
conditions, enabling water supply managers to plan irrigation and
flood control. This is especially important to the multi-billion
dollar agricultural economies of our western states, where mountain
runoff provides an estimated 70 percent of the water supply.
Satellite ice monitoring helps extend the shipping season on the
Great Lakes into the winter months, generating extra economic
activity for middle America and neighboring Canadian provinces.
S E V E R E S T O R M S U P P O R T
Geostationary satellites continuously watch atmospheric conditions
that breed tornadoes, squall lines, and other severe storms. The
"triggers" for such events often can be detected by satellites before
the actual storms develop. When they do develop, the satellites
monitor storm life cycles, and track movements. The value of this
information is increasing steadily as new applications and small
interactive computer systems are developed from the partnership of
government, industry, and our high schools and universities.
R A I N F A L L
Imagery from space is also used to estimate rainfall during
thunderstorms and hurricanes for flash flood warnings. Using GEOS
satellite data, interactive computer technologies estimate the
precipitation amounts associated with severe weather. During
Hurricane Diana, for example, calculations indicated nearly 20 inches
of rainfall over North Carolina in a 2-day period. Actual rainfall
reached 18 inches. Interactive computer technology is also used to
estimate snowfall accumulations and overall extent of snow cover.
Such data help meteorologists issue winter storm warnings and spring
snow melt advisories. Satellite sensors also detect ice fields, and
map the movements of sea and lake ice. By also monitoring the
southward progression of freezing seasonal temperatures, satellite
imagery has often allowed forecasters enough time to warn crop
growers to light smudge pots or take other measures to protect crops
from damage.
V O L C A N I C M O N I T O R I N G
Far above the surface, satellites provide remarkable views of
volcanic eruptions. Both the polar orbiting and GOES satellites
monitor eruptions when they occur, allowing scientists to analyze and
track dust clouds.
So far, numerous volcanic eruptions have been detected with sensors
on polar-orbiting satellites. The satellites send this data to the
Smithsonian Scientific Event Alert Network where it is disseminated
to the scientific community, federal and state agencies, and foreign
countries.
F I R E D E T E C T I O N
Blazes can be located by their smoke plumes and the heat registered
on infrared sensors. Sensors on both the polar orbiters and the GOES
have located wilderness fires in remote and difficult-to-reach areas,
enabling emergency forces to better contain the flames.
O C E A N O G R A P H Y
Large-scale weather patterns are affected by temperatures and
currents in the oceans. Polar-orbiting sensors compute some 20-40
thousand global sea temperatures daily. In areas around the United
States, these readings reveal ocean currents and features such as the
Gulf Stream, its wall and associated eddies, upwellings off the West
Coast, and the Gulf of Mexico loop currents.
Ocean surface temperatures help meteorologists study ocean influences
on climate. Such an influence is El Nino, a widespread warming of the
waters off the west coast of South America. In 1982-83, El Nino and
its associated weather-pattern reversal over the coast of South
America brought severe flooding to Ecuador and northern Peru, while
it left a drought in other areas of the world.
V E G E T A T I O N I N D E X M A P P I N G
Since 1982, NOAA has used satellites to look at the progress of crops
and to study vegetation growth. Since 1984, vegetation index maps
have been sent to the Weather Service field offices to help farmers
monitor the growing season. The multispectral images of the
polar-orbiting satellites make this possible.
By measuring the greenness of plant chlorophyl, scientists can
determine not only the health of crops, but can also pinpoint areas
of drought, desert creep, or deforestation the world over. Weather
scientists of many developing nations have been trained in this
technique under a program conducted by NOAA for the Agency for
International Development.
F I S H E R I E S
Commercial fishery operations have also benefited from the data
supplied by weather satellites. Determining the currents and sea
temperatures can help locate schools of tuna or salmon and can assist
in tracking the movement of fish eggs and larvae. Satellite data can
be used to study hypoxia, a severe lack of oxygen at deep sea levels
that can completely block the growth and development of sea life.
* * *
(PART II)
NASA FACTS, SENTINELS IN THE SKY: WEATHER SATELLITES
N E W S A T E L L I T E S , S E N S O R S , S Y S T E M S
In the near future, satellite research and applications will resolve
a global view of the atmosphere, ocean and land in unprecedented
breadth and depth. This will mean a big shift in emphasis for the
weather satellite system. Originally driven by space hardware, the
system is increasingly being driven by information. Data processing
and distribution will get more emphasis as this information explosion
develops.
Three new polar-orbiting satellites are to be launched beginning in
1992, carrying a new, eagerly awaited instrument called an Advanced
Microwave Sounding Unit. For the first time, it will give forecasters
global profiles of temperature and moisture inside cloudy regions
over the world's oceans and continents. The instruments will provide
new data every six hours, so that when the satellite passes over an
area of severe storms, local forecasters will, for the first time,
have information about atmospheric stability close to, and inside, a
storm system. This is important in predicting storms because air mass
instability feeds tornadoes, hurricanes, and other severe weather
conditions.
Five new geostationary satellites of an entirely new design, carrying
improved instruments, are to appear at intervals beginning in 1989.
For two generations all the GOES spacecraft have been spinning like
tops as they move along their orbits. The new GOES will be
non-spinning and their instruments will be able to view the Earth
continuously, rather than "eyeing" a scene once with each revolution.
Both the satellite and its solar photovoltaic-panel powerplant will
be larger.
This new design is expected to improve dramatically the performance
of both imagers and sounders. When this new satellite appears it will
no longer be necessary to turn off a sounder while an imager is
operating. An unusual new sensor will constantly take pictures of the
Sun, detecting solar X-rays. This will enable scientists to peer
deeper into the Sun's surface to spot energy flares that can affect
Earth's weather.
Another new sensor will observe lightning flashes on Earth. By
counting these flashes, scientists hope to learn more about how
electrical exchange affects weather.
For the period into the 21st century and beyond, plans are being
drawn by NASA and NOAA for astronaut-tended, polar-orbiting
meteorological platforms. These will function as part of NASA's Space
Station. Whereas present weather satellites have life expectancies of
only a few years, the repairable platforms are expected to last for
many years.
NASA plans a serviceable geostationary platform for early in the 21st
century. Discussion of a project to develop a microwave sounder for
geostationary flight is already in progress. Planners anticipate that
many "active" instruments will join the array of existing sensors.
Active instruments, such as radar, send out a pulse or series of
pulses to "illuminate an area of interest." A wind sensor using laser
pulses is another potential gain for forecasters.
Already weather satellites present an excellent example of
international cooperation in space. In the years ahead, many more
nations are expected to participate in weather satellite programs.
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I N S T R U M E N T A T I O N
C A R R I E D B Y
W E A T H E R S A T E L L I T E S
Both the polar-orbiting and GOES satellites carry instruments to
measure and monitor activities other than weather. For example, they
monitor solar winds and flares, and collect and relay data picked up
by river and tide gages, seismometers, buoys, ships, airplanes, and
automatic weather stations. GOES satellites transmit their data to
the National Weather Service as well as directly to amateur and
professional users.
Future GOES satellites will carry improved instruments for concurrent
imaging and atmospheric sounding and provide additional information
on atmospheric movements of water vapor and the remapping of picture
elements to permit better calculation of winds based on cloud motion.
EACH POLAR-ORBITING SATELLITE CARRIES SIX PRIMARY SYSTEMS.
ADVANCED VERY HIGH RESOLUTION RADIOMETER. This instrument senses
clouds over both ocean and land, using the visible and infrared parts
of the spectrum. It stores measurements on tape, and later plays them
back to NOAA's command and data acquisition stations. The satellites
also broadcast in "real time" (this means transmissions are
simultaneous with the observations of the instruments). These
real-time broadcasts are available in both high-resolution and
low-resolution picture images and can be received by anyone in the
world equipped with a receiving station. Over the years, such
receiving stations have been built and operated by foreign weather
services, commercial American weather services, and high schools and
colleges throughout the world.
TIROS OPERATIONAL VERTICAL SOUNDER. This instrument combines data
from three complementary instrument units to provide temperature and
moisture data from the Earth's surface up through the atmosphere.
ARGOS DATA COLLECTION AND PLATFORM LOCATION SYSTEM. The instruments
in this French-provided system collect data from sensors placed on
fixed and moving platforms, including ships, buoys, and weather
balloons, and transmits data to a ground station antenna. Because
ARGOS also determines the precise location of these moving sensors,
it can serve wildlife managers by monitoring and tracking sensors
placed on birds and animals.
SPACE ENVIRONMENT MONITOR. This equipment measures energetic
particles emitted by the Sun over essentially the full range of
energies and magnetic field variations in the Earth's near-space
environment. Readings made by these instruments are invaluable in
measuring the Sun's radiation activity.
SEARCH AND RESCUE TRACKING (COSPAS/SARSAT). This system has already
proven invaluable in saving human life. Search and rescue
transmission equipment on board weather satellites receives emergency
signals from persons in distress. The satellites transmit these
signals to ground receiving stations in the U.S. and overseas.
Signals are forwarded to the nearest rescue coordination center.
These centers compute the location of the signals and give a rescue
team (usually within a few miles) the coordinates of the emergency
site. Both the U.S. and Soviet Union cooperate in the Search and
Rescue Tracking system: SARSAT is the American acronym for "Search
and Rescue Satellite Assisted Tracking"; COSPAS is the Soviet
equivalent.
EARTH RADIATION BUDGET EXPERIMENT. This instrument is a radiometer.
It is designed to measure all radiation striking the Earth as well
as all radiation leaving . Such monitoring enables scientists to
measure the loss or gain of terrestrial energy to space. Shifts in
this energy "budget" affect the Earth's average temperatures, in
which even slight changes can affect climatic patterns.
GEOS SATELLITES CARRY FOUR BASIC SENSOR SYSTEMS.
VISIBLE-INFRARED SPIN-SCAN RADIOMETER AND ATMOSPHERIC SOUNDER. This
radiometer provides visible infrared and sounding measurements of the
Earth. These images, together with images received from the
polar-orbiting satellites, are processed on the ground and then
radioed back up to the GOES for broadcast in graphic form as a
"Weather Facsimile," or WEFAX. WEFAX images are received by ground
stations on land as well as on ships. Currently, the GOES WEFAX
transmissions are received from western Europe to eastern Australia.
GOES satellites transmit their data to the National Weather Service
as well as to the amateur and professional users as far away as
Australia. There are over 1,000 known WEFAX users who avail
themselves to this free service.
SPACE ENVIRONMENT MONITOR. This instrument is almost identical to the
sensors aboard the polar orbiters. They measure the condition of the
Earth's magnetic field, the solar activity and radiation around the
spacecraft, and transmit these data to a central processing facility.
THE DATA COLLECTION SYSTEM. Similar to the Data Collection and
Platform Location System on the polar orbiters, this system also
gathers and relays readings made by sensors placed on various objects
(both mobile and stationary) at various locations.
SEARCH AND RESCUE TRANSPONDERS. This instrument is carried on GOES
East, orbiting at 75 degrees west longitude, and on GOES West, which
is located at 135 degrees west longitude. The GOES satellites can
relay distress signals at all times, but cannot locate them. Only the
low altitude polar-orbiting satellites can compute a signal's
location. The two satellites work together to create a search and
rescue system, allowing a message to be intercepted by a GOES and
relayed even though a polar orbiter may be temporarily outside radio
"line of sight." [See "Search and Rescue"]
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S E A R C H A N D R E S C U E
The concept for a satellite-aided search and rescue project (SARSAT)
began almost as soon as the first satellites were placed in Earth
orbit. Experimental equipment had been placed on the early Nibus
satellites, and the first operational system was on TIROS. In 1976,
the effort became an international project, with the United States,
Canada, and France participating.
In 1980, the Soviet Union agreed to equip COSMOS satellites with
COSPAS repeaters. Other nations have since joined in. The
COSPAS/SARSAT satellites monitor the entire surface of the Earth,
listening for distress signals from downed airplanes, capsized boats,
and persons in other emergencies.
These signals are transmitted to special ground receiving stations in
the United States and overseas. The location of the signal is
computed and the nearest rescue coordination center is notified. When
an air or sea rescue team goes out, it has a "fix" within a few miles
of the actual emergency. Satellite search has cut recovery time from
days to a few hours.
The program has been instrumental not only in saving hundreds of
lives but also in saving millions of dollars in search efforts. The
system is proving increasingly valuable as additional enhancements
and improvements are made.
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P O L A R - O R B I T I N G S A T E L L I T E S
For normal weather coverage, NOAA operates two polar-orbiting
satellites. They circle the globe in a north-south orbit, passing
over both the North and South poles. One crosses the equator in the
morning and the other in the afternoon. They circle in a
"Sun-synchronous" orbit of approximately 810 - 850 kilometers, and
each observes the entire Earth twice a day. Because they are
Sun-synchronous, these satellites circle the Earth so that they cross
the equator at the same time daily. The morning satellite crosses
southward over the equator at 7:30 am and the afternoon satellite
crosses northward at about 2:30 pm. Operating together as a pair,
these satellites assure that measurements for any region of the
Earth are no more than six hours old.
These polar orbiters provide visible and infrared radiometer data
that are used for imaging purposes, radiation measurements, and
vertical temperature profiles, and can help calculate water vapor
content at several atmospheric levels. They send some 16,000 global
measurements daily to NOAA's Weather Service computers, adding
valuable information to forecasting models, especially for remote
ocean areas, where conventional data are lacking.
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G O E S S A T E L L I T E S
The current Geostationary Operational Environmental Satellite (GOES)
system consists of GOES East and West, which orbit at 75 degrees west
and 135 degrees west longitude. Each views almost a third of the
Earth's surface: GOES East monitors North and South America and most
of the Atlantic Ocean, while GOES West looks down at North America
and the Pacific Ocean basin. The two operate together to send a
full-face picture of the Earth every 30 minutes, day and night.
Pictures of smaller areas can be sent more often should a storm need
monitoring. These satellites give meteorologists nearly continuous
viewing of storms and cloud patterns, as well as measurements of wind
fields at cloud altitudes.
The GOES satellites circle the Earth in a "geosynchronous" orbit.
This means they orbit the equatorial plane of the Earth at a speed
and altitude that allows them to hover continuously over one
position on the surface. The geosynchronous plane is about 35,800
kilometers above the Earth, high enough to allow satellites orbiting
there to have a full-disc view of the Earth.
NASA launched the first geostationary weather satellite in 1966 and
followed it with another the next year. So far, NASA has placed eight
of NOAA's GOES satellites into orbit. Two of these, GOES East and
GOES West, are fully operational.
These GOES satellites complement the TIROS polar-orbiting satellites.
Both view remote areas and relay their data to instruments at NOAA's
ground stations.
By remaining stationed over the same spot both day and night, the
GOES can provide the kind of continuous monitoring necessary for
intensive data analysis and weather predictions. They look for such
catastrophic events as hurricanes, tornadoes, and other severe storms
and relay data to ground receiving antennas. These satellites
duplicate some of the functions on the polar-orbiting satellites, but
from a distant broader perspective, they add the advantage of
maintaining a constant vigil.
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D E F I N I T I O N S
RADIOMETER - A satellite instrument that measures radiation
(reflected sunlight and heat, or thermal radiation).
SOUNDER - A special kind of radiometer which measures changes of
atmospheric temperature with height, and changes in
water vapor content of the air at various levels.
IMAGER - A satellite instrument that measures and maps
sea-surface temperatures, cloudtop temperatures, and
land temperatures. Imager data are converted by
computer into pictures.
INFRARED-ONLY - A small part of the electro-magnetic "spectrum" is
visible to us as light. The rest of it can be sensed
as heat, or infrared radiation. Infrared sensors serve
as satellite "eyes" during periods of darkness.
REMAPPING - When the spherical Earth is photographed by satellites,
areas lying near the outer edge of the picture are
distorted. Remapping is done to flatten the Earth into
a standard projection.
RESOLUTION - The value of a telephoto lens is permitting the
photographer to get a larger picture of the subject
(sharper resolution) but less of the surroundings.
It's the same with satellites. Resolution of today's
satellite "picture elements" ("pixels") can vary from
10 meters (30 feet) for surveying uses to 1 km (3000
feet) for weather satellites.
WEFAX - Telegraphic abbreviation for "weather facsimile," a
system for transmitting via radio visual reproductions
of weather forecast maps, temperature summaries, clouds
analyses, etc. Most of these WEFAX transmissions are
relayed by GOES spacecraft today.
PLATFORMS - Weather satellites are often called "space platforms"
because they serve as emplacements in space for various
instrument systems. The same term is applied to
automatic weather data transmitters installed on buoys,
balloons, ships, and planes, and mounted in remote
areas. Weather satellites collect this data and feed it
into the daily world weather analysis.
EL NINO - A warming of the surface waters of the eastern equato-
rial Pacific that occurs at irregular intervals of 2 to
7 years and lasts for 1 to 2 years. The SOUTHERN
OSCILLATION is a global-scale seesaw in atmospheric
pressure between Indonesia-North Australia, and the
southeast Pacific. Together, they are interacting parts
of a single global system of climate fluctuations
popularly known as ENSO--The El Nino/Southern
Oscillation, so named because it first was associated
with Christmas, the time of the Christ Child.
NEPHANALYSIS Using cloud pictures to study the relationship between
cloud forms and storm centers. In classical mythology
Nephele was a woman Zeus formed from a cloud.
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NASA FACTS, SENTINELS IN THE SKY: WEATHER SATELLITES, Haynes, NF-152,
NASA/NOAA