Landsat Glossary | U.S. Geological Survey

A

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.

Anomoly

(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

Direction a satellite is traveling relative to the Equator. An ascending node implies a northbound Equatorial crossing.

Asychronous

Not synchronized.

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.

B

Band (channel)

A slice of wavelengths from the electromagnetic spectrum. Landsat ETM+ has eight bands that collect radiation from different parts of the electromagnetic spectrum. Of the eight bands, three bands are visible light, one band is panchromatic, three bands are infrared, and one band is thermal infrared.

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.

C

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

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 7 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 LP DAAC 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

A one-way communications link. (also see Band)

Coherent noise

A spurious, periodic pattern of noise within an image, generally of electronic origin.

D

Data continuity

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

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.

E

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+ is the sensor aboard Landsat 7 that picks up solar radiation reflected by or emitted from the Earth. 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. 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.

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.

F

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.

G

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

The mean solar time of the meridian of Greenwich, England, used as the prime basis of standard time throughout the world.

H

High gain/low gain antenna

A high gain antenna is highly focused, whereas a low gain antenna receives or transmits over a wide angle.

I

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 ETM+ instrument on board the Landsat 7 spacecraft. The raw wideband data collected during an interval consists of a contiguous set of WRS scenes.

L

Landsat

Land Remote Sensing Satellite Program managed by the USGS. The Landsat Satellite series began in 1972 to gather information about land surface features of the planet. 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 7 contact period

The time duration between the start and end of raw wideband data transmissions from the Landsat 7 spacecraft to a ground station [e.g., the Landsat 7 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 or files

The generic term used to denote the grouping of band, MSCD, PCD, and calibration data files for a single subinterval.

Level or 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 or quality and accounting data

The data quality and accounting information collected by the Landsat 7 Processing System (LPS), on a subinterval basis, from processing of the ETM+ major frames constructed from the wideband Virtual Channel Data Units (VCDUs) received during a Landsat 7 contact period.

Longitude

The angular distance East or West between the meridian of a particular 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.

M

Mercator

Of, relating to, or drawn on the Mercator projection (a conformal map projection with the meridians drawn parallel to each other and the parallels of latitude drawn as straight lines whose distance from each other increases with their distance from the equator).

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

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 usually 4 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

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, 4, and 5, the MSS had four spectral bands in the visible and near infrared with an IFOV of 80 meters. Landsat 3 had a fifth band in the thermal infrared with an IFOV of 240 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.

N

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.

O

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 7 is about 1.5 hours.

Oscillator

A device for producing alternating current.

P

Panchromatic

Sensitive to all or most of the visible spectrum, between 0.4 and 0.7 micrometers. Landsat 7 has 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. ETM+ is a passive remote sensing system.

Payload correction data

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 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.

Pseudorandom

Being or involving entities (as numbers) selected by a definite computational process that satisfy one or more standard tests for statistical randomness.

Q

Quantization

1) To subdivide into small but measurable increments. 2) To calculate or express in terms of quantum mechanics

R

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.

S

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).

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.

Scan

An image data line produced from a single detector of a band during a scan.

Scan line

A series of spacecraft pointings in one dimension.

Scanning mirror

Landsat TM?s 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.

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 7 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 7 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 ETM+ 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 to be as long as a full WRS scene, it will contain approximately 24 seconds worth of ETM+ 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-7 is sun-synchronous, always passing overhead at approximately 10:00 am local time.

Synchronous

The instantaneous alignment of two or more events in time. Events may occur at irregular intervals

T

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.

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."

Thermal infrared

Electromagnetic radiation with wavelengths between 3 and 25 micrometers.

U

Universal transverse mercator (UTM) projection

A widely used map projection that employs a series of identical projections around the world in the mid-latitude areas, each spanning six degrees of longitude and oriented to a meridian. This projection is characterized by its conformality; that is, 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 projection.

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.

V

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.

W

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.

Worldwide reference system

A global indexing scheme designed for the Landsat Program based on nominal scene centers defined by path and row coordinates.

X

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.

Z

Zenith

The point on the celestial sphere directly above the observer. Opposite the nadir.