As airborne hyperspectral and thermal mapping and 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.


Across Track

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.

Active Remote Sensing

Remote sensing techniques or technologies that involve actively emitting some form of energy and then measuring that which is returned after reflection or back scattering from the target surface or object. Inferences about the surface may then be made based on the measured returns, such as heights or changes in density. Examples of active remote sensing techniques include airborne Lidar, Ifsar, ground-based seismic refraction, or sonar.
See also Passive Remote Sensing.


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. See also Average Terrain Height; ASL.

Air Speed

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.

Along Track

Defines a direction parallel to the direction of flight (`along the track of the aircraft’). Along track spatial pixel resolution is related to the aircraft flying speed (m/s) and integration time (ms) being used.

Aperture Setting

The aperture is the part of the lens through which light is allowed to pass into the optical train of the sensor head. The CASI is the only ITRES instrument to have multiple aperture settings. The CASI has six numbered calibrated aperture settings corresponding roughly to the following f-stops:
Closed (aperture 0), f/11 (aperture 1), f/8 (aperture 2), f/5.6 (aperture 3), f/4 (aperture 4), f/2.8 (aperture 5).
All other ITRES imagers (SASI, MASI, TASI, and TABI) only feature a single open aperture setting.

ApplAnix Corporation

A leading provider of position and orientation solutions (POS) for precision commercial applications. Manufacturers of the POS/AV (POS for Airborne Vehicles), enabling the real-time measurement of precise position and attitude data. One of the precise geocorrection system options available for use with the ITRES instruments. Headquartered in Richmond Hill, Ontario, Canada. Telephone: 905-709-4600.
At the time of writing, information about ApplAnix can be found on the web at:

Area of Interest

Refers to the specific target or area on the ground to be covered by 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’.


Above Sea Level. A term used to define the height above the average height of the surface of the sea. Usually measured in feet when specifying flying altitude. See also AGL; Mean Sea Level.

Atmospheric Correction

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 Edge Brightening; Path Radiance.


With reference to airborne data collection, attitude refers to roll, pitch, and heading motions of the aircraft. Measured using an IMU/INS system (50 Hz or 200 Hz).

Attitude Data

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 ITRES 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.

Average Terrain Height

Used in flight planning, represents the average height of the imaged terrain above mean sea level along a single flight line. Usually measured in feet when defining flying altitude. The average terrain height must be added to the height AGL to define the actual flying height. See also Flying Height; Mean Sea Level.


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.

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 ITRES 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.

Band Width

As related to an Instrument, the width of a selected spectral band or channel, in nanometers (nm).


Distance between the kinematic or rover (airborne) GPS receiver and a static GPS receiver (base station) when used for differential GPS corrections. Shorter baselines (<100 km) result in more precise positioning solutions.


A GPS receiver set up at a ground location for which the coordinates are known, used to collect static data for differentially correcting rover (kinematic) GPS measurements. Because the two GPS data streams are coincident, the basestation measurements can be used to calculate the errors for each satellite signal. These errors may then be removed from the measurements made by the moving receiver located within the same geographic area as the basestation, thereby improving the positional accuracy of its data. This process is referred to as differential correction.

Batch File (*.bat)

A user-created ASCII text file used in Windows that lists the switches and corresponding arguments necessary to run the ITRES 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.

Baud Rate

The number of bits per second transmitted across a communications port. The baud rate typically used to transfer GPS data to the ITRES 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.


See Band Interleaved by Pixel.


A blackbody is a theoretical “object” that is a perfect absorber and emitter of energy; no reflection or transmission occurs, and all energy absorbed is emitted for a given temperature. In reality, any surface will always emit less energy than it has absorbed. Comparing the amount of energy emitted by a given surface to that of a blackbody at the same temperature (and with the same area) results in a measure of the emissivity of that surface or object.
See also Emissivity.

Broadcast GPS Week Number

This is the GPS week number broadcast by GPS receivers (contained within the ephemeris file). Since the GPS week rollover in August 1999, 1024 weeks may need to be added to the broadcast GPS week number to arrive at the correct value. As of February 3-9, 2002, the broadcast week number is 128. The actual week number used in processing is (128 + 1024 = 1152). See also GPS Week Number.

Buffered Serial Port

A serial port is a hardware communication socket interface that is used to transmit data between two serial devices. To reduce chance of interrupting the incoming data stream at higher data transfer rates, ITRES uses a hardware modification to buffer the incoming data at a rate of 256 bytes per second. Current instrument control units have three buffered serial ports (currently only the first two are enabled). BSP1 is typically used to transmit GPS data from a receiver to the instrument control unit.

Bundle Adjustment

The process of estimating and removing the linear and angular offsets that exist between the instruments installed in the aircraft (the GPS antenna, sensor head, and IMU). In doing so, the final positional accuracy of the geocorrected imagery is improved.
To perform a bundle adjustment, a geometric calibration target site (for which good ground control exists) is first flown using multiple overlapping flight lines. During the bundle adjustment processing, common features (tie points) are identified in the overlap regions of the processed flight lines, and ground control points are identified in the imagery. This information is then input to the bundle adjustment software. This program estimates the instrument offsets, represented as a set of values that are applied to the navigation data. A corrected navigation file results from this process, which is then used as the navigation file input to the geocorrection.
Bundle adjustment are performed on all ITRES imagers as a standard practice.

C/A Code

Also known a Coarse Acquisition Code. A spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the distance to the transmitting GPS satellite. The C/A code modulates only the L1 frequency, in comparison to P-Code, which uses both L1 and L2. C/A code allows a receiver to quickly lock onto a satellite. See also P-Code.


The referencing of the image data collected by an instruments sensor head to a standard light source, black body, or laser (instrument type-dependent). This allows correction factors (contained in the *.rad files as radiant sensitivity coefficients for each pixel on the array) to be created for each aperture. These can then be applied to the raw imagery to convert from raw DN to either spectral radiance units or temperature based on the standard used. See also SRU.

Carrier Phase

From the Ashtech website ( The fraction of a cycle, often expressed in degrees, where 360 degrees equals a complete cycle. Carrier phase can also mean the number of complete cycles plus a fractional cycle. In a survey-grade GPS receiver, the receiver can lock on to a satellite and, keeping track of the number of whole cycles of the carrier, creates a cumulative phase of the signal which is often referred to as integrated Doppler.
Also: The cumulative phase of either the L1 or L2 carrier of a GPS signal, measured by a receiver while locked-on to the signal (also known as integrated Doppler).

Carrier Phase Processing

A post-processing technique which measures and integrates the phase of the incoming GPS signal from each tracked satellite to more precisely determine the velocities and positions than are available from the C/A code alone. Requires the rover GPS data to be acquired in an uninterrupted stream from the start of the flight to the end. Carrier phase processing is typically conducted when the POS is used with the instrument for measuring attitude data.

Instrument Configuration File – Instrument

An instrument file configured by ITRES for the TASI, SASI, and TABI and by the user with the CASI. This file specifies the combination of spectral bands and frame/integration time to be used to collect data.
See also Bandset.

CASI (Commercially available since 1989)

Compact Airborne Spectrographic Imager. High performance, CCD-based VNIR hyperspectral sensor based on the pushbroom imaging concept. The CASI combines the better features of aerial photography and digital satellite imaging with the analytic capabilities of a spectrometer. Spectrally programmable (up to 288 bands) across 380-1050nm, wide-array CASI-1500 features 1500 across-track imaging pixels and features true diffraction-limited, custom optics. Designed and produced commercially by ITRES Research Limited of Calgary, Alberta, Canada. Tel: 403-250-9944; Web:

CCD Array

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. Used with the CASI VNIR imager.
See also MCT Array.

CCD Signal

This signal is composed of a series of additive components. These include the photo signal, dark current, scattered light, frame shift smear, and electronic offset. The non-target related components (all but the photo signal) must be subtracted from the CCD signal before multiplying by the Radiant Sensitivity Coefficients (contained in the calibration files) in order to determine spectral radiances.


See Spectral Band.

Choke Ring

Typically used with a GPS basestation receiver to reduce or eliminate multipath scattering. The choke ring can appear as a circular disk of metal, ranging between approximately 20-30 cm in diameter, on top of which is installed the GPS antenna. Sometimes referred to as a ground plane.


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.

Correct (radcorr)

The removal of the non-signal-related additive components (dark current, frame shift smear, electronic offset, scattered light) of the raw CASI image data. Other ITRES instruments are corrected for dark current and electronic offset only. Accomplished through the use of

. The correction of image data leaves only the target-related signal.

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 track.

Crab Angle

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.

Cycle Slip

The loss of signal lock between the GPS receiver and a satellite.

Dark Current

One of the additive components to the signal measured by ITRES instruments 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. See also Dark Data; Electronic Offset; Frame Shift Smear; Internal Scattered Light.

Dark Data

Refers to the 200 scan lines of data collected with the lens aperture/shutter closed, both prior to and immediately after the acquisition of each image data file. Because incoming light is prevented from entering the optical train, any signal measured will be dark current signal generated internally. The measured dark current data varies according to the internal and ambient temperature, IT, and pixel location on the sensor. This dark data is accessed, averaged, and subtracted from the raw image data during radiometric correction. Dark data is captured for all ITRES instruments. See also Dark Current.

Datum (Geodetic)

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 ITRES instrument processing output is WGS84. Referencing coordinates to the wrong datum can result in position errors of hundreds of meters. See also Geoid.


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 pseudorange errors measured from a receiver at a known location (static basestation) 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 tendency of an aircraft to be unable to maintain the planned course over ground when flying in crosswind conditions unless corrections are made.

Drift Angle

See Crab Angle.


Digital Number. For all ITRES instruments, the raw image data are measured in units of DN. Each pixel is represented by raw integer intensity values with a depth of 14bits (0-16383).


Earth Centered Earth Fixed. A cartesian coordinate system where points are referenced (x, y, and z) in meters from the center of the earth’s mass. In addition to UTM coordinates, the navigation files output by the processing software include references to this coordinate system.

Edge Brightening

A path radiance effect sometimes seen in airborne imagery before atmospheric correction where the outermost edge of a flight line is brighter than the nadir pixels. For a given flying height and field of view, as you move from the nadir pixel to an edge pixel, the distance the reflected sunlight must travel from the ground to the lens increases (as does the amount of atmosphere the radiance of light must travel through). This leads to a corresponding increase in the possibility/magnitude of path radiance atmospheric scattering. See also Atmospheric Correction; Path Radiance.

Electromagnetic Spectrum

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.

Electronic Offset

One of the additive components to the signal measured by an Instrument that is not target-related. These components are measured by the instrument during data collection, and are removed from the raw image data during radiometric processing. Electronic offset is a uniform signal deliberately added to the sensor’s raw signal before digitization and data recording. Adding this offset helps to ensure that noise will never drive the output signal to negative values. See also Dark Current; Frame Shift Smear; Internal Scattered LIght.


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. See also Ellipsoidal Height; Geoid Height; Mean Sea Level.


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. See also Datum; Ellipsoidal Height; Geoid.

Ellipsoidal Height

The height of an object above the reference ellipsoid in use. GPS systems measure ellipsoidal height. To arrive at a measure of elevation, the geoidal height at that location must be subtracted from this value.


Across infrared wavelengths, in addition to reflection, energy may be emitted by surfaces as well. The amount of energy radiated from a surface is dependent on both the temperature and its material composition. Emissivity describes a material’s ability to emit the thermal energy it has absorbed as compared to a perfect energy radiator at the same temperature and with the same area, known as a blackbody. Emissivity is a non-dimensional value that varies between 0 and 1. A perfect blackbody would have a value of 1.0
See also Blackbody.

Ephemeris File (.epp)

An ASCII GPS file containing the predictions of the current positions of satellites `visible’ to the GPS receiver. Also contains information on the `health’ of each satellite, as well as the current date, time and broadcast GPS week number. See also Broadcast GPS Week Number.

Ferry Time

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.

Fixed-Point Number

A number whose decimal place is in a specified position. Like floating point numbers, fixed-point numbers can have a fractional part, but operations on fixed-point numbers are usually much faster. See also Floating-Point Number.

Flight Block

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 sidelap. Contains one or more areas of interest. Not to be confused with Mosaic. See also Flight Line; Area of Interest.

Flight Line

Typically used to refer to a single linear path (usually preplanned) along which image data is acquired. A flight block is made up of multiple and adjacent flight lines with sidelap. Not to be confused with a scan line. See also Flight Block.

Floating-Point Number

Also referred to as exponential or scientific notation. Unlike fixed-point numbers, floating-point numbers can have their decimal point in any position. Floating-point numbers can be used to represent very large or very small numbers. Consists of two parts: a mantissa which contains the digits of the number, and an exponent, which specifies the magnitude of the number. For example, 235433 may be represented as the floating-point number 2.35433E5, while 0.0000034231 is represented by 3.4231E-6. More computationally intensive than fixed-point numbers. See also Fixed-Point Number.

Flying Altitude

See Flying Height.

Flying Height

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 ASL. Note that pilots often require this value to be specified ASL, 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 ITRES instruments have a 10,000 foot operating altitude limit based on air pressure limitations of the recording devices and video monitor. See also AGL; ASL.


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 ITRES 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.


See Scan Line.

Frame Shift Smear (FSS)

One of the additive components to the signal measured only by the CASI imager only, that is not target-related. These components are measured by the instrument during data collection, and are removed from the raw image data during radiometric processing. FSS is image smear that occurs during frame transfer on the CASI’s CCD array. See also Dark Current; Frame Shift Smear; Internal Scattered LIght.

Free Spectral Range

The wavelength range over which the ITRES instruments are sensitive and to which they have been calibrated.

Frame Time

See Integration Time.


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.

Geodetic Coordinates

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.

Geodetic Height (Ellipsoidal Height)

This is the vertical height of an object measured above an ellipsoidal surface. The difference between an object’s ellipsoidal height and the orthometric height is the height of the geoid.

Geoid Height (Orthometric Height)

Vertical height of the object above the geoid in use. Also called orthometric height.


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. Typically refers to the NAVSTAR (Navigation Satellite Timing and Ranging) space-based radio navigation system. NAVSTAR consists of 24 continuously orbiting satellites in 6 orbital planes (space segment), the associated ground support for control and tracking (control segment), and the user segment (GPS receivers). Measurement data from the NAVSTAR satellite constellation is used to provide positional information used in the geocorrection of image data. The NAVSTAR system is controlled by the US Department of Defence.

GPS Accuracy

Outside of other factors that affect GPS accuracy (baseline length, ionosphere, multipath…) GPS accuracy roughly equates to 1/100 x wavelength of the GPS signal.
A quick comparison of CA code and carrier phase processing accuracy:
CA Code: wavelength ~300 m; accuracy ~3 m
Carrier Phase: wavelength ~20 cm; accuracy ~0.02 cm

GPS Time

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).

GPS Week

The GPS week (after which the GPS Week Number is based), begins on Sunday morning, at 00:00:00.0 hours). See also GPS Week Number.

GPS Week Number

This is the number of weeks elapsed since the `Zero GPS Epoch’ (i.e. 1980/Jan 06/00:00:00). For example, `GPS Week 1131′ begins on Sunday, September 9 at 00:00:00 and ends on Saturday September 15 at 23:59:59. See also Broadcast GPS Week Number, GPS Week.

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 Plane

See Choke Ring.

Ground Resolution Element

The smallest visually resolvable unit (L x W) on the ground for a given pixel size. Also referred to as Ground Spacing Distance.

Ground Speed

The horizontal speed of an aircraft measured relative to the ground. Not to be confused with Air Speed. Ground speed = Air speed – Wind speed.

Ground Truth

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.

Hertz (Hz)

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.


Method of counting where the base number is 16 rather than 10. The sequence is as follows 0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f. The GPS data contained in the *.att file is in hexadecimal format.

Hyperspectral Mode

The second of two operational modes used by the CASI imager (the other is Spatial Mode, sometimes referred to as Spectral Mode). Hyperspectral Mode is often chosen when maximizing the number of spectral bands to be acquired is desired, sometimes at the expense of spatial pixel resolution or swath width. In this mode, a user-specified row summation factor is used to `divide’ the spectral dimension (rows) of the CCD array into discrete bands of equal spectral width (all bands are adjacent and gaps between bands are not allowed). Up to 288 non-overlapping and adjacent spectral bands may be acquired in this mode (1 row summed). Using a row summation factor of 12 results in 24 adjacent bands of equal widths to be acquired.
Regarding swath width, while Spatial Mode automatically acquires the full number of across-track imaging pixels on the array (1500 for the CASI-1500), Hyperspectral Mode allows the user to optionally reduce the swath width coverage. Although doing so results in narrower flight lines (and necessitates more flying to cover the same flight block width), the flexibility of reducing the integration time is achieved. This is often important when trying to meet along-track pixel resolution constraints. Even when acquiring the full data cube in this mode (288 spectral bands, 1500 across-track imaging pixels), a pixel resolution of ~2 m is possible using a typical fixed wing aircraft. By manipulating the row summation factor and number of imaging pixels, it is still possible to acquire sub-meter pixels resolutions as low as 15-20 cm in this mode, as is also the case with Spatial mode.
The potentially higher number of spectral channels often acquired using this mode lengthens integration times, and by extension, along-track pixel sizes.See also Row Summation; Spatial Mode.


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. See also SHU;.


Incident Light Sensor. An optional sensor used with the CASI or SASI to measure downwelling irradiance at the roof of the aircraft during data acquisition. Comprised of a cosine diffuser probe and a fibre optic cable, the ILS allows for the correction of downwelling irradiance variations in order to convert the CASI and SASI radiance data to units of spectral reflectance.

Image Analysis

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.

Image Data

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.


Inertial Measurement Unit.


A precise, self-contained navigation system using inertial detectors, which automatically provides aircraft position, heading, and velocity. See ApplAnix, NovAtel.

Integrating Sphere

Used in the spectral calibration of the CASI and SASI instruments. The definition from the Photonics Dictionary ( 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.’

Integration Time (IT)

In general terms with the CASI imager, the integration time is the time needed to collect a single scanline of data. Measured in milliseconds, the IT is positively correlated with both the number of spectral bands and the number of look directions being collected. The IT is analogous in function to shutter speed used to capture a single photo frame from a 35mm camera. In most cases, the IT is longer than the MIT (it is never shorter).
In operational terms, the IT is the amount of time used by the CASI to both collect and record a single scan line of data, while permitting the acquisition of a specific along-track pixel size for a given flying speed.
It is important to note the difference in how the CASI treats the concept of integration time and frame time (FT) as compared to other ITRES imagers. Owing to the CASI’s detector array and readout system, both values are considered to be the same. The integration time alone is programmable for this sensor, and doing so affects both along-track pixel resolution for a given aircraft speed as well as data signal levels.
Conversely, both frame time and integration time are treated as separate variables for the SASI, MASI, TABI, and TASI imagers. For these, the frame time and aircraft speed affects along-track pixel resolution, while the integration time affects signal level. Because frame time is fixed for these latter imagers, the sensor operator only needs to adjust the aircraft speed to change the acquired along-track pixel size. Should signal levels be too high or too low during data collection, changing the integration time allows these levels to be optimized.

Internal Scattered Light

One of the additive components to the signal measured by an instrument that is not target-related. These components are measured by the instrument during data collection, and are removed using

from the raw image data during radiometric processing. Internal scattered light is the signal derived from light which has been scattered inside the sensor head optical train. See also Dark Current; Electronic Offset; Frame Shift Smear.


In-Flight Processing System. Optional system offered by ITRES for in-flight hyperspectral and thermal geocorrection and orthocorrection (uses existing DEM).The operator sees the georeferenced and radiometrically corrected images and mosaics on-screen during flight. Allows fast access to processed data for time critical applications where rapid mapping response and quickly pinpointing GPS locations are necessary. Modular processor approach also allows some forms of automated analysis to be conducted on the data during flight.


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.


Innovation, Technology, Research, Excellence, and Science. Manufacturer of the CASI, SASI, TABI and TASI airborne imager. Headquartered in Calgary, Alberta, Canada. For further information, contact us via or via the World Wide Web at Tel: 403-250-9944.


Aircraft vibration transmitted to the sensor head unit during data acquisition. The visible effect of this vibration on the image data may be reduced by smoothing or processing of the attitude data or through the use of a gyro stabilized mount. See Gyro Stabilized Mount.


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 pushbroom-style instruments using diffraction gratings. See also Spectral Smile.

Kinematic Mode

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.

Knot (kt)

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


Denotes an extension added to the start of a planned flight line during flight planning before the area of interest or flight block is reached. A one kilometer lead-in is usually added in order to provide the pilot and instrument operator with a buffer of time and distance before the flight block is reached. This helps the pilot to ensure that the designated speed, altitude, and course over ground has been achieved before being on-line. Lead-in ensures that the GPS is receiving satellite signals if any loss of lock has occurred during a sharp bank to get on-line. It also provides the operator with time to ensure the instrument is finished collecting reference data (dark and/or uniformity data in the case of the CASI imager) and is recording data normally before entering the block. Can also be used as lead-out if the planned flight direction has been changed for the line. See also lead-out, dark data, and uniformity data.


Denotes an extension added to the end of a planned flight line during flight planning after the area of interest or flight block has been exited. A one kilometer lead-out is usually added. This provides the instrument operator with a buffer of time and distance after the flight block has been exited to ensure that a small buffer of data has been recorded outside of the flight block to ensure full coverage. Can also be used as lead-in if the planned flight direction has been changed for the line. See also Lead-In.


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 ITRES 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. All ITRES imagers are Lidar-compatible.
See also Active Remote Sensing; Passive Remote Sensing.

Log File

Some of the ITRES programs used in the standard processing of ITRES instrument imagery allow the user to optionally output an ASCII log file. This file serves as a record of the program’s output as displayed on the computer monitor during operation. It is recommended that where possible, log files be used, reviewed, and archived to be certain that errors did not occur during processing. Typically given a *.txt or *.log extension for differentiation from other file types.

Look Directions

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.

MASI (Commerically available since 2008)

Midwave Airborne Spectrographic Imager. High performance, MCT-based MWIR hyperspectral sensor based on the pushbroom imaging concept. The MASI combines the better features of aerial photography and digital satellite imaging with the analytic capabilities of a spectrometer. Acquires 64 bands of spectral coverage across 3-5 microns, the MASI-600 features 600 across-track imaging pixels and features true diffraction-limited, custom optics. Designed and produced commercially by ITRES Research Limited of Calgary, Alberta, Canada. Tel: 403-250-9944; Web:


The specific term used with Ashtech GPS receivers to denote measurement data that contains the raw data as received directly from the satellites being tracked. All GPS receivers transmit measurement records. Along with PBEN records, this data is sent to the instrument where it is recorded to the image frame headers. It is the time stamps associated with the arrival times of these records at the instrument that are used to assist in the time synchronization of the GPS data with the image data. The order that the measurement and position records are sent to the instrument sometimes depends on the receiver. The length of time required to record a measurement record will vary according to the number of satellites viewed by the receiver at each epoch. See also PBEN.

MCT Array

Mercury Cadmium Telluride array. An optical imaging detector type used in various ITRES imagers. Proportions of the three constituent elements can be customized during manufacture to provide sensitivity to either midwave infrared (typically 3-5 microns) or longwave 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.


An SI unit of length equal to 10E-6 metres. See also Nanometer.

Millisecond (ms)

One thousandth of a second. 15 ms = 0.015 seconds.

Minimum Integration Time (MIT)

In general terms, the MIT for a specific spectral configuration is the time required by the instrument and its recording device to collect a single scanline of CASI data. There are two measured values for the MIT provided by the CASI: the ICU MIT and the hard drive MIT.
Operationally, the user typically adds at least 2-3 ms to the MIT to arrive at the shortest Integration Time (IT) possible for a specific spectral configuration. Remember that the flying speed and desired along-track pixel size must also be taken into account when determining the final IT value to be used for the data acquisition. Operating the CASI using an IT no less than 2-3 ms above the MIT ensures that no data frames are skipped (lost) during recording. If it is deemed necessary to operate the CASI right at the stated MIT, it is strongly recommended that test files be recorded on the system prior to project use to ensure that no frames are lost. See also Integration Time.


Refers to a single geocorrected (usually orthorectified) image file made up of multiple, adjacent flight lines with sidelap 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.


Multiple Sensor Instrument Controller. An ITRES option provided to users wishing to simultaneously operate multiple imagers covering different spectral regions using the same aircraft. Up to five imagers (VNIR, SWIR, MWIR, hyperspectral thermal, broadband thermal) may be operated simultaneously to provide multiple spectral region coverage of the same target surface.


Midwave Infrared. A spectral region covering infrared wavelengths between ~3 to 5 microns. The hyperspectral ITRES MASI and broadband midwave thermal TABI are both MWIR imagers.


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.


With reference to the ITRES instruments, nadir refers to the point on the ground located vertically below the center of the objective lens.

nanometer (nm)

An SI unit of length equal to 10E-9 metres used to define wavelengths in the visible and NIR portions of the electromagnetic spectrum. See also Micrometer.

Navigation File

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.

Nearest Neighbor

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.

Near-Infrared (NIR)

The shortest wavelengths of the infrared region, nominally 750 to 3 nm. With regards to the CASI, 750-1050 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.

Non-Differential GPS

GPS data collected without the use of either real-time or post-processing improvements to positional accuracy. See also DGPS.


A leading provider of position and orientation solutions (SPANTM) for precision commercial applications. The SPAN (Synchronized Position Attitude & Navigation) system, enables the real-time measurement of precise position and attitude data, one of the precise geocorrection system options available for use with an ITRES imager. Headquartered in Calgary, Alberta, Canada. Telephone: 403-295-4900.
At the time of writing, information about Novatel precise positioning products can be found on the web at:

On-Line Time

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.

Orthometric Height

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 ITRES imagers are passive remote sensing technologies.
See also Active Remote Sensing.

Path Radiance

The component of reflected sunlight sensed by the instrument that was not reflected by the ground surface. Path radiance is light scattered towards the sensor by the atmosphere. This has the effect of attenuating the measured light reflected by the target, and leads to an overall brightening of the image. See also Edge Brightening.


The specific term used with Ashtech GPS receivers to denote position data that the GPS receiver calculates from the raw data contained in its measurement records.
All GPS receivers transmit position records. Along with MBEN records, this data is sent to the instrument where it is recorded to the image frame headers. It is the time stamps associated with the arrival times of these records at the ICU that are used to assist in the time synchronization of the GPS data with the image data. The order that the measurement and position records are sent to the instrument depends on the receiver. The length of time required to record a measurement record will vary according to the number of satellites viewed by the receiver at each epoch.
See also MBEN.

Pitch Angle

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.


Position Orientation System/Airborne Vehicle. An integrated intertial/GPS system that generates accurate instantaneous position (latitude, longitude, altitude) and orientation (roll, pitch, heading) measurements using carrier-phase GPS information. The POS AV is used for airborne survey/mapping applications, such as those with the ITRES instruments. Manufactured by ApplAnix Corporation, of Markham Ontario, Canada. Telephone: 905-709-4600.
At the time of writing, information on the POS/AV can be found on the web at:
ITRES has been using the POS/AV with ITRES instruments since 1995.

Position Data

Refers to GPS data collected during image acquisition relating to the simultaneous 3-D position (X, Y, and Z) of the aircraft.

Post-Processed DGPS

Differential corrections performed on airborne GPS data after acquisition. Requires that a nearby, autonomous basestation GPS receiver be used to collect simultaneous GPS data on the ground over a point with a known location. As the distance (baseline) between the basestation and the rover (airborne) GPS receivers increases, the corresponding positional accuracy improvements to the DGPS solution will decrease. Baselines of under 100 km are desirable when geocorrecting imagery, although distances of up to 300 km may give acceptable positional results depending on the spatial resolution of the imagery. See also Differential Correction; Real-Time DGPS.

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 general term referring to the use of software programs to modify and/or improve in some way the already acquired digital data. Post-processing is typically performed after data collection, but with ITRES’ introduction of the IPS, radiometric, geometric, and orthocorrection can optionally be performed on the raw image data in near real-time, during flight. Data outputs from this processing are said to be post-processed results. With regards to ITRES imagers, most commonly the term is limited in use to refer either to differential GPS processing or processing of the precision GPS/IMU attitude data to further improve positional accuracy.


Pulse Per Second. A signal sent by the GPS receiver (the rover unit) used to synchronize external devices (such as the CASI, SASI, MASI, TASI or TABI) with receiver time. The PPS is generated by default once every second. Either the rising or falling edge of this pulse may be programmed to be synchronized with receiver time.

PPS Mark

The time mark of the PPS signal. GPS receivers can be programmed to send a PPS signal to the serial port of another instrument. When an instrument receives a PPS signal from the GPS receiver, it will mark the arrival time of the signal in instrument time. Note that ITRES instruments are “aware” only of their internal clock time, and are not aware of time systems such as UTC or GPS. Conversions to these time systems are performed during navigation data processing.

Precision Code

Also called `P-Code’. A spread spectrum direct sequence code on both the L1 and L2 frequencies that is used primarily by military GPS receivers to determine the range to the transmitting satellite.

Processed Block

See Mosaic.

Projection (Map)

A mathematical means of portraying the earth (or a portion of the earth) on a flat surface. Distortions related to conformality, distance, direction, area, and scale often arise to varying degrees, based on the type of projection. A commonly used projection in the production of ITRES 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.


Also pseudorange measurement. The measured distance between the receiver’s GPS antenna and the GPS satellite. This measurement is calculated based on time differences between the sending of the GPS signal and the reception of the same signal by the receiver. This time difference is related to both the distance travelled and the speed of the transmission.


A qualitative way of looking at the reflectance-like properties of a scene. Not a measure of true reflectance where atmospheric-related effects have been removed from the measured radiance signal. Measures of pseudoreflectance allow for the general comparison of reflectance properties of spectra taken across multiple flight lines acquired on a given flight. See also Reflectance.

Pushbroom Scanner

All ITRES 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). See also CCD and CMOS.


A Windows based utility program (also accessible on the ICU) 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 ITRES 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 CASI features five open aperture settings, hence five different rad files are generated during instrument calibration. All other ITRES imagers (SASI, MASI, TABI, and TASI) 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.

Radiant Sensitivity Coefficient (RSC)

The coefficients applied to the raw image data during the radiometric correction process. These coefficients are found in the calibration files (*.rad), and are generated during the calibration of the ITRES instrument. The RSC for each pixel in the array is unique. See also Rad File.

Radiometric Correction

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, frame shift smear (CASI only), internal scattered light, and electronic offset) 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.

Raw Data

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. See also Post-Processed Navigation Solution.


Rectification is the process of warping an image to control points. With ITRES instrument, this is accomplished in a semi-automated manner through the application of a bundle adjustment procedure. Tie points and ground control points are used only in this procedure, making the use of similar points in the acquired data unnecessary.


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.

Reflected Light

Light reflected off of a ground target and passively sensed and measured by the VNIR (CASI), SWIR (SASI), or MWIR (MASI) imagers.

Region of Interest (ROI)

See `Area of Interest’.

Remote Operation System

An option that allows ITRES airborne imagers to be operated remotely from the ground without the need for an on-board operator. Utilizes an existing TCP-compatible RF data down link. Depending on available bandwidth capability, three RF control modes are available: full remote control plus full data download and archive; full remote control plus real-time visual download; or blind remote control.
See also RF Data Downlink.

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 ITRES 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.


With reference to an ITRES instrument, 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. All ITRES imagers are capable of acquiring data with sub-metre pixel resolutions, some as small as 10-15 cm. See also Pixel; Ground Resolution Element.

RF Data Down Link

Wireless, radio frequency link used to broadcast or transmit communications data. With regards to ITRES imagers, an existing TCP-compatible RF wireless data down link is used with the ITRES remote operation system. This option allows real-time ground-based control and monitoring of hyperspectral and thermal IR mapping data acquired in a small aircraft, without the need for an on-board operator. See also Remote Operation System.


(R)eceiver (IN)dependent (Ex)change format. An optional GPS data output format made up of a set of standard definitions and formats used to aid in the use of GPS data in any software package, independent of receiver type. Contains both ephemeris and observation data. See also Ephemeris.

Roll Angle

The angle formed between the aircraft’s right wing and the horizontal plane. Synonymous with Bank Angle. See also Pitch Angle; Heading.

Row Summation

Pertains to the spectrally programmable VNIR CASI imager. Refers to the on-chip summation of measured charge from adjacent spectral rows on the CCD array in forming individual spectral bands. When using the CASI’s Hyperspectral Mode, the user specifies a row summation factor that automatically divides the array’s rows evenly to form spectral bands. As might be expected in this case, the number of rows summed must be evenly divisible into 288, the number of spectral rows on the array. As a result, row summation factors of 1, 2, 3, 4, 6, 8, 9, 12, etc. are permissible in Hyperspectral Mode.
By summing the charges of adjacent CCD rows in this way, the number of spectral bands is reduced, integration time is shortened (allowing smaller along-track pixel resolutions for a given aircraft speed), and the width of each band is increased (increasing signal).
The relationship between the row summation factor in Hyperspectral Mode operation of the CASI and number of spectral bands is as follows:

Rows Summed CASI Spectral Bands
1 288
2 144
3 96
4 72
6 48
8 36
9 32
12 24

See also Hyperspectral Mode.


Roll, pitch, and heading.

Run Screen

This is the program window area within the instrument controller software used by the operator to control data collection in real-time. This screen contains the image and signal displays, and important diagnostic values used to monitor the quality of the data being acquired.

SASI (Commercially available since 2003)

Shortwave Airborne Spectrographic Imager. High performance, MCT-based SWIR hyperspectral sensor based on the pushbroom imaging concept. The SASI combines the better features of aerial photography and digital satellite imaging with the analytic capabilities of a spectrometer. Acquires 100 bands of spectral coverage across 0.95 to 2.45 microns, overlapping spectrally with the VNIR CASI. The SASI-600 features 600 across-track imaging pixels and features true diffraction-limited, custom optics. Designed and produced commercially by ITRES Research Limited of Calgary, Alberta, Canada. Tel: 403-250-9944; Web:

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.

Scattered Light

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 CASI that is removed during radiometric correction.


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.


Spectral and Radiometric Calibration software provided by ITRES with the hyperspectral and thermal calibration system. GUI-based, SpaRCalTM guides the user through the processing of lab-based calibration data collected using a given ITRES imager, towards the creation of the radiometric sensitivity coefficient (RSC) files. These files (known as rad files; *.rad) are later applied during standard processing to radiometrically correct and calibrate acquired raw image data. See also RSC; Radiometric Correction.

Spatial Mode (or Spectral Mode)

Unlike other ITRES imagers, the CASI can operate in two modes, Spatial or Hyperspectral. Spatial mode (sometimes called Spectral Mode) is most often chosen when maximum spectral programmability is desired. This mode allows the operator to program each spectral band uniquely, choosing the width and placement of each separately. Overlapping bands are not allowed. Bandwidths may vary, and data gaps between bands are possible. Between 1 to 288 bands may be programmed separately and the full number of available across-track imaging pixels (1500 for the CASI-1500) are acquired by default.
Because fewer spectral bands may be programmed for data collection in this band as compared to Hyperspectral Mode, shorter integration times can result, enabling smaller pixel resolutions to be acquired for a given aircraft speed. See also Hyperspectral Mode; Integration Time.

Spectral Band

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).

Spectral Mode

See Spatial Mode.

Spectral Radiance Units (SRU)

The units used for radiometrically calibrated instrument image data, where:

1 SRU = 1 W cm-2 sr-1 nm-1 and
1 SRU = 1000 DN
See also Spectral Irradiance Unit (SIU).

Spectral Signature

The unique profile for a specific target type (e.g. 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. See also SRU.

Spectral Smile

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.

Standard Processing

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. The ITRES SASI is a hyperspectral SWIR imager.

TABI (Commercially available since 2003)

Thermal Airborne Broadband Imager. High performance, MCT-based midwave thermal IR broadband sensor based on the pushbroom imaging concept. The TABI combines the better features of aerial photography and digital satellite imaging. Acquires a single spectral band across 3-5 microns, the TABI-1800 features 1800 across-track imaging pixels and features true diffraction-limited, custom optics. Designed and produced commercially by ITRES Research Limited of Calgary, Alberta, Canada. Tel: 403-250-9944; Web:

TASI (Commercially available since 2006)

Thermal Airborne Spectrographic Imager. High performance, MCT-based hyperspectral thermal sensor based on the pushbroom imaging concept. The TASI combines the better features of aerial photography and digital satellite imaging with the analytic capabilities of a spectrometer. Acquires 32 bands of spectral coverage across 8-11.5 microns, the TASI-600 features 600 across-track imaging pixels and features true diffraction-limited, custom optics. Designed and produced commercially by ITRES Research Limited of Calgary, Alberta, Canada. Tel: 403-250-9944; Web:

Tie Point

A feature identifiable in the overlapping regions of two adjacent georeferenced flight lines used in generating a bundle adjustment solution for a specific instrument installation. Tie points do not necessarily have a known ground coordinate. See also Ground Control Point.

TIN (Triangulated Irregular Network)

A representation of a surface as a set of contiguous, non-overlapping triangles. The vertices of the triangles form irregularly spaced nodes. Unlike a grid, the TIN allows the presentation of dense information in complex areas, and sparse information in simpler or more homogeneous areas.
Each sample point in the TIN has an X and Y coordinate and a surface, or Z value. These points are connected by edges, forming the triangles that represent the surface.


Thermal Infrared. A spectral region covering longwave infrared wavelengths between ~10 to 12 microns. Rather than reflection, what is measured in this wavelength region is emissivity. The ITRES TASI is a hyperspectral TIR imager.


See Course Over Ground.


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.

UTM (Grid)

Universal Transverse Mercator. A two dimensional, horizontal grid coordinate system defining positions measured in metres east and north (or south) of a reference point. Based on the Transverse Mercator projection. This is the default coordinate system used to georeference ITRES 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.

Visible Wavelengths

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).
See also VNIR, SWIR, MWIR, and Thermal IR


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). The ITRES CASI is a hyperspectral VNIR imager.


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.

Zero GPS Epoch

The reference time from which GPS time is measured. This date is January 6, 1980, 00:00:00. See also GPS Week Number; GPS Time.