absorption - The process by which electromagnetic radiation (EMR) is assimilated and converted into other forms of energy, primarily heat. Absorption takes place only on the EMR that enters a medium, and not on EMR incident on the medium but reflected at its surface. A substance that absorbs EMR may also be a medium of refraction, diffraction, or scattering; however, these processes involve no energy retention or transformation and are distinct from absorption.
absorption band - A range of wavelengths (or frequencies) of electromagnetic radiation that is assimilated by a substance.
ACCA - Automated Cloud Cover Assessment
AGS - Alaska Ground Station, Poker Flats AK.
albedo - (1) The ratio of the amount of electromagnetic energy reflected by a surface to the amount of energy incident upon it, often expressed as a percentage. Example: the albedo of the Earth is 34 percent. (2) The reflectivity of a body as compared to that of a perfectly diffusing surface at the same distance from the Sun, and normal to the incident radiation. Albedo may refer to the entire solar spectrum or merely to the visible portion.
algorithm - (1) Any method of computation consisting of a comparatively small number of steps, which are specifically adapted to the solution of a problem of some particular type. The steps are taken in a predetermined order, usually involving iteration. In computer terminology, an algorithm is a detailed logical procedure, or statement, which represents the solution of a particular problem. Most commonly, the term is used to indicate an analysis procedure such as that used for evaluation of a square root or for sorting a data file.
altitude - height above a datum, the datum usually being mean sea level. Not the same as elevation, for altitude generally refers to points above the Earth's surface rather than those on it. Compare with elevation.
analog - A physical variable, which remains similar to another variable insofar as the proportional relationship between the two, remain the same over a specified, and usually continuous, range. A temperature, for example, may be represented by a voltage level, which would be its analog. Contrast with digital.
ancillary - Auxiliary; accessory. in remote sensing, ancillary data are secondary data, pertaining to the area or classes of interest, such as topographic, demographic, or climatological data. Ancillary data may be digitized and used in conjunction with the primary remote sensing data.
angle of drift - The angle between the heading of the axis of a craft and its ground track.
annotation - Any marking on illustrative material for the purpose of clarification, such as numbers, letters, symbols, and signs. On digital Landsat products, the annotation (such as date of coverage, sun angle, scene identification number, and so forth) is represented in digital form in special records devoted to this purpose.
anomaly - A deviation from the norm.
aperture - An opening that admits electromagnetic radiation to a film or detector. An example would be the lens diaphragm opening in a camera.
apogee - The point in the orbit of a heavenly body, especially of a manmade satellite, at which it is farthest from the Earth.
aspect ratio - (1) Of pixels, the numerical ratio of the width of a pixel
to its height. (2) Of images, the ratio of
two perpendicular axes scales, or the ratio of image length to width.
atmospheric correction - The correction made to remotely sensed radiance (external link) to reduce or normalize for the intervening atmosphere (external link) between the Earth's surface and the satellite. The product of an atmospheric correction is the conversion of at-satellite spectral radiance to the innate reflectance or bidirectional reflectance of the surface. Also see radiometric reflective band calibration and reflectance. (Source: Dr. John Barker)
attenuation - The reduction in the intensity of radiation, with distance from its source due to atmospheric absorption and/or scattering; it does not include the inverse-square decrease of intensity of radiation with distance from the source. In a general sense, attenuation can be taken to mean any reduction of a physical variable; see also transmittance.
attitude - The angular orientation of a spacecraft as determined by the relationship between its axes and some reference line or plane or some fixed system of axes. Usually, "y" is used for the axis that defines the direction of flight, "x" for the "cross-track" axis, perpendicular to the direction of flight, and "z" for the vertical axis. Roll is the deviation from the vertical (the angle between the z-axis of the vehicle and the vertical axis, or angular rotation around the y-axis). Pitch is the angular rotation around the x-axis. Yaw is rotation around the z-axis.
azimuth - The arc of the horizon measured clockwise from the north point to the point referenced. Expressed in degrees. Azimuth indicates direction, and not location.
background - Any effect in a sensor or other apparatus or system above which the phenomenon of interest must manifest itself before it can be observed. See background noise.
background noise - (1) In recording and reproducing, the unwanted disturbance within a useful frequency band, independent of whether or not a signal is present. The signal is not to be included as part of the disturbance. (2) In receivers, the random oscillation in the absence of signal modulation on the carrier. Ambient oscillations detected, measured, or recorded with the signal become part of the background noise. Included in this definition is the interference resulting from primary power supplies.
band, spectral - An interval in the electromagnetic spectrum defined by two wavelengths, frequencies, or wave numbers. With Landsat, bands designate the specific wavelength intervals at which images are acquired. I.e., a band is a slice of wavelengths from the electromagnetic spectrum. Landsat ETM+ has eight bands which collect radiation from different parts of the electro-magnetic spectrum. Of the eight bands, three bands are visible light, one band is panchromatic, three bands are infrared, and one band is thermal infrared.
banding - Radiometric artifacts that affect all detectors in a spectral band, rather than the relative correction for striping of a single detector in the image of a band. When forward and reverse scans are radiometrically different, this uncorrected radiometric artifact is an example of banding. Also see noise and scan-correlated-shift. (Source: Dr. John Barker)
batch processing - Pertaining to the technique of executing a set of computer programs such that each program of the set is completed before the next program of the set is started; loosely, sequential processing.
bias - (1) In image processing, a persistent difference (not due to sampling error) between the true value of a population characteristic and the value obtained through the estimator used resulting in a systematic distortion in the results. This may be due to a flaw in the measurement, the method of sample selection, or the technique of estimation. (2) A steady voltage inserted in series with an element of an electronic device to signal a desired operation.
bidirectional reflectance - A unitless measure of the ratio of incoming to outgoing radiation created from converting a radiometrically calibrated image to an innate characteristic of the target being observed. After removing the atmospheric component of calibrated at-satellite spectral radiance, bidirectional reflectance distribution functions (BRDFs), bidirectional reflectance, and bidirectional reflectance factors (BRF) attempt to take into account target-related differences in reflectance as a function of four sources of variability of non-Lambertian surfaces: solar zenith and azimuthal irradiance angles and sensor viewing zenith and azimuthal angles. Also see planetary albedo and reflectance. (Source: Dr. John Barker)
bit - (1) An abbreviation of binary digit. (2) A unit of memory corresponding to the ability to store the result of a choice between two alternatives, used especially in connection with digital computing devices.
black body - An ideal body which, if it existed, would be a perfect absorber and a perfect radiator, absorbing all incident radiation, reflecting none, and emitting radiation at all wavelengths. In remote sensing, the exitance curves of black bodies at various temperatures can be used to model naturally occurring phenomena like solar radiation and terrestrial emittance.
brightness - The attribute of visual perception in accordance with which an area appears to emit more or less light. One of three psychological dimensions of color perception by which visual stimuli are ordered continuously and which is correlated with light intensity. See hue and saturation.
brightness value - (Landsat usage) A number in a range of 0-63, 0-127, or 0-255 that is related to the amount of radiance in watts per square centimeter striking a detector in either the Multispectral Scanner, the Thematic Mapper, or the Enhanced Thematic Mapper Plus.
buffer - A temporary storage device or circuitry which retains data for transmission between two equipment units, usually in order to compensate for differing data handling speeds of the units.
bumper mode - Satellite scanners operating in backup bumper mode. Landsat-type sensors such as the Thematic Mapper (TM) and the Enhanced Thematic Mapper Plus (ETM+) are designed to have a fixed line length on every scan controlled by the Scan Angle Monitor (SAM). When the wear on the bumpers is sufficiently large that it is no longer possible to maintain control in the SAM-mode, then the instrument can be put in a backup bumper mode where the mirror is allowed to scan at a fixed frequency of 14 Hz as a free pendulum rather than being controlled on each scan. The calibration shutter is still synchronized with the scan mirror in this mode so that calibration pulse can be obtained at the end of each scan. However, the variability of the scan length increases in bumper mode. The Landsat 5 TM was placed in the bumper mode in 2002. Also see scan line length. (Source: Dr. John Barker).
byte - A group of consecutive bits (usually 8 bits) that is operated upon as a single unit of information in a digital computer; usually shorter than a word.
calibration - The adjustment or systematic standardization of the output of a quantitative measuring instrument or sensor.
calibration data - In remote sensing, measurements pertaining to the spectral or geometric characteristics of a sensor or radiation source. Calibration data are obtained through the use of a fixed energy source such as a calibration lamp, a temperature plate, or a geometric test pattern. The application of calibration data to restore measurements to their true values is called rectification.
calibration lamps - Lamps carried into orbit on imaging sensors for the purpose of measuring the responsivity of reflective band detectors as a function of time under the assumption that controlling or monitoring the current or spectral radiance of the lamp will result in a constant or known irradiance on the detectors. There were three such lamps on each of the Thematic Mapper (TM) sensors on the Landsat 4/5 satellites that were cycled in an 8-step sequence whenever the instruments were turned on. There were two lamps on the Enhanced Thematic Mapper Plus (ETM+) imager on Landsat 7; the primary one was used on almost every acquisition, while the redundant one was only used a few times per year. Also see detector responsivity, internal calibrator, and radiometric reflective band calibration. (Source: Dr. John Barker)
calibration parameter file - Computer files of fixed parameters used in the ground processing system to convert raw uncalibrated bits from a spacecraft imager to radiometrically calibrated, artifact-reduced and geometrically-referenced or re-sampled products. For example, the USGS Landsat Ground Processing System (LGPS) at EROS Data Center uses the current version of the Calibration Parameter File (CPF) to convert input Level-0R in its archive to radiometrically corrected Level-1Rs and radiometrically and systematically corrected Level-1Gs or topographically corrected (Level-1G terrain) products. Each CPF is stamped with applicability dates associated with the date of acquisition of the imagery being processed. The Image Assessment System (IAS) updates the CPF as necessary. (Source: Dr. John Barker)
cluster - In image processing, a homogeneous group of units, which are very "like" one another. "Likeness" among units is usually determined by the association, similarity, or distance separating the measurement patterns associated with the units.
coherent noise - Systematic periodic noise in image or calibration data which can be characterized in terms of frequency, phase and amplitude. It is often quantified using Fourier analysis. Coherent noise is usually electronic, coming from sources such as power supplies, transmitters, and clocks. It is desirable to remove, isolate or characterize sources of instrumental coherent noise before launch. It is possible to characterize signal-independent coherent noise most easily on dark background data such as night scenes. It is also visible in dark radiometrically homogeneous regions of a scene such as water. If the coherent noise is well enough characterized and stable with time then it can be significantly reduced with notch filtering using inverse Fourier transforms as a part of radiometric processing to produce the user’s product on the ground. Also see noise. (Source: Dr. John Barker)
collimate - (1) To align the axes of instruments, especially optical axes of multiple lenses, so that they have the same spatial orientation along a common line. (2) To make parallel rays of light by means of a lens or concave mirror.
color - That property of an object which is dependent on the wavelength of the light it reflects or, in the case of a luminescent body, the wavelength of the light it emits. If, in either case, this light is of a single wavelength, the color seen is a pure spectral color, but, if the light of two or more wavelengths is emitted, the color will be mixed. White light is a balanced mixture of all the visible spectral colors.
color composite (multiband imagery) - A color image produced by the combination of (usually) three individual monochrome images each of which is reproduced in a given color. With Landsat TM and ETM+ imagery, a natural color image is created by assigning the color blue to band 1 (0.45-0.52 µm), green to band 2 (0.52-0.60 µm), and red to band 3 (0.63-0.69 µm).
contrast - The difference between highlights and shadows; the ratio of reflecting power between the highlights and shadows is the contrast of the image. It is possible to change the contrast of Landsat images, when in digital form, by performing a linear stretch of the image grey levels as much as possible to fill the complete dynamic range of the display medium.
cross-calibration - The radiometrical comparison of one sensor to another sensor on different satellites. Imagers are often considered to be cross-calibrated if they are calibrated to a common source such as an integrating sphere before launch. However, testing for changes with time requires on-orbit cross-calibration being performed by looking at the same target at as close to the same time as possible to avoid possible changes in the target scene, especially atmospheric changes. The best target for this is the Moon because it has no atmosphere and is spectrally flat. The common assumption in cross-calibrations is that the spectral band-passes of the two sensors are identical or that there are no spectral features in the target, including the atmosphere. Even with sensors designed to have nearly identical band-passes, differences in relative spectral response (RSR) curves can result in uncorrectable radiometric differences of 5-15% due to real spectral differences in the target viewed by the two sensors. Also see relative spectral response and spectral striping. (Source: Dr. John Barker)
cubic convolution - A high-order resampling technique in which the brightness value of a pixel in a corrected image is interpolated, from the brightness values of the 16 nearest pixels around the location of the corrected pixel.
data compression - Any technique that condenses the available data so as to make data storage or transmission more efficient. Data compression can be lossy in which some amount of information (data) is lost or lossless in which no information (data) is lost.
decompression - A reversal of the process of data compression.
density slicing - A general class of electronic or digital techniques used to assign linage points or data vectors to particular classes based on the density or level of the response in a single image or channel; classification by thresholds.
detector responsivity - The radiometric response of an electronic detector that converts photons into voltage or current. Detector responsivity includes the end-to-end optical and electrical path, since radiometric calibration involves converting the digital signal into scientific units such as at-satellite radiance. Typically before launch, detectors within each spectral band of an imaging sensor are exposed to full aperture illumination from a uniform source such as a large integrating sphere, which itself has been calibrated against standards that are traceable to National Institute of Standards and Technology (NIST) standards. Absolute detector responsivity is in units of digital number per radiance unit (such as watts per square meter per steradian), or in units of digital number per spectral radiance unit (such as watts per square meter per steradian per micron). Once on-orbit, the detector responsivity is often monitored for possible change with time. Relative gain of the detector in comparison to a reference detector or to an average of detectors within a band can be made from image data by histogram equalization. Absolute gain can be monitored using internal lamps, or external sources such as the Sun, Moon or nearly homogeneous areas on Earth after correcting for atmospheric effects. Also see calibration lamps, NIST traceability, radiometric emissive band calibration, and radiometric reflective band calibration. (Source: Dr. John Barker).
DFCB - Data Format Control Book
digital image - A digital image, or digitized image, or digital picture function of an image, is an image represented numerically in digital form and is obtained by partitioning the area of the image into a finite two-dimensional array of small, uniformly shaped, mutually exclusive regions, called resolution cells or picture elements (pixels), and by assigning a representative grey shade to each such spatial region. A digital image may be abstractly thought of as a function whose domain is the finite two-dimensional set of resolution cells and whose range is the set of grey shades.
distortion - A change in scale from one part of an image to another.
DN - digital number
dwell time - Refers to the momentary time interval during which a detector is able to, or allowed to, sense incoming electromagnetic radiation within its intended instantaneous field of view.
dynamic range - the range between the maximum and minimum amount of input radiant energy that an instrument can measure.
electromagnetic radiation - Energy emitted as a result of changes in atomic and molecular energy states and propagated through space at the speed of light, i.e., energy transfer in the form of electromagnetic waves or particles that propagate through space at the speed of light. The term radiation is used commonly for this type of energy, although it actually has a broader meaning. Also called electromagnetic energy. See electromagnetic spectrum.
electromagnetic spectrum - (1) A system that classifies, according to wavelength, all energy that moves, harmonically, at the constant velocity of light. (2) A continuum that is conventionally broken into arbitrary segments (as ultraviolet, visible, radio). The entire range of electromagnetic radiation. The spectrum usually is divided into seven sections. From the longest wavelengths to the shortest: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma ray radiation.
elevation - Vertical distance from the datum, usually mean sea level, to a point or object on the Earth's surface. Not to be confused with altitude, which, refers to points or objects above the Earth's surface. Compare with altitude.
emission - With respect to electromagnetic radiation, the process by which a body emits electromagnetic radiation as a consequence of its kinetic temperature only.
emissivity - Ratio of radiation emitted by a surface to the radiation emitted by a black body at the same temperature under similar conditions. May be expressed as total emissivity (for all wavelengths), spectral emissivity (as a function of wavelength), or goniometric emissivity (as a function of angle).
EMR - electromagnetic radiation
EMS - electromagnetic spectrum
Enhanced Thematic Mapper Plus (ETM+) - The sensor aboard Landsat 7 that picks up solar radiation reflected by, or emitted from the Earth.
enhancement - The process of altering the appearance of an image so that the interpreter can extract more information. Some types of digital enhancements commonly applied to Landsat imagery include edge enhancement, noise reduction (filtering), haze removal, and contrast stretching.
EOSAT - Earth Observation Satellite Company
ephemeris - Any tabular statement of the assigned places of a celestial body (including a manmade satellite) for regular intervals. Ephemeris data help to characterize the conditions under which remote sensing data are collected and may be used to correct the sensor data prior to analysis.
EROS - Earth Resources Observation Systems. Also, Earth Resources Observation and Science, the USGS Data Center in Sioux Falls, SD.
ETM+ - Enhanced Thematic Mapper Plus
FAC - Full Aperture Calibrator
focal plane - In an instrument such as the Enhanced Thematic Mapper Plus (ETM+), the plane occupied by the detectors, and on which the radiances to be sensed are incident.
focal point - The point toward which light rays converge to form an image after passing through a lens or having been reflected by mirrors. The condition of sharpest imagery.
frame - Any individual member of a continuous sequence of images.
full aperture solar calibrator (FAC or FASC) - One of two on-board external solar calibrators on Landsat 7 ETM+. The FASC consists of a nearly white panel, which is extended in front of the full aperture of the ETM+ as it ascends over the North Pole from the night side so the Sun’s irradiance can be reflected into the aperture. FASC measurements are typically made once a month. The gain or responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles relative to the solar panel, known distance from the Sun, and pre-launch measurements of the nearly diffuse bidirectional reflectance factor of the solar panel. Also see PASC and IC. (Source: Dr. John Barker)
fully processed - In reference to Landsat images, those scene data, which have been both radiometrically, corrected and geometrically corrected through the ground processing system, labeled “Level-1G or L1G” data.
gain - A general term used to denote an increase in signal power in transmission from one point to another. Gain is usually expressed in decibels.
GCP - Ground Control Point
geodetic coordinates - Quantities which define the position of a point on the spheroid of reference (for example, the Earth) with respect to the planes of the geodetic equator and of a reference meridian. Commonly expressed in terms of latitude and longitude.
geometric correction - The transformation of image data, such as Landsat data, to match spatial relationships as they are on the Earth. Includes correction for band-to-band offsets, line length, Earth rotation, and detector-to-detector sampling delay. For Landsat, a distinction is made between data that have been geometrically corrected using systematic, or predicted, values (Level-1G) and data that have been geometrically corrected using more precise ground control point data (orthorectified, Level-1G-Terrain).
geometric image calibration - The location of every pixel in a 2-dimensional satellite image to a specific spatial location on the ground. Geometric calibration may be done by either re-sampling the image in a specific projection or by storing the geometric location information with each pixel in the image. If pre-launch values of the instrument “MTF” have been accurately enough made prior to launch, then an MTF correction can also be made as part of the geometric ground processing. Resampling for TM-type imagery is usually done using cubic convolution algorithms with an accuracy of ±50 m for 30 m ETM+ pixels from inputs that include “definitive” ephemeris and known characteristics of the non-uniform scan rate. This geodetic accuracy refers to how close the knowledge of the pixel’s location is to the actual location on the Earth. MODIS sensors on the Terra and Aqua satellites store calibrated imagery as geolocated information for each pixel. Systematic geometric corrections correct for sensor-related factors, either with or without the use of ground control points for the specific image. Such systematic products can be significantly improved if topographic information is available and used. In TM-era products the approximate 8-bit radiometric precision was reduced with each geometric re-sampling, often by simple rotation and translation, to a new 8-bit product, and therefore it was desirable when radiometrically or geometrically re-calibrating an image, to normalize it to best estimates, to start from raw uncalibrated imagery. Also see modulation transfer function and scan line length. (Source: Dr. John Barker)
geosynchronous - An Earth satellite orbit in which the satellite remains in a fixed position over a geographic location on the Earth. This requires that the orbital plane be in the same plane as the equator, and that the satellite's altitude be high enough for the satellite to revolve about the Earth at a speed equal to that of the Earth's rotation. Geosynchronous orbits are common for most communications satellites, such as the Tracking and Data Relay Satellite System.
GeoTIFF - Geographic Tagged Image File Format
GMT - Greenwich Mean Time
grey level - (1) A shade of grey representing a given radiometric level, or intensity, on the image. (2) A number or value assigned to a position (x, y) on the image proportional to the integrated output, reflectance, or transmittance of a small area, usually called a resolution cell or pixel, centered on the x, y position. Grey level can be measured or expressed as: transmittance, reflectance, brightness, radiance, luminance, density, voltage, current, or brightness value.
ground control point (GCP) - A geographic feature of known location that is recognizable on images and can be used to determine geometric corrections to those images.
ground data - Supporting data collected on the ground, and information derived therefrom, as an aid to the interpretation of remotely sensed data. Ground data typically pertain to weather, soils, and vegetation types and conditions.
ground resolution cell - (1) The area of terrain that is covered by the instantaneous field of view of a detector. The altitude of the remote sensing system and the instantaneous field of view of the detector determine the size of the ground resolution cell. (2) The smallest area on the ground that can be resolved on a Landsat image. Compare with resolution cell.
ground track - The vertical projection of the actual flight path of an aerial or space vehicle onto the surface of the Earth.
ground truth/ground observations - Observations made on the ground at a site that is being imaged from space for the purpose of verifying either the absolute radiometric and/or geometric calibration of the imagery or the classified product from the image. These data which are acquired from field checks, high-resolution remote sensing data, or other sources of "known" data are used as the basis for making decisions on training areas and evaluating classification results. Instrument ground truthing made during field trips is often called vicarious calibration when experimental measurements are made of such factors as solar irradiance, atmospheric transmittance, and reflectance of either natural or calibrated homogeneous or gridded targets. Classification ground truthing can be done (1) by visiting the sites to identify what is on the ground, (2) by referring to classification of the area from other sources such as thematic maps, or (3) by classifying higher spatial resolution imagery from satellites or aircraft into classes that can be observed in the coarser resolution imagery. Also see ground data and vicarious calibration. (Source: Dr. John Barker)
histogram - A graphical representation of a frequency distribution by means of lines or rectangles that represent class intervals along the x-axis, and corresponding class frequencies along the y-axis; a frequency distribution plot, often of a classification training area.
hue - the attribute of a color that differentiates it from grey of the same brilliance and that allows it to be classed as blue, green, red, or intermediate shades of these colors. See brightness and saturation.
IAS - Image Assessment System
IFOV - Instantaneous Field of View
IGS - International Ground Stations
image - (1) The recorded representation of an object produced by optical, electro-optical, optical-mechanical, or electronic means. It is the term generally used when the electromagnetic radiation emitted or reflected from a scene is not directly recorded on photographic film. (2) The optical counterpart of an object produced by a lens, mirror, or other optical system.
image enhancement - Any one of a group of operations which improves the interpretability of an image or the detectability of targets or categories in the image. These operations, in the case of Landsat images, include: contrast stretch, edge enhancement, spatial filtering, noise suppression, image smoothing, and sharpening of image detail (often called pan-sharpening).
image processing - Encompasses all the various operations which can be applied to photographic or image data. These include, but are not limited to: image compression, image restoration, image enhancement, preprocessing, quantization, spatial filtering, and pattern recognition techniques. The term usually refers to the application of such operations by digital means.
image restoration - A process by which a degraded image is restored to its original condition. Image restoration is possible only to the extent that the degradation transform is mathematically invertible.
image transformation - A function or operator that takes an image as its input and produces an image as its output. Depending on the transform chosen, the input and output images may appear entirely different and have different interpretations. Fourier transforms, principal component analysis (also called Karhunen-Loeve analysis), and various spatial filters, are examples of frequently used image transformation procedures.
imagery - The products of image-forming instruments (analogous to photography). Used loosely, but acceptably, to refer to Landsat image data products.
infrared (IR) - Pertaining to or designating the portion of electromagnetic spectrum with wavelengths from the red end of the visible spectrum to the microwave portion of the spectrum, or from 0.7 µm to 1mm. Infrared waves are not visible to the human eye. Longer infrared waves are called thermal infrared waves.
instantaneous field of view (IFOV) - (1) The solid angle through which a detector is sensitive to radiation. In a scanning system this refers to the solid angle subtended by the detector when the scanning motion is stopped. instantaneous field of view is commonly expressed in milliradians. (2) The ground area covered by this solid angle. See ground resolution cell.
internal calibrator (IC) - A specific type of lamp-based calibration system carried on-board an imager in space for the purpose of monitoring for possible radiometric changes in detector responsivity with time. The internal calibrators (ICs) on Landsat 4/5 TM consisted of 3 lamps that were sequentially cycled through 8 possible radiation levels. The IC on Landsat 7 ETM+ consisted of a single primary lamp, which was used during almost all acquisitions and a secondary or backup lamp. On both imagers, background and IC pulse values were taken on each scan as a shutter, which was synchronized with the scan mirror, obscured the ground image and projected a pre-launch calibrated radiance from the IC lamps onto the focal planes. Also see other on-board external full aperture and partial aperture solar calibrators. Also see calibration lamps. (Source: Dr. John Barker)
isarithm - A line connecting points of equal quantity on a map. A contour line is an isarithm.
jitter - Small rapid variations in a variable (such as a waveform) due to deliberate or accidental electrical or mechanical disturbances or to changes in the supply voltages in the characteristics of components, etc. Jitter effects arising from the oscillating mirrors and other movable components aboard Landsat are often a cause of certain anomalies in the image data received and must be compensated for by the ground processing system.
K band - A portion of the electromagnetic spectrum in the microwave region of frequencies ranging between 12 and 93 gigahertz (GHz). The 18-40 GHz range is used for radar and general communications. The 12-18 GHz region is used for satellite communications. (Source: Rich Irish)
Kilobyte (Kb) - In decimal systems kilo stands for 1,000, but in binary systems a kilo is 1,024 (2 to the 10th power). Technically a kilobyte is 1,024 bytes but is often used loosely as a synonym for 1,000 bytes. The term is commonly used to quantify the amount of a computer's core memory storage. (Source: Rich Irish)
L band - A frequency between 390 MHz and 1.55 GHz which is used for satellite communications and for terrestrial communications between satellite equipment. (Source: Rich Irish)
L0R - Level 0 Reformatted Landsat data product. The Level 0R product is reformatted, raw data. Reformatting includes shifting pixels by integer amounts to account for 1) the alternating forward-reverse scanning pattern of the Enhanced Thematic Mapper Plus (ETM+) sensor, 2) the odd-even detector arrangement within each band, and 3) the detector offsets inherent to the focal plan array engineering design. Pixels are neither resampled nor are they geometrically corrected or registered, i.e. the pixels are NOT aligned per scan line. (Source: USGS)
L1G - Level 1 Geometrically Corrected Landsat data product. The L1G product is radiometrically and geometrically corrected (systematic) to the user-specified parameters including output map projection, image orientation, pixel grid-cell size, and resampling kernel. The correction algorithms model the spacecraft and sensor using data generated by onboard computers during imaging. Sensor, focal plane, and detector alignment information provided by the Image Assessment System (IAS) in the Calibration Parameter File (CPF) is also used to improve the overall geometric fidelity. The resulting product is free from distortions related to the sensor (e.g., jitter, view angle effect), satellite (e.g., attitude deviations from nominal), and Earth (e.g., rotation, curvature). Residual error in the systematic L1G product is less than 250 meters (1 sigma) in flat areas at sea level. The systematic L1G correction process does not employ ground control or relief models to attain absolute geodetic accuracy. (Source: USGS)
LIR - Level 1 Radiometrically Corrected Landsat data product. The Level 1R product is a radiometrically corrected L0R product. This product 1) corrects detector artifacts such as coherent noise, 2) improves cosmetic artifacts such as banding, striping, and dropped lines or pixels, and 3) is calibrated to radiance units, i.e. color corrected, as integer values. Radiometric corrections are not reversible. Pixels are neither resampled nor are they geometrically corrected or registered, i.e. the pixels are NOT aligned per scan line. (Source: USGS)
laser (Light Amplification by Stimulated Emission of Radiation) - A device for producing light by emission of energy stored in a molecular or atomic system when stimulated by an input signal.
LGS - Landsat Ground Station, Sioux Falls S.D.
light, transmitted - Light that has traveled through a medium without being absorbed or scattered.
line scanner - A scanning radiometer which by use of a rotating or oscillating plane mirror can scan a path normal to the movement of the radiometer. The mirror directs incoming radiation to a detector, which converts it into an electric signal.
linear quantizing - The range of grey shades from maximum to minimum is divided into contiguous intervals, each of equal length, and each grey shade is assigned to the quantized class which corresponds to the interval within which it lies.
linear spatial filter - A spatial filter for which the grey shade assignment at coordinates (x, y) in the transformed image is made by some weighted average (linear combination) of grey shades located to a particular spatial pattern around coordinates (x, y) of the domain image. The linear spatial filter is often used to change the spatial frequency characteristics of the image. For example, a linear spatial filter, which emphasizes high spatial frequencies (high-pass), will tend to sharpen, the edges in an image. A linear spatial filter, which emphasizes the low spatial frequencies (low-pass), will tend to blur the image and reduce noise.
lookup table - A file of values from which functions corresponding to a given argument can be obtained. A table lookup procedure is employed.
LPS - Landsat Processing System
map projection - Any systematic arrangement of meridians and parallels portraying the curved surface of a sphere or spheroid upon a plane.
mask - Any material or digital procedure used to obscure or define a part of the image.
micrometer (µm) - Sometimes expressed using the obsolete term micron, a micrometer is one-millionth of a meter and also can be expressed as 10 -6 meter, one thousandth of a millimeter, one 25-thousandth of an inch. (Source Rich Irish)
MOC - Mission Operations Center
modulate - To vary, or control, the frequency, phase, or amplitude of an electromagnetic wave or other variable.
modulation transfer function (MTF) - The geometric description of a detector’s instantaneous field-of-view (IFOV) from the satellite. The modulation transfer function (MTF) is a frequency-based characterization of the IFOV of the plane area of a specific detector that is both sensitive to and exposed to radiation from the imaging optics. The full-width-at-half-maximum (FWHM) of a 2-dimenstional line-spread function is a spatial representation of IFOV. MTF is the ratio of the contrast of the output to the input image as a function of frequency. MTF is defined as the magnitude of the Fourier transform of the line spread function. Common units of IFOV are radians, steradians (sr) or degrees. Because of the common optics and the similar size, detectors within a spectral band are usually assumed to have identical IFOVs. An IFOV expressed in spatial rather than angular units, such as 30 m for a TM detector, is altitude-dependent. An instrumental in-vacuum MTF is modulated further by the atmosphere, which broadens the nominal instrumental IFOV, such that it is not possible to resolve features as easily on the ground. If the structural aspects of the optics of the imager are changing with time from factors such as expansion or contraction from outgassing of water in space, then the IFOV or MTF will also change. Also see geometric image calibration and point spread function. (Source: Dr. John Barker)
mosaic, controlled - A mosaic oriented and scaled to horizontal ground control, usually assembled from rectified photographs or geolocated images.
multiband system - A system for simultaneously recording electromagnetic radiation from the same scene in several bands from essentially the same spectral region, such as the visible or visible and near infrared. May be applied to cameras with different film/filter combinations or scanning radiometers that use disparate optics to split wavelength bands apart for viewing by several filtered detectors.
multispectral image - A remote sensing image created using data collected from more than one band.
multispectral - Generally denotes remote sensing in two or more spectral bands, such as visible and infrared.
Multispectral Scanner (MSS) - (1) A non-photographic imaging system, which utilizes an oscillating mirror and fiber optic sensor array. The mirror sweeps from side to side, transmitting incoming energy to a detector array which sequentially outputs brightness values (that is, signal strengths) for successive pixels one swath at a time. The forward motion of the sensor platform carries the instrument to a position along its path where an adjacent swath can be imaged (2) For Landsats 1,2,3, 4, and 5 the Multispectral Scanner sensed radiation simultaneously by an array of six detectors in each of four spectral bands from 0.5 to 1.1 µm. See radiometer.
nadir - That point on the celestial sphere vertically below the observer (i.e. the point on the Earth directly below an orbiting satellite), or 180° from the zenith.
near infrared (NIR) - The preferred term for the shorter wavelengths in the infrared region (the entire infrared region extends from about 0.7 µm (visible red) to about 3 µm). The longer wavelength end grades into the middle infrared, sometimes called solar infrared, as it is only available for use during the daylight hours. Also known as the shortwave infrared (SWIR).
NDVI - Normalized Difference Vegetation Index. NDVI is the most commonly used vegetation index for satellite imagery. The difference in reflectance from the near infrared and red bands is divided by the sum of the two reflectances. This compensates for different amounts of incoming light and produces a number between 0 and 1. The typical range of actual values is about 0.1 for bare soils to 0.9 for dense vegetation.
NIST traceability - The attempt to trace radiometric units and calibrations back to a common radiance source at the United States National Institute of Standards and Technology (NIST) in order to compare measurements taken by different methodologies in absolute terms. The stability and precision requirements for remote sensing from space are often higher than that of (1) the transfer calibration from NIST to transfer lamps, (2) the calibrated radiometers for vicarious measurements and (3) the calibrated on-board sources. For this reason, some sensors reference their calibration to presumably more quantifiable and more stable solar irradiance rather than to NIST-traceable pre-launch calibration of on-board calibrators. However, in the absence of any absolute national radiance standard other than at NIST, it is still necessary to reference radiometric calibrations to NIST standards. Also see radiometric reflective band calibration. (Source: Dr. John Barker)
noise - Any unwanted disturbance affecting a measurement (as of a frequency band), especially that which degrades the information-bearing quality of the data of interest. Noise determines the precision with which a radiometric measurement can be made. The standard deviation of a measurement is a common method for defining noise. Noise includes systematic or random sources. Systematic noise is constant or modelable with time and includes coherent noise, scan-correlated-shift, banding and striping and others, which reduce the ability to extract information from images. Systematic noise is potentially reducible with ground processing. Random noise, or white noise, is not correctable, but the uncertainty of estimates of the mean value can be reduced by multiple measurements, which are subject only to random noise. The potential degradation of signal from variations in the analog reference signal from space are reduced by using analog-to-digital converters and then adding error correction code to the digital signal to allow the exact original digital number to be recovered in ground processing even if it was degraded in transit. Also see banding, coherent noise, background noise, scan-correlated-shift and signal-to-noise ratio. (Source: Dr. John Barker)
nominal - (1) In name only; so-called. (2) Loosely, a rough designation or an approximation. (3) According to plan or design: a nominal flight check.
nonsystematic distortion - Geometric irregularities on images that are not constant and cannot be predicted from the characteristics of the imaging system. For example: parallax distortions caused by topography.
observatory - A place or structure, such as the Landsat satellite, equipped and used for making observations of astronomical or other natural phenomena; a place or structure affording an extensive view.
optical axis - An imaginary line drawn directly through the optical center of the camera lens to the geometric center of the film. For Landsat ETM+, TM, or MSS imagery, the point in the image where geometric distortion is at a minimum.
orbital node - Either of the two points at which the orbit of a heavenly body intersects a given plane, especially the plane of ecliptic. With respect to Landsat, the orbital nodes occur at the equator, one on the descending, or daylight, track of the orbit and the other on the ascending, or nighttime, track.
orbital period - The interval in time between successive passages (orbits) of a satellite through a reference plane. The orbital period of Landsat 7 is about 1.5 hours.
PAC - Partial Solar Calibrator
panchromatic - Describing films or detectors that are sensitive to broadband electromagnetic radiation (the entire visible part of the spectrum). Landsat 7 has a 15 m "panchromatic" band that extends into the near-IR and covers the spectral region between 0.52-0.9 µm.
pan-sharpening - The practice of using Landsat 7’s 15 m panchromatic band in conjunction with the other 30 m spectral bands to increase the apparent resolution of a multiband (color) Landsat 7 image to 15 m.
parameter - Any quantity of a problem that is not an independent variable. More specifically, a term which distinguishes, from dependent variables, quantities which are constants or which may be assigned more or less arbitrary values for purposes of the problem at hand. In computing, parameters passed to a function subroutine are more normally called arguments.
partial aperture solar calibrator (PAC or PASC) - One of two on-board external solar calibrators on Landsat 7 ETM+. The PASC consists of structure in front of the ETM+ aperture that has four independent pinholes for reflecting solar irradiance on a presumably parallel path to the optical axis of ETM+ as it comes over the North Pole and into sunlight. As compared to FASC measurements, PASC irradiance from a single pinhole at a time only fills a small part of the defining aperture. PASC measurements were made approximately once a day prior to failure of the Scan-Line-Corrector (SLC), and once every other day subsequently. The SLC-failure also necessitated rewriting the software for PASC processing by the Image Assessment System (IAS) at EROS Data Center in Sioux Falls, SD. Unfortunately, there are two major problems with PASC data: 1) the elliptical PASC images are saturated in several bands, and 2) an as-yet unknown cyclic variation has made them unusable in an absolute sense. The relative gain or responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles. Also see full aperture solar calibrator and internal calibrator. (Source: Dr. John Barker)
partially processed - In reference to Landsat images, those scene data which have only been radiometrically, corrected through the ground processing system, labeled Level 1R or L1R data.
passive sensor - One type of remote sensing instrument, a passive sensor picks up radiation reflected or emitted by the Earth. The ETM+, TM, and MSS sensors are all examples of a passive remote sensing system.
path - The longitudinal center line of a Landsat scene corresponding to the center of an orbital track. Sequential numbers from east to west are assigned to 233 nominal satellite tracks for Landsat 4, 5, and 7. Path numbers can be used with row numbers to designate nominal scene center points. See row and scene center, nominal.
payload - Originally, the revenue producing portion of an aircraft's load (passengers, cargo, mail); by extension, that which a spacecraft carries that is separate from the equipment or operations necessary to maintain the spacecraft in orbit.
perigee - The point in the orbit of heavenly body, especially of a man-made satellite, at which it is nearest the Earth.
perturbation - Irregular variation in, or deviation from, what is usual or expected, as in the orbital motion of a satellite being affected by extraordinary pull.
picture element (pixel) - A unit whose first member is a resolution cell and whose second member is the grey shade assigned to that resolution cell by a digital count. A Landsat MSS pixel represents about 0.44 hectares (1.09 acres) on the ground. One Landsat MSS frame contains about 7.36 x 106 pixels, each described by one of 64 radiance values. A Landsat ETM+ or TM pixel represents about 0.09 hectares (0.22 acres) on the ground. Each Landsat TM pixel is described by one of 256 radiance values. (All values given for DN (digital number) data only.)
pitch - The rotation of a spacecraft about the horizontal axis normal to its longitudinal axis (in the along-track direction) so as to cause a nose-up or nose-down, attitude. The pitch axis is referred to as the "x” axis. See attitude.
pixel - Abbreviation of picture element.
planetary albedo - A unitless measure of the ratio of outgoing to incoming solar radiation created by converting a radiometrically calibrated image observed from orbit to the apparent reflectance of the planet being observed (assuming the planet is acting as a perfect Lambertian reflector). Conversion to planetary albedo assumes knowledge of the solar irradiance for a specific spectral band, distance from the Sun and solar zenith angle. Also see reflectance and bidirectional reflectance. (Source: Dr. John Barker)
point spread function - The inferring of spatial characteristics of the instrument from the collected image of a point source such as a star. Also see modulation transfer function. (Source: Dr. John Barker)
polar orbit - A satellite orbit that passes over, or very close to, both poles of the Earth. Landsat 4, 5, and 7 all had/have a near-polar orbit at 438 miles (705 kilometers) above the Earth.
primary color - One of the three colors, either additive (blue, green, and red) or subtractive (cyan, yellow, and magenta) that may be combined to produce the full range of colors.
quantize - (1) To restrict a variable to discrete values, each of which is normally an integral multiple of the same quantity. (2) To process a range of grey shades, from maximum to minimum, such that the entire range is divided into contiguous intervals of normally equal lengths, each being assigned an integer value unique to the grey shade corresponding to it. Output voltages from the ETM+ and TM sensors aboard Landsat 4, 5 and 7 are quantized into 256 discrete values prior to transmission to the ground. A total of 256 brightness values are therefore possible in each band of ETM+ and TM imagery.
quantization level - The number of numerical values used to represent a continuous quantity.
radar - Short for "radio detection and ranging," radar sends out short pulses of microwave energy and records the returned signal's strength and time of arrival.
radian - A unit of angle subtended by an arc of a circle equal in length to the radius of the circle.
radiance-based calibration - The empirical calibration of satellite sensors by measurement of upwelling radiation from targets that are viewed on the ground or from aircraft at the same time as from space. The at-satellite radiance is calculated by using radiative transfer models of the atmosphere along with measurements of the atmospheric transmittance or optical thickness for the spectral band-pass being calibrated. Also see ground truth/ground observations, radiometric reflective band calibration, and reflectance-based calibration. (Source: Dr. John Barker)
radiation - The process by which electromagnetic energy is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space. This concept is to be distinguished from conduction and convection. Also, the process by which energy is propagated through any medium by virtue of the wave motion of that medium, as in the propagation of sound waves through the atmosphere. Also called radiant energy and electromagnetic radiation.
radiometric emissive band calibration: brightness temperature - Equations converting unitless digital numbers (DN) from the satellite imager to brightness temperature, a measure of the amount of energy being radiated by the target. Brightness temperature is not the internal (kinetic) temperature of the target except when the emissivity of the target is unity. The at-satellite brightness temperature includes atmospheric radiance so it does not represent the brightness temperature of the surface. (Source: Julia Barsi).
radiometric reflective band calibration: radiance - Calibration equations to convert unitless values from an optical satellite imager in Digitals Numbers (DNs) to engineering or scientific units such as at-satellite radiance or spectral radiance. Radiance is power (energy per unit time) per unit area per unit solid angle, in units such as watts per steradian per square meter (W sr-1 m-2). Spectral radiance is radiance per unit wavelength, in units such as watts per steradian per square meter per micron (W sr-1 m-2 mm-1) for a specific spectral band-pass. The coefficients for such calibration equations are normally developed by laboratory tests of the instrument prior to launch with reference to sources that are traceable to National Institute of Science and Technology (NIST) radiance standards. Also see NIST traceability. (Source: Dr. John Barker).
range, brightness - Variation in light intensity from maximum to minimum. This generally refers to a subject to be imaged or photographed.
range, dynamic - The difference between maximum measurable signal and minimum detectable signal. The upper limit usually is set by saturation and the lower limit by noise.
raster - Refers to the two-dimensional array of pixels in an image.
ratio image - An image prepared by processing digital multispectral data. For each pixel the value for one band is divided by that of another. The resulting digital values are displayed as an image. The term ratioing refers to the process by which a ratio image is produced. Often used ratios: NDVI, VI.
real time - Time in which the recording of an event is simultaneous with the event. The real time of a satellite is that in which it simultaneously reports on its environment as it encounters it.
rectification - Process by which a tilted or oblique image is projected onto a horizontal reference plane, the angular relation between the image and the plane being determined by ground reconnaissance. For example if the image is taken of an equally spaced rectangular grid pattern, then the rectified image will be an image of an equally spaced rectangular grid pattern.
reflectance - A measure of the ratio of outgoing to incoming radiation calculated by converting a radiometrically calibrated image to an innate characteristic of the target being observed. Calibrated at-satellite spectral radiance is converted to unitless reflectance by separating out the atmospheric component of the reflective band radiance and assuming that the target is a Lambertian reflector, re-radiating incident solar radiation equally in all directions. In general, reflectance is a function of incident angle of the energy, viewing angle of the sensor, spectral wavelength, and bandwidth, and the nature of the object. Also see planetary albedo, bidirectional reflectance and atmospheric correction. (Source: Dr. John Barker)
reflectance-based calibration - The field calibration of satellite sensors by measurement of reflected solar radiation from targets of known or measured reflectance that are viewed on the ground at the same time as from space. The at-satellite radiance is calculated by using radiative transfer models of the atmosphere along with measurements of the atmospheric transmittance or optical thickness for the spectral band-pass being calibrated. Also see ground truth/ground observations, radiance-based calibration, and radiometric reflective band calibration. (Source: Dr. John Barker)
registration - The translation-rotation rectification alignment process by which two images of like geometries and of the same set of objects are positioned coincident with respect to one another so that corresponding elements of the same ground area appear in the same place on the registered image. In this manner, the corresponding grey shades of the two images at any (x, y) coordinate or resolution cell will represent the sensor output for the some object over the full image frame being registered. In digital registration, the digital grids also must be superimposed at the same time the ground elements are superimposed.
relative spectral response (RSR) - The relative spectral transmittance as a function of wavelength. Relative spectral response (RSR) measurements are assumed to be constant for all detectors covered by a common filter and are normalized to unity at peak response. There are currently no methods to check spectral stability with time from either on-orbit or ground measurements. Also see spectral striping. (Source: Dr. John Barker)
remote sensing - In the broadest scene, the measurement or acquisition of information of some property of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object or phenomenon under study; for instance, the utilization at a distance (as from aircraft, spacecraft, or ship) of any instrument and its attendant recording and display devices for gathering information pertinent to the environment such as measurements of force fields; electromagnetic radiation, or acoustic energy. The technique employs such devices as cameras, lasers, radio frequency receivers, radar systems, sonar seismographs, gravimeters, magnetometers, multispectral scanners, and scintillation counters.
resolution, spatial - The ability of an imaging system to distinguish closely spaced objects in the subject area. Can be expressed as the spacing, in line-pairs per unit distance, of the most closely spaced lines that can be distinguished. See instantaneous field of view and resolution cell.
resolution cell - The smallest, most elementary areal constituent of the grey shades considered by an investigator in an image. A resolution cell is referenced by its spatial coordinates. The resolution cell or formations of resolution cells can sometimes constitute the basic unit for pattern recognition in image data.
return beam vidicon (RBV) - As used on Landsat 1 and 2, a camera system which operated by shuttering three independent cameras simultaneously, each sensing a different spectral band in the range of 0.48 to 0.83 µm. The RBV system for Landsat 3 contained two identical cameras, which operated in the spectral band from 0.50 to 0.75 µm. The cameras were aliened to view adjacent nominal 99 by 99 km square ground scenes with a 15 km sidelap yielding 183 by 99 km scene pairs. Two successive scene pairs nominally overlapped an MSS scene.
roll - The rotation of a spacecraft about its longitudinal axis (in the along-track direction) so as to cause a side-up or side-down attitude. The roll axis is referred to as the "y" axis. See attitude.
row - The latitudinal (nominal) centerline of a Landsat scene. Row 60 corresponds to lat. 0° (the equator), row 1 is at lat. 80°47' N, and row 122 is at lat. 81°51' S. There are 248 rows, altogether, for Landsat 4, 5, and 7, the same as for the earlier Landsats 1,2, and 3.
S band - A radio frequency band extending from approximately 2.0 to 4.0 gigahertz. It is part of the microwave portion of the electromagnetic spectrum.
saturation - (1) In general, the point at which a further increase in input yields no further increase in output. (2) (optics) The presence of the maximum number of wavelengths over the spectral region contributing to a particular color. Contrast with hue (tint) and brightness (intensity), the other two components of a color.
scale - (1) The ratio of a distance on an image or map to its corresponding distance on the ground. The scale of an image varies from point to point because of tilt and relief, but is usually taken as f/H where f is the principal distance (focal length) of the sensor, and where H is the height of the sensor above mean ground elevation. Scale may be expressed as a ratio, e.g. 1:24,000; a representative fraction, e.g. 1/24,000; or an equivalence, e.g. 1 in = 2,000 ft.
scan-correlated-shift (SCS) - A systematic radiometric noise observed on both TM instruments on the Landsat 4 and 5 satellites. Since this SCS noise had only a finite set a values and was constant within any given scan, it was possible to correct for SCS noise during ground processing by measuring the value of the step-change in the background taken during shutter obscuration of the image at the end of each scan and inferring the SCS state for that detector and band. Since all detectors within a band switch states at the same time, the effect of SCS noise is to produce patterns of banding where different numbers of adjacent scans can be in the same state. The frequency of change of states became less and less with the time on orbit. See banding and noise. (Source: Dr. John Barker)
scanner - Any device that systematically breaks up an image into picture elements (or pixels) and records some attribute of each picture element. The sweep of a scanner's mirror, prism, antenna, or other element across the track (direction of flight) may be straight, circular, or another shape.
scan line - The ground trace of a narrow strip that is recorded by the instantaneous field of view of a detector in a scanner system.
scan line length - The time of a forward or reverse scan on an imager such as TM or ETM+. In order to make the geometric calibration easier, there is an active scan time (AST) and a turn-around time (TAT) for each scan. The Scan-Line-Corrector rotates two mirrors on the optical axis to keep the scan nearly perpendicular to the path of the satellite on the ground. The active scan time (AST) is controlled as long as the scanner is in the Scan-Angle-Monitor (SAM) mode, rather than the bumper mode. At a Landsat satellite altitude of 705 Km, an active scan of about 6320 pixels covers approximately 189 Km on the ground and is cropped so that all the bands cover a common swath width of about 180 Km. Each scan on a TM is a little less than 0.5 Km wide. It is the knowledge of the AST and first and second half scan errors on each scan that allows the TM sensors to be systematically corrected when resampled such that only correction in rotation and translation need to be made for the image as a whole from ground control points. While in the SAM mode the wear on the bumpers results in increases in TAT but no change in AST. After twenty years in orbit, Landsat 5 TM lost its ability to stay in the SAM mode and had to be operated in the bumper mode. Also see bumper mode and geometric image calibration. (Source: Dr. John Barker)
scan center, nominal - The center of the cluster of scene centers of scenes imaged over the same area but at different times. Useful as an indexing tool in view of the fact that slight changes in the Landsat orbit, in addition to difficulties in timing or framing individual Landsat scenes, cause scenes acquired on different dates over the same area to coincide inexactly.
scenes - Each Landsat image collected is called a scene. Each Landsat scene is dimensionally 115 x 106 (185 x 170 km) miles. The globe is divided into 57,784 scenes, and each Landsat 7 scene has about 3 billion bytes of data.
SCS - Scan Correlated Shift
sensor - Any device that gathers energy and presents it in a form suitable for obtaining information about the environment. Passive sensors, such as the ETM+, TM, and MSS, utilize electromagnetic radiation produced by the surface or object being sensed. Active sensors such as radar, supply their own energy source.
SGS - Svalbard Ground Station, Norway
sidelap - The extent of lateral overlap between images acquired over adjacent ground tracks. Sidelap increases with distance from the equator.
signal - The effect (for example, a pulse of electromagnetic energy) conveyed over a communications path or system. Aboard Landsat, signals from the scene being sensed are received by the sensors and converted to another form prior to transmission to the ground.
Signal-to-Noise Ratio (SNR) - The ratio of the level of the information-bearing signal power to the level of the noise power. SNR determines the precision of a radiometric measurement. The signal-to-noise ratio (SNR) is defined as the value of a measurement over the standard deviation of the measurement and is therefore a unitless variable. Typically, sets of visible reflective bands or sets of emissive bands on a scanner are designed to have approximately equal SNRs, with exceptions being made for poorer SNR in a panchromatic sharpening band as a cost of the higher spatial resolution. SNR is often taken as a time average over both random and systemic variations, however it can be a measure of either total or random noise only. SNR usually increases with increasing signal. Also see noise. (Source: Dr. John Barker)
signature - Any characteristic or series of characteristics by which a material may be recognized. Used in the sense of spectral signature, as in photographic color reflectance. A category is said to have a signature only if the characteristic pattern is highly representative of all units of that category. See spectral signature.
skew - The degree of asymmetry or deviation from symmetry of a distribution. Distribution skewed to the right has a positive skew. Distribution skewed to the left has a negative skew.
skew correction - A geometric correction made on Landsat imagery to offset the skewing effect of the Earth's rotation during scene acquisition.
SLC - Scan Line Corrector
SLC-off - Scan Line Corrector-off. On May 31, 2003, the Scan Line Corrector (SLC), which compensates for the forward motion of Landsat 7, failed. Subsequent efforts to recover the SLC were not successful, and the failure appears to be permanent. Without an operating SLC, the Enhanced Thematic Mapper Plus (ETM+) line of sight now traces a zig-zag pattern along the satellite ground track. As a result, imaged area is duplicated, with width that increases toward the scene edge. The Landsat 7 ETM+ is still capable of acquiring useful image data with the SLC turned off, particularly within the central part of any given scene. The Landsat 7 ETM+ therefore continues to acquire image data in the "SLC-off" mode. All Landsat 7 SLC-off data are of the same high radiometric and geometric quality as data collected prior to the SLC failure. The SLC-off effects are most pronounced along the edge of the scene and gradually diminish toward the center of the scene (Figure 2). The middle of the scene, approximately 22 kilometers wide on a Level 1 (L1G, L1Gt, L1T) product, contains very little duplication or data loss, and this region of each image is very similar in quality to previous ("SLC-on") Landsat 7 image data. Landsat 7 data collected after May 31, 2003 is know as "SLC-off data." (Source: USGS)
slope (topography) - Inclination of the terrain from horizontal. It is expressed in convenient units, such, as percent, feet per mile, etc.
SMA - Scan Mirror Assembly
SME - Scan Mirror Electronics
SNR - Signal to Noise Ratio, also abbreviated S/N ratio.
Space Oblique Mercator (SOM) - A variation on the basic Mercator map projection based on the dynamics of satellite motion. The movements of the Landsat platform, sensors, and the Earth, expressed as functions of time, are used to calculate which latitudes and longitudes on the Earth correspond to locations in the projection plane. Thus, a continuous projection of the entire area of coverage is obtained. SOM is conformal and nearly totally distortion-free; it was developed by John Parr Snyder. See Universal Transverse Mercator.
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) coordinate 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 coordinates (x,y).
spectral band - An interval in the electromagnetic spectrum defined by two wavelengths, frequencies, or wave numbers, e.g. Landsat 7 ETM+ Band 1 covers and area between 0.45 and 0.515 µm.
spectral interval - The width, expressed either in wavelength or frequency, of a particular portion of the electromagnetic spectrum. A given sensor, such as radiometer detector or camera film, may be designed to measure or be sensitive to energy from a particular spectral interval. Also termed spectral band.
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 - The quantitative measurement of the properties of an object at one or several wavelength intervals. Spectral signature analysis techniques use the variation in the spectral reflectance or emittance of objects as a method of identifying the objects, e.g. mineral detection.
spectral striping - Non-uniform detector-to-detector variations that are due to different spectral responses of the adjacent detectors. Spectral striping is rarely considered a problem within a single band of an instrument because filters that cover all the detectors of a band are chosen for their homogeneity. However, spectral striping is a serious problem when trying to cross-calibrate two imagers because there is not a way to remove the differences with a single radiometric correction if there are real differences in the spectral bands and real differences in the target, including the atmosphere. Also see cross-calibration and relative spectral response. (Source: Dr. John Barker)
SSR - Solid State Recorder
standard deviation (SD) - The square root of the variance. The value is expressed in the units of measure in which the observations were taken.
steradian - A unit of measure of solid angles. Formally, it is the angle subtended at the center of the sphere by a portion of the surface whose area is equal to the square of the radius of the sphere. There are 4π steradians in a sphere.
sun angle - The angle of the Sun above the horizon. Both the quantity (lumens) and the spectral quality of light being reflected to a remote sensor are influenced by Sun angle. Also called Sun elevation and Sun elevation angle.
sun-synchronous - An Earth satellite orbit in which the orbital plane remains at a fixed angle with respect to the Sun, precessing through 360° during the period of a year. Landsat satellites are in near-polar orbit of this type and maintain an orbital altitude such that each pass over a given latitude on the Earth's surface occurs at the same mean Sun time. Compare with geosynchronous.
SWIR - Short Wave Infrared
synoptic view - The ability to see large areas at one time, such as an entire metropolitan area.
swath - A strip, belt, or long narrow extent of anything. Specifically refers to the ground track, or trace, followed by Landsat satellites which image a continuous swath 189 km wide.
telemetry - The science of measuring a quantity or quantities, transmitting the measured value to a distant station, and there interpreting, indicating, or recording the quantities measured. A telemetry link, such as the Tracking and Data Relay Satellite System, (TDRSS) is a system for transmitting data over long distances using radio techniques.
temporal - Pertaining to, concerned with, or limited by time.
temporal resolution - the revisit time of a satellite over a given geographic location. The revisit time for Landsat 4, 5, and 7 is 16 days. For Landsat 1, 2, and 3 it was 18 days.
TDRSS - Tracking Data and Relay Satellite System
TM - Thematic Mapper
thermal band - A general term for intermediate and long wavelength infrared-emitted radiation, as contrasted to short wavelength reflected (solar) infrared radiation. In practice, generally refers to infrared radiation emitted in the 3 to 5 µm and 8 to 14 µm atmospheric windows.
thermal infrared - The preferred term for the middle wavelength ranges of the infrared region extending roughly from 3 µm at the end of the near infrared, to about 15 or 20 µm where the far infrared commences. In practice the limits represent the envelope of energy emitted by the Earth behaving as a greybody with a surface temperature around 290° K (27° C). Seen from any appreciable distance, the radiance envelope has several brighter bands corresponding to windows in the atmospheric absorption bands. The thermal band most used in remote sensing extends from 8 to 14 µm.
Thematic Mapper (TM) - One of the two Earth-sensing payloads carried aboard Landsat 4 and 5, the Multispectral Scanner being the other on 4 and 5. It is a non-photographic imaging system, which utilizes an oscillating mirror and seven arrays of detectors, which sense electromagnetic radiation in seven different spectral bands. The Thematic Mapper is a derivative of the Multispectral Scanner generation of sensors, but it achieves much greater ground resolution, better spectral separation, improved geometric fidelity, and greater radiometric accuracy and resolution.
threshold - The boundary in spectral space beyond which a data point, or pixel, has such a low probability of inclusion in a given class that it is excluded from that class.
time, Greenwich mean - Mean solar time of the meridian of Greenwich, England (known as the prime meridian, longitude 0°), used by most navigators and adopted as the prime basis of standard time throughout the world. Abbreviated GMT.
time, mean Sun - The mean Sun time at a given location on the Earth is determined by the distance in longitude from the Greenwich meridian. The mean Sun time at any location is determined by dividing the difference in longitude from Greenwich (in degrees, moving east) by 15 and adding the result to the current GMT. This will be the mean Sun time relative to Greenwich expressed in hours.
tone - Each distinguishable variation of shade from black to white.
Tracking and Data Relay Satellite System (TDRSS) - A system of geosynchronous communication satellites launched for the purpose of receiving and relaying data, command, and telemetry signals to and from all NASA orbiting satellites, including the Space Shuttle. The TDRSS system reduces the number of ground stations needed and simplifies the handling of a growing volume of satellite telecommunications traffic.
transformation - (1) The process of projecting an image (mathematically or graphically) from its plane onto another plane by translation, rotation, and/or scale change. (2) The process of defining such a plane through mathematical operations. See image transformation.
transmittance - The ratio of the energy per unit time per unit area (radiant power density) transmitted through an object to the energy per unit time per unit area incident on the object. In general, transmittance is a function of the incident angle of the energy, viewing angle of the sensor, spectral wavelength and bandwidth, and the nature of the object.
truncate - To shorten by or as if by cutting off.
ultraviolet radiation - Electromagnetic radiation of shorter wavelength than visible radiation but longer than X-rays; roughly, radiation in the wavelength interval between 10 and 4,000 angstroms.
umbra -The complete or perfect shadow of an opaque body, as a planet, where the light from the source of illumination is completely cut off.
uncontrolled mosaic - A mosaic made without correction for distortion or displacement of any type.
uniform grid - Square, rectangular, or, more rarely, hexagonal lattice for recording geographical data, The simpler grids are usually not related to geodetic coordinate systems.
Universal Transverse Mercator (UTM) - A widely used map projection employing a series of identical projections around the world in the intermediate latitudes, each covering 6° of longitude and oriented to a meridian. The UTM projection is characterized by its property of conformality, meaning that it preserves scale and angular relationships well, and, by the ease with which it allows a useful rectangular grid to be superimposed on it. Extensively used in navigational applications, the UTM has been the most common projection used with Landsat data. It is sometimes called the Gauss-Kruger projection.
USGS - United States Geological Survey
vector - In image processing, the coordinate in n space (where n is the number of features, bands, or channels), determined by the intensity value of each band.
VI - Vegetation Index. A vegetation index is a quantitative measure used to measure biomass or vegetative vigor, usually formed from combinations of several spectral bands, whose values are added, divided, or multiplied in order to yield a single value that indicates the amount or vigor of vegetation. The simplest form of vegetation index is a ratio between near infrared and red reflectance. For healthy living vegetation, this ratio will be high due to the inverse relationship between vegetation brightness in the red and infrared regions of the spectrum. (Source: RangeView, external link)
vicarious calibration - In-orbit radiometric calibration of a satellite sensor by a method independent of that used to perform the initial laboratory calibration. Two methods are commonly used when making vicarious measurements of ground truth: the radiance-based method in which data from the sensor are compared with radiance measured by a sensor mounted on an aircraft, and the reflectance-based method in which a radiative transfer model is used to estimate the top-of-atmosphere radiance from a ground target of known reflectance. Also see ground truth/ground observations, radiance-based calibration and reflectance-based calibration. (Source: Dr. John Barker)
vidicon - A storage-type, electronically scanned, photoconductive, television camera tube, which, often has a response to radiation beyond the limits of the visible region. Particularly useful in space applications, as no film is required. See return beam vidicon.
visible radiation - Electromagnetic radiation of the wavelength interval to which the human eye is sensitive; the spectral interval from approximately 0.4 to 0.7 µm (4,000 to 7,000 angstroms).
VNIR - Visible & Near Infrared
wave - A disturbance which is propagated in a medium in such a manner that at any point in the medium the quantity serving as the measure of the disturbance is a function of time, while at any instant the displacement at a point is a function of the position of the point. Any physical quantity having the same relationship to some independent variable (usually time) that a propagated disturbance has, at a particular instant, with respect to space, may be called a wave. An electromagnetic wave is one in which the disturbance is the change in the electric and magnetic field intensities from their equilibrium values in space.
wavelength - Wavelength (symbol λ) = 1/frequency. In general, the mean distance between maximums (or minimums) of a roughly periodic pattern. Specifically, the shortest distance between particles moving in the same phase of oscillation in a wave disturbance. Optical and infrared wavelengths are measured in nanometers (10-9, abbr. nm), micrometers (10-6, abbr. µm), and angstroms (10-10, symbol Å).
Worldwide Reference System (WRS) - A global indexing system for Landsat data, first developed in Canada, which is based on nominal scene centers defined by path and row coordinates. See path, row.
WRS - Worldwide Reference System
X band - A radio frequency band extending from approximately 8.0 to 12.5 gigahertz. It is part of the microwave portion of the electromagnetic spectrum and is used for some communications satellites and by X-band radar primarily for science and technology applications.
yaw - The rotation of a spacecraft about its vertical axis so as to cause the spacecraft's longitudinal axis to deviate left or right from the direction of flight. The yaw axis is referred to as the "Z" axis. See attitude.
zenith - The point in the celestial sphere that is exactly overhead. Contrast with nadir.