S band: Radar bandwidth from 1.55-4.2 GHz (75-150 mm).
SACRS: South Australian Centre for Remote Sensing.
SAR: Synthetic-Aperture Radar.
Satellite: (1) A body that revolves around another body of higher
mass. (2) A man-made object that orbits a space body, usually the Earth.
Saturation: A subjective term which usually refers to the differences
of a hue from a grey of the same value; also called chroma.
Scale: The ratio of a distance on a photograph or map to its corresponding
distance on the ground. The scale of a photograph varies from point to point
because of displacements caused by tilt and relief, but is usually taken
as f/H, where f is the principal distance (focal length) of the camera and
H is the height of the camera above mean ground elevation. Scale may be
expressed as a ratio 1:24000; a representative fraction, 1/24 000; or an
equivalence, 1 inch = 2000 feet.
Scan line: The narrow strip on the ground that is swept by the instantaneous
field of view of a detector in a scanner system.
Scanner: (1) Any device that scans, and thus produces an image (see
scanning radiometer). (2) A radar set incorporating a rotatable antenna,
or radiator element, motor drives, mounting, etc. for directing a searching
radar beam through space and imparting target information to an indicator.
Scanning: Process of reading data in regular horizontal sweeps to
cover the entire image or screen. Scan conversion refers to the process
of putting data into grid format for display on a raster device.
Scanning radiometer: A radiometer, which by the use of a rotating
or oscillating plane mirror, can scan a path normal to the movement of the
radiometer.
Scattering: (1) The process by which small particles suspended in
a medium of a different index of refraction diffuse a portion of the incident
radiation in all directions, eg, atmospheric scattering. (2) The process
by which a rough surface re-radiates electromagnetic radiation incident
upon it.
Scatterometer: A device for recording the microwave scattering properties
of surfaces.
Scatter plot: Also called a scatter diagram or crossplot. Shows a
two-variable frequency distribution by plotting a dot or symbol at each
data point. Sometimes a line or curve is added to show the correlation (if
there is one) between the variables represented on the two axes.
Scene: (1) In a passive remote sensing system, everything occurring
spatially or temporally before the sensor, including the Earth's surface,
the energy source, and the atmosphere, that the energy passes through as
it travels from its source to the Earth and from the Earth to the sensor.
(2) Image formed across the full swath width of a sensing system, eg, Landsat
scenes.
Seasat: Polar-orbiting, US oceanographic satellite briefly active
in 1978; carried active and passive microwave sensors.
Seed: Central radiance values, which together with a tolerance, are
used to define classes of spectrally similar pixels (that is, similar to
the seed). The term 'seed' refers to the growth of the class in the classification
process.
Sensitivity: The degree to which a detector responds to electromagnetic
energy incident upon it.
Sensor: Any device that gathers energy, electromagnetic radiation
or other, converts it into a signal and presents it in a form suitable for
obtaining information about the environment.
Sensor histogram: Separate histograms can be accumulated for each
MSS sensor over a subset or set of subsets by sampling pixels along each
sixth line of the image(s) (starting on an appropriate line for each of
the six sensors). These histograms may be used in reducing the sensor imbalance
striping by matching each one to the average histogram and modifying the
lines scanned by each sensor separately.
Shadow: (1) Obscurity within the area or space from which direct
electromagnetic radiation from a source is excluded by an interposed opaque
body. (2) radar: A no-return area extending in range from an object that
is elevated above its surroundings; the object obstructs the radar beam,
preventing illumination of the area behind it. As the name implies radar
shadows are analogous to shadows caused by visible light.
Shuttle Imaging Radar (SIR): Synthetic aperture radar experiments
carried out aboard the NASA Space Shuttle.
Shuttle Multi-spectral Infrared Radiometer (SMIRR): A non-imaging
spectroradiometer carried by the NASA Space Shuttle covering ten narrow
wavebands in the 0.5-2.4 µm range.
SI units: Standardised measurement units of the Systeme Internationale
d'Unites.
Side-looking radar: An all-weather, day/night remote sensor which
is particularly effective in imaging large areas of terrain. It is an active
sensor, as it generates its own energy which is transmitted and received
to produce a photo-like picture of the ground; also referred to as Side-Looking
Airborne Radar (SLAR).
Signal: The effect (such as pulse of electromagnetic energy) conveyed
over a communication path or system. Signals are received by the sensor
from the scene and converted to another form for transmission to the processing
system.
Signal-to-noise (S/N) ratio: The ratio of the level of the information-bearing
signal power to the level of the noise power.
Signature: Any characteristic or series of characteristics by which
a material may be recognised in an image, photograph, or data set (see also
spectral signature).
Signature analysis techniques: Techniques that use the variation
in the spectral reflectance or emittance of objects as a method of identifying
the objects.
SIR: Shuttle Imaging Radar.
Six line striping: A radiometric problem which occurs in Landsat
MSS imagery. This occurs when the six sensors, which scan six image lines
in each swath, are miscalibrated, causing the same ground reflectance to
be recorded as different image values.
Skew: Distortion from a true or symmetrical form. A characteristic
westerly skew distortion occurs in Landsat data which is caused by the movement
of the Earth beneath the satellite as the satellite moves along its track.
Skylight: Scattered light reaching the Earth's surface (see diffuse
light).
Slant range: For radar images this term represents the distance measured
along a line between the antenna and the target.
SLAR: Side-Looking Airborne Radar.
SMMR: Scanning Multi-channel Microwave Radiometer; carried on Nimbus
satellite.
Smoothing: The averaging of densities in adjacent areas to produce
more gradual transitions. In image processing, this effect can be achieved
by image filtering with appropriate filter weights.
SMS: Synchronous Meteorological Satellite.
S/N: Signal-to-Noise ratio.
Software: The computer programs that drive the hardware component
of a data processing system; includes system monitoring programs, programming
language processors, data handling utilities, and data analysis programs.
Solar angle: The angle made by the intersection of the sun's azimuth
and a line through the Earth's true north; varies with the time of day,
time of year, and geographic position on the Earth's surface.
SOM: Space Oblique Mercator projection (see Hotine Oblique Mercator
).
Spacecraft: Device, manned or unmanned, designed to be placed into
an orbit about the Earth, or into a trajectory to another celestial body;
generally considered to be manoeuvrable, as contrasted to satellites, which
are placed in fixed orbits.
Space shuttle: Series of manned US spacecraft.
Spatial data: Locational data. Usually refers to distribution of
a variable or the relationships between variables in a geographic region.
Demographic features, marketing distributions, energy resource data and
topographic data are examples of information readily represented spatially,
that is, on a map.
Spatial filter: An image transformation, usually a one-to-one operator
used to lessen noise or enhance certain characteristics of the image. For
any particular (x,y) co-ordinate on the transformed image, the spatial filter
assigns a grey shade on the basis of the grey shades of a particular spatial
pattern near the co-ordinates (x,y).
Spatial-frequency filtering: The analysis of the spatial variations
in image values and the separation or suppression of selected frequency
ranges.
Spatial resolution: The level of spatial detail depicted in an image.
Spectra: Data that result from spectral scanning; measurements of
the variations in spectral response over a range of wavelengths for a single,
constant viewing area.
Spectral band: An interval in the electromagnetic spectrum defined
by two wavelengths, frequencies, or wavenumbers.
Spectral class: A class which is developed on the basis of the pixel
spectral, or radiance, data and/or channels derived from radiance data.
Spectral class overlay (or theme mapping): All the pixels which fall
into a spectral class are interactively overlaid with a distinct colour
to aid in interpretation.
Spectral colour: Colour of a single wavelength on the visible portion
of the electromagnetic spectrum. Refraction of white light yields the spectral
hues ranging from violet with the shortest wavelength through blue, green,
yellow, orange, and ending with red at the longest wavelength. Black, white,
and colours which are mixtures of wavelengths are not spectral colours.
Spectral energy distribution: Diagram showing the component wavelengths
of a colour, as measured by a spectrophotometer.
Spectral irradiance (El): Energy at a specific wavelength radiated
onto an object of unit area per unit time. Standard measurement unit W m-2
µm-1.
Spectral radiance (Ll): Energy at a specific wavelength radiated
by an object of unit area per solid angle of measurement. Standard measurement
units W m-2 sr-1 µm-1.
Spectral radiant exitance (Ml): Energy at a specific wavelength radiated
from an object of unit area per unit time. Standard measurement unit W m-2
µm-1.
Spectral reflectance: The reflectance of electromagnetic energy at
specified wavelength intervals.
Spectral regions: Conveniently designated ranges of wavelengths subdividing
the electromagnetic spectrum; eg, the visible region, X-ray region, infrared
region, middle infrared region.
Spectral resolution: Range of wavelengths incorporated into each
image channel and/or number of channels in image.
Spectral response: The response of a material as a function of wavelength
to incident electromagnetic energy, particularly in terms of the measurable
energy reflected from and emitted by the material.
Spectral signature: Quantitative measurement of the properties of
an object at one or several wavelength intervals.
Spectrometer: A device to measure the spectral distribution of electromagnetic
radiation. This may be achieved by a dispersive prism, grating, or circular
interference filter with a detector placed behind a slit. If one detector
is used, the dispersive element is moved so as to sequentially pass all
dispersed wavelengths across the slit. In an interferometer-spectrometer,
on the other hand, all wavelengths are examined all the time, the scanning
effect being achieved rapidly by oscillating two, partly reflective (usually
parallel) plates so that interference fringes are produced. A Fourier transform
is required to reconstruct the spectrum.
Spectrophotometer: A photometer which measures the intensity of electromagnetic
radiation as a function of the frequency (or wavelength) of the radiation.
This is usually used for the visible portion.
Spectroradiometer: A device which measures the energy reflected or
radiated by materials in narrow electromagnetic wavebands.
Spectrum: (1) In physics, any series of energies, arranged according
to wavelength (or frequency). (2) The series of images produced when a beam
of radiant energy is subject to dispersion. A rainbow-coloured band of light
is formed when white light is passed through a prism or a diffraction grating.
This band of colours results from the fact that the different wavelengths
of light are bent in varying degrees by the dispersing medium and is evidence
of the fact that white light is composed of coloured light of various wavelengths.
Specular: In sensitometry, applied to a measurement made by collimated
or essentially parallel light rays; referring to reflection, or transmission
without scattering or diffusion.
Specular reflection: The reflectance of electromagnetic energy without
scattering or diffusion, as from a surface that is smooth in relation to
the wavelengths of incident energy; also called mirror reflection.
SPOT: Système Probatoire d'Observation de la Terre; French
polar- orbiting, Earth resources satellite.
SPOT Image: Commercial company responsible for the distribution of
SPOT data and products.
sr: Abbreviation for steradian.
SSU: Stratospheric Sounding Unit; part of TOVS package carried on
NOAA satellites.
Standard false colour composite: For Landsat MSS data, such an image
would be formed by assigning band 4 (that is, visible green) to the blue
primary, band 5 (visible red) to green and band 6 or 7 (near infrared) to
red.
Stefan-Boltzmann constant (s): Constant value relating total energy
emitted by a blackbody to the fourth power of its absolute temperature (T).
Steradian (sr): The solid angle that cuts unit area from the surface
of a sphere of unit radius centred at the vertex of the solid angle. There
are 4(pi )sr in a sphere.
Stereoscopic image: That mental impression of a three-dimensional
object which results from stereoscopic vision (stereo viewing).
Sub-sampling: (1) Image scanning process in which the image optical
pixel size is smaller than the geometric pixel size. (2) Rewriting the geometry
of an image so that the output image contains fewer pixels than the input
image.
Subset: A section of a full image scene which can be conveniently
stored and processed.
Subtractive colour process: A method of creating essentially all
colours through the subtraction of light of the three subtractive colour
primaries (cyan, magenta and yellow) in various proportions through use
of a single white light source.
Subtractive primary colours: The colours cyan, magenta and yellow,
the subtraction of which from white light in different proportions allows
all colours to be created.
Sun angle: The angle of the sun above the horizon; both the quantity
and spectral quality of light being reflected to an aerial camera or sensor
are influenced by sun angle.
Sun-synchronous: An Earth satellite orbit in which the orbital plane
is near polar and the altitude such that the satellite passes over all places
on Earth having the same latitude at the same local sun time.
Supervised classification: A computer-implemented process through
which each measurement vector is assigned to a class according to a specified
decision rule, where the possible classes have been defined on the basis
of representative training samples of known identity.
SVF: Single Variable Format; the raster data format used in Arc/Info.
Swath: A strip of terrain or ocean, recorded by a sensor.
Swath width (total field of view): The overall plane angle or linear
distance covered by a multi-spectral scanner in the across-track direction.
Synchronous satellite: An equatorial west-to-east satellite orbiting
the Earth at an altitude of 34,900 km at which altitude it makes one revolution
in 24 hours synchronous with the Earth's rotation.
Synoptic view: The ability to see or otherwise measure widely dispersed
areas at the same time and under the same conditions, such as the overall
view of a large portion of the Earth's surface which can be obtained from
satellite altitudes.
Synthetic antenna: The effective antenna produced by storing and
comparing the Doppler signals received while the aircraft travels along
its flight path. This is many times longer than the physical antenna, thus
sharpening the effective beam width and improving azimuth resolution.
Synthetic-Aperture Radar (SAR): Radar scanning system in which the
physical antennae length is effectively lengthened by processing return
signals according to their Doppler Shifts.
Système Probatoire d'Observation de la Terre (SPOT): A French
satellite carrying two pushbroom imaging systems, one for three wavebands
in the visible and VNIR with 20 m resolution, the other producing panchromatic
images with 10 m resolution. Each system comprises two devices which are
pointable so that off-nadir images are possible, thereby allowing stereoscopic
viewing.