Glossary

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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. 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 the atmosphere or other substance.

acquisition. (1) Image captured by satellite sensor. (2) The process of searching for and locking onto a received signal.

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. (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.

alignment data. Angular measurement of the physical position of the optical axis with respect to the primary space vehicle reference axes.

altitude. Height above a datum, the datum usually being mean sea level. Refers to point above the Earth's surface rather than those on it (elevation).

analog-to-digital conversion. The process of sampling continuous analog signals in order to convert them into a stream of digital values. ETM+ data undergo such a conversion prior to downlinking. Abbreviated as A/D conversion.

angular velocity. Also called rotational velocity, it is the amount of rotation that a spacecraft undergoes per unit time. For Landsat 7 it is equal to 1.059 mrad/sec ((233 paths/cycle * 2*pi*1000 mrad/path) / (16 days/cycle * 86400 sec/day)).

angle of drift. The angle between the heading of the axis of a craft and its ground track.

anomaly. A deviation from the norm.

aperture. An opening that admits electromagnetic radiation to a detector or film. 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.

ascending node. The point at which the orbit of an earth satellite intersects the plane of the equator going from south to north.

at-aperture-radiance. The radiance at the aperture of the sensor.

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.

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 crosstrack 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, 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.

band sequential. A format that arranges the data by band such that all of the data from band 1 followed by all of the data from band 2, etc.

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.

BCH. An error detection and correction scheme named after its inventors Bose, Chanduri, and Hochergan.

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 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 emmitance.

brightness value. In Landsat parlance, a number in the range of 0-255 that is related to the amount of planetary radiance striking a sensor's detector.

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.

coherent noise. The noise associated with periodic signals arising from power supplies, transmitters and clock signal typically.

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. A color image produced by the combination of three individual monochrome images in which each is assigned a given color. For ETM+ data, if blue is assigned to band 1, green assigned to band 2, and red assigned to band 3, a true color image will result.

cubic convolution. A high-order resampling technique is 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 capture. The receipt and storage of return link mission data at the CADU level.

data continuity. A NASA requirement to ensure that Landsat 7 data are compatible to those obtained by earlier Landsat satellites.

data granule. The increment of image data stored in the archive, i.e. an interval, swath, or WRS scene.

data loads. Data and command transfers from the MOC to the onboard computer.

dark shutter image data. The image data obtained from ETM+ detectors when the calibration shutter obscures the detectors from incident electromagnetic radiation.

descending node. The point at which the orbit of an earth satellite intersects the plane of the equator going from north to south.

detector. The composite circuitry supporting the development of a single output data sample.

detector sample. The process of determining the transfer characteristics (detector mean output as a function of incident exposure) for each detector element.

digital terrain elevation data (DTED). Digital information produced by DMA which provides a uniform matrix of terrain elevation values. DTED is commonly used to terrain correct Landsat data.

distortion. A change in scale from one part of an image to another.

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 ratio of the maximum signal to the smallest measurable signal.

EDC. Earth Resources Observation System Data Center is a national archive, production, distribution and research facility for remotely sensed data and other geographic information. (see EROS)

electromagnetic radiation. Energy emitted as result of changes in atomic and molecular energy states and propagated through space at the speed of light.

electromagnetic spectrum. The system that classifies, according to wavelength, all energy (from short cosmic to long radio) that moves, harmonically, at the constant velocity of light.

elevation. Vertical distance from the datum, usually mean sea level, to a point or object on the Earth's surface.

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).

Enhanced Thematic Mapper Plus (ETM+). The ETM+ is a fixed-position nadir viewing whisk-broom instrument. The viewing swath is produced by means of an oscillating mirror system that sweeps across track as the sensor field of view moves forward along-track due to satellite motion.

ETM+ scene. A set of ETM+ observations that covers 170 km in width by 185 km in length and is centered about a WRS vertex.

engineering data. All data available on-board about health, safety, environment or status of the platform and instruments.

ephemeris. A set of data that provides 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. The Earth Resources Observation System was established in the early 1970?s under the Department of Interior? U.S. Geological Survey, to receive, process and distribute data from the United States? Landsat satellite sensors and from airborne mapping cameras.

field-of-view. The solid angle through which an instrument is sensitive to radiation. See effective resolution element, instantaneous field of view, resolution.

focal length. In a camera, the distance measured along the optical axis from the optical center of the lens to the plane at which the image of a very distant object is brought into focus.

focal plane. In a sensor, the plane occupied by the detectors, and on which the radiances sensed are incident.

frame. For Landsat 7, a frame is one Virtual Channel Data Unit with a frame synchronizer pattern (frame marker) attached. This is the same as a Channel Access Data Unit (CADU).

geocentric. Any coordinate frame whose origin is relative to the Earth's center of mass.

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 ETM+ data, a distinction is made between data that have been geometrically corrected using systematic, or predicted, values and data that have been geometrically corrected using precise ground control point data and elevations models.

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.

geodetic accuracy. A measure of how closely a point on the Earth can be located relative to its true absolute location.

geosynchronous. An Earth satellite orbit in which the satellite remains in a fixed position over a geographic location on Earth.

Global Position System (GPS). A constellation of satellites that can be used to determine accurately the orbit data of satellites.

ground control point (GCP). A geographic feature of known location that is recognizable on images and can be used to determine geometric correction functions for those images.

ground track. The vertical projection of the actual flight path of a plane or space vehicle onto the surface of the Earth.

ground truth. Data which are acquired from field checks, high-resolution remote sensing data, or other sources of known data. Ground truth is used as the basis for making decisions on training areas and evaluating classification results.

housekeeping data. All data available onboard about health, safety, environment, or status of the platform and instruments.

hue. The attribute of a color that differentiates it from gray of the same brilliance and that allows it to be classed as blue, green, red, or intermediate shades of these colors.

image. 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.

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 include contrast enhancement, edge enhancement, spatial filtering, image smoothing, and image sharpening.

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 invertable.

image-to-image registration. The registration between images taken at different times.

image transformation. A function or operator which takes an image as 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 intrepretations. Fourier, Hadamard, and Karhunen-Love transforms as well as various spatial filters, are examples of frequently used image transformation procedures.

infrared. Pertaining to energy in the 0.7 - 100 μm wavelength region of the electromagnetic spectrum. For remote sensing, the infrared wavelengths are often subdivided into near infrared (0.7 - 1.3 μm), middle infrared (1.3-3.0 μm), and far infrared (7.0 - 15 μm). Far infrared is sometimes referred to as thermal or emissive infrared.

instantaneous field of view (IFOV). 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. IFOV also refers to the ground area covered by this solid angle.

international ground station (IGS). Any Landsat ground station not belonging to the United States.

interval. Is a scheduled ETM+ image period along a WRS path, and may be from 1 to 90 full scenes in length.

irradiance. The measure, in units of power, of radiant flux incident on a surface.

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 of voltages, in the characteristics of components. Jitter effects arising from the oscillating mirrors and other movable parts aboard the Landsat spacecraft are often a cause of certain anomalies in the image data received and must be compensated for by the ground processing software.

K-band. A radio frequency band extending from approximately 12.5 to 36 gigahertz.

kernel. In the spatial domain, a kernel is a MxM operator which can be used in the convolution or multiplication with a NxN image to accentuate certain features or properties of an image. A kernel can also be represented in the frequency domain as a Fourier transform.

L-band. A radio frequency band extending from approximately 1.0 to 2.0 gigahertz.

Landsat 7. Consists of the spacecraft and the ETM+ payload.

level 0. Space vehicle or instrument data at full space-time resolution with space-to-ground communication artifacts removed.

light, transmitted. Light that has traveled through a medium without being absorbed or scattered.

long term acquisition plan. The tasking of the sensor using cloud predictions to optimize the acquisition of cloud free scenes.

lookup table. An array of values from which functions corresponding to a given argument can be obtained.

major frame. For ETM+, a major frame period is one complete scan of the ETM+ scan mirror (either direction), which includes not only the period during a scan but also the turnaround interval when the scan mirror changes direction for the next scan.

map projection. Any systematic arrangement of meridians and parallels portraying the curved surface of a sphere or spheroid upon a plane.

metadata. An archived set of descriptive information about a scene and the parent sub-interval that provides a user with geographic coverage, date of acquisition, sun angles, could cover, gain states, and other quality measurements.

minor frame. For ETM+ major frames are partitioned into minor frames which is the most fundamental element of the data stream structure in which specific data measurands (e.g. imagery, PCD, time codes) are extracted.

mirror scan correction data. This data includes scan start time, first half scan time error, second half scan time error, scan direction, and any other data which is required to perform mirror scan correction.

modulate. To vary, or control, the frequency, phase, or amplitude of an electromagnetic wave or other variable.

modulation transfer function (MTF). The modulation transfer function of an imaging system measures the spatial frequency modulation response of the system. As an imaging system processes or records an image, the contrast modulation of the processed or recorded image is different from the input image. The MTF can be thought of as a curve, indicating for each spatial frequency the ratio of the contrast modulation of the output image to the contrast modulation of the input image. It is formally defined as the magnitude of the Fourier transform of the line spread function of the imaging system.

mosaic. An image made by piecing together individual images covering adjacent areas.

multiplexer. An electronic device which permits the transmission of multiple messages simultaneously on one communication channel.

multispectral. Generally denotes remote sensing in two or more spectral bands, such as visible and infrared.

nadir. That point on the celestial sphere vertically below the observer, or 180° from the zenith.

narrowband data. The data includes the command or forward ranging in the narrowband forward link, and the telemetry or return ranging in the narrowband return link.

near infrared. The preferred term for the shorter wavelengths in the infrared region extending 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).

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.

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.

Nyquist interval. The maximum time interval between equally spaced samples of a signal that will enable the signal waveform to be completely determined. The Nyquist interval is equal to the reciprocal of twice the highest frequency component of the sampled signal.

Nyquist's theorem: A theorem, developed by H. Nyquist, which states than an analog signal waveform may be uniquely reconstructed, without error, from samples taken at equal time intervals. The sampling rate must be equal to, or greater than, twice the highest frequency component in the analog signal.

optical transfer function (OTF). A mathematical statement that describes the relationship between the input and the output of an imaging system. When the transfer function operates on the input, the output is obtained. Given any two of these three entities, the third can be obtained.

orbit adjust. The adding to or taking away of orbital velocity. This is normally done to maintain altitude or orbit phasing relationships.

orbital period. The interval in time between successive passages (orbits) of a satellite through a reference plane.

orthorectified. Describing an image in which terrain relief distortions have been removed.

panchromatic. A single band covering a broad range of wavelengths; usually used in context of collecting information from the whole visible spectrum.

parallax. The apparent change in the position of one object, or point, with respect to another, when viewed from different angles.

path. The longitudinal center line of a Landsat scene 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 7. Path numbers are used with row numbers to designate nominal scene center points.

payload. That part of a spacecraft (e.g. ETM+) that is separate from the equipment or operations necessary to maintain the spacecraft in orbit.

payload correction data. Image support data imbedded in the wideband data stream. Includes satellite attitude, ephemeris, time, angular displacement sensor (ADS) data and payload state.

perigee. The point in the orbit of a heavenly body (e.g. satellite) at which it is nearest the Earth.

pixel. Picture element provided by a single detector scene sample output.

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.

polar stereographic. An azimuthal stereographic projection commonly used with Landsat data acquired about 65° latitude. In this projection, the meridians are straight lines converging at the pole (central point), and lines of latitude are concentric circles about this point. Like the UTM projection, the polar stereographic is a conformal projection, meaning that angular relationships are preserved.

pole wander. The apparent motion in the poles of the Earth relative to inertial coordinate system. Changes in moments of inertia are due to changes in moments of density due primarily to tides and liquid mass. The National Imager and Mapping Agency (NIMA) generates pole wander data which are used by the Landsat 7 system in the conversion of downlinked ephemeris from inertial to fixed reference, during Level 0R processing.

precision correction. Post-processed geometric correction of satellite data using ground control points to correlate the spacecraft's predicted position with its actual geodetic position.

prime meridian. Meridian of longitude 0 degrees, used as the origin for measurements of longitude. The meridian of Greenwich, England, is the internationally accepted prime meridian on most charts.

quantization level. The number of numerical values used to represent a continuous quantity.

quaternion. A vector of four components; the position is contained in the first three components and an associated scalar rater is located in the last component of this four element vector.

radian. The angle subtended by an arc of a circle equal in length to the radius of the circle: 57.3°

radiance. Measure of the energy radiated by an object. In general, radiance is a function of viewing angle and spectral wavelength and is expressed as energy per solid angle.

Rayleigh scattering. Selective scattering of light in the atmosphere by particles that are small compared with the wavelength of light.

reflectance. The ratio of the radiant energy reflected by a body to that incident upon it. In general, reflectance 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.

registration. The process of geometrically aligning two or more sets of image data such that resolution cells for a common ground area can be digitally or visually superimposed.

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.

row. The latitudinal (nominal) center line of a Landsat scene. Row 1 is at latitude 80° 47'N, row 60 is at the equator, and row 122 is at latitude 81° 51'S. In total there are 248 rows.

sampling rate. The number of samples taken per unit time, i.e., the rate at which signals are sampled for subsequent use, such as for modulation, coding, and quantization.

saturation. The condition where energy flux exceeds the sensitivity range of a detector.

S band. A radio frequency band extending from approximately 2.0 to 4.0 gigahertz.

sidelap. The extent of lateral overlap between images acquired over adjacent ground tracks.

signal-to-noise ratio. The ratio of the level of the information-bearing signal power to the level of the noise power. More precisely, the signal-to-noise ratio of the mean DN to the standard deviation in DN. This is a temporal noise definition in that the mean DN is the time averaged value and the standard deviation in DN is the standard deviation in the time series.

space oblique mercator. A variation on the basic mercator map projection based on the dynamics of satellite motion. The movements of the satellite, sensor, 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.

spectral band. An interval in the electromagnetic spectrum defined by two wavelengths, frequencies, or wave numbers.

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.

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 4 pi steradians in a sphere.

subinterval. Is a contiguous segment of raw wideband data received during a Landsat 7 contact period. Subintervals are caused by breaks in the wideband datastream due to communication dropouts and/or the inability of the spacecraft to transmit a complete observation (interval) within a single Landsat 7 contact period. The largest possible subinterval is 35 full scenes long with a partial scene preamble and postamble. The smallest possible subinterval is a single ETM+ scene.

sun elevation angle. The angle of the Sun above the horizon.

solar zenith angle. Reciprocal of the 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.

swath. Refers to the 185 kilometer wide ETM+ imaging ground track.

telemetry. The science of measuring a quantity, transmitting the measured value to a distant station, and there, interpreting or recording the quantity measured.

temporal. Pertaining to, concerned with, or limited by time.

temporal resolution. The expected repeat time between measurements over the same location.

thermal band. A general term for intermediate and long wavelength infrared-emitted radiation, as contrasted to short wavelength reflected infrared radiation. In practice, generally refers to infrared radiation emitted in the 3-5 μm and 9-14 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 m where the far infrared commences. In practice the limits represent the the envelope of energy emitted by the Earth behaving as a graybody with a surface temperature around 290 K. Seen from space, 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 15 μm.

time, Greenwich mean. Mean solar time of the meridian of Greenwich, England (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 mean Sun time relative to Greenwich, expressed in hours.

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.

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.

universal transverse mercator. A widely used map projection employing a series of identical projections around the world in the intermediate latitudes, each covering 6 degrees 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. The UTM projection is most commonly used with landsat data.

UT1-UTC time correction data. Universal Time (UT) 1 is determined from observations of stellar transits to determine local mean sidereal time corrected to remove the effects of polar motion. Universal Time Coordinated (UTC) is defined to be equal to that of the International System used for atomic time, but it is kept with .9 seconds of UT1 by periodic leap-second adjustments.

virtual channel data unit (VCDU). The CCSDS protocol data unit consisting of a fixed length data structure. It is used for bidirectionally space/ground communications on a CCSDS virtual channel.

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.

wavelength. Wavelength = 1/frequency. In general, the mean distance between maximums (or minimums) of roughly periodic pattern. Specifically, the shortest distance between particles moving in the same phase of oscillation in a wave disturbance.

world geodetic system (WGS). The reference Earth model used by the Landsat 7 system.

worldwide reference system. A global indexing system for Landsat data which is based on nominal scene centers defined by path and row coordinates.

X-band. A radio frequency band extending from approximately 8.0 to 12.5 gigahertz.

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.

zenith. The point in the celestial sphere that is exactly overhead.