As airborne hyperspectral, thermal mapping, and new technologies have developed over past decades, so has the terminology used in their description. The following glossary is by no means complete, but covers many of the concepts and terms specifically related to our imaging technologies.
Wavelength interval within which EM radiation is absorbed by the atmosphere or by other substances.
Above Ground Level. A term used to define the flying height above the ground surface required to obtain a desired spatial across-track pixel resolution. Heights provided by the flight calculator (accessed from the instrument main menu during operation) are measured AGL, and do not take into account the average terrain height above mean sea level. Usually measured in feet.
The horizontal speed of an aircraft relative to the surrounding air mass. Not to be confused with ground speed. Air speed – wind speed = Ground speed.
AREA OF INTEREST-AOI
Refers to the specific target or area on the ground to be covered during data acquisition. Also referred to as ‘region of interest’ or ‘ROI’. A single flight line or flight block may contain one or more areas of interest. Same as ‘region of interest’.
The process of attempting to remove atmospheric-related effects such as path radiance from the measured radiance to leave the signal that is more closely related to surface characteristics. See also Calibrating & Processing.
With reference to airborne data collection, attitude refers to roll, pitch, and heading motions of the aircraft. Measured using an IMU/INS system
Files containing measurements of the roll and pitch (and possibly heading) motions experienced by the aircraft during data acquisition. Source of attitude data used with AISA imagers precision GPS/IMU system (measures roll, pitch, and heading) such as systems manufactured by ApplAnix (POS AV) or NovAtel (SPAN). Attitude data is recorded separately and synchronized with the image data during post-processing.
Horizontal direction expressed as the angular distance (or bearing) between a fixed point and true or magnetic north. May be referenced to true north or grid north. See also Bearing.
A discrete portion of the electromagnetic spectrum over which data is acquired by a sensor.
BAND INTERLEAVED BY LINE-BIL
A raster-based data storage file format that references the data primarily by line, for each band and pixel in the image. This is the format by which the AISA raw instrument data is recorded. If the pixels of bands A, B, C, and D are denoted a, b, c, and d, then the BIL format organizes data like: aaaaaaaaa….. Band 1, Line 1 bbbbbbbbb….. Band 2, Line 1 cccccccccc….. Band 3, Line 1 ddddddddd….. Band 4, Line 1 aaaaaaaaa….. Band 1, Line 2….. See also Band Interleaved by Pixel; Band Sequential.
BAND INTERLEAVED BY PIXEL-BIP
A raster-based data storage file format that references the data primarily by pixel, for each band and line in the image. If the pixels of bands A, B, C, and D are denoted a, b, c, and d, then the BIP format organizes data like: abcdabcdabcdabcd….. Line 1 abcdabcdabcdabcd….. Line 2 abcdabcdabcdabcd….. Line 3 See also Band Interleaved by Line; Band Sequential.
BAND SEQUENTIAL (BSQ)
A raster-based data storage file format that references the data primarily by spectral band, for each pixel and line in the image. If the pixels of bands A, B, C, and D are denoted a, b, c, and d, then the BSQ format organizes data like: aaaaaaaaa….. Line 1, Band 1 aaaaaaaaa….. Line 2, Band 1 aaaaaaaaa….. Line 3, Band 1 bbbbbbbbb….. Line 1, Band 2 bbbbbbbbb….. Line 2, Band 2 bbbbbbbbb….. Line 3, Band 2 cccccccccc….. Line 1, Band 3 cccccccccc….. Line 2, Band 3 cccccccccc….. Line 3, Band 3 See also Band Interleaved by Pixel; Band Interleaved by Line.
A general term referring to the specific configuration of spectral bands, widths, and locations, used in acquiring Instrument data for a particular flight line. See also configuration file.
As related to an Instrument, the width of a selected spectral band or channel, in nanometers (nm).
BATCH FILE (*.BAT)
A user-created ASCII text file used in Windows that lists the switches and corresponding arguments necessary to run the AISA standard processing software. Batch files are used as a means of both standardizing the operation of the software, as well as providing a record (when printed) of the processing. Text editors such as word are used to create batch files.
The number of bits per second transmitted across a communications port. The baud rate typically used to transfer GPS data to the AISA instruments from a GPS receiver is 19,200.
The horizontal direction or angle of a point relative to true north (true bearing), grid north (grid bearing), or another point or object (relative bearing). Measured in degrees clockwise from the specified point. Not to be confused with heading, which specifies a compass direction. See also Azimuth.
BINARY CODED DECIMAL (BCD)
A binary data format used to represent decimal numbers where each digit is represented by four bits. The number 723 would be represented in the following way: 0111 0010 0011. Unlike binary format, there is no number size limitation. Used to prevent rounding errors in calculations.
Binning is the averaging of neighboring pixels in the spatial and/or the spectral axes. Binning will reduce the frame size (by reducing spatial and/or spectral resolution), and increase the signal to noise. Many cameras perform binning inside the camera, thus the reduced amount of data to be communicated increases the maximum
frame rate. The chart on the right shows a typical maximum frame rate of a CCD-based hyperspectral camera as a function of binning in both dimensions. Some cameras have different speed improvements depending on binning in x or y directions.
See Band Interleaved by Pixel.
SpecTIR’s history in hyperspectral data collections and operations has led to a wealth of experience in radiometric and spectral calibration. SpecTIR’s standard radiometric calibration is achieved through the use of a Labsphere USS-2000-V uniform source. This 20-inch diameter integrating sphere is equipped with three internal 45-watt and one 75 watt externally mounted halogen light sources. Each lamp is powered by separate DC regulated constant current power supplies and the addition of a variable attenuator, provides even more precise control of light levels. Luminance output is variable from 0 to 4000 foot-lamberts and measured uniformity is > 98% over the entire 8-inch exit port. This sphere carries a NIST traceable spectral radiance calibration from 400 nm to 2500 nm at a sampling interval of 5nm. The resultant calibration allows SpecTIR to provide data that is within +/- 5% of absolute radiance.
Wavelength calibration is generated through an Oriel Cornerstone 130 1/8m monochromator. This automated, computer controlled monochromator provides calibrated and repeatable wavelength outputs of 1nm channels in the VNIR and 3nm in the SWIR range. The central wavelength locations of this output is known and certified within 0.5nm accuracy. Additionally, data QA/QC processing routines utilize well-documented atmospheric features such as the Oxygen Fraunhoffer line at 760 nm to ensure that accurate wavelength mapping is maintained.
Charge Coupled Device. A light-sensitive, silicon-based semiconductor device. Made up of an array of light-sensitive photocells, the CCD measures the incoming light and converts it to an electrical charge for each cell or pixel, which may then be measured and recorded.
The process of assigning individual pixels in an image into one of a number of categories, based on its reflectance characteristics. This technique is useful for determining land use, and remotely mapping habitats, amongst other things.
Complimentary Metal-Oxide Semiconductor. A light-sensitive, silicon-based semiconductor device. Made up of an array of light-sensitive photocells, the CCD measures the incoming light and converts it to an electrical charge, which may then be measured and recorded.
COURSE OVER GROUND (COG)
The actual path over the ground flown by the aircraft. May not be the same as the planned path. Also referred to as flight track.
An angle of rotation in the z-axis formed between the aircraft track or course over ground and the longitudinal axis of the aircraft fuselage. Commonly, a crab angle occurs when flying in crosswinds as the pilot uses the aircraft rudder to avoid drift. Large crab angles (e.g. >10 degrees) may lead to geocorrection problems. Also referred to as yaw or drift angle.
Defines a direction perpendicular to the direction of flight (`across the aircraft’s track’). Across track spatial pixel resolution is related to the FOV of the instrument lens, and the flying height of the aircraft.
One of the additive components to the signal measured by an AISA instrument that are not target-related. These components are measured by the instrument during data collection, and are removed during radiometric correction processing of the raw image data. Dark current is a current that flows in a photodetector when there is no optical radiation incident on the detector and operating voltages are applied.
A model of the earth used for geodetic calculations. Composed of an ellipsoid (defining the size and shape of the earth) and a reference frame. A geodetic datum is used to define the reference coordinate system (e.g. UTM, lat/long) that describes geographic position. An ellipsoid is a geometrical approximation of the earth’s geoid or sea level surface. The most commonly used datum for AISA instrument processing output is WGS84. Referencing coordinates to the wrong datum can result in position errors of hundreds of meters.
Digital Elevation Model. Also referred to as Digital Terrain Models (DTM). ASCII raster file representation of terrain elevations. Uses the format: counter, x, y, and z (elevation). Often incorporated during geocorrection of image data to improve the geometric accuracy of the processed imagery by correcting for terrain height variations.
Differential GPS. A technique used to improve GPS accuracy whereby pseudo range errors measured from a receiver at a known location (static base station) is used to correct the data from a receiver at an unknown location (but in the same general geographical vicinity – with regards to an instrument this is the kinematic rover GPS). DGPS may either be performed real-time or during post-processing. DGPS solutions are always more accurate than non-DGPS solutions. See also Post-Processed DGPS; Real-Time DGPS.
The amount of light or other radiant energy directed toward the earth’s surface from the sun or the atmosphere,
The tendency of an aircraft to be unable to maintain the planned course over ground when flying in crosswind conditions unless corrections are made.
The entire range of frequencies over which electromagnetic radiation can be propagated. This spectrum encompasses, in general terms, frequencies associated with (from longest wavelength to the shortest): heat, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
The height of an object or point on the ground, usually referenced to mean sea level. It is important to note if the elevation is being referenced to the ellipsoid or to the geoid.
A mathematical model that best describes the shape of the earth’s surface, calibrated to mean sea level. In comparison, geoid height uses the earth’s gravitational field to describe the shape of the earth’s surface. Ellipsoidal models have been developed to approximate the geoid in local areas. GPS uses the WGS84 earth model which is based on the GRS80 ellipsoid.
Electromagnetic radiation. Radiation such as visible light, microwaves, and infra-red, which is composed of interacting electric and magnetic fields.
FALSE COLOR IMAGE
An image in which parts of the non-visible EM spectrum are assigned to the red, green, and blue components of the image, so that the colors produced do not match the colors that we see with the naked eye.
The time needed to fly to the flight line or flight block to begin data collection. The main ferry is the flying time and distance from the home base of operations to the field base of operations. Local ferries are the flying time and distance between the field base of operations and the flight block, or between flight blocks.
An optical fiber is a thin, flexible, transparent element that acts as a waveguide, or “light pipe”, to transmit light between the two ends of the fiber. They can be used in communication or for illumination that carries light a long distance.
An area on the ground, identified during flight planning, for which imagery data is to be acquired using either a single flight line or multiple and adjacent flight lines with side lap. Contains one or more areas of interest. Not to be confused with Mosaic. See also Flight Line; Area of Interest.
Typically used to refer to a single linear path (usually pre-planned) along which image data is acquired. A flight block is made up of multiple and adjacent flight lines with side lap. Not to be confused with a scan line. See also Flight Block.
The required height at which the aircraft must fly to obtain a specified across-track pixel size. Varies given the FOV of the instrument lens. Can be specified AGL or MSL. Note that pilots often require this value to be specified MSL, meaning that the both the height AGL and the average terrain height must be added together. Also referred to as flying altitude. Also note that in unpressurized aircraft, the AISA instruments have a 60K operating altitude limit based on air pressure limitations of the recording devices and video monitor.
The ratio of the focal length to the lens aperture. It is a measure of the brightness of a lens, the smaller F-number is brighter.
FOCAL PLANE ARRAY-FPA
Cameras Image sensing devices consisting of an array (typically rectangular) of light-sensing pixels at the focal plane of a lens. FPAs operate by detecting photons at particular wavelengths and construct an image of the object or scene.
The main objective lens located at the front of a camera for viewing a pre-determined field of view.
Field of View. The angle between two rays passing through the perspective center of a camera lens to the two opposite sides of the image area. With the AISA instruments, the FOV defines the width of the area viewable through the lens in the across-track direction. This value may be measured in degrees or in pixels. The FOV directly influences the required flying height of the aircraft or platform for a given across-track pixel size.
Frames per second, used to describe camera speed.
See Scan Line.
FREE SPECTRAL RANGE
The wavelength range over which the AISA instruments are sensitive and to which they have been calibrated.
Full width Half Maximum.
Is the process of assigning X and Y coordinates to a point on the image.
Application of aircraft motion using IMU measurements and application of GPS geographic position data to geocode imagery. Final product is an geocoded image map. If a DEM is used, the image is considered orthorectified. See also Geocoded; Orthorectification.
An equipotential surface model of the earth’s gravity field that best approximates mean sea level, even projected under land masses. In other words, the exact `shape’ of sea level as opposed to ellipsoidal height, which is based on a mathematical model. The `true’ shape of the sea level surface of the earth can vary by as much as 80 meters above and 60 meters below the corresponding ellipsoidal model. Heights above sea level are orthometric or geoidal heights. See also Datum; Ellipsoid Height; Orthometric Height.
A coordinate system where points are referenced in units of latitude, longitude, and geodetic height. The prime meridian and equator are the reference planes for this system.
The registration of an image to a specific map coordinate system and datum, often with the inclusion of a DEM for terrain height correction. Also referred to as geocoding.
Greenwich Mean Time. This is the mean solar time determined by the meridian that runs through Greenwich, England. GMT was defined in terms of the earth’s motions, which fluctuates in rate by a few thousandths of a second per day. It was the official time reference for the world until 1972. During daylight savings time (April to October), Calgary, Alberta is 6 hours behind GMT; otherwise the time difference is 7 hours.
Global Positioning System. A method of accurately determining your position using signals from a ‘constellation’ of satellites orbiting the Earth.
The time reference used by GPS satellites and receivers. Based on the `Zero GPS Epoch’, defined as midnight of Sunday, 1980/Jan/06, and is synchronized to UTC. Unlike UTC, GPS time is a linear time system (i.e. GPS is NOT adjusted for leap seconds). As of January 1, 2002, GPS time was ahead of UTC by 13 seconds (UTC = GPS – 13 seconds).
GROUND CONTROL POINT (GCP)
A resolvable ground feature, either artificial or natural, for which (at a minimum) horizontal survey coordinates (x,y) are known and which can also be identified within one or more georeferenced flight lines. Used in generating a bundle adjustment solution for a specific instrument installation. See also Tie Point.
GROUND SAMPLE DISTANCE-GSD
The smallest visually resolvable unit (L x W) on the ground for a given pixel size.
The horizontal speed of an aircraft measured relative to the ground. Not to be confused with Air Speed. Ground speed = Air speed – Wind speed.
Relating direct field observations of ground surface phenomena to corresponding features in remotely-sensed data for verification of accuracy.
Graphical User Interface.
GYRO STABILIZED MOUNT
A specialized platform mount used to provide precise, real-time motion compensation, leveling, and drift control to the installed imager sensor head in the aircraft. Dampens aircraft vibrations and reduces attitude changes transmitted to the sensor head, providing imagery that is less prone to effects of smear or jitter. The mount Incorporates three axis (roll, pitch, yaw) corrections from a precision GPS/IMU system and can extend the operational survey window under conditions of turbulence. The use of a gyro stabilized mount may also allow for faster survey flight speeds, if this is permissible within the imager’s operational resolution constraints.
A horizontal direction or angle referenced to a magnetic compass direction. Not to be confused with bearing, which refers to the relative horizontal direction or angle between two points. See also Bearing.
The SI unit of frequency. The number of cycles per second of an electromagnetic wave. 1 Hz represents 1 cycle/second; 2 Hz represents 2 cycles/second; 50 Hz represents 50 cycles/second.
The 3-D data set that results from a scan with a hyperspectral camera. The three dimensions are two spatial (x, y) and one spectral.
An sensor is classed as hyperspectral if it is capable of imaging an area in many (e.g. hundreds) of bands simultaneously.
The process of measuring a complete spectrum at each spatial point of a sample with a hyperspectral camera.
Instrument Control Unit. This is the instrument component containing the system’s primary electronic systems and recording device(s). It is into this device that the Sensor Head Unit (SHU), and POS are connected.
The extraction of characteristic information inherent within the processed imagery for the imaged target. The information derived from spectral and/or spatial image analysis is used to produce value-added products used by decision makers for resource management. See also Standard Processing.
Radiance measurements of the target as measured and recorded by an instrument from reflected or emitted radiation in the visible, NIR, SWIR, MWIR, or LWIR portions of the electromagnetic spectrum. These radiance measurements are made on a scan line by scan line basis across multiple, programmable spectral wavelengths. The generation of a recognizable image of the target is made by the successive buildup and display of these image scan lines.
See definition for Spectrograph.
Inertial Measurement Unit.
A precise, self-contained navigation system using inertial detectors, which automatically provides aircraft position, heading, and velocity. See ApplAnix, NovAtel.
Used in the spectral calibration of the AISA family of instruments. The definition from the Photonics Dictionary (www.labsphere.com) is: `A hollow sphere coated internally with a white diffusing material and provided with openings for incident beam, specimen, and detector used for measuring the diffuse reflectance or transmittance of objects.
The rate of emitted energy from unit surface area through unit solid angle.
A measurement of the radiant energy received for per unit area on a surface. Measured over an area perpendicular to the direction of the incident radiation. See also Radiance.
A change in optical magnification with wavelength. It is a measure of the spatial shift between pixels at different wavelengths. Keystone, and its spectral equivalent, Spectral Smile, are present to varying degrees with push broom-style instruments using diffraction gratings. See also Spectral Smile.
With reference to an instrument, this is the mode in which GPS data is recorded while the airborne receiver is moving (i.e. during flight). This is in contrast to static mode where a base station receiver is placed on a stationary point during GPS data collection.
A unit of speed used in aviation. One knot = 1.85 km/hr = 0.5144 m/s = 1.15 m.p.h (or):
knots x 0.5144 = m/s
knots x 1.15 = miles per hour
Light Detection and Ranging. An “active” optical remote sensing technology that uses laser scanning to measure height or elevation data, thereby creating a detailed 3-dimensional representation of the ground surface (DEM, or Digital Elevation Model). Lidar data provides a perfect complement to AISA hyperspectral and thermal imaging technologies. Its data is used not only during orthocorrection of the image data (to remove terrain-related image distortions), but also during data analysis and modeling. Hyperspectral or thermal image data may be “draped” over the Lidar DEM, providing more powerful data visualization and modeling opportunities than is already possible using either the imaging or laser technology alone. SpecTIR offers advanced processing solutions (for HSI/UDAC Fusion). See GAIA.
LONG WAVE INFRARED-LWIR
Thermal Infrared. A spectral region covering long wave infrared wavelengths between ~10 to 12 microns. Rather than reflection, what is measured in this wavelength region is emissivity. The AISA is a hyperspectral TIR imager.
The number of columns (or spatial pixels) on the x-axis of the CCD used to record imagery determined as part of the setup of configuration files. Assuming a look direction spacing of zero pixels, the number of look directions multiplied by the pixel resolution determines the swath width of a flight line.
Mercury Cadmium Telluride array. An optical imaging detector type used in various AISA imagers. Proportions of the three constituent elements can be customized during manufacture to provide sensitivity to either mid wave infrared (typically 3-5 microns) or long wave infrared (10-12 microns) spectral regions. See also CCD Array.
MEAN SEA LEVEL (MSL)
The average height of the surface of the sea. Used as a reference for measures of height. See also Ellipsoidal Height; Geoid Height.
MERCURY CADMIUM TELLURIDE-MCT-FPA
A two-dimensional detector array for the short-wave infrared (SWIR) region. The MCT SWIR camera in this catalog detects light in the 1000-2500 nm range. Mid-infrared (LWIR) MCT arrays also used for hyperspectral imaging
An SI unit of length equal to 10E-6 meters. See also Nanometer.
One thousandth of a second. 15 ms = 0.015 seconds.
Refers to a single geocorrected (usually orthorectified) image file made up of multiple, adjacent flight lines with side lap that have been stitched together. Sometimes referred to as a processed (or georeferenced) block. Because of the ~2Gb file size limitation in UNIX/LINUX (there is no limit while processing in the windows environment), a single flight block may be covered by multiple mosaic files during geocorrection. The mosaic file often becomes the unit on which image analysis is based.
A sensor is multispectral if it simultaneously acquires images of the same area in a number of different wavelength ranges (bands).
An image that contains intensity information at more than one wavelength. A RGB image is a multispectral image because it includes information from three different wavelengths. The term hyperspectral image is used when many wavelengths covering an entire wavelength region (visible, NIR, etc.) are included.
Mid wave Infrared. A spectral region covering infrared wavelengths between ~3 to 5 microns. The hyperspectral SEBASS Sensor contains both, HWIR & LWIR.
NAD27 – North American Datum of 1927. Many older topographic maps use this datum This dated datum ellipsoid has distortions that vary across North America.
NAD83 – North American Datum of 1983. As this datum was developed using satellite measurements, distortions are no longer a problem. Many newer maps use this datum.
Nadir refers to the point on the ground located vertically below the center of the objective lens.
An SI unit of length equal to 10E-9 meters used to define wavelengths in the visible and NIR portions of the electromagnetic spectrum. See also Micrometer.
A binary file that contains the merged GPS and attitude data. May be thought of as a geocoded grid into which the image data is resampled. See also Corrected Navigation File.
A means of resampling where the DN value for the pixel to be mapped is assigned based on the DN of the closest pixel in the input image. The nearest neighbor approach has the benefit over other methods (e.g. bilinear interpolation, cubic convolution) in that it does not alter the original input pixel values, and is computationally simple. See also Resample.
The shortest wavelengths of the infrared region, nominally 750 to 1000 nm.
Spurious, unwanted energy generated internally within an electronic system or from outside interference. Tends to limit the useful range of a system. See also Signal-to-Noise Ratio.
GPS data collected without the use of either real-time or post-processing improvements to positional accuracy. See also DGPS.
Original Equipment Manufacturer. This term is also used to refer to the uncased hyperspectral cameras in this catalog.
The time actually spent with the instrument acquiring data over the planned flight line and region of interest. Does not include time spent in turns between lines, or ferry time to the flight block.
Vertical height, usually MSL, based on a geoid model. In other words, the height of an object above the geoid. See also geoid height.
Is the process of warping an image using ground control points and a DEM that corrects for relief displacement and distortions caused by terrain. An orthorectified image is one in which distortions are similar throughout its extent.
PASSIVE REMOTE SENSING
Remote sensing techniques or technologies that measure naturally reflected or radiated energy in order to make inferences about the surface without physical contact. The key difference between these technologies and those of active remote sensing systems (such as Lidar or Ifsar) lies in the origin of the measured energy. While active systems actively emit energy towards an object and then measure what returns, passive systems measure naturally occurring energy from the sun that is reflected or emitted from an object. All AISA imagers are passive remote sensing technologies. See also Active Remote Sensing.
The vertical angle formed between the aircraft’s longitudinal axis and the horizontal plane. Fixed wing aircraft often have a positive pitch angle (nose up) during normal flight. See also Roll Angle, Heading.
Short for picture element. The smallest visual element that a digital image can be divided into. The resolution of the image is dependent on the number of pixels it contains. For an image with a given physical dimension, the more pixels used there are, the higher the resolution. See also Resolution.
Refers to GPS data collected during image acquisition relating to the simultaneous 3-D position (X, Y, and Z) of the aircraft.
POST-PROCESSED NAVIGATION SOLUTION
Used to refer to the navigation data file (from a precision GPS/IMU system) that has undergone post-processing to achieve an increased level of accuracy. This contrasts with the real-time navigation solution. See also Real-Time Navigation Solution.
A mathematical means of portraying the earth (or a portion of the earth) on a flat surface. Distortions related to con-formality, distance, direction, area, and scale often arise to varying degrees, based on the type of projection. A commonly used projection in the production of AISA instrument image maps is the Transverse Mercator (TM) projection, which forms the basis of the Universal Transverse Mercator (UTM) grid coordinate system. The TM projection results from projecting a sphere onto a cylinder tangent to a central meridian.
All AISA instruments are pushbroom scanners, meaning that the use line of detectors to scan over a two dimensional scene. The ground swath covered in a single pass of the instrument is related to the chosen pixel size and the number of pixels in each row of the sensor array. Pushbroom scanners rely on the forward motion of the platform relative to the imaged target to provide a recognizable image that is built up scan line by scan line, according to the chosen integration time. Because a two dimensional sensor is used, one dimension represents the spatial dimension (x), and the other the spectral range (y).
A Windows based utility that allows raw imagery to be played back from a removable hard drive for review purposes.
RAD FILE (*.RAD; RSC)
Binary files containing the calibration matrices for each aperture setting (f-stop) which describe the radiant sensitivity of each sensor pixel. These files are generated during the spectral calibration of AISA instruments, and are specific to that instrument and calibration. The calibration matrices are then applied to each image data file during radiometric correction. The appropriate *.rad file must be applied to each image file according to the aperture used during its acquisition. The AISA features five open aperture settings, hence five different rad files are generated during instrument calibration. All other AISA imagers feature only a single open aperture, so only one rad file is needed. Referred to as calibration files, Radiant Sensitivity Coefficient (RSC) files, or rad files. See also Radiant Sensitivity Coefficients.
Radiant intensity measured in a specific direction per unit projected area. Measured in watts/steradian/m2. See also Irradiance.
Under most circumstances, the term radiometric correction generally implies that two major processes have been performed on raw image data: correction and calibration. Radiometric correction involves the removal of additive instrument-related components (dark current, etc.) from the raw image data. This leaves only the actual target-related component to the signal. Correction also scales the raw image data from 14 to 16 bits in dynamic range. Radiometric calibration involves the application of instrument and aperture-specific coefficients (contained in the *.rad files generated during instrument calibration) to the raw image data, converting it from raw DN to units of radiance. See also Rad File; Radiance.
Acquired imagery which has not gone through standard processing. With reference to attitude and position data, data which has not changed significantly from its acquired format.
REAL-TIME DIFFERENTIAL CORRECTION
Differential corrections performed on airborne GPS data at the time of acquisition. Requires the use of a differential DGPS decoder such as that produced by Racal or Omnistar. See also Differential Correction; Post-Processed Differential GPS.
REAL-TIME NAVIGATION SOLUTION
A term used to refer to GPS/IMU navigation and attitude data files as acquired in the aircraft. These files have not undergone post-processing to improve their accuracy.
The ratio of a given wavelength of light reflected by a surface to the light incident on a surface, expressed as a percentage. To arrive at a measure of reflectance, the instrument radiance data must be atmospheric correction to remove as much of the effect of the atmosphere as possible.
Light reflected off of a ground target and passively sensed and measured by the imagers.
The change in direction of travel of an electromagnetic wave as it passes between two areas which have a different refractive index.
REGION OF INTEREST (ROI)
See `Area of Interest’.
Acquisition of information of an object or phenomenon, by the use of device(s) that are not in physical or intimate contact with the object (such as by way of aircraft, spacecraft, satellite, or ship). Hyperspectral cameras may be used in ground-based or airborne remote sensing applications.
Spectral Imaging A combination of imaging and spectroscopy. In a spectral image each spatial point contains information about two or more spectral regions.
REMOVABLE HARD DRIVE
A hard drive unit installed in an enclosure that permits it to be inserted and removed without requiring a reboot of the computer (hot swappable drive). Removable hard drives are used in AISA instruments as means of recording acquired data.
Used during geocorrection in the process of removing both systematic and random geometric distortions from an image. To accomplish this, a transformation function is used to `map’ the original pixel values into a geocoded (and distortion-reduced) grid or matrix. The cell values between the original and the transformed matrix will not exactly match. The intensity value of a cell to be mapped into the output matrix is based on the intensity values which surround it in the original image.
Resolution defines the smallest ground element that can be identified in the imagery. An image with pixels at 1m x 1m resolution in theory allows for the visual identification of targets that are at least 1m square. The smaller the pixel size, the higher the resolution of the data. High resolution capabilities are aircraft and speed dependent.
The angle formed between the aircraft’s right wing and the horizontal plane. Synonymous with Bank Angle.
Synthetic Aperture Radar. A method of active illumination remote sensing in which the instrument emits microwaves, and uses the range and Doppler shift of the returns to locate ground points. SAR instruments are mounted on the ERS 1 and 2 satellites. More information can be found.
SCAN LINE (FRAME)
Data collected during one frame time of the sensors. A scan line may be visualized as having two spatial dimensions and one spectral dimension. A pushbroom sensor’s image is made up of a continuous series of many scan lines or frames, successively built up throughout the data acquisition in a `waterfall’ style display. A single flight line can be made up of thousands of scan lines. Not to be confused with the term flight line.
Light scattered internally within the optical train of a VNIR system. This light is scattered by reflections from the edges of optical components or mounts, or from irregularities on the optical surfaces. Scattered light is one of the additive components to the signal measured by the AISA that is removed during radiometric correction.
The multiple reflection of EM radiation off surfaces or particles. This leads to a decrease in the energy being transmitted.
Sensor Head Unit. This is the camera component of the instrument that contains the optical system and the digitization electronics. See also ICU.
Also called side overlap. Sidelap defines the planned percentage of overlap between two adjacent and parallel flight lines. Sidelap is necessary to avoid data coverage gaps caused by turbulence, terrain height changes, or flying height inaccuracies. The amount of sidelap planned for will depend on the target; sidelap of 20% may suffice for flying in calm conditions over flat topography, while 40% sidelap may be necessary if flying in areas of rugged topography and variable turbulence.
SIGNAL-TO-NOISE RATIO (SNR)
The ratio of the target information related to the signal measured by an instrument to the undesirable noise present in the absence of a signal. A very general rule of thumb is: If the spectral feature, or spatial contrast, that you are trying to reliably resolve is one part in N, then your data needs to have a SNR of at least 2 1/2 X N.
Sometimes referred to as a channel or band. A set or user-defined range, contained in a *.ccf file, of contiguous rows on the sensor used to measure reflected or emitted radiation. Spectral bands are programmable using the CASI imager (in terms of number, width, and placement), and are fixed for the SASI, MASI, TABI, and TASI imagers. See also Channel; Free Spectral Range.
SPECTRAL IRRADIANCE UNITS (SIU)
The units used for radiometrically calibrated instruments and ILS data, where: 1 SRU = 1 W cm-2 nm-1 For the ILS to be calibrated, an `i’ series of calibration files (*.rad) must be used in radcorr. Also, the ILS data is calibrated in a separate run of radcorr, with the dark data correction disabled (refer to the Processing Program Reference for more information).
Performance of spectral devices to provide true wavelength information by separating the wavelengths properly.
The unique profile for a specific target type (e.g. minerals,water, tree species, grass, concrete…) based on its measured spectral response across the a portion of the electromagnetic spectrum. The difference between unique spectral signatures often provides the basis for powerful analysis techniques to be used in extracting useful information from hyperspectral image maps.
A measure of spectral distortion, or wavelength shift in the across-track direction of the array. As a result, the sensor does not respond to the exactly same wavelength across an individual scan line. Typically this distortion is low, on the order of ±0.25 to ±0.8 pixels (depending on the sensor type) and is ameliorated during calibration. The term arises from the general appearance of a plot of wavelength versus spatial pixel, which approximates the curve of a shallow smile with upturned ends. Spectral smile, and its spatial equivalent, Keystone, are present to varying degrees with pushbroom-style instruments using diffraction gratings. See also Keystone.
A device which separates incoming light into its wavelength components producing spectra.
A device for measuring the intensity of absorbed, reflected, or emitted electromagnetic radiation as a function of wavelength.
The study of light as a function of wavelength (spectrum) which has been transmitted, emitted, or reflected from a solid, liquid or gas sample.
Intensity of light as a function of wavelength, which can be displayed on an emission, reflectance, absorption, or transmission scale.
Staring Array Hyperspectral Imaging A type of camera or method of hyperspectral imaging in which the object and camera are stationary and the entire object is imaged at the same time. To acquire spectral information at multiple wavelengths, a tunable filter is used to scan through the wavelengths. Please see the Hyperspectral Imaging chapter for a more detailed explanation and a comparison to the push-broom method.
Also referred to as data processing. The use of specific software programs to convert acquired raw image data to a calibrated and corrected image product on which analysis may begin. This data conversion involves radiometric correction and calibration, and most often includes the application of synchronized attitude and position data to yield a geocorrected or orthorectified image product. See also Raw Data, Radiometric Calibration, Geocorrection, Orthorectification, and Image Analysis.
The area on the ground covered by a single flight line. Swath width is simply a function of the pixel resolution x the number of look directions. The larger the pixel size (and the higher the plane flies), the wider the imaged ground swath.
Shortwave Infrared. A spectral region covering infrared wavelengths between ~1 to 2.45 microns.
See Course Over Ground.
TRUE COLOR IMAGE
An image in which the bands which most closely represent visible red, green, and blue have been assigned to red green and blue, thus producing an image which is similar in color to what we see with the naked eye.
Universal Time Coordinated. UTC is a time scale that couples Greenwich Mean Time (GMT; based solely on the earth’s rotation rate) with International Atomic Time (TAI; based on the precisely measured vibrations of a cesium atom). Like GMT, UTC is normalized to the local time at 0 degrees longitude (i.e. on the prime meridian which runs through Greenwich, England). Like TAI, UTC runs at the rate of atomic clocks. It is now the basis of most legal and radio time scales. UTC is not a `linear’ time scale. In particular, when the difference between TAI and GMT approaches one second, a one-second adjustment (i.e. leap second subtraction) is made to UTC. These adjustments are usually made after the 60th second of the last minute of either June 30 or December 31. Generally, one adjustment is made every 12 to 18 months.
An optical device used for diffracting a beam of light into its wavelength components; the main component of line scan spectrographs.
Universal Transverse Mercator. A two dimensional, horizontal grid coordinate system defining positions measured in meters east and north (or south) of a reference point. Based on the Transverse Mercator projection. This is the default coordinate system used to georeference AISA instrument imagery. Using this system, the world is divided into longitudinal strips starting at the 180th meridian and moving eastward. Each strip, called a UTM grid zone, is 6 degrees wide. Coordinates are listed using eastings and northings within a specific UTM grid zone. Eastings are measured from a central meridian within each 6 degree grid zone (each zone has a 500 km false easting to ensure positive coordinates). Northings are measured from the equator (a 10,000 km false northing is used for positions south of the equator). UTM world coverage extends from 84 degrees North latitude to 80 degrees South latitude. Example: Easting 706000 Northing 5662300 Grid Zone 11 defines the Olympic Speed Skating Oval in Calgary, Alberta, Canada. See also Projection.
Spectral wavelengths of light (visible radiation) of which the human eye is sensitive, ranging between approximately 400 to 700 nm. Corresponds roughly to the continuous variation of color in the series: violet, blue (~450 nm), green (~550 nm), yellow, orange, and red (~650 nm).
Visible Near-Infrared. A spectral region covering wavelengths between ~0.38 to 1.05 microns. Within this range are found visible wavelengths (~0.4 – 0.7 microns) and near infrared wavelengths (~0.7 to 1.05 microns).
Electromagnetic energy is transmitted in the form of a sinusoidal wave. The wavelength is the physical distance covered by one cycle of this wave; it is inversely proportional to frequency.
WGS84 – World Geodetic System 1984. Almost the same as NAD83. For purposes of geocorrection we can assume them to be the same.