7#,;|;(****:VVhxr *mL0 GLOSSARY  Absorption: Penetration of electromagnetic waves into a medium and their subsequent reduction in strength governed by dielectric properties of the material. AIRSAR: An acronym for the NASA/JPL airborne imaging radar system. This imaging radar is capable of simultaneously collecting data at three different frequencies (C-, L- and P-band) and at all different polarizations. Alluvial fan: A fan-like surface layer of eroded rocks and sand, usually found at the foot of a mountian range. Altitude: Vertical elevation of an object above a surface or sea level. Amplitude: Measure of the strength of a signal (units in volts). Antenna: Part of the radar system which transmits and/or receives electromagnetic energy. ASI: An acronym for the Italian Space Agency. Aspect Angle: A description of the geometric orientation in the horizontal plane of the object with respect to the illuminating or transmitted radar beam. AVHRR: An acronym for the Advanced Very High Resolution Radiometer, a low-resolution (1kmx1km pixels) infra red imaging satellite launched by NASA.. Azimuth: The angular position of an object within the field of view of an antenna in the plane intersecting the moving radar's line of flight. The term is commonly used to indicate linear distance or image scale in the direction parallel to the radar flight path. (In an image, azimuth is also known as the along-track direction).  Backscatter: The (microwave) signal reflected by elements of an illuminated scene back towards the radar. It is named to make clear the difference between energy scattered in arbitrary directions, and that which returns to the radar and therefore may be received and recorded by the sensor. Bandwidth: A measure of the span of frequencies available in the signal, or passed by the band limiting stages of the system. Bandwidth is a fundamental parameter of any imaging system, and determines the ultimate resolution available. Beamwidth: An angular measure of the width of the radiation pattern or beam of an antenna. For imaging radar, both the vertical beamwidth (affecting the width of the illuminated swath) and the horizontal or azimuth pattern (which determines, indirectly, the azimuth resolution) are frequently used concepts. BFPQ: An acronym for Block Floating Point Quantizer, a form of data compression used to reduce the number of bits per sample, notably on Magellan and SIR-C. Brightness: Property of a radar image in which the strength of the radar reflectivity is expressed as being proportional to a DN or digital number (digital image file) or to a gray scale (photographic image) which for a photographic positive show 'bright' as 'white'.  Cartography: The making of maps. C-band: Microwave band in which the wavelengths are at or near 5.6 cm (5.3 GHz). CD-ROM: Compact Disc - Read Only Memory, a convenient way of storing image data files, with a capacity of 600 Mbytes. Circulator: A switch used in many types of radar systems. Classification: The process of assigning classes or types to image pixels. As an example, if the DN value of a pixel is greater than 128, it may be assigned to class A; if the DN value is less than or equal to 128, it may be assigned to class B. Applying this process to an entire image, it is classified into classes A and B. Corner Reflector (dihedral): Combination of two orthogonal intersecting reflecting surfaces that combine to enhance the signal back in the direction of the radar (see Double-bounce). Corner Reflector (trihedral): Intersection of three mutually orthogonal reflecting surfaces. Returns are very strong.  DARA: An acronym for the German Space Agency. Deforestation: Cutting down of forests. Desertification: The process in which land turns into desert. Detection: Processing stage at which the strength of the signal is determined. Detection removes phase information from the data. DLR: An acronym for the German Institute for Aerospace Research. DN: Digital Number, quantifying the brightness of a pixel. Doppler (frequency): Shift in frequency caused by relative motion along the line of sight between the sensor and the observed scene. Double-bounce: A type of scattering, in which the electromagnetic wave is reflected by two surfaces at right angles to each other. Dynamic Range: The ratio of largest measurable signal to minimum detectable signal.  Electromagnetic Spectrum: The ordered array of known electromagnetic energy extending from the shortest rays, through gamma rays, X rays, UV, visible, infra red and including microwave and all other wavelengths of radio energy. Electromagnetic Wave: A wave described by variations in electrical and magnetic fields. All such waves move through the atmosphere at the speed of light, 3.0 x 108 m per second. EOS SAR: Satellite proposed by JPL to carry a three frequency (L, C and X-bands) polarimetric SAR for the Earth Observation Satellite series. If approved by NASA, it would be operational after the year 2000. ERS-1: Satellite launched by ESA in July 1991. The main instrument (AMI) includes a C-band SAR, VV polarization and 23o incidence angle. ESA: European Space Agency, with headquarters in Paris, France.  Footprint: The area on the ground that is illuminated by a beam of electromagnetic radiation. Foreshortening: Distortions related to the imaging geometry of the radar system through which terrain slopes facing the antenna appear smaller (compressed) in the resulting imagery. Frequency: Rate of oscillation of a wave. In the microwave region, frequencies are on the order of .3 GHz - 300 GHz having wavelengths of 1 mm - 1 m respectively.  Galileo: a NASA space probe to Jupiter, launched in 1989. Geology: The study of rocks and minerals. GHz: Gigahertz (109 cycles per sec). A measure of frequency of electromagnetic energy. Ground Truth: Data collected on the ground, often at the same time as or close to the time the area is imaged.  Histogram: A plot of the frequency of occurrence of values (DN or so) in an image. Hydrology: The study of water on land and underground.  Image: A pictorial representation acquired in any wavelength of the electromagnetic spectrum. For radar, the image pixel values represent the radar reflectivity of the scene. Incidence Angle: Defined as the angle between the line of sight (between the radar and object) and the vertical. The local incidence angle takes into account the slope of the terrain at the object's location. Incidence angle can have an important influence on radar backscatter. Infra red: A type of electromagnetic radiation, usually associated with heat. Interferometer: Device such as an imaging radar that uses two different paths for imaging, and deduces information from the coherent interference between the two signals. In SAR applications, spatial interferometry has been demonstrated to measure terrain height, and time delay interferometry is used to measure movement in the scene such as oceanic currents. Isotropic: In all directions.  J-ERS-1: Japanese satellite launched February, 1992. L-band SAR, HH polarization, 38.5o incidence angle and optical sensor.  L-band: Microwave band in which the wavelengths are at or near 23.5 cm (1.0 GHz). LANDSAT: A series of optical/infra red imaging satellites launched by NASA. Layover: Displacement of the top of an elevated feature with respect to its base on the radar image (Extreme form of foreshortening). Look Direction: The angle between geographic North and the direction in which the radar beam is pointing i.e. perpendicular to the flight direction.  Macsigma0: A software program for displaying and analyzing radar images on the Macintosh computer. Magellan: A NASA/JPL space probe to Venus which mapped over 98% of the planet's surface using imaging radar. Meteorology: The study of weather. MHz: Megahertz (106 cycles per sec). A measure of frequency of electromagnetic energy. Microwave: Electromagnetic wavelength 1 m - 1 mm (.3 GHz - 300 GHz). The most common imaging radars operate at frequencies between 24 cm - .85 cm (1.25 GHz - 35.2 GHz).  Nadir: Point on the surface of the Earth directly below the radar source. NASA: National Aeronautics and Space Administration, headquarters in Washington, DC, USA. Near Polar Orbit: Orbital plane within 10o of a plane containing true North (90o latitude). NIH Image: A general purpose image processing program for the Macintosh computer. Normalized RCS (so): A measure of backscatter used by radar scientists (dimensionless). Non-isotropic: towards a particular direction or region.  Oceanography: The study of the oceans. Optical: Electromagnetic radiation at wavelengths visible to the human eye.  P-band: Microwave band in which the wavelengths are at or near 68 cm (440 MHz). Phase: A particular appearance or state in a regularly recurring cycle of changes. In electromagnetic waves, the phase ranges from -180o to + 180o. Photon: A particle of light. Pixel: Any of the small discrete elements that together constitute an image. A resolution cell on satellite imagery having both spatial and spectral characteristics and represented by a digitized value; an abbreviation of 'picture element'. Polarization: Orientation and ellipticity of the electric (E) vector in a propagating electromagnetic wave. Imaging radars are able to generate and receive with the same or different polarization. HH - horizontal transmit/horizontal receive VV - vertical transmit/vertical receive HV - horizontal transmit/vertical receive VH - vertical transmit/horizontal receive Pulse: A short burst of electromagnetic energy.  Radar: An acronym for Radio Detection and Ranging. The science of locating and/or identifying distant objects by means of radio techniques. Radar depends on two processes: the reflection of radar waves by material energy, and the use of short pulses of high frequency energy to make possible the accurate measurement of distance. Radar Cross Section (RCS): A measure of backscatter used by radar scientists (units are square meters). Radar Images: A pictorial representation of the reflected microwave energy received at the antenna. The image pixel values represent the amount of reflected energy received from the surface. Radar Pulse: A packet or burst of microwave radiation transmitted by a radar. Radar Shadow: A dark area of no return on a radar image resulting from an elevated object, such as a mountain ridge, because the object cuts off the radar beam and prevents illumination of the area behind it. RADARSAT: Canadian remote sensing C-band SAR satellite to be launched in 1995. RAM: random access memory. Range: Line of sight distance between the radar and each illuminated scatterer. In imaging radar, the term is also applied to the dimension of an image at right angles to the flight path of the radar. Slant range is the distance as measured by the radar directly, in effect along each line perpendicular to the flight vector and directly connecting the radar and each scatterer. Ground range is the same distance projected (through a geometrical transformation) onto the ground. Reflectivity: Property of illuminated objects and surfaces to re-radiate a portion of incident energy. Remote Sensing: A technique of acquiring information about an area or object from a distance. Resolution: The smallest discernible unit or the smallest unit represented. In satellite imagery it refers to the smallest object that can be discerned. Also referred to as spatial resolution. Resolution in an imaging radar system differs in two directions: the azimuth or along-track direction and the range or across-track direction.  SAR: Synthetic Aperture Radar: A type of imaging radar. SAR systems use the motion of the aircraft/satellite and Doppler frequency shift to electronically synthesize a large antenna in order to obtain high resolution. Scattering: Reflection of an electromagnetic wave in many directions. Scattering Mechanism: The way in which electromagnetic waves are scattered. SEASAT: NASA satellite that was in operation July - September of 1978. SEASAT was the first (civilian) imaging radar satellite. It operated at L-band, using horizontal polarization at 22o incidence angle. Data from Seasat is still important for applications and processing technique development. SIR (-A and -B): NASA sponsored radar missions flown on the Shuttle, each lasting about one week. SIR-A (November 1981) was an L-band, HH polarization imaging radar, with nominally 50o incidence angle and was optically processed. SIR-B (October 1984) was also at L-band, HH polarization, offered a variety of incidence angles from about 20o to 50o, and was digitally processed. SIR-C/X-SAR: A Shuttle radar being built for missions in 1994 and 1995. It will carry a SAR with different polarizations at C- and L-bands, and an X-band HH polarized SAR (contributed by Germany and Italy). It will offer a variety of incidence angles, band selections, resolutions and polarization modes. Slotted Waveguide: a type of antenna used in radars, notably X-SAR. Specular: A specular surface is one that is smooth at the wavelength of illumination, having the qualities of a mirror. SPOT: An acronym for the Optical/Infra-red imaging satellite operated by the French Space Agency. Notable for its high resolution (10m) and stereo capabilities. SRL: An acronym for NASA's Spaceborne Radar Laboratory, which includes SIR-C/X-SAR. Stereo: A technique for combining image pairs of the same site, viewed from different directions, to produce a 3-D image (by finding parallax). Stokes Matrix: Data format for NASA/JPL AIRSAR radar images. Radar images of different polarizations can easily be generated from this data format. Supersite: A site selected by the SIR-C/X-SAR Science Team for priority coverage. Swath (width): Total field of view. The overall plane angle or linear ground distance (width) of the imaged scene in the range or across-track direction.  TOPSAT: An imaging radar mission proposed by JPL to NASA for an interferometric radar to map topography over the whole earth. Trajectory: The path followed by a spacecraft. Transmission: Energy sent by the radar, normally in the form of a sequence of pulses, to illuminate a scene of interest.  Volume Scattering: Scattering occurring at more than one interface, such as in a forest canopy or by buried objects. The relative importance of volume scattering is controlled by the dielectric properties of the material.  Wavelength: The distance travelled by a photon (particle of light) during one complete cycle (wave).  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