This list of words is designed to include most of the specialized terminology arising in basic synthetic aperture radar (SAR) applications and image interpretation. Linked words indicate cross references within the glossary.

ALMAZ

Synthetic aperture radar (S-band) satellite launched by the USSR in May, 1991, which operated until October 1992.

ABSORPTION

Reduction in strength of an electromagnetic wave propagating through a medium, determined by dielectric properties of the material.

ALONG-TRACK

Dimension parallel to the path of the vehicle carrying the radar, sometimes called the cross range or azimuth direction for side-looking radars.

AMPLITUDE

Measure of the strength of a signal, and in particular the strength or "height" of an electromagnetic wave (units of voltage). Amplitude may imply a complex signal, including both magnitude and the phase.

ANTENNA

Device to radiate electromagnetic energy on transmission by a radar, and to collect such energy during reception. An antenna pattern is designed with spatial directivity, which concentrates the energy into a beam in both the vertical (elevation) and the horizontal (azimuth) directions. The electrical losses of an antenna together with its directivity determine the antenna gain. In general, the beamwidth in any plane is inversely proportional to the aperture width in that plane, and directly proportional to the wavelength of the radiation. Polarization on transmit and on receive is determined by the antenna.

ATTENUATION

Decrease in the strength of a signal, usually described by a multiplicative factor in the mathematical description of signal level. A signal is attenuated by application of a gain less than unity. Common causes of attenuation of an electromagnetic wave include losses through absorption and by volume scattering in a medium as a wave passes through.

ASPECT ANGLE

Description of the geometric orientation in the horizontal plane of an object in the scene with respect to the illuminating wavefront. (See incidence angle.)

AUTOMATIC GAIN CONTROL

Adaptive change in radar gain in the along-track direction, to compensate changes in average scene reflectivity.

AZIMUTH

The relative 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 commonly is used to indicate linear distance or image scale in the along-track direction.

AZIMUTH AMBIGUITIES

(See DOPPLER frequency).

AZIMUTH RESOLUTION

Resolution characteristic of the azimuth dimension, usually applied to the image domain. Azimuth resolution is fundamentally limited by the Doppler bandwidth of the system. Excess Doppler bandwidth is usually used to allow extra looks, at the expense of azimuth resolution.

BACKSCATTER

The (microwave) signal reflected by elements of an illuminated scene back in the direction of the radar. It is so named to make clear the difference between energy scattered in arbitrary directions, and that which returns to the radar and thus may be received and recorded by the sensor.

BANDWIDTH

A measure, according to a standard definition (see width), of the span of frequencies available in a signal or other distribution, or of the frequency limiting stages in the system. Typical bandwidths in the range channel of a SAR are on the order of 20 Megahertz, and in the azimuth channel are on the order of 1 Kilohertz. Bandwidth is a fundamental parameter of any imaging system, and determines the ultimate resolution available. For any pulse, the basic parameter that describes its structure is the time bandwidth product.

BEAMWIDTH

A measure, according to a standard definition, of the width of the radiation pattern of an antenna. For SAR applications, 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. Beamwidth may be measured in the one-way or two-way form, and in either voltage or power.

BETA NOUGHT ()

Radar brightness coefficient; the reflectivity per unit area in slant range, dimensionless. (See sigma nought).

BRAGG SCATTERING

Enhanced backscatter due to coherent combination of signals reflected from a rough surface having features, with periodic distribution in the direction of wave propagation, and whose spacing is equal to half of the wavelength as projected onto the surface.

BRIGHTNESS

Property of a radar image (digital or optical) in which the observed strength of the radar reflectivity is expressed as being proportional to a digital number (digital image file) or to a gray scale mapping, which, for a photographic positive, shows "bright" as "white".

CALIBRATION

Process whereby one may relate the digital numbers describing an image to physical quantities such as reflectivity, geometry (position or size), or phase.

C-BAND

Microwave band in which the wavelengths are at or near 5.6 cm.

CCRS

Canada Centre for Remote Sensing, Ottawa, Ontario. CCRS is the leading centre in Canada for the development of imaging radar and other remote sensing applications and technology.

CCT

Computer compatible tape.

CHIRP

Typical phase coding or modulation applied to the range pulse of an imaging radar designed to achieve a large time-bandwidth product. The resulting phase is quadratic in time, which has a linear derivative

such coding is often called linear frequency modulation, or linear fm.

COHERENT

Property of a signal or data set in which the phase of the constituents is measurable, and plays a significant role in the way in which several signals or data combine. The power of coherent signals combine according to the sum of the signals, magnitude squared, . (See incoherent.)

COHERENT REFLECTOR

Simple or complex surface (such as a corner reflector) from which reflected wave components are coherent with respect to each other, and thus combine to yield larger effective power than would be observed from a diffuse scattering surface of the same area.

COMPLEX (number)

In the sense of radar systems, this implies that the representation of a signal or data file needs both magnitude and phase measures. In the digital SAR context, a complex number is often represented by an equivalent pair of numbers, the in-phase (I) component and the quadrature (Q) component. For any complex number a, the relationships are , where , and . In the exponential notation, r is the magnitude and is the phase of the number a, which is the complex amplitude (sometimes, simply called "amplitude" which could be confused with "magnitude"). For coherent systems such as SAR, the role of complex numbers is an essential part of the signal, since signal phase is used in the processor to obtain high resolution.

CONDUCTIVITY

Property of a material to allow electrical current to flow with very little loss. For natural surfaces, conductivity in general is increased with increased moisture content.

CONSERVATION OF CONFUSION

Principle, for imagery derived from a given SAR, that the amount of "information" in the data is a constant. One expression of this rule is that the product of the range and the azimuth resolution divided by the number of statistically independent looks is a constant, which serves as a figure of merit of the system. (In this context, "information" is related to the statistical degrees of freedom in the data ensemble, and not necessarily to knowledge about objects in the scene.)

CONSERVATION OF COORDINATES

Principle, for synthetic aperture radar imagery, that image position is not changed by pitch, roll, or yaw rotations of the radar, since range is determined by the speed of light, and azimuth is determined by the along track radar velocity.

CONSERVATION OF ENERGY

Principle, assuming that all available data is used for each case, that the average value of the estimated reflectivity from a scene is a constant for a given SAR and processor, independent of the number of looks used, and independent of any time varying noncoherence in the scene (such as from a moving surface of water) or in the radar/processor combination.

CONTRAST

Difference between the tone of two neighbouring regions.

CORNER REFLECTOR

Combination of two or more intersecting specular surfaces that combine to enhance the signal reflected back in the direction of the radar. Strongest reflection is obtained when the materials are good conductors.

CSA

Canadian Space Agency, St. Hubert, Québec, Canada.

DECIBEL (dB)

Measurement of signal strength, properly applied to a ratio of powers. For a signal power P compared by ratio to a reference power , the formal definition is . Example power ratio of 1/2 corresponds to "-3 dB", derived from log10(0.5)=0.3010. DeciBels often are used in radar, such as in measures of reflectivity, for which the dynamic range may span several factors of ten. The unit is named in honour of Alexander Graham Bell, inventor of the telephone.

DEGREE OF POLARIZATION

Ratio of the power in the polarized part of an electromagnetic wave to the total power; in terms of the Stokes parameters.

DEPRESSION ANGLE

Usually refers to the line of sight from the radar to an illuminated object as measured from the horizontal plane at the radar. For image interpretation, use of the term is not recommended because it does not account for the effects of Earth curvature, and it does not conveniently include effects of local slope in the scene. It is more appropriate for engineering description of the vertical antenna pattern at the radar itself. (See incidence angle.)

DETECTION

Processing stage at which the strength of the signal is determined for each pixel value. Detection removes phase information from the data file. The preferred detection scheme uses a magnitude squared method, , which is energy conserving, and has units of voltage squared per pixel. (See image.)

DIELECTRIC

Material which has neither "perfect" conductivity nor is perfectly "transparent" to electromagnetic radiation. The electrical properties of all intermediate materials, such as ice, natural foliage, or rocks, may be described by two quantities relative dielectric constant; and loss tangent. Reflectivity of a smooth surface and the penetration of microwaves into the material are determined by these two quantities.

DIELECTRIC CONSTANT

Fundamental (complex) parameter, also known as the complex permittivity, that describes the electrical properties of a lossy medium. (See permeability.) By convention, the relative dielectric constant of a given material is used, defined as the (absolute) dielectric constant divided by the dielectric constant of "free space". The (relative) dielectric constant is usually defined as (It is common practice to refer to the real component as "the dielectric constant", whose partner, the loss tangent, accounts for .)

DIFFUSE

Reflection typically made up of many individual reflections having random phase with respect to each other, such as from a natural forest canopy or agricultural field. The term is also used to describe a surface that reflects (microwave) illumination in this fashion. (The opposite term is specular or coherent.)

DIGITAL NUMBER (DN)

Numerical number, between zero and 255 for example, assigned to each spatial grid position in the file representing the brightness levels of an image. The digital numbers may be related to sigma nought of scene elements through the process of calibration.

DIHEDRAL

Corner reflector formed by two surfaces orthogonally intersecting. For enhanced backscatter, the dihedral must be open to the radar, and have the axis of intersection at right angles to the direction of illumination.

DISTRIBUTED SCATTERERS

Elements of a scene consisting of many small scatterers of random location, phase, and reflectivity in each resolution cell. (See diffuse.)

DISTRIBUTION

General purpose mathematical description of a signal characterized by values with magnitude significantly larger than zero over only a relatively small span in time or distance. A distribution may have extensive low level tails or sidelobes. Examples of distributions include the pulse transmitted by a radar, and the description in space of the pattern of an antenna.

DOPPLER (frequency)

Shift in frequency caused by relative motion along the line of sight between the sensor and the observed scene. In SAR, it is more formally the first derivative of the signal phase in the azimuth direction. The span of useful Doppler frequencies illuminated by the antenna must be smaller than the azimuth pulse repetition frequency (prf), otherwise false image features (azimuth ambiguities) will occur.

DYNAMIC RANGE

A description of the variety of signal amplitudes (or power levels) available in a system, or present in a data file. Dynamic range is specified either i) to be within minimum and maximum values, or ii) with respect to the ratio of maximum to minimum values. The most important specification is linear dynamic range over which signals combine according to the property of linearity.

ELECTROMAGNETIC (EM) WAVE

A wave described by variations in electric and magnetic fields, elegantly formulated by J. C. Maxwell in 1873. Light waves, radio waves, and microwaves are well known examples. All such waves propagate at the speed of light in "free space", which includes most realistic atmospheric conditions. Three material parameters are necessary and sufficient to describe em waves in a given medium: dielectric constant (or permittivity); permeability; and conductivity.

ELEVATION DISPLACEMENT

Image distortion in the range direction of a side and downward looking radar caused by terrain features in the scene being above (or below) the reference elevation contour, and thus in fact being closer to (or further from) the radar than their planimetric position. The effect may be used to create radar stereo images (see parallax). It may be removed from an image through independent knowledge of the terrain profile. In many applications, an approximate correction may be derived through shape from shading techniques.

ENERGY

For a waveform of time-limited duration such as a radar pulse reflected by an object, the pulse energy is given by the power of the signal integrated over the duration of the signal (Units of watt-seconds = joules).

EOS SAR

Satellite proposed by JPL to carry a three frequency quadrature polarimetric SAR for the Earth Observation Satellite series. If approved by NASA, it would be operational only after the year 2000.

ERS-1

Satellite launched by ESA in July 1991. The main payload (AMI) instrument includes a SAR at C-band, VV polarization and 23° incidence angle.

ESA

European Space Agency, with headquarters in Paris, France.

FORESHORTENING

Spatial distortion whereby terrain slopes facing a side-looking radar's illumination are mapped as having a compressed range scale relative to its appearance if the same terrain were level. Foreshortening is a special case of elevation displacement. The effect is more pronounced for steeper slopes, and for radars that use steeper incidence angles. Range scale expansion, the complementary effect, occurs for slopes that face away from the radar illumination.

FOURIER TRANSFORM

Mathematical operation used to derive the frequency domain description of a distribution. An efficient digital implementation is the "fast Fourier transform", or FFT. The inverse Fourier transform returns a frequency domain description to the original distribution. The digital inverse form is known as the IFFT.

FREQUENCY

Rate of oscillation of a wave. In the microwave region, frequencies are on the order of 1 GHz (Gigahertz) to 100 GHz. ("Giga" implies multiplication by a factor of .) For electromagnetic waves, the product of wavelength and frequency is equal to the speed of propagation, which, in free space, is the speed of light.

FREQUENCY DOMAIN

For every distribution f in time there is an equivalent representation F whose independent variable is frequency. The frequency domain representation is the Fourier transform of the original distribution. F and f are equivalent in the sense that they carry the same information, but expressed in an alternative way. The concept is often generalized to distributions in the space domain, for which the Fourier transform is in the spatial frequency domain, having units of cycles per unit length. The azimuth frequency domain is also known as the Doppler domain.