Satellite
An object in orbit around a celestial body.

Saturation
In the IHS system, represents the purity of color. Saturation is also the condition where energy flux exceeds the sensitivity range of a detector.

Scale
Ratio of distance on an image to the equivalent distance on the ground.

Scan line
Narrow strip on the ground that is swept by IFOV of a detector in a scanning system.

Scanner
An imaging system in which the IFOV of one or more detectors is swept across the terrain.

Scanner distortion
Geometric distortion that is characteristic of cross-track scanner images.

Scan skew
Distortion of scanner images caused by forward motion of the aircraft or satellite
during the time required to complete a scan.

Scattering
Multiple reflections of electromagnetic waves by particles or surfaces.

Scattering coefficient curves
Display of scatterometer data in which relative backscatter is shown as a function of incidence angle.

Scatterometer
Nonimaging radar device that quantitatively records backscatter of terrain as a function of incidence angle.

Scene
Area on the ground that is covered by an image or photograph.

Scotopic vision
Vision under conditions of low illumination, when only the rods are sensitive to light. Visual acuity under these conditions is highest in the blue part of the spectrum.

Sensitivity
Degree to which a detector responds to electromagnetic energy incident on it.

Sensor
Device that receives electromagnetic radiation and converts it into a signal that can be recorded and displayed as either numerical data or an image.

Sidelap
Extent of lateral overlap between images acquired on adjacent flight lines.

Side-looking airborne radar (SLAR)
An airborne side scanning system for acquiring radar images.

Side-scanning sonar
Active system for acquiring images of the seafloor using pulsed sound waves.

Signal
Information recorded by a remote sensing system.

Signal to noise ratio (S/N)
The ratio of the level of the signal carrying real information to that carrying spurious information as a result of defects in the system.

Silver halide
Silver salts that are especially sensitive to visible light and convert to metallic silver when developed.

Skylight
Component of light that is strongly scattered by the atmosphere
and consists predominantly of shorter wavelengths.

Slant range
In radar, an imaginary line running between the antenna and the target.

Slant
Range distance-Distance measured along the slant range.

Slant-range distortion
Geometric distortion of a slant-range image.

Slant-range image
In radar, an image in which objects are located at positions corresponding to their slant-range distances from the aircraft path. On slant-range images, the scale in the range direction is compressed in the near-range region

SLAR
Side-looking airborne radar.

Smooth criterion
In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will scatter the incident radar pulse in a smooth or intermediate fashion.

Software
Programs that control computer operations.

Sonar
Acronym for sound navigation ranging. Sonar is an active form of remote sensing that employs sonic energy to image the seafloor.

Space Shuttle
U.S. manned satellite program in the 1980s, officially called the Space Transportation System (STS).

Space Station
A planned series of three polar-orbiting, sun-synchronous satellites to be launched by NASA, the European Space Agency, and the Japanese Space Agency in the 1990s. They will carry a large range of remote-sensing devices.

Spatial-frequency filtering
The analysis of the spatial variations in DN of an image and the separation or suppression of selected frequency ranges.

Specific heat
The ratio of the heat capacity of unit mass of a material to the heat capacity of unit mass of water.

Spectral Band
Band representing a small interval of frequencies, or wave lengths, in a spectrum (e.g. electro-magnetic spectrum).

Spectral hue
A hue which is present in the spectral range of white light analysed by a prism or diffraction grating.

Spectral reflectance
Reflectance of electromagnetic energy at specified wavelength intervals.

Spectral sensitivity
Response, or sensitivity, of a film or detector to radiation in different spectral regions.

Spectral vegetation index
An index of relative amount and vigor of vegetation. The index is calculated from two spectral bands of AVHRR imagery.

Spectrometer
Device for measuring intensity of radiation absorbed or reflected by a materiel as a function of wavelength.

Spectroradiometer
A device which measures the energy reflected or radiated by materials in narrow EM wavebands.

Spectrum
Continuous sequence of electromagnetic energy arranged according to wavelength or frequency.

Specular
Refers to a surface that is smooth with respect to the wavelength of incident energy.

SPOT
Systeme Probatoire d'Observation del la Terre. Unmanned French remote sensing satellite orbiting in the late 1980s.

Stefan
Boltzmann constant- 5.68 x 10 -12 W . cm-2 .K-4. Boltzmann law-States that radiant flux of a blackbody is equal to the temperature to the fourth power times the Stefan-Boltzmann constant.

Stereo base
Distance between a pair of correlative points on a stereo pair that are oriented for stereo viewing.

Stereo model
Three-dimensional visual impression produced by viewing a pair of overlapping images through a stereoscope.

stereo pair
Two overlapping images or photographs that may be viewed stereoscopically.

Stereopsis
The ability for objects to be perceived in three dimensions as a result of the parallax differences produced by the eye base.

Stereoscope
Binocular optical device for viewing overlapping images or diagrams. The left eye sees only the left image, and the right eye sees only the right image.

Stereo- Restitution
Process that uses two-dimensional information contained in a pair of images to recreate the shape and position of objects.

Subscene
A portion of an image that is used for detailed analysis.

Subtractive primary colors
Yellow, magenta, and cyan. When used as filters for white light, these colors remove blue, green and red light, respectively.

Sunglint
Bright reflectance of sunlight caused by ripples on water

Sun-synchronous
Earth satellite orbit in which the orbit plane is nearly polar and the altitude is such that the satellite passes over all places on earth having the same latitude twice daily at the same local sun time.

Supervised classification
Digital-information extraction technique in which the operator provides training-site information that the computer uses to assign pixels to categories.

Surface phenomenon
Interaction between electromagnetic radiation and the surface of a material.

Surface roughness
See roughness.

Synthetic-aperture radar (SAR)
Radar system in which high azimuth resolution is achieved by storing and processing data on the Doppler shift of multiple return pulses in such a way as to give the effect of a much longer antenna.

Synthetic stereo images
Stereo images constructed through digital processing of a single image. Topographic data are used to calculate parallax.

System
Combination of components that constitute an imaging device.

Systematic distortion
Geometric irregularities on images that are caused by known and predictable characteristics.