Radar Glossary

Glossary of Radar Terminology

GAIN: Change in signal level due to processing functions that increase the magnitude of the signal. Examples include: signal amplification in a radar receiver; processing gain in the processor; and antenna gain, a result of the directivity of the pattern.
GAUSSIAN: The classical distribution characterized by a "bell-shaped" curve, it plays several roles in SAR. For example, it is the "normal" probability distribution that describes the in-phase and the quadrature components of the signal corresponding to a purely diffuse scattering surface, which are sometimes described as Gaussian scatterers.
GROUND RANGE: Range direction of a side-looking radar image as projected onto the nominally horizontal reference plane, similar to the spatial display of conventional maps. For spacecraft data, an Earth geoid model is used, whereas for airborne radar data, a planar approximation is sufficient. Ground range projection requires a geometric transformation from slant range to ground range, leading to relief or elevation displacement, foreshortening, and layover unless terrain elevation information is used.

HDDT: High density digital tape.
HERTZ: Named after H. R. Hertz, a 19th century German physicist, it is the standard unit for frequency, equivalent to one cycle per second.
HISTOGRAM: Graph which plots number of samples versus digital number (the statistical distribution of brightness) of data selected from a region of an actual image file.

IMAGE: Mapping of the observed radar reflectivity of a scene. For radars with digital image processing, the image consists of a file of digital numbers assigned to spatial positions on a grid of pixels, and presented either as hard copy (such as a photographic print) or soft copy (such as a digital data record). All radar images are subject to statistical variations, mainly speckle and noise, which must be accommodated in either visual or numerical image interpretation. The most commonly used image formats occur after detection. After calibration (and compensation for speckle and noise effects), image files from magnitude squared detection are proportional, on average, to sigma nought . Magnitude scaling (formed by taking the square root of the detected, look-summed file to yield an image proportional to is the "standard" for most SAR image files. A magnitude image often yields a photographic copy that is more readily interpreted visually, and requires less dynamic range and data storage space. A digital SAR image file may be retained in complex format (before detection) for specialized applications.
IMPULSE RESPONSE: Also known as the point spread function, it is the two-dimensional brightness pattern in an image (after processing) corresponding to the signal reflected by an object whose sigma falls within the dynamic range of the system, and for which the width of the imaged pattern is determined by the radar and processor rather than by the size of the object. (A trihedral corner reflector is the most commonly used object for generating an impulse response in a test image.) A "good" impulse response has a relatively large value for the pixel that maps the point scatterer location, and very small values for all surrounding pixels. The impulse response is a basic building block in describing a given radar's imaging performance, since an image is built up from the linear combination of impulse responses from all individual scatterers illuminated by the radar. The impulse response width (IRW, or resolution) of the central peak is the most important characteristic of the impulse response, together with the shape of the impulse response distribution both close to and remote from its centre.
INCIDENCE ANGLE: Angle between the line of sight from the radar to an element of an imaged scene, and a vertical direction characteristic of the scene. The definition of "vertical" for this purpose is important. One must distinguish between the (nominal) "incidence angle" determined by the large scale geometry of the radar and the Earth's geoidal surface, and the local incidence angle which takes into account the mean slope with each pixel of the image. Smaller incidence angle refers to viewing line of sight being closer to the (local) vertical, hence "steeper". (See aspect angle.) In general, reflectivity from distributed scatterers decreases with increasing incidence angle.
INCOHERENT (or noncoherent): Property of a signal or data set in which the phases of the constituents are not statistically correlated, or systematically related in any fashion. The power of incoherent signals si combine according to the power of each of the individual signals, (See coherent.)
IN-PHASE (I): Component of the signal that has the same phase as the complex reference frequency.
INTENSITY: Strength of a field or of a distribution, such as an image file, proportional to magnitude, squared.
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.

J-ERS-1: Satellite launched by Japan in February 1992. It includes an L-band SAR, HH polarization and 38.5o incidence angle. [See Proceedings of the IEEE, June 1991].
JPL: Jet Propulsion Laboratory, Pasadena, California. Over the years JPL and their airborne radar systems have established themselves as one of the world's leaders in civilian SAR technology development.

L-BAND: Microwave band in which the wavelengths are at or near 23.5cm.
LAYOVER: Extreme form of elevation displacement or foreshortening in which the top of a reflecting object (such as mountain) is closer to the radar (in slant range) than are the lower parts of the object. The image of such a feature appears to have fallen over towards the radar. The effect is more pronounced for radars having smaller incidence angle.
LINEARITY: Property according to which an operation on a sum of signals is equivalent to the same operation applied to each of the signals individually, and the resulting numbers added together. If C is a multiplicative constant, then a linear operation on any two numbers x and y satisfies . (The additive constant is needed to account for realistic behaviour of many practical systems that may impose a constant offset onto the sum.) Linearity, over the dynamic range of the system, is an essential attribute of most measurement devices such as an imaging radar.
LOOKS: Each of the sub-images used to form the output summed image, implemented in a SAR processor. Speckle, the radiometric uncertainty in each estimate of the scene's reflectivity, is reduced by the averaging implied by adding together different detected images of the same scene. For N statistically independent looks (which may be implemented in various ways), the standard deviation of each estimate is reduced by . Multiple looks may be generated by averaging over range and/or azimuth resolution cells. For an improvement in radiometric resolution using multiple looks there is an associated degradation in spatial resolution. Note that there is a difference between the number of looks physically implemented in a processor, and the effective number of looks as determined by the statistics of the image data.
LOSS TANGENT: Ratio of the imaginary part of the dielectric constant to the real part, written as tan . Low loss materials satisfy .

MAGNITUDE: One of three parameters required to describe a wave. Magnitude is the amplitude of the wave irrespective of the phase. For a complex signal described by in-phase (I) and quadrature (Q) components, the magnitude is given by . For complex amplitude a, magnitude is, by definition, . (See detection.)
MATCHED FILTER: Mathematical model of the detailed structure of a specific two-dimensional distribution, applied in a processor to cancel the phase structure of the desired set of signals. The matched filter (first derived by North in 1942) maximizes the signal-to-noise ratio of the processor output when the input is a known signal against an additive noise background.
MICROWAVE: An electromagnetic wavelength in (or near) the span 1-100 cm.
MULTI-LOOK: (See looks.)
MOTION COMPENSATION: Adjustment of a radar system and/or the recorded data to remove effects of radar platform motion, including rotation and translation, and variations in along track velocity. Motion compensation is essential for aircraft SARs, but usually is not needed for spacecraft SARs.

NADIR: Locus of points on the surface of the Earth directly below the radar as it progresses along its line of flight.
NASA: National Aeronautics and Space Administration, with headquarters in Washington D.C., USA.
NOISE: Any unwanted or contaminating signal competing with the desired signal. In a SAR, two common kinds of noise are additive (receiver) noise and signal dependent noise, usually either additive or multiplicative. The relative amount of additive noise is described by the signal-to-noise ratio. Signal dependent noises, such as azimuth ambiguities or quantization noise, arise from system imperfections, and are dependent on the strength of the signal itself. "Good" SAR systems usually keep these noise levels below acceptable levels, by design. (Speckle is sometimes considered to be a kind of signal dependent multiplicative noise in a SAR system.)
NOISE EQUIVALENT SIGMA NOUGHT: A measure of the sensitivity of a given SAR. It describes the strength of the (additive) system noise in terms of the equivalent (average) power in the image domain that would result from an idealized distributed scatterer of the stated reflectivity. Smaller noise equivalent sigma nought values are better. Within physical limitations, smaller may be achieved by increasing the power of the radar transmitter, or by decreasing the noise figure of the electronics.
NOISE FIGURE: Factor that describes the noise level in a radar receiver relative to the that in a theoretically perfect receiver. The noise figure, which is always larger than one, is typically two or more, and is usually expressed in decibels.

ONE-WAY: The radar illuminates the scene through the transmit pattern of the antenna. It receives the backscattered energy through the receive pattern of the antenna. Thus the received pulse must travel two ways, out to each object at range R, and back again the same distance. Numbers relating to only one direction of propagation are denoted as "one-way", and the corresponding numbers that include the round trip are called "two-way". The difference is important in measuring effective antenna pattern widths, in signal phase, and in the relationship between two-way delay time t and range distance R, such that R = ct/2. (See speed of light.)

P-BAND: As has been adopted by the SAR community, the microwave band in which the wavelengths are at or near 75cm.
PARALLAX: Apparent change in the position of an object due to an actual change in the point of view of observation. For a SAR, true parallax occurs only with viewpoint changes that are away from the nominal flight path of the radar. In contrast to aerial photography, parallax cannot be created by forward and aft looking "exposures". Parallax may be used to create stereo viewing of radar images.
PENETRATION: Act of (microwaves) entering a medium such as dry sand or forest leaf canopy. Microwave penetration, in general, is proportional to the wavelength, and inversely proportional to the loss tangent. The penetration depth for most natural materials (except highly conductive media such as water) encountered in radar remote sensing is given by , where is the wavelength, and tan is the loss tangent.
PERIOD: Time duration of one cycle of a wave, or of one cycle of any regularly recurring pattern. Period is inversely equal to frequency. (Units of time, seconds).
PERMEABILITY: Parameter that describes the magnetic properties of a material. For remote sensing applications, (magnetic) permeability is essentially the same for all materials of interest, and plays an insignificant role in image interpretation.
PERMITTIVITY: (See dielectric constant.)
PHASE: The angle of a complex number.
PITCH: Vertical rotation of a sensor platform, in the "nose up" plane.
PIXEL: Term derived from "picture element" in a digital representation to indicate the spatial position of a sample of an image file, which consist of a spatial array of digital numbers. A two-dimensional ensemble of pixels forms the geometric grid on which an image is built. The fundamental parameter describing this grid is the inter-pixel spacing in each of the two image directions. (To confuse matters, pixel spacing is often referred to as "pixel" or "pixel size" in the literature. Pixel "size" is to be avoided.)
POLARIZATION: Orientation of the electric (E) vector in an electromagnetic wave, frequently "horizontal" (H) or "vertical" (V) in conventional imaging radar systems. Polarization is established by the antenna, which may be adjusted to be different on transmit and on receive. Reflectivity of microwaves from an object depends on the relationship between the polarization state and the geometric structure of the object. Common shorthand notation for band and polarization properties of an image file is to state the band, with a subscript for the receive and the transmit state of polarization, in that order. Thus, for example, LHV indicates L-band, horizontal receive polarization, and vertical transmit polarization. Possible states of polarization in addition to vertical and horizontal include all angular orientations of the E vector, and time varying orientations leading to elliptical and circular polarizations. (See quadrature polarization.)
POST-PROCESSING: Steps that may be applied to digital SAR image files to adjust selected attributes of the image, such as geometric accuracy or radiometric corrections, including speckle reduction and contrast enhancement, or any other form of value-added processing.
POWER: For a given signal, proportional to the magnitude, squared, per unit time. (Units are Watts.)
PROCESSING: Sometimes denoted "preprocessing", it is the means of converting the received reflected signal into an image. Processing consists of image focusing through matched filter integration, detection, and multi-look summation. The output files of a SAR processor usually are presented with unity aspect ratio (so that range and azimuth image scales are the same). Images may be either in slant range or ground range projection. Both of these spatial adjustments require resampling of the image file.
PROPAGATION: The movement of energy in the form of waves through space or other media. Electromagnetic waves move at thespeed of light c in free space, but the speed v of propagation through other materials is reduced according to the dielectric constant of the material in question, according to .
PULSE: Group of waves with a distribution confined to a short interval of time. Such a distribution is described in the time domain (or in spatial dimensions) by its width and its amplitude or magnitude, from which its energy may be found. In radar, use is made of modulated or coded pulses which must be processed to decode or compress the original pulse to achieve the impulse response observed in the image. Coded pulses have a time-bandwidth product that is much larger than unity. The resolution that may be achieved after processing is determined by the bandwidth of the original pulse.
PULSE REPETITION FREQUENCY (PRF): Rate of recurrence of the pulses transmitted by a radar.

QUADRATURE (Q): Signal component that is 90o out of phase with respect to the reference frequency.
QUADRATURE POLARIZATION ("QUAD POL") RADAR: System designed to simultaneously collect imaging data of a scene in two orthogonal polarization states on transmit and the same two polarization states on receive. From such a data set a complete scattering matrix of the reflectivity of the scene may be synthesized, leading to the concept of polarization signature. The best known example of a "quad pol" radar is the AirSAR of JPL.