Landsat Glossary
This glossary endeavors to assist users with common terms in Landsat publications. These definitions are as they pertain to Landsat acquisition, processing, and distribution.
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Acquisition - (1) An image captured by a satellite sensor. (2) The process of searching for and locking onto a received signal.
Acquisition Date - The date the image was acquired. Format: YYYY/MM/DD.
Acquisition of Signal - The time a site receives a signal from a spacecraft.
Algorithm - In the context of remote sensing, algorithms generally specify how to determine higher-level data products from lower-level source data. For example, algorithms prescribe how atmospheric temperature and moisture profiles are determined from a set of radiation observations originally sensed by satellite sounding instruments.
Altitude - The angle, in degrees, above the level horizon where an object in the sky appears (the height in space).
Analog - A form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analog signal is used to transmit audio (such as voice, radio, stereo, and control tones) and is responsive to changes in light, sound, heat and pressure.
Analog-to-Digital Conversion - The process of converting analog signals to a digital representation.
Anomaly - (1) A deviation from the norm. (2) The angular distance between the position of a planet and its last perihelion (point nearest to the sun), or between that of a satellite and its last perigee (point nearest to the center of the earth).
Antenna - A device for transmitting and receiving radio waves.
Aperture - The diameter of an opening; the diameter of the primary lens or mirror of a telescope. A cross-sectional area of the antenna that is exposed to the satellite signal.
Ascending Node (AN) - Direction a satellite is traveling relative to the Equator. An ascending node implies a northbound Equatorial crossing.
Asychronous - When the relationship between two or more related objects or events do not depend on or exist at the same time to function.
Attitude - The angular orientation of a remote sensing system with respect to a geographical reference system. The orientation of the sensor along with information about the accuracy and precision with which this orientation is known. This data is required to perform proper calibration of instrument data. The attitude is usually stored in Euler angle or quaternion form and may be 1) calculated by the on-board computer and telemetered to the ground or 2) calculated by ground processing facilities using a variety of attitude sensor data.
Band (channel) - A slice of wavelengths from the electromagnetic spectrum. Landsat sensors have multiple bands that collect radiation from different parts of the electromagnetic spectrum. Some record in visible light, one band is panchromatic, some bands are infrared, and there are thermal infrared bands as well.
Band Gain - The band gain condition detected at the start of a WRS scene. H indicates band acquired in high gain mode. L indicates band acquired in low gain mode.
Bit - A single digital unit of information.
Bit Error Rate - The fraction of a sequence of message bits that are in error (a bit error rate of 10-6 means that there is an average of one error per million bits).
Browse data - A reduced data volume representation of an image scene that can be viewed to determine general ground area coverage and cloud coverage. Browse data typically consist of three spectral bands.
Calibration - The act or process of comparing certain specific measurements in an instrument with a standard.
Calibration Data - In remote sensing, measurements pertaining to the spectral or geometric characteristics of a sensor or radiation source. Calibration data are obtained by using a fixed energy source such as a calibration lamp, a temperature plate, or a geometric test pattern.
Calibration Parameter File (CPF) - The Calibration Parameter File (CPF) supplies the radiometric and geometric correction parameters required during Level 1 processing to create superior products of uniform consistency across the Landsat system. They fall into one of three major categories: geometric parameters, radiometric parameters, or artifact removal parameters. Each CPF is stamped with applicability dates; then sent to the Hierarchical Storage Management (HSM) for storage and eventual bundling with outbound Level 0 products. The Image Assessment System (IAS) updates and distributes the CPF at least every 90 days.
Channel - In remote sensing, a channel is also known as a band. It is a portion of the electromagnetic spectrum that is used to detect specific wavelengths. (See also Band).
Coherent Noise - A spurious, periodic pattern of noise within an image, generally of electronic origin.
Collections - In 2016 the USGS reorganized the Landsat archive into a tiered collection management structure titled Landsat Collection 1. Landsat Collections ensure that all Landsat Level-1 products provide a consistent archive of known data quality while controlling continuous improvement of the archive and access to all data as they are acquired. The implementation of collections ensures consistent quality through time and across all instruments. Collection 1 contained data acquired since 1972 to December 2022 from Landsat 1 through Landsat 8. Collection 2 (2021-present) contains all Landsat data from the sensors onboard Landsat 1 through Landsat 9 satellites.
Constellation - Landsat satellites work in tandem as a system. Each satellite circles the Earth every 16 days, with their orbits offset temporally so that every location on the planet is imaged every eight days. The satellite constellation of Landsat 8 and Landsat 9 is in a near-polar orbit on the World Reference System-2.
Data Continuity - A NASA requirement to ensure that currently orbiting Landsat sensors and data are compatible to those obtained by earlier Landsat sensors.
Data set - A logically meaningful grouping or collection of similar or related data.
Descending node - Direction a satellite is traveling relative to the Equator. A descending node implies a southbound Equatorial crossing.
Digital - A means for encoding information in a communications signal by using bits (binary digits). Conversion of information into bits of data for transmission through wire, fiber optic cable, satellite, or over air techniques. Method allows simultaneous transmission of voice, data or video.
Downlink - A communications channel for receiving transmissions from a spacecraft.
Dynamic Range - The range between the maximum and minimum amount of input radiant energy that an instrument can measure.
Electromagnetic - Relating to the interplay between electric and magnetic fields.
Electromagnetic radiation - Energy transfer in the form of electromagnetic waves or particles that propagate through space at the speed of light.
Electromagnetic spectrum - The entire range of radiant energies or wave frequencies from the longest to the shortest wavelengths--the categorization of solar radiation. Satellite sensors collect this energy, but what the detectors capture is only a small portion of the entire electromagnetic spectrum. The spectrum is usually divided into seven sections: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray radiation.
Enhanced Thematic Mapper Plus (ETM+) - The sensor aboard Landsat 7 that picks up solar radiation reflected by or emitted from the Earth.
Ephemeris - A table of satellite orbital locations for specific time intervals. The ephemeris data help to characterize the conditions under which remotely sensed data are collected and are commonly used to correct the sensor data before analysis.
Equator - An imaginary circle around a body that is everywhere equidistant from the poles, defining the boundary between the northern and southern hemispheres.
EROS Center - Earth Resources Observation and Science Center is a national archive, production, distribution, and research facility for remotely sensed data and other geographic information.
ETM+ Format 1 - The ETM+ Format 1 major frames contain all data (e.g., imaging and calibration) from and associated with Bands 1-6. The MSCD and PCD data are duplicated in both ETM+ formats.
ETM+ Format 2 - The ETM+ Format 2 major frames contain all data (e.g., imaging and calibration) from and associated with Bands 6-8. The MSCD and PCD data are duplicated in both ETM+ formats.
Full WRS Scene - A full WRS scene with overlap is 375 ETM+ scans. A nominal scene without overlap is 335 ETM+ scans. Due to a movement of the bumpers, which resulted in a larger turn-around interval and an increase in the nominal number of minor frames per major frame (from 7,423 to 7,473), the nominal size of a scene without overlap may decrease. Further bumper wear changes may decrease the number of scans without overlap. The number of overlap scans will increase to compensate for the decrease in the non-overlap scene size.
Gain - (1) A general term used to denote an increase in signal power in transmission from one point to another; usually expressed in decibels. (2) An increase or amplification. (3) A measure of amplification expressed in dB.
Gaussian Noise - Statistically random radio noise characterized by a wide frequency spectrum that is continuous and uniform over a specified frequency band.
Geostationary orbit - "Describes an orbit in which a satellite is always in the same position (appears stationary) with respect to the rotating Earth. The satellite travels around the Earth in the same direction, at an altitude of approximately 35,790 km (22,240 statute miles) because that produces an orbital period equal to the period of rotation of the Earth (actually 23 hours, 56 minutes, 04.09 seconds). A worldwide network of operational geostationary meteorological satellites provides visible and infrared images of Earth's surface and atmosphere."
Gimbal - A device with two mutually perpendicular and intersecting axes of rotation, thus giving free angular movement in two directions, on which an object may be mounted.
Greenwich Mean Time (GMT) - The mean solar time of the meridian of Greenwich, England, used as the prime basis of standard time throughout the world.
High Gain/Low Gain Antenna - A high gain antenna is highly focused, whereas a low gain antenna receives or transmits over a wide angle.
Infrared radiation - Electromagnetic radiation with wavelengths between about 0.7 to 1000 micrometers. Infrared waves are not visible to the human eye. Longer infrared waves are thermal infrared waves.
Interval - The time duration between the start and end of an imaging operation (land observation) by the instrument on board the Landsat spacecraft. The raw wideband data collected during an interval consists of a contiguous set of WRS scenes.
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Landsat - Landsat is a partnership between the National Aeronautics and Space Administration (NASA) and the U.S. Geological Survey (USGS). Since 1972, Landsat data have provided a unique resource for those who work in agriculture, geology, forestry, regional planning, education, mapping, and global-change research.
Data from the satellites have been used for monitoring land cover conditions, geological / mineralogical exploration, urban growth, and cartography. Global coverage is available and data sets are provided by the USGS at the cost of reproduction.
Landsat Contact Period - The time duration between the start and end of raw wideband data transmissions from the Landsat spacecraft to a ground station [e.g., the Landsat Ground Station (LGS).
Latitude - The angular distance North or South from the Earth's equator measured in degrees with the equator at 0o and the poles 90o N and 90o S.
Level 0R Files - The generic term used to denote the grouping of band, MSCD, PCD, and calibration data files for a single subinterval.
Level 0R product - A US data product in which the data has been spatially reformatted but the data values remain unchanged. No radiometric or geometric corrections have been performed on the data. The reformatting is fully reversible. The data is band sequential. Attached to the image data are radiometric calibration image data, payload correction data, quality data, and metadata.
Level 0R Quality and Accounting Data - The data quality and accounting information collected by the Landsat Processing System (LPS), on a subinterval basis, from processing of the major frames constructed from the wideband Virtual Channel Data Units (VCDUs) received during a Landsat contact period.
Longitude - The distance East or West between the meridian of a place on Earth and that of Greenwich, England, expressed in degrees or time.
Loss of Signal - The inability to receive a satellite signal because the satellite's orbital path has taken it below the antenna's horizon.
Lower Scene Corners - The corners associated with the leading edge (last scan) of a scene. For descending path scenes, the lower left corner corresponds to the southwest corner of a scene and the lower right corner corresponds to the southeast corner of a scene. For ascending path scenes, the lower left corner corresponds to the northeast corner of a scene and the lower right corner corresponds to the northwest corner of a scene. These mappings hold for the band file geolocation fields and the metadata file.
LPS (output) Files - The generic term used to denote the grouping of Level 0R, browse, and metadata files for a single subinterval. L0R files contain image data, cal data, PCD, and MSCD. Multi-band browse scenes file and metadata file contained in the LPS Output File Set.
Mercator Projection - Cylindrical projection derived mathematically where the meridians are drawn parallel to each other distorting latitudes at the poles. The parallels of latitude are drawn as horizontal straight lines equidistant from each other.
Metadata - A set of descriptive information about the scene data contained in the archive. The information is sufficient for a user, during the process of scene query and selection, to determine at a minimum geographic coverage, date of collection sensor gain mode, time of acquisition, cloud cover, and other quality measurements.
Missions Operations Center (MOC) - The place where the satellite or spacecraft receives coded instructions and delivers data. The coordination place (usually on Earth) for any space mission.
Modulation - The variation of a property of an electromagnetic wave or signal, such as its amplitude, frequency, or phase.
Mosaicking - The assembling of photographs or other images whose edges are cut and matched to form a continuous photographic representation of a portion of the Earth's surface.
Multispectral - Sensing in distinct wavelength bands (equivalent to colors, not all of which are visible to the human eye). Because the data handling capacity of the sensor is spread over these different wavelengths, this usually translates to lower resolution than panchromatic.
Multispectral image - A remote sensing image created using data collected from more than one band.
Multispectral Scanner (MSS) - A line-scanning instrument flown on Landsat satellites that continually scans the Earth in a 185 km (100 nautical miles) swath. On Landsats' 1, 2, 3 the MSS had four spectral bands in the visible and near infrared with an instantaneous field of view (IFOV) of 60 meters. Landsat 3 had a fifth band in the thermal infrared with an IFOV of 240 meters that failed on launch. Landsat 4 and 5 had an IFOV of 30 meters. The MSS is a non-photographic imaging system that utilizes an oscillating mirror and fiber optic sensor array. The mirror sweeps from side to side, transmitting incoming energy to a detector array that sequentially outputs brightness values (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 it can image an adjacent swath.
Nadir - Point on Earth directly beneath a satellite, the opposite of zenith.
NASA - National Aeronautics and Space Administration (NASA) was established in 1958 through the National Aeronautics and Space Act as an outgrowth of the National Advisory Committee for Aeronautics.
Noise - Any unwanted and unmodulated energy that is always present to some extent within any signal.
OLI - Operational Land Imager is a sensor built by Ball Aerospace and Technologies Corporation that measures in visible, near infrared, and short wave infrared portions of the electromagnetic spectrum. This sensor is on Landsat 8 and Landsat 9.
Orbit - The path of a body acted upon by the force of gravity.
Orbital period - The time it takes a satellite to complete one revolution (orbit) around the Earth. The orbital period of Landsat is about 1.5 hours.
Oscillator - A device for producing alternating current.
Panchromatic - Sensitive to all or most of the visible spectrum, between 0.4 and 0.7 micrometers. Landsat 7-9 have a panchromatic band.
Partial Scene - A partial scene (less than 375 scans) may exist at the beginning or end of a subinterval because imaging events do not always start or end on WRS scene boundaries. If generated, browse and scene metadata for these occurrences accurately reflect their partial scene nature and geographic extent.
Passive Sensor - A type of remote sensing instrument, a passive sensor picks up radiation reflected or emitted by the Earth. Landsat satellites are passive remote sensing systems.
Path - Longitudinal grid for collecting Landsat data. Day-time acquisitions are ascending orbit night-time acquisitions are descending. These paths are then cut into rows that follow latitudes creating Landsat scenes.
Payload Correction Data (PCD) - Imaging support data imbedded in the wideband data stream. Includes satellite attitude, ephemeris, time, angular displacement sensor (ADS) data, and payload state.
Pixel - An abbreviation of picture element. The minimum size area on the ground detectable by a remote sensing device. The size varies depending on the type of sensor.
Polar orbit - An orbit with an orbital inclination of near 90 degrees where the satellite ground track will cross both polar regions once during each orbit. The term describes the near-polar orbits of spacecraft.
Pseudorandom - Being or involving entities (as numbers) selected by a definite computational process that satisfy one or more standard tests for statistical randomness.
Push Broom - also known as along track scanners, use detectors that are arranged perpendicular to the direction of flight of the spacecraft. As the spacecraft orbits, images are collected one line at a time with all pixels scanned simultaneously. If not calibrated perfectly stripes can result from this type of sensor.
Quantization - 1) To subdivide into small but measurable increments. 2) To calculate or express in terms of quantum mechanics.
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."
Radiometer - A device that detects and measures electromagnetic radiation.
Radiometric - Relating to, using, or measured by a radiometer. The measurement of radiation.
Raster Data - An abstraction of the real world where spatial data is expressed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels. With the raster data model, spatial data is not continuous but divided into discrete units. This makes raster data particularly suitable for certain types of spatial operations (e.g., overlays or area calculations). Unlike vector data, there are no implicit topological relationships.
Raster Graphics - Graphics in which an image is generated by scanning an entire screen or page and marking every point as black, white, or another color (as opposed to vector graphics).
Rasterize - The process of converting vector points, lines, and areas into raster image format.
Raw Data - Numerical values representing the direct observations output by a measuring instrument transmitted as a bit stream in the order they were obtained.
Real-time - Time in which reporting on events or recording of events is simultaneous with the events. For example, the real time of a satellite is the time in which it simultaneously reports its environment as it encounters it.
Remote Sensing - (1) In the broadest sense, the measurement or acquisition of information about 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. (2) Instruments that record characteristics of objects at a distance, sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or reflected radio waves emitted by the spacecraft.
Resampling - Modifying the geometry of an image (which may be from either a remotely sensed or map data source). This process usually involves rectification and/or registration.
Resolution - (1) A measure of the amount of detail that can be seen in an image; the size of the smallest object recognizable using the detector. (2) Intensity or rate of data sampling. In remotely sensed imagery, resolution is significant in four measurement dimensions: spectral, spatial, radiometric, and temporal.
Retrograde - (1) Of or relating to the orbital revolution or axial rotation of a planetary or other celestial body that moves clockwise from east to west, in the direction opposite to most celestial bodies. (2) Of or relating to the brief, regularly occurring, apparently backward movement of a planetary body in its orbit as viewed against the fixed stars, caused by the differing orbital velocities of Earth and the body observed.
Revolution - Orbital motion about a point located outside the orbiting body.
Rotation - Motion around an axis passing through the rotating body.
Row - (1) The latitudinal (nominal) center line of a Landsat scene. (2) The scan lines that constitute an image.
Satellite - Any body, natural or artificial, in orbit around a planet. The term is used most often to describe moons and spacecraft. A man-made satellite is a spacecraft that orbits another body, such as a planet or the Sun. A natural satellite is another term for a moon.
Saturation - The intensity of a color. A highly saturated color is a vivid, brilliant color; to dull a color (decrease its saturation), you add small amounts of its complement, making it closer to gray.
S-Band - A range of microwave radio frequencies in the neighborhood of 2 to 4 GHz, used for communicating with piloted space missions (~2 Ghz).
Scan - An image data line produced from a single detector of a band during a scan.
Scan Line - Single row or line imaged by a spacecraft sensor that composes a (in Landsat’s case) horizontal raster.
Scanning Mirror - Landsat scanning mirror collects data on both the forward and reverse scans.
Scenes - Each Landsat image collected is a scene. Each Landsat scene is 115 x 106 miles long. The globe is divided into 57,784 scenes, and each Landsat scene has about 3 billion bytes of data.
Single Event Upsets - A Single Event Upset (SEU) occurs when an energetic particle travels through a transistor substrate and causes electrical signals within the transistor. This is a known phenomenon that usually occurs in near-earth orbit to spacecraft passing through the Van Allen belts, especially the northern and southern auroral zones and the south Atlantic anomaly.
Site - The physical location of an International Ground Station (IGS) or the Mission Operations Center (MOC).
Spatial Data - Any information about the location, shape of, and relationships among geographic features. This includes remotely sensed data as well as map data.
Spatial Resolution - The area on the ground that an imaging system (such as a satellite sensor) can distinguish.
Spatio Temporal Asset Catalog (STAC) - A method to provide a universal standard and common language to describe geospatial information. For Landsat, STAC standardizes the way geospatial metadata is structured to better enable data querying. The USGS provides STAC metadata for every Landsat product accessible through the Amazon Web Services (AWS) cloud platform.
Spectral Range - The wavelength difference between two wavelengths in adjacent orders at the same angle of diffraction.
Spectral Response - The relative amplitude of the response of a detector vs. the frequency of incident electromagnetic radiation.
Spectrometer - An optical instrument that splits the light received from an object into its component wavelengths by means of a diffraction grating, then measuring the amplitudes of the individual wavelengths.
Subinterval - A segment of a raw wideband data interval received during a Landsat contact period. Subintervals are caused by breaks in the wideband data stream due to communication dropouts and/or the inability of the spacecraft to transmit a complete observation (interval) within a single Landsat contact period. The largest possible subinterval can be as long as a full imaging interval (a set of contiguous WRS scenes) transmitted during an uninterrupted contact period. The smallest possible subinterval can be as small as a set of a few contiguous scans (a partial WRS scene). The smallest size of a subinterval is an operator modifiable parameter in LPS. If the smallest subinterval size is chosen as a full WRS scene, it will contain approximately 24 seconds worth of data or 335 scans (without the 20 overlapping scans each.
Sun-synchronous orbit - An orbit in which a satellite is always in the same position with respect to the rotating Earth at the same time of day. The satellite travels around the Earth in the same direction, at an altitude of approximately 438 miles (705 kilometers). Landsat is sun-synchronous, always passing overhead at approximately 10:00 am local time.
Synchronous - The instantaneous alignment of two or more related events in time.
Telemetry - (1) Radio signals from a spacecraft used to encode and transmit data to a ground station. (2) The science of measuring a quantity, transmitting the measured value to a distant station, and there, interpreting or recording the quantity measured.
Thematic data - Thematic data layers in a data set are layers of information that deal with a particular theme. These layers are typically related information that logically goes together.
Thermal infrared - Electromagnetic radiation with wavelengths between 3 and 25 micrometers.
Thematic Mapper - "A Landsat multispectral scanner designed to acquire data to categorize the Earth's surface. Particular emphasis was placed on agricultural applications and identification of land use. The scanner continuously scans the surface of the Earth, simultaneously acquiring data in seven spectral channels. Overlaying two or more bands produces a false color image. The ground resolution of the six visible and shortwave bands of the Thematic Mapper is 30 meters, and the resolution of the thermal infrared band is 120 meters. Thematic mappers were flown on Landsat 4 and 5."
TIRS - Thermal Infrared Sensor on Landsat 8 and Landsat 9 measures land surface temperature in two thermal bands. This sensor, built by NASA, measures infrared light emitted at 10.6-11.19 micrometers for Band 10 and 11.5-11.51 for Band 11.
U.S. Geological Survey (USGS) - A bureau of the Department of the Interior. USGS was established in 1879 following several Federally sponsored independent natural resource surveys of the West and Midwest. The Department of the Interior has responsibility for most of our nationally owned public lands and natural resources. The USGS monitors resources such as energy, minerals, water, land, agriculture, and irrigation. The resulting scientific information contributes to environmental-policy decision-making and public safety. For example, USGS identifies flood- and landslide-prone areas and maintains maps of the United States.
Universal Transverse Mercator (UTM) - A widely used coordinate system based on the Transverse Mercator projection. This system allows for the elliptical planet to conform to a rectangular grid. The UTM system consists of 60 zones, each zone is 6-degrees of longitude in width oriented to a meridian. The zones are numbered 1-60, zone 1 begins at 180-degrees longitude, and increasing moving to the east. This coordinate system is characterized by its conformality; it preserves angular relationships and scale and it easily allows a rectangular grid to be superimposed on it. Many worldwide topographic and planimetric maps at scales ranging between 1:24,000 and 1:250,000 use this coordinate system.
Uplink - A connection through which signals transmit to a satellite.
Upper Scene Corners - The corners associated with the trailing edge (first scan) of a scene. For descending path scenes, the upper left corner corresponds to the northwest corner of a scene and the upper right corner corresponds to the northeast corner of a scene. For ascending path scenes, the upper left corner corresponds to the southeast corner of a scene and the upper right corner corresponds to the southwest corner of a scene. These mappings hold for the band file geolocation fields and the metadata file.
Vector Data - Vector data, when used in the context of spatial or map information, refers to a format where all map data is stored as points, lines, and areas rather than as an image or continuous tone picture. These vector data have location and attribute information associated with them.
Visible radiation - The electromagnetic radiation that humans can see as colors. The visible spectrum is composed of wavelengths between 0.4 to 0.7 micrometers. Red is the longest and violet is the shortest. Landsat 7 has three visible bands in red, green, and blue.
Wavelength - The distance from crest to crest, or trough to trough, of an electromagnetic or other wave. Wavelengths are related to frequency: The longer the wavelength, the lower the frequency.
Whisk Broom - also called across track scanners use a mirror to reflect light onto a single detector. The mirror swings back and forth collecting reflected light one pixel at a time. The constant motion of this kind of scanner can cause parts to wear out.
Worldwide Reference System (WRS) - A global indexing scheme designed for the Landsat Program based on nominal scene centers defined by path and row coordinates.
X-band - A nominal frequency range from 12.5 to 8 GHz (2.4 to 3.75 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. X-band is a suitable frequency for several high-resolution radar applications and is used for both experimental and operational airborne systems.
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Zenith - The point on the celestial sphere directly above the observer. Opposite the nadir.