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Words begining with "S"
Sampling
The process of obtaining a sequence of discrete digital values
from a continuous sequence of analog
data.
SAR
See synthetic aperture radar.
SARSAT
Search and Rescue Tracking System
carried on NOAA polar-orbiting
satellites that 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 which computes the location from which the
emergency signals came and provides the coordinates of the emergency
site to a rescue team.
Satellite
A free-flying object that orbits the Earth, another planet, or the
sun.
Satellite Dish (aka Parabolic Reflector)
Bowl shaped antennas that collect and focus the signals that a
satellite beams down to Earth. The dish reflects the incoming radio
frequency energy to a focal point where it can be picked up by a
feedhorn antenna to
transfer the RF energy to a
transmission line. The bigger the dish, the greater will be the
intercepted RF energy and hence, the gain.
For example, a satellite dish is used to receive GOES WEFAX
imagery.
Satellite Operations Control Center (SOCC)
NOAA National Environmental Satellite Data and Information Service
(NESDIS) Satellite Operations
Control Center located in Suitland, Maryland. A principal operating
feature of the NOAA system is the
centralized remote control of the satellite through command and data
acquisition (CDA) stations. The CDA stations transmit command
programs to the satellite, and acquire and record meteorological and
engineering data from the satellite. Data is transmitted from CDA to
Suitland NESDIS Data Processing Services Subsystem (DPSS). DPSS is
responsible for data processing and timely generation of
meteorological products and distribution of these products.
Satellite Orbital Elements
See Keplerian
elements.
Satellite Positioning
A procedure by which satellites are used to locate precise objects
or particular points on Earth
Satellite Revolution
The time from one perigee (the point of an elliptical orbit path
where a satellite is closest to Earth) to the next.
S-Band
One of the segments or bands into which the radio frequency
spectrum above 1000 MHz is divided, designated by letters. Signals
from GOES and other geostationary
spacecraft transmitting on or near 1691 MHz are transmitting on
S-Band.
Scanner
A system thal optically scans its detector(s) across a scene and
records or stores the data in a two-dimensional format to form an
image.
Scanning Radiometer
An imaging system consisting of lenses, moving mirrors, and
solid-state image sensors used to obtain observations of the Earth
and its atmosphere. Scanning radiometers,
which are the sole imaging systems on all current operational weather
satellites, have far better long-term performance than the vidicon TV
camera tubes used with earlier spacecraft.
Scattering
The process by which electromagnetic radiation interacts with and
is redirected by the molecules of the atmosphere, ocean, or land
surface. The term is frequently applied to the interaction of the
atmosphere on sunlight, which causes the sky to appear blue (since
light near the blue end of the spectrum is scattered much more than
light near the red end).
Screaming Eagles
Cloud pattern so named because some observers maintain they can
see the head of an eagle facing west in these cloud patterns. The
pattern is similar to a comma, only the pattern is disorganized and
not solid. Weather associated with screaming eagles consists of rain
showers and gusty surface winds up to about 25 knots. The eagles can
intensify and enlarge when moving into areas east of troughs, in that
case, intense thunderstorms can develop. Screaming eagles are common
in the Pacific Ocean between Hawaii and the equator, and are uncommon
in the western Atlantic.
Sea Breeze
Local coastal wind that blows from the ocean to land. Sea breezes
usually occur during the day, because the heating differences of land
and sea cause pressure differences. Cooler, heavier air from the sea
moves in to replace rising warm air on the coastline. See land
breeze.
Sea Level
The datum against which land elevation and sea depth are measured.
Mean sea level is the average of high and low tides.
Search and Rescue
International satellite-aided search and rescue project. COSPAS/
SARSAT satellites monitor the entire surface of the Earth, and
transmit distress signals to special ground receiving stations. The
receiving stations compute the location of the signal, and notify the
nearest rescue coordination center. Satellite search has cut recovery
time from days to hours, and has aided downed airplanes, capsized
boats, and persons in other emergencies.
SEM
See Space Environment Monitor, and TIROS.
Semi-major Axis (aka a)
One of the six Keplerian
elements, it indicates the size of an orbit. The semi-major axis
is one-half of the longest diameter of an orbital ellipse,
e.g., one-half of the distance between the apogee
and perigee of an Earth orbit.
(The semi-major axis is related to the orbital period and mean motion
by Kepler's third law. See Kepler's
three laws of motion.)
Sensor
Device that produces an output (usually electrical) in response to
stimulus such as incident radiation. Sensors aboard satellites obtain
information about features and objects on Earth by detecting
radiation reflected or emitted in different bands of the electromagnetic
spectrum. Analyzing the transmitted data provides valuable
scientific information about Earth.
Weather satellites commonly carry radiometers,
which measure radiation from snow, ice, clouds, and bodies of water.
Spaceborne radars are used for Earth observations, bouncing radar
waves off land and ocean surfaces to study sea-surface conditions,
ice thickness, and land surface features. A wind scatterometer is a
special type of radar designed to measure ocean surface winds
indirectly by bouncing signals off the water and measuring them from
various angles. Infrared (IR) detectors
measure heat generated by Earth features in the IR band of the
spectrum.
Photographic reconnaissance sensors in their simplest form are
large telescope-camera systems used to view objects on Earth's
surface. The bigger the lens, the smaller the object that can be
detected. Camera-telescope systems now incorporate all sorts of
sophisticated electronics to produce better images, but even these
systems need cloudless skies, excellent lighting, and good color
contrast between objects and their surroundings to detect objects the
size of a basketball. Some of the satellites produce film images that
must be returned to Earth, but a more convenient method is to record
the image as a series of digital code numbers, then reconstruct the
image from the electronic code using a computer at a ground station.
Sensor Calibration
The relationship between input and output for a given measurement.
Signal
Electrical impulses, sound or picture elements, etc., received or
transmitted. Signals can exist in many different forms and media
(electrical/wires, acoustic/air, light/transparent fibers, etc. ).
but all signals will vary with time.
The signal shape plotted as a function of time is called the
waveshape or waveform. Some waveforms are repetitive or periodic,
that is, a small segment of the waveform repeats itself regularly.
Other waveforms, such as noise, are nonperiodic or aperiodic. All
waveforms can be distilled into the combination of pure waves called
sine waves. The frequency
of a sine wave is the rate at which the fundamental shape repeats
itself.
Most signals occupy a limited range of frequencies between a lower
limit and an upper limit. This range or band of frequencies occupied
by a signal is called the bandwidth
of the signal.
Communication medium or channel can pass only a specific range or
band of frequencies, which is called the bandwidth of the channel.
The bandwidths of the channel and the signal determine the number and
types of signals that can be transmitted by a particular
communication channel. Signals often are too small and need to be
made larger through a process called amplification. The amount of
amplification is measured in decibels.
However, amplification is an imperfect process, and inadvertently
introduces various distortions, noise, and bandwidth limitations.
Often, multiple signals must share the same medium. One way the
sharing can be accomplished is to place each signal in its own band
of frequencies within the total band of the medium. The combining of
a number of signals to share a medium by dividing it into different
frequency bands for each signal is called frequency-division
multiplexing.
Frequency-division multiplexing requires the ability to move
signals around so that each multiplexed siqnal occupies its own band.
This is accomplished through a process called modulation,
in which a high-frequency sine wave carries the signal into the
specified band. Either the amplitude or the frequency of the
carrier wave can be varied, or
modulated, in synchrony with the information-bearing signal. These
methods are called amplitude modulation (AM)
and frequency modulation (FM). FM is
the more complex process of the two, and the bandwidth of the FM
carrier can be many times that of the modulating signal. The process
of demodulating a frequency-modulated signal eliminates much of the
deleterious effects of additional noise. (The trade-off between
bandwidth and noise immunity characterizes most communication
systems. Both are analog modulation schemes for multiplexing signals
in the frequency spectrum.)
Digitizing a signal requires a number of steps and results in a
binary digital signal that takes on one of two discrete values. This
process results in considerable immunity to additive noise, but
requires a considerable increase in bandwidth.
Signal-to-Noise Ratio (SNR)
In decibels (dB), the difference
between the amplitude of a desired radio frequency (RF}
signal and the internal or external RF noise level in a system. A
negative SNR indicates the signal is below the system noise level and
unusable. The greater the positive SNR, the less effect noise will
have on the final quality. SNR of at least +12dB is necessary to
produce imagery with minimal noise effects.
Sine Wave
A smoothly varying wave that repeats itself; its frequency is the
rate at which the fundamental shape repeats itself. Any waveform can
be distilled into a combination of pure sine waves of varyinq
frequencies and amplitudes.
Sink
The process of providing storage for a substance. For example,
plants - through photosynthesis - transform carbon dioxide in the air
into organic matter, which either stays in the plants or is stored in
the soils. The plants are a sink for carbon dioxide.
Skylab
The first U.S. space station, launched unmanned in May 1973 and
soon after occupied in succession by three crews through November
1973.
SNR
See siqnal-to-noise ratio.
SOCC
See Satellite Operations Control Center
Software
The programs, data, or routines used by a computer, distinguished
from the physical components (e.g., hardware).
Solar Backscatter Ultraviolet Radiometer (SBUV)
Instrument that measures the vertical distribution and total ozone
in the Earth's atmosphere. Data is used for the continuous monitoring
of ozone distribution to estimate long-term trends. SBUV instruments
are flown on NOAA polar-orbiting
satellites.
Solar Constant
Aka total solar irradiance. The constant expressing the amount of
solar radiation reaching the Earth from the sun, approximately 1370
watts per square meter. It is not, in fact, truly constant and
variations are detectable.
Solar Cycle
Eleven-year cycle of sunspots and solar flares that affects other
solar indexes such as the solar output of ultraviolet radiation and
the solar wind. The Earths magnetic field,
temperature, and ozone levels are affected by this cycle.
Solar Radiation
Energy received from the sun is solar radiation. The energy comes
in many forms, such as visible light (that which we can see with our
eyes). Other forms of radiation include radio waves, heat (infrared),
ultraviolet waves, and x-rays. These forms are categorized within the
electromaqnetic spectrum.
Solar Wind
A continuous plasma stream
expanding into interplanetary space from the suns corona. The solar
wind is present continuously in interplanetary space. After escaping
from the gravitational field of the sun, this gas flows outward at a
typical speed of 400 km per second to distances known to be beyond
the orbit of Pluto. Besides affecting Earth's weather, solar activity
gives rise to a dramatic visual phenomena in our atmosphere. The
streams of charged particles from the Sun interact the Earth's
magnetic field like a generator to create current systems with
electric potentials of as much as 100,000 volts. Charged electrons
are energized by this process, sent along the magnetic field lines
towards Earth's upper atmosphere, excite the gases present in the
upper atmosphere and cause them to emit light which we call the
auroras. The auroras are the northern (aurora borealis) and southern
(aurora Australis) lights.
Sounder
A special kind of radiometer
that measures changes in atmospheric temperature with height, as well
as the content of various chemical species in the atmosphere at
various levels. The High Resolution Infrared Radiation Sounder
(HIRS), found on NOAA
polar-orbiting satellites, is a passive
instrument.
Space Environment Monitor (SEM)
Instrument that measures the condition of the Earth's magnetic
field and the solar activity and radiation around the spacecraft, and
transmits these data to a central processing facility. NOAA
polar-orbiting and geostationary satellites both carry SEMs. See
TIROS
Spacelab
A manned laboratory module built by the European Space Agency
(ESA) that accommodates dozens of experiments on each flight, mainly
in the categories of materials science and life science
Spacelink
NASA electronic database for educators, with information stored on
a computer at the Marshall Space
Flight Center . Via computer, educators communicate with NASA
education specialists and access the following menus:
- current NASA news
- aeronautics research, U.S.
- Space Program historical information
- aerospace research in the 1980s and beyond
- overviews of NASA and its Centers
- NASA educational services
- classroom materials
- space program spin-offs
The computer access number is 205-895-0028 the data word format is
8 data bits, no parity, and I stop bit - 300, 1200, or 2400 baud
modem required. Callers with Internet access may reach NASA
Space Link here.
Space Physics
Scientific study of magnetic and electric phenom- ena that occur
in outer space, in the upper atmosphere of the planets, and on the
sun.
Space Shuttle
NASAs manned, recoverable spacecraft designed to be used as a
launch vehicle for Earth-orbiting experiments and as a short-term
research platform.
Spectral Band
A finite segment of wavelengths in the electromagnetic
spectrum.
Spectrum
- The series of colored bands diffracted and arranged in the
order of their respective wave lengths by the passage of white
light through a prism or other diffracting medium and shading
continuously from red (produced by the longest visible wave) to
violet (produced by the shortest visible wave).
- Any of various arrangements of colored bands or lines,
together with invisible components at both ends of the spectrum,
similarly formed by light from incandescent gases or other sources
of radiant energy, which can be studied by a spectroqraph.
- In radio, the range of wave lengths of radio waves, from 3
centimeters to 30,000 meters, or of frequencies of radio waves,
from 10 to 10,000,000 kilocycles. Also radio
spectrum.
- The entire range of radiant energies. See electromagnetic
spectrum.
SPOT
Systeme Pour l'Observation de la Terre. French, polar-orbiting
Earth observation satellitets) with ground resolution of 10 meters.
SPOT images are available commercially and are intended for such
purposes as environmental research and monitoring, ecology
management, and for use by the media, environmentalists, legislators,
etc.
SPOT Image
Company that markets data gathered by the SPOT satellite
worldwide.
Start Tone
Five seconds of 300 Hz black to white squarewave modulation
of the WEFAX subcarrier signaling
the start of a frame transmission (the beginning of a direct
readout imaqe).
Stop Tone
Five seconds of 450 Hz black to white squarewave modulation
of the WEFAX subcarrier, signaling
the stop of a frame transmission (end of a direct
readout image).
Stratosphere
Region of the atmosphere between the tropsphere
and mesosphere, having a lower
boundary of approximately 8 km at the poles to 15 km at the equator
and an upper boundary of approximately 50 km. Depending upon latitude
and season, the temperature in the lower stratosphere can increase,
be isothermal, or even decrease with altitude, but the temperature in
the upper stratosphere generally increases with height due to
absorption of solar radiation by
ozone.
Subcarrier
The 2400 Hz audio tone transmitted by APT
and WEFAX spacecraft. Amplitude
modulation of this tone is used to convey video information.
Subsatellite Point
Point where a straight line drawn from a satellite to the center
of the Earth intersects the Earth's suface.
Subsatellite Track
See ground track.
Subsystem
- A subunit of either the physical climate system (e.g., ocean
dynamics) or the biogeochemical cycles (e.g., terrestrial
ecosystems).
- A subunit of a spacecraft, e.g., the telemetry subsystem, the
power subsystem, the sensor subsystem, etc.
Sun
The closest star to Earth (149,599,000 km away on average). The
sun dwarfs the other bodies in the solar system, representing
approximately 99.86 percent of all the mass in the solar system. One
hundred and nine Earths would be required to fit across the Sun's
disk, its interior could hold over 1.3 million Earths.
The source of the Sun's energy is the nuclear reactions that occur
in its core. There, at temperatures of 15 million degrees Celsius (27
million degrees Fahrenheit) hydrogen atom nuclei, called protons, are
fused and become helium atom nuclei. The energy produced through
fusion at the core moves outward, first in the form of
electromagnetic radiation called photons.
Next, energy moves upward in photon heated solar gas - this type of
energy transport is called convection. Convective motions within the
solar interior generate magnetic fields that emerge at the surface as
sunspots and loops of hot gas called prominences. Most solar energy
finally escapes from a thin layer of the Sun's atmosphere called the
photosphere - the part of the Sun observable to the naked eye.
The sun appears to have been active for 4.6 billion years and has
enough fuel for another 5 billion years or so. At the end of its
life, the Sun will start to fuse helium into heavier elements and
begin to swell up, ultimately growing so large that it will swallow
Earth. After a billion years as a "red giant," it will suddenly
collapse into a "white dwarf." It may take a trillion years to cool
off completely
Sun Synchronous
Describes the orbit of a satellite that provides consistent
lighting of the Earth-scan view. The satellite passes the equator and
each latitude at the same time each day. For example, a satellite's
sun-synchronous orbit might cross the equator twelve times a day,
each time at 3:00 p.m. Iocal time. The orbital plane of a
sun-synchronous orbit must also precess
(rotate) approximately one degree each day, eastward, to keep pace
with the Earths revolution
around the sun.
Survey Mode
Refers to observational emphasis upon frequent global coverage,
usually with restricted spatial and spectral resolution, aimed at
developing a consistent, long-term data product for later
interpretation.
Swath
The area observed by a satellite as it orbits the Earth.
Synoptic Chart
Chart showing meteorological conditions over a region at a given
time; weather map.
Synoptic View
The ability to see large areas at the same time.
Synthetic Aperture Radar (SAR)
A high-resolution ground-mapping technique that effectively
synthesizes a large receiving antenna by processing the phase of the
reflected radar return. The along-track resolution is obtained by
timing the radar return (time gating) as for ordinary radar. The
crosstrack (azimuthal) resolution is obtained by processing the
Doppler phase of the radar return. The cross-track dimension of the
antenna is a function of the length of time over which the Doppler
phase is collected. See Doppler
effect.
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