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Words begining with "S"


The process of obtaining a sequence of discrete digital values from a continuous sequence of analog data.


See synthetic aperture radar.


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.


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.


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.


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.


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.


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.)


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.


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.


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.


The first U.S. space station, launched unmanned in May 1973 and soon after occupied in succession by three crews through November 1973.


See siqnal-to-noise ratio.


See Satellite Operations Control Center


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.


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


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


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.


  1. 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).


  2. 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.


  3. 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.


  4. The entire range of radiant energies. See electromagnetic spectrum.


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).


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.


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.


  1. A subunit of either the physical climate system (e.g., ocean dynamics) or the biogeochemical cycles (e.g., terrestrial ecosystems).


  2. A subunit of a spacecraft, e.g., the telemetry subsystem, the power subsystem, the sensor subsystem, etc.


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.


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

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