Radar Glossary

Glossary of Radar Terminology

RADAR: Electromagnetic sensor characterized by RAdio Detection And Ranging, from which the acronym RADAR is derived. Predicted in the early part of the 20th century, the first important system was built in England in 1938. Basic building blocks of a radar are the transmitter, the antenna (normally used for both transmission and for reception), the receiver, and the data handling equipment. A synthetic aperture radar system, by implication, includes an image processor, even though it may be remotely located in time or space from the radar electronics.
RADAR CROSS SECTION (RCS): Measure of radar reflectivity, expressed in terms of the physical size of an hypothetical uniformly scattering sphere that would give rise to the same level of reflection as that observed from the sample target. (See sigma.)
RADAR EQUATION: Mathematical expression that describes the average received signal level (or, sometimes, the image signal level), compared to the additive noise level, in terms of system parameters. Principal parameters include transmitted power, antenna gain, noise power, and radar range R. The range effect is sometimes called the spreading factor, since effective power decreases significantly with a small increase in range. All else equal, the power received by a SAR per image pixel is proportional to .
RADARSAT: Satellite to be launched by Canada in 1995. It will carry a C-band SAR, HH polarization, and incidence angles spanning (20° - 60°) selectable in a variety of modes. [See Proceedings of the IEEE, June 1991].
RADIANS: Convention for describing the size of an angle with respect to =3.141592..., rather than with respect to "degrees". Conversion between degrees and radians uses the equivalence .
RADIATION: Act of giving off electromagnetic energy, particularly by the antenna of a radar when excited by the transmitter.
RADIOMETRIC RESOLUTION: The expected spread of variation in each estimate of scene reflectivity as observed in an image. Smaller radiometric resolution is "better". Radiometric resolution for a given radar may be improved by averaging, but at the cost of spatial resolution. (See looks.)
RANGE: Line of sight distance between the radar and each illuminated scatterer (see one-way). In SAR usage, the term also is applied to the dimension of an image away from the line of flight of the radar. (See slant range and ground range.)
RANGE AMBIGUITIES: Unwanted echoes that fall into the image from ranges that in fact are outside of the intended swath, due to the range sampling operation of the radar. Range ambiguities may be minimized by antenna pattern and imaging mode control, are observed only rarely in imagery from well designed systems.
RANGE CURVATURE: Describes the changing distance between the radar and an object during the time that the object is illuminated by the antenna. Range curvature is more important for long range systems such as satellite SARs, and must be compensated in the processor as a part of image focusing.
RANGE RESOLUTION: Resolution characteristic of the range dimension, usually applied to the image domain, either in the slant range plane or in the ground range plane. Range resolution is fundamentally determined by the system bandwidth in the range channel.
REFLECTIVITY: Property of illuminated objects to reradiate a portion of the incident energy. Reflectivity, in general, is larger in the specular direction for smaller surface roughness. For side looking radars, backscatter is the observable portion of the energy reflected. Backscatter, in general, is increased by greater surface roughness. In general, reflectivity is increased for higher conductivity of the scattering surface. The relative strength of radar reflectivity is tabulated by sigma, for discrete objects, and by sigma nought for natural terrain surfaces.
REFLECTIVITY, COMPLEX COEFFICIENT OF: Ratio of the complex amplitude of the reflected electric component to the incident electric component of an electromagnetic wave at a surface orthogonal to the incoming illumination.
RELIEF DISPLACEMENT: Alternative term for elevation displacement.
RESOLUTION: Generally (but loosely) defined as the width of the "point spread function", the "Green's function", or the " impulse response function", depending on whether one has an optics, a physics, or an electronic systems background. More properly, "resolution" refers to the ability of a system to differentiate two image features corresponding to two closely spaced small objects in the illuminated scene when the brightness of the two objects in question are comparable and fall within the dynamic range of the radar in question. (Definition adapted from Lord Rayleigh [1879]). "Higher resolution" refers to a system having a smaller impulse response width.
RESOLUTION CELL: A three-dimensional cylindrical volume surrounding each point in the scene. The cell range depth is slant range resolution, its width is azimuth resolution, and its height, which is conformal to the illumination wavefront, is limited only by the vertical beam width of the antenna pattern. Resolution cell often is defined with respect to the local horizontal. (See ground range).
ROLL: Rotation of a sensor platform around the flight vector, hence in a "wing down" direction.
ROUGHNESS: Variation of surface height within an imaged resolution cell. A surface appears "rough" to microwave illumination when the height variations become larger than a fraction of the radar wavelength. The fraction is qualitative, but may be shown to decrease with incidence angle.

SAR: Synthetic Aperture Radar, so-called because azimuth resolution is achieved through computer operations on a set of (coherently recorded) signals such that the processor is able to function like a large antenna aperture in computer memory, thus realizing azimuth resolution improvement in proportion to aperture size. The SAR concept was introduced by C. Wiley (USA) in 1951.
S-BAND: Microwave band in which the wavelengths are at or near 10cm.
SCATTERING MATRIX: Array of four complex numbers that describe the transformation of the polarization of a wave incident upon a reflective medium to the polarization of the backscattered wave. It is the polarization vector counterpart to the coefficient of reflectivity.
SCENE: Object space illuminated by the radar.
SEASAT: NASA satellite that was in operation July-September of 1978. Seasat was the first (civilian) satellite to carry a SAR. It operated at L-band, using horizontal polarization at 22° incidence angle. Data from Seasat is still important for applications and processing technique development.
SENSITIVITY TIME CONTROL (STC): Pre-programmed change in radar amplitude due to weaker backscatter from greater ranges and varying incidence angles across the imaged swath.
SHADOW: From an optical point of view as seen from the position of a radar, a region hidden behind an elevated feature in the scene would be out of sight. This region corresponds to that which does not get illuminated by the radar energy, and thus is also not visible in the resulting radar image. The region is filled with "no reflectivity", which appears as small digital numbers, or a dark region in hard copy.
SIDELOBES: Non-zero levels in a distribution that are separated from the desired central response. Sidelobes arise naturally in antenna patterns, for example, although in general they are a nuisance, and must be suppressed as much as possible. Large side-lobes may lead to unwanted multiple images of a single feature.
SIGMA (): The conventional measure of the strength of a radar signal reflected from a geometric object (natural or manufactured) such as a corner reflector. Sigma specifies the strength of reflection in terms of the geometric cross section of a conducting sphere that would give rise to the same level of reflectivity. (Units of area, such as metres squared). (See radar cross section.)
SIGMA NOUGHT (): Scattering coefficient, the conventional measure of the strength of radar signals reflected by a distributed scatterer, usually expressed in dB. It is a normalized dimensionless number, comparing the strength observed to that expected from an area of one square metre. Sigma nought is defined with respect to the nominally horizontal plane, and in general has a significant variation with incidence angle, wavelength, and polarization, as well as with properties of the scattering surface itself. (See speckle, statistics.)
SIGNAL: Generalized terminology used to signify a mathematical description of a wave, pulse, or other sequence of interest. It often suggests the ensemble of data corresponding to observed scattering from the scene, either before reception, within the radar or processor, or in the image file. Normally there is a distinction between "signal" and noise.
SIGNAL-TO-NOISE RATIO: Quantitative basis for comparing the relative level of a desired signal, such as a SAR image, to unwanted elements, traditionally taken to be additive noise. The signal-to-noise ratio may be increased (improved) by increasing the amount of signal power.
SIR-A and SIR-B: NASA sponsored radar missions in the Shuttle, each lasting about one week. SIR-A (November 1981) was at L-band, HH polarization, nominally 50° 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 20° to 50°, and was digitally processed.
SIR-C/X-SAR: A Shuttle radar being built for missions in 1993, 1994, and 1996. It will carry a quadrature polarimetric SAR at C- and L-bands, and an X-band HH polarized SAR (contributed by Germany and Italy). It will offer a variety of incidence angle, wavelength band selection, resolution, and polarization modes.
SLANT RANGE: Image direction as measured along the sequence of line-of-sight rays from the radar to each and every reflecting point in the illuminated scene. Since a SAR is looks down and to the side, the slant range to ground range transformation has an inherent geometric scale which changes across the image swath. (See ground range.)
SPECKLE: Statistical fluctuation or uncertainty associated with the brightness of each pixel in the image of a scene. A single look SAR system achieves one estimate of the reflectivity of each resolution cell in the image. Speckle may be reduced, at the expense of resolution, in the SAR processor by using several looks. Speckle appears as a multiplicative random process whose variance and spatial correlation are determined primarily by the SAR system.
SPECULAR: Coherent reflection from a smooth surface in a plane normal to the surface at an angle opposite to the local incidence angle. (From speculum, mirror in Latin.)
SPEED OF LIGHT (c): Approximately 300,000,000 metres per second, the speed of light in "free space", a condition typical of electromagnetic propagation through most atmospheric conditions found on Earth. Denser media, such as the atmosphere of Venus, that have a low loss dielectric constant, retard the speed of propagation according to their material properties.
STATISTICS: Set of numbers that describes average properties of a random process, such as sigma nought, the reflectivity observed from a nominally uniform scattering surface distributed in two dimensions, say x and y. Each observation of is a sample function having a variety of values at each location due to speckle, whose probability distribution function is determined primarily by the number of independent looks used in the processor. The average value of the corresponding image brightness, for calibrated data, is (mean) reflectivity , and the average spread in brightness values is given by the standard deviation, approximately given by .
STOKES MATRIX: 4x4 array of real numbers that describes the transformation of the Stokes parameters of the incident wave into the Stokes parameters of the electromagnetic wave reflected by each element of a scene illuminated by a radar. The Stokes matrix describes the complete polarization signature of the reflective medium.
STOKES PARAMETERS: Set of four real numbers that together describe the state of polarization of an electromagnetic wave.
SWATH: Width of the imaged scene in the range dimension, measured either in ground range or in slant range.

TEXTURE: Second order spatial average of brightness. Scene texture is the spatial variation of the average reflectivity. For areas of nominally constant average reflectivity, image texture consists of scene texture multiplied by speckle.
TIME-BANDWIDTH PRODUCT: Parameter (TBP) found from the width of a distribution in the time (or space) domain multiplied by the width of the same distribution observed in the frequency domain. (Typically, the azimuth (Doppler modulated) signal and the range chirp coded pulse each have TBP larger than 100.)
TONE: First order spatial average of image brightness, often defined for a region of nominally constant average reflectivity.
TRANSMISSION: Energy sent by the radar, normally in the form of a sequence of pulses, to illuminate a scene of interest.
TRIHEDRAL: Corner reflector formed from three mutually orthogonal surfaces.

VOLTAGE: Standard unit of magnitude of an electrical signal, named after Count A. Volta, inventor of the battery (about 1800).
VOLUME SCATTERING: Multiple scattering events occurring inside a medium, generally neither dense nor having a large loss tangent, such as the canopy of a forest. The relative importance of volume scattering is governed by the dielectric properties of the material.

WAVE: Propagating periodic displacement of an energy field. A surface wave on the water serves to visualize the key properties of an electromagnetic wave. At any instant of time, a wave is described by its "height" (amplitude) and its "length" (wavelength). Equally important is the phase of the wave, which is the number that describes the position of the "crests" or "troughs" with respect to a given reference position. At any specific location in space, propagation of the wave occurs. From this perspective, its frequency may be observed. A wave propagates within a given medium at a speed given by the product of its wavelength and its frequency. In radar, waves are very well represented by families of sinusoidal functions, so-called harmonic oscillation.
WAVEFRONT: Three dimensional surface in space for which the field radiated by an antenna has the same phase at all points. At a large distance R from an antenna, the wavefront is a spherical surface with radius R over the angular window established by the antenna pattern. For most geometries encountered in remote sensing, the wavefront may be approximated by a plane tangent to the spherical surface, within a tolerance of much less than a wavelength over a spatial scale of several resolution cells.
WAVELENGTH: Minimum distance between two events of a recurring feature in a periodic sequence, such as the crests in a wave. (Units of length, such as metres).
WAVENUMBER (k): By convention, the ratio , where is the wavelength.
WIDTH, EQUIVALENT RECTANGLE: A standard definition to measure the effective width of a distribution. The width is that of a rectangular distribution with the same amplitude as the maximum of the distribution, and having the same area in the rectangle as is in the measured distribution.
WIDTH, 3dB: Width of a distribution equal to the distance between the outer two points on the distribution having power level half of that at the peak.

X-BAND: Microwave band with wavelength at or near 3cm.
YAW: Rotation of a sensor platform in the horizontal plane, hence in a "nose right" direction.