Sea and Space

# Glossary

A/D

  Analogue/Digital

AGC

  see Automatic Gain Control

ALT

  see Radar Altimeter

AMI

  see Active Microwave Instrument

ASF

  Alaska SAR Facility

ATSR-M

  Along Track Scanning Radiometer and Microwave Sounder

AVHRR

  Advanced Very High Resolution Radiometer

Absorption

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

Active Microwave Instrument (AMI)

  Instrument on the ERS satellites operating at 5.3 GHz frequency (C-band). It consists of 2 separate radars with three modes of operation - SAR for image and wave mode, and a three-antennae scatterometer for wind measurements.

Advective motion

  Mass motion in fluids and gases whereby heat, for example, is transported in horizontal direction.

Almaz-1

  Almaz (= "rough diamond"); a Synthetic Aperture Radar (S-band) satellite launched by the former USSR on 31 March 1991, and operated until 17 October 1992. Almaz-1b is planned for launch at the end of 1996.

Along Track Scanning Radiometer (ATSR)

  Consists of two instruments: an Infrared Radiometer (IRR) and a Microwave Sounder (MWS). On ERS-1 the IRR is a 4-channel (1.6, 3.7, 10.8 and 12 µm) radiometer providing SST and cloud top temperatures. On ERS-2 the ATSR-2 instrument has a total of 7 visible and infrared channels. The two-channel Microwave Sounder (23.8 and 36.5 GHz) provides water vapour content information.

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, whereas the term magnitude is not ambiguous.

Ancillary data

  Data other than instrument data needed for the processing and correct interpretation of the instrument's science data (e.g., spacecraft orbit/attitude, instrument pointing information).

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.

Anticyclonic

  Sense of rotation about the local vertical that is clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere and udefined at the Equator.

Archive

  A facility providing long storage and preservation of data sets and associated documentation.

Archived data

  Data received from the ERS satellites and any other data types necessary for later data processing or investigation. Data is copied to archive media, maintained and stored in a library.

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).

Atmosphere boundary layer

  The layer of air directly above the Earth' s surface (land and ocean). Over open ocean, the layer usually has a vertical depth of less than 2 km.

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.

Automatic Gain Control (AGC)

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

Auxiliary data

  Data other than ancillary data and instrument data needed for processing the science data produced by the instrument (e.g., instrument calibration data coefficients).

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 to allow extra looks, at the expense of azimuth resolution.

BDDN

  Broadband Data Dissemination Network

Backscatter

  The (microwave) signal reflected by elements of an illuminated scene back in the direction of the radar. The term 'backscatter' was choosen to make a clear distinction between energy scattered in arbitrary directions, and energy which returns to the radar and thus is 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 MHz, and in the azimuth channel are on the order of 1 KHz. The azimuth frequency domain is also known as the Doppler domain. Bandwidth is a fundamental parameter of any imaging system, and determines the ultimate resolution available.

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.

Bragg scattering

  Enhanced backscatter due to coherent combination of signals reflected from a rough surface having linear features orthogonal to the illuminating wavefront and whose spacing is equal to half of the wavelength as projected onto the surface.

Brash ice

  Accumulation of floating ice made up of fragments not more than 2 m across (small ice cakes), the wreckage of other forms of ice.

Break up

  A general expression applied to the formation of a large number of fractures through a compact ice cover, followed by a rapid diverging motion of the separate fragments.

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".

Buoy

  A platform at, or below the sea surface for carrying instruments to measure oceanographical and meteorological parameters. Buoyancy frequency Maximum freqency of internal waves

C-band

  Microwave band with wavelengths at or near 5.6 cm.

CCRS

  see Canada Centre for Remote Sensing

CCT

  Computer Compatible Tape

CEOS

  Committee on Earth Observation Satellites. Members are current and prospective operators of Earth observation systems.

CEOS-WGD

  CEOS Working Group on Data

CNES

  Centre National d'Études Spatiales

CRPE

  Centre de Recherche en Physique de l'Environment

CUS

  see Central User Service at ESRIN

Calibration

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

Canada Centre for Remote Sensing (CCRS)

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

Capillary waves

  Waves at the surface of the sea caused by surface tension. The wavelength of capillary waves is of order 1 cm.

Catalogue

  An ordered collection of concise describers and pointers, permitting the easy location of relevant item(s): in the ERS case the catalogue permits the identification of data products complying with specific requirements, such as geographical area and time coverage, quality, etc.

Central User Service (CUS)

  Part of the ESRIN ERS Central Facility (EECF) dedicated to the planning and supervision of all data acquisition, product generation and dissemination activities, as well as to providing an interactive interface between the ERS system and its users.

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.

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.

Commissioning phase

  The initial phase (A) after the launch of an ERS satellite. This period is used for satellite and payload verification and instrument calibration.

Compactness

  The ratio of the area of the sea surface actually covered by ice to the total area of the sea surface under consideration. Therefore, a compactness of 0 corresponds to ice free and a compactness of 1 to compact ice.

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 () component and the quadrature () component. For any complex number , the relationships are , where , , and . In the exponential notation, is the magnitude and is the phase of the number , 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.

Concentration (ice concentration)

  The ratio in tenths of the sea surface actually covered by ice to the total area of the surface, both ice covered and ice free at a specific location or over a defined area. May be expressed in the following terms: Compact pack ice: Concentration 10/10, no water visible. Consolidated ice: Concentration 10/10, floes frozen together. Very close pack ice: Concentration 9/10 to less than 10/10. Close pack ice: Concentration 7/10 to 8/10, floes mostly in contact. Open pack ice: Concentration 4/10 to 6/10, many leads and polynyas, floes generally not in contact. Very open pack ice: Concentration 1/10 to 3/10.

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.

Convective motion

  Motion in liquids and gases whereby heat, for example, is transported in a vertical direction.

Coriolis force

  A force due to the Earth's rotation that acts on the motion of air and water masses.

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.

Crack

  Any ice fracture which has not yet parted.

Current front

  The boundary between two areas of different current speed and direction, often associated with different water masses.

Cycle

  The interval of time between exact repeats of an orbit (e.g., in the 35-day cycle, orbits repeat exactly every 35 days).

Cyclonic

  Sense of rotation about the local vertical that is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.Data centreAn institutionally supported facility providing convenient access to and distribution of data sets (including supporting information and expertise) for a wide community of users. It has a long term charter (not tied to the lifetime of a specific project).

D-PAF

  German Processing and Archiving Facility (see DLR)

DDN

  Data Dissemination Network

DEM

  Digital Elevation Model (see also DTM)

DESC

  Display ERS SAR Coverage software

DFD

  Deutsches Fernerkundungs-Datenzentrum (German Remote Sensing Data Centre at DLR, Oberpfaffenhofen)

DIBAG

  Institute for Image Processing and Computer Graphics, Graz

DLR

  Deutsche Forschungsanstalt für Luft- und Raumfahrt (German Aerospace Research Establishment)

DMOP

  Detailed Mission Operation Plan

DMSP

  see Defense Meteorological Satellite Program

DN

  see Digital Number

DQL

  Digital Quick-Look Images

DRA

  Defence Research Agency (formerly Royal Aerospace Establishment, RAE)

DTM

  Digital Terrain Model (see also DEM)

Data product

  1) A homogeneous set of information resulting from raw data processing: it includes annotated, transcribed or decommuted raw data.2) General term to indicate raw data, validation data, auxiliary data, fast delivery, regenerated, or precision products.3) A uniformly processed and formatted data set, portion of a data set, or transformed representation of data (e.g., plot, photograph); may be produced by, or for, a project or by a data centre.

Defense Meteorological Satellite Program (DMSP)

  Meteorological program of the U.S. Department of Defense.

Deformed ice

  A general term for ice which has been squeezed together and in places forced upward (and downward). Forms of deformation include rafting, ridging and hummocking.

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 a complex number. (It is common practice to refer to the real component as "the dielectric constant", whose partner, the loss tangent, accounts for the immaginary one.)

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.

Dissemination

  Transmission of data from one facility to another or to many other facilities.

Dissemination schedule

  A project plan which designates at a fixed time in the future the transmission of data from one facility to other facilities.

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.

Divergence

  The divergence can be considered as the change in area per unit area at a given point. The word is also used to indicate a general diverging motion in the ice.

Doppler, 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.

Draft

  The distance, measured normal to the sea surface, between the lower surface of the ice and the water level.

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 eitheri) to be within minimum and maximum values, orii) 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.

EECF

  see ESRIN ERS Central Facility

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.

EPO

  Earthnet Programme Office

ERIM

  Environmental Research Institute of Michigan

ERS

  European Remote Sensing Satellite. ERS-1 was put into orbit in July 1991 by an Ariane 4 launcher from Kourou, French Guiana, ERS-2 April 1995. The satellites have a sun-synchronous, near-polar, quasi-circular orbit with a mean altitude of 785 km. Instruments: Active Microwave Instrument (AMI), operating at C-band with VV polarization, 23º incidence angle; Radar Altimeter (RA); Along Track Scanning Radiometer (ATSR); Precise Range And Range-rate Equipment (PRARE); Laser Retro-Reflectors (LRR); Global Ozone Monitoring Experiment (GOME), on ERS-2 only.

ERS product catalogue

  A database containing information identifying all the raw data collected and archived, and all the products produced and archived for ERS during the satellites' operational lifetime and follow-on. Specific information stored will include the type of data, geographic coverage, time acquired, and reference to product source.

ERS-1

  European Remote Sensing Satellite - 1, launched on 17 July 1991.

ERS-2

  European Remote Sensing Satellite - 2, launched on 21 April 1995.

ESA

  European Space Agency, with headquarters in Paris, France.

ESA ERS team

  All the ESA management, scientific and support staff involved in various aspects of the ERS missions.

ESOC

  European Space Operations Centre. ESA Establishment in Darmstadt, Germany.

ESRIN

  European Space Research Institute. ESA Establishment in Frascati, Italy.

ESRIN ERS Central Facility (EECF)

  The EECF is located in Frascati, Italy, and carries out all user interface functions, including cataloguing, handling of user requests, payload operating planning, scheduling of data processing and data dissemination, quality control of data products and system performance monitoring.

ESTEC

  European Space Research and Technology Centre. ESA Establishment in Noordwijk, The Netherlands.

ETOPO5

  Global Digital Elevation Model of 5 arcmin resolution

Eddies

  Circular ocean currents having a horizontal scale of between 10 to 100 km.

Eddy orbital motion

  Rotational velocity of an eddy

Electromagnetic 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 electromagnetic 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).

F-PAF

  French Processing and Archiving Facility (see IFREMER)

FAO

  Food and Agriculture Organization

FD

  Fast Delivery

FD/RA

  Fast Delivery Radar Altimeter

FDC

  Fast Delivery Copy

FDP

  Fast Delivery Product

Facility

  A collection of hardware, software, personnel, and infrastructure (centralised or decentralised) with an identified functionality necessary for the support of the ERS missions.

Fast delivery

  Fast Delivery Products (FDP): products generated and distributed from the ground stations within one day for SAR image mode and 3 hours for LBR data.

Fast ice

  Sea ice of any origin which remains fast (attached with little horizontal motion) along a coast or to some other fixed object.

First year ice

  Sea ice of not more than one winter's growth, developing from young ice. Thickness 0.3-3 m. May be subdivided into thin first year ice/white ice (0.3-0.7 m), medium first year ice (0.7-1.2 m) and thick first year ice (over 1.2 m).

Flaw

  A narrow separation zone between pack ice and fast ice, where the pieces of ice are in a chaotic state, that forms when pack ice shears under the effect of a strong wind or current along the fast ice boundary.

Flaw lead

  A lead between pack ice and fast ice.

Floe

  Any relatively flat piece of sea ice 20 m or more across. Floes are subdivided according to horizontal extent: Giant floe: more than 10 km across. Vast floe: 2-10 km across. Big floe: 0.5-2 km across. Medium floe: 100-500 m across. Small floe: 20-100 m across.

Foreshortening

  Spatial distortion whereby terrain slopes facing a side-looking radar's illumination are mapped as having range extent reduced from their true size, which is a special case of elevation displacement. The effect is more pronounced for steeper slopes, and for radars that use steeper incidence angles.

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.

Fracture

  Any break or rupture through very close, compact or consolidate pack ice, fast ice or a single floe resulting from deformation processes (see lead). Fractures may contain brash ice and be covered with nilas or young ice. The length may be a few meters or many kilometers.

Frazil ice

  Fine spicules or plates of ice, suspended in water.

Freeboard

  The distance, measured normal to the surface, between the upper surface or the ice and the water level.

Frequency

  Rate of oscillation of a wave. In the microwave region, frequencies are on the order of 1 GHz to 100 GHz. 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 in time there is an equivalent representation whose independent variable is frequency. The frequency domain representation is the Fourier transform of the original distribution. and 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.

Front

  A sloping surface of discontinuity in the troposphere, separating air masses of different density or temperature.

Frontal disturbance

  Perturbation of a front.

GAP

  see Global Activity Plan

GARS

  German Antarctic Receiving Station

GCP

  Ground Control Point

GEC

  Geocoded Ellipsoid Corrected Image (ERS SAR product)

GEOS

  GEOcoding System (SAR geocoding system at D-PAF)

GIM

  Geocoded layover, shadow and Incidence angle Mask

GIS

  Geographic Information System

GMT

  Greenwich Meridian Time (see also UTC)

GOME

  The Global Ozone Monitoring Experiment. This instrument is a nadir-viewing spectrometer onboard ERS-2 measuring a range of atmospheric trace constituents and in particular ozone.

GPS

  Global Positioning System

GSFC

  NASA Goddard Space Flight Center

GTC

  Geocoded Terrain Corrected Image (ERS SAR product)

GTC01

  Geocoded Terrain Corrected Image without GIM

GTC02

  Geocoded Terrain Corrected Image with GIM

GTS

  Global Telecommunication System

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.

Geodetic phase

  An ERS-1 mission phase (E), with a 168-day repeat cycle.

Geophysical product validation

  Verification that ERS geophysical products are consistent with best available independent geophysical measurements.

Geostrophic velocity

  Motion of air and water masses caused by the balance between the Coriolis force and the pressure gradient force in the horizontale plane.

Geostrophic wind

  A wind whose direction and speed are determined by a balance of the pressure-gradient force and the force due to the Earth's rotation.GHzGigahertz (109 Hz)

Global Activity Plan (GAP)

  The GAP covers the future operation of the different payload instruments. Sections of the GAP are constantly modified and updated based on orders received from users.

Gradient

  A vector obtained from a real function f whose components are the partial derivatives of f; this measures the maximum rate of change of f in a given direction.

Gravity waves

  Waves in the atmosphere, at the ocean surface and in the interior of the ocean (internal waves).

Grease ice

  A stage of freezing, in which the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the sea a matte appearance.

Grey ice

  Young ice, 10-15 cm thick, less elastic than nilas, usually rafts under pressure.

Grey-white ice

  Young ice, 15-30 cm thick. Under pressure, it is more likely to ridge than to raft.

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.

Grounded ice

  Floating ice (e.g. ridge, hummock and ice island) which is aground (stranded) in shoal water.

HDDR

  High Density Digital Recorder

HDDT

  High Density Digital Tape

HF

  High Frequency

HH

  Horizontally polarized transmitted wave, Horizontally polarized received wave

HRV

  High Resolution Visible

HV

  Horizontally polarized transmitted wave, Vertically polarized received wave

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.

Housekeeping data

  Engineering data used exclusively for managing the operation, health and safety of a spacecraft, platform, instrument or equivalent.

Hummock

  The raised area of multiyear ice formed by the ablation of the surrounding ice. Also, a hillock of broken ice which has been forced upward by pressure. May be fresh or weathered. The submerged volume of ice under the hummock is called a bummock.

Hydrodynamic modulation

  A mechanism at the sea surface responsible for the varying backscatter of SAR microwaves.

Hydrophobic oil

  Water-repellent oil.

I-PAF

  Italian Processing and Archiving Facility (see ASI)

IFOV

  Instantaneous Field of View

IFREMER

  Institut Français de Recherche pour l'Exploitation de la Mer

IGARSS

  International Geoscience and Remote Sensing Symposium

IGN

  Institut Géographique National (French mapping agency)

II16

  Intermediate Image 16 bit

IR

  Infrared

IRR

  Infrared Radiometer

ISLR

  Integrated Sidelobe Ratio

ISS

  Interface Subsystem. Element of the EECF which handles all EECF interfaces and the telecommunications links with the other ground segment and user facilities.

ITC

  International Training Center

Ice boundary

  The demarcation at any given time between fast ice and pack ice or between areas of pack ice of different concentrations (see ice edge).

Ice cake

  Any relatively flat piece of sea ice less than 20 m across (see floe). If less than 2 m across, it is a small ice cake.

Ice cover

  The ratio of an area of ice of any concentration to the total area of sea surface within some large geographic locale; this locale may be global, hemispheric, or prescribed by a specific oceanographic entity, such as Baffin Bay or the Barents Sea.

Ice edge

  The demarcation at any given time between the open sea and sea ice of any kind, whether fast or drifting.

Ice field

  Area of pack ice greater than 10 km across, consisting of floes of any size. Subdivided as follows: Large ice field: more than 20 km across Medium ice filed: 15-20 km across. Small ice field: 10-15 km across.

Ice free

  No sea ice present. There may, however, be some icebergs present (see also open water).

Ice limit

  Climatological term referring to the extreme minimum or extreme maximum extent of the ice edge in any given month or period, based on observations over a number of years.

Ice phases

  The ERS-1 mission phases (B and D), with a 3-day repeat cycle, which will operate twice during the mission, to ensure highly repetitive coverage of ice zones during Arctic winters.

Ice shelf

  A floating ice sheet of considerable thickness, showing 2-50 m or more above sea level, attached to the coast. Usually of great horizontal extent and with a level or gently undulating surface. Nourished by annual snow accumulation and often by the seaward extension of land glaciers. Parts of it may be aground. The seaward edge is called an ice front.

Iceberg

  A massive piece of ice of greatly varying shape with a freeboard of more than 5 m, which has broken away from a glacier and may be afloat or aground.

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 and 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.

Imagette

  10 km x 6 km images located at intervals of 200 km along-track, produced by the SAR operating in wave mode.

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 width (resolution) of the impulse response 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.

In-phase ()

  Component of the signal that has the same phase as the complex reference frequency.

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.

Instrument

  A hardware integrated collection of one or more sensors and associated hardware/software controls contributing data to an investigation.

Instrument data

  Data produced and transmitted by the science and engineering sensors of an instrument.

Instrument engineering data

  Data produced by an instrument's engineering sensor(s) (e.g., instrument temperature).

Instrument science data

  Data produced by an instrument's science sensor(s).

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.

Internal waves

  Waves in the deep ocean, in SAR images such waves having a wavelength of 1 to 5 Km are often observable.

Inversion layer

  A layer of the atmosphere in which the temperature increases with height, this is the reverse of the normal situation where the temperature decreases with height.

Isobar

  Lines joining points of equal atmospheric pressure.

JERS-1

  Japanese Earth Resources Satellite-1. Satellite launched by Japan in February 1992. It includes an L-band SAR with HH polarization and 38.5º incidence angle.

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.

JRC

  Joint Research Centre. Research establishment of the European Union, located in Ispra, Italy.

Jet

  Narrow and intense current or wind.

KHz

  Kilohertz (103 Hz)

Keel

  The underside of a ridge that projects downward below the lower surface of the surrounding sea ice.

L-band

  Microwave band with wavelengths at or near 23.5 cm.

LBR data

  Low bit-rate data from: SAR operating in wave mode, wind scatterometer, radar altimeter and ATSR.

LBRFDP

  LBR Fast Delivery Processor

LEOP

  Launch and Early Orbit Phase

LRDDF

  Low Rate Data Dissemination Facility

LRDPF

  Low Rate Data Processing Facility

LRDTF

  Low Rate Data Transcription Facility

LRR

  see Laser Retro-Reflectors

LUT

  see Look-Up Table

Landsat

  Land Satellite. American remote sensing satellite series, previously called ERTS (Earth Resources Technology Satellite), with ERTS-1 launched in 1972. Since then, 4 further Landsat satellites have been launched.

Laser Retro-Reflectors (LRR)

  Passive optical instrument operating in the infra-red to permit ranging of the satellite by the use of laser ranging stations. Used for the calibration of Radar Altimeter altitude measurements and the improvement of the satellite orbit determination.

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.

Lead

  Any fracture or passage through sea ice that is generally too wide to jump across. A lead may contain open water (open lead) or be ice covered (frozen lead).

Lee waves

  Gravity waves created in the atmosphere by the perturbation of air masses flowing over a mountain.

Level ice

  Sea ice which has been unaffected by deformation.

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 is a multiplicative constant, then a linear operation on any two numbers and satisfies . (The additive constant is needed to account for realistic behaviour of many practical systems.) Linearity, over the dynamic range of the system, is an essential attribute of most measurement devices such as an imaging radar.

Look-Up Table (LUT)

  Tables of data containing reference and calibration parameters for fast delivery processing.

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 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 at . Low loss materials satisfy << 1.

MMCC

  see Mission Management and Control Centre

MPH

  Main Product Header

MSAR

  Multisensor SAR Processor (D-PAF)

MSS

  Multispectral Scanner

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 () and quadrature () components, the magnitude is given by . For complex amplitude , 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.

Meander

  A deviation of the flow pattern of a current.

Melt pond

  An accumulation of meltwater on the surface or sea ice that, because of appreciable melting of the ice surface, exceeds 20 cm in depth, is embedded in the ice (has distinct banks of ice), and may reach several tens of metres in diameter.

Mesoscale currents

  Ocean currents appearing in patterns, ranging in scale from tens to hundreds of Kilometers. MHzMegahertz (106 Hz)

Microwave Sounder (MWS)

  One of the two instruments which comprise the ATSR. A nadir-viewing passive radiometer providing measurements of the total water content of the atmosphere within a 20 km footprint.

Microwaves (MW)

  Electromagnetic waves at wavelengths in the order of 1 to 100 cm.

Mission Management and Control Centre (MMCC)

  Located at the European Space Operations Centre in Darmstadt, Germany, the MMCC carries out all satellite operations control and functional management, including overall satellite and payload operational scheduling.

Mission plan segments

  Specific time periods within an orbit which identify the operational mode of the ERS satellites' payload. They are generated from the user requests received by the Central User Service at the ESRIN ERS Central Facility, the national requirements, responses to the Announcement of Opportunity studies, calibration and validation campaigns, and ESA requirements.

Mission plans

  A series of plans containing details on instrument operations, acquisition and processing schedules of various levels of detail and the principals, rules and guidelines related to the ERS satellites operation

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.

Multi-look

  see Looks

Multidisciplinary phase

  An ERS-1 mission phase (C), with a 35-day repeat cycle, which will operate for the majority of the mission.

Multiyear ice

  Old ice 3 m or more thick, which has survived at least two summers' melt. The hummocks are even smoother than in second year ice, and the ice is almost salt-free. The color, where bare, is usually blue. The melt pattern consists of large inter-connecting irregular puddles and melt ponds, and a well-developed drainage system.

NERSC

  Nansen Environmental and Remote Sensing Center, Bergen, Norway

NFS

  National and Foreign Stations

NOAA

  National Oceanic and Atmospheric Administration

NRSC

  National Remote Sensing Centre, United Kingdom

Nadir

  Locus of points on the surface of the Earth directly below the radar as it progresses along its line of flight.

Natural film

  A thin layer at the sea surface consisting of biochemical material.NASANational Aeronautics and Space Administration, with headquarters in Washington D.C., USA.

Near-real time data

  Data from the source that are transmitted with propagation delays and minimal delays due to buffering.

New ice

  A general term for recently formed ice, which includes frazil ice, grease ice, slush and shuga. These types of ice are composed of ice crystals which are only weakly frozen together (if at all) and have a definite form only while they are afloat.

Nilas

  A thin elastic crust of ice up to 10 cm thick, with a matte surface. Bends easily under pressure, thrusting in a pattern of interlocking "fingers" (finger rafting). Dark nilas, up to 5 cm thick is very dark in color; light nilas, 5-10 cm thick, is rather lighter in color.

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. Additive receiver noise is always present in any system, and may be reduced, relative to the signal level, by increasing the power of the radar. 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 is the strength of a hypothetical imaged distributed terrain feature that would have the same average image level as does the (additive) system noise inherent to the radar. 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.

Nominated centres

  Scientific institutions and/or engineering establishments receiving ERS data and nominated at national level to provide coordination between ESA/ERS programme and end users for specific data requirements or to play a significant role in the ERS research related activities.

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 non-coherent signals combine according to the power of each of the individual signals, (see coherent).

ODP

  Ocean Drilling Program

OPR

  Ocean Product (ERS RA product)

Old ice

  Sea ice which has survived at least one summers melt. Most topographic features are smoother than first year ice. May be subdivided into second year ice and multiyear ice.

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 in two ways, out to each object at range 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 tow-way delay time and range distance , such that (see speed of light).

Open water

  A large area of freely navigable water in which sea ice is present in less than 1/10 concentration (see also ice free).

Orographic interaction

  The modification of the characteristics of air flowing over mountains, valleys and other topographic features. Pack iceAny accumulation of of sea ice, other than fast ice, no matter what form it takes or how it is disposed (see also concentration).

P-band

  Microwave band with wavelengths at or near 75 cm.

PAF

  Processing and Archiving Facility

PCS

  see Product Control Service

PDF

  Probability Density Function

PEP

  Preferred Exploitation Plan

PPI

  Plan Position Indicator

PRARE

  Precise Range And Range-rate Equipment. All-weather microwave ranging system designed to perform high-precision two-way microwave range and range-rate measurements to ground-based transponder stations for orbit determination at decimeter levels of accuracy and for geodetic applications. Unfortunately non-functional on ERS-1 due to a fatal failure at the beginning of the mission.

PRC

  Precise orbit (ERS orbit product

PRF

  see Pulse Repetition Frequency

PRI

  SAR Precision Image (ERS SAR product)

PRS

  PRARE System

PSLR

  Peak Sidelobe Ratio

Pancake ice

  Predominantly circular pieces of ice from 0.3-3 m in diameter, and up to about 10 cm in thickness, with raised rims due to the pieces striking against one another. It may be formed on a slight swell from grease ice, shuga or slush, or as a result of the breaking up of ice rind, nilas, or under serve conditions of swell or waves, grey ice. It also sometimes forms at some depth, at an interface between water bodies of different physical characteristics, and floats to the surface. It may rapidly cover wide areas of water.

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.

Path

  One orbit within a cycle. Equivalent to track.

Penetration

  Act of (microwave) 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 wavenumber, and is the (relative) dielectric constant of the medium.

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 a insignificant role in image interpretation.

Phase

  The angle of a complex number.

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.)

Playback data

  Data that are stored on a spacecraft, platform, or other carrier that are transmitted at a later time. The data may be transmitted in a chronological or reverse time order and may have delays due to processing.

Polarization

  Orientation of the electric (E) vector in an electromagnetic wave, frequently "horizonal" (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 transmit and the receive state of polarization, in that order. Thus, for example, LHV indicates L-band, horizontal transmit polarization, and vertical receive 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).

Polynyas

  Any nonlinear shaped opening enclosed in ice. Polynyas may contain brash ice or be covered with new ice, nilas or young ice. If it is limited on one side by the coast, it is called shore polynya; if it is limited by fast ice, it is called a flaw polynya. If it is found in the same place every year, it is called a recurring polynya.

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 (units are watts).

Precise orbit

  The most accurate estimate of the actual satellite orbit; generated by the German PAF and available a few months after satellite pass.

Preliminary orbit

  The first estimate of the actual satellite orbit; generated by the German PAF and available one week after satellite pass.

Pressure ridge

  A general expression for any elongated (in plain view) ridgelike accumulation of broken ice caused by ice deformation.

Processing (SAR)

  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.

Product

  Final result from the application of data processing algorithms by computer to the raw data acquired by the satellite. The user can select from a range of products. The product media consists of hard copy (e.g., photographic), telecommunication links and various forms of computer compatible media (e.g., CCT).

Product Control Service (PCS)

  Part of the EECF, which carries out tasks associated with the monitoring and control of ERS data products.

Product assurance

  Element of the EECF which is responsible for the tasks associated with monitoring and controlling of ERS product quality, system performance and algorithm development.

Propagation

  The movement of energy in the form of waves through space or other media. Electromagnetic waves move at the speed of light in free space, but the speed 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.

Q/L

  Quick-Look

Quadrature ()

  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.

R/V

  Research Vessel

RA

  see Radar Altimeter

RADARSAT

  Satellite to be launched by Canada in 1995. It will carry a C-band SAR, HH polarization, and incidence angles spanning (20º - 60º) selectable in a variety of modes.

RAE

  Royal Aerospace Establishment (see also DRA)

RAW

  SAR Annotated Raw Data (ERS SAR product)

RCS

  see Radar Cross Section

RF

  Radio Frequency

RSL

  Remote Sensing Laboratories, University of Zürich, Switzerland

RTM

  see Roll Tilt Mode

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 Altimeter (RA)

  A Ku-band (13.8 GHz) nadir-pointing active microwave sensor designed to measure the time return echoes from ocean and ice surfaces providing information on significant wave height (SWH), surface wind speed, sea surface elevation and various parameters over sea ice and ice sheets. SWH measurements in range 1 to 20 m, to 0.5 m accuracy or 10%, whichever is greater. Altimetric precision to within 10 cm over oceans and smooth ice, and 40 cm over rough ice.

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 . 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 proportion to .

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. Small radiometric resolution is "better". Radiometric resolution for a given radar may be improved by averaging, but at the cost of spatial resolution (see looks).

Rafting

  Process whereby one piece of ice overrides another. Most obvious in new and young ice but common in ice of all thickness.

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 system.

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.

Raw data

  1) Instrument data or housekeeping data in the same format as transmitted from the spacecraft or collected on a storage medium on the carrier (e.g., tape recorder, optical disk).2) The data received from the satellite prior to the application of any on-ground data processing algorithms. The raw data media will be as for products, i.e. hard copy and various forms of computer compatible media.

Real time data

  Data from the source that are transmitted with only propagation delays.

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.

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 beamwidth of the antenna pattern.

Ridging

  The process whereby ice is deformed into ridges.

Roll Tilt Mode (RTM)

  ERS-1 operating with a radar mid-swath incidence angle of 35º as opposed to the normal 23º. This incidence angle is of special interest for some land applications.

Roll vortices

  Cloud streaks observed in visual satellite images over open ocean caused by convergence and upward motion between the circulating helices in the atmospheric layer. The rolls are oriented parallel to the main wind 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.

Rubble field

  An area of sea ice that has essentially all been deformed. Unlike hummock field, does not imply any specific form of the upper or lower surface or the deformed ice.

S-band

  Microwave band with wavelengths at or near 10 cm.

S/N

  see Signal-to-Noise ratio

SAR

  see Synthetic Aperture Radar

SARFDP

  SAR Fast Delivery Processor

SAW

  Surface Acoustic Wave

SCAT

  Scatterometer

SIR-A, SIR-B

  NASA sponsored radar missions on the Space Shuttle, each lasting about one week. SIR-A (November 1981) was at L-band, HH polarization, 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 that was built for missions in 1993, 1994 and 1996. It carries a quadrature polarimetric SAR at C- and L-bands, and an X-band HH polarized SAR (contributed by Germany and Italy). It offers a variety of incidence angle, wavelength band selection, resolution and polarization modes.

SIZEX 92

  see Seasonal Ice Zone Experiment 1992

SLAR

  Side-Looking Airborne Radar

SLC

  SAR Single Look Complex image (ERS SAR product (quarter scene))

SLCF

  SAR Single Look Complex image (ERS SAR product (full scene))

SMMR

  see Scanning Multichannel Microwave Radiometer

SPOT

  Système Probatoire d'Observation de la Terre

SSH

  Sea Surface Height (ERS RA product)

SSM/I

  see Special Sensor Microwave Imager

SST

  Sea Surface Temperature

SWH

  Significant Wave Height

SWM

  SAR Wave Mode

Scanning Multichannel Microwave Radiometer (SMMR)

  Sensor onboard NIMBUS-7 for the observation of sea ice parameters, ocean surface conditions, atmospheric conditions, land parameters, and glacial features.

Scene

  Object space illuminated by the radar.

Seasat

  First satellite designed for oceanography. Launched on 26 June 1978, failed on 10 October 1978. Polar orbiting at 800 km altitude. Instruments: Radar Altimeter, Scatterometer, Scanning Multichannel Microwave radiometer, SAR and Visible and Infra-red Radiometer. Seasat, built by NASA, was the first (civilian) satellite carrying a SAR. It operated at L-band, using horizontal polarization at 22º incidence angle. Data from Seasat is still important for applications and processing technique development.

Seasonal Ice Zone Experiment 1992 (SIZEX 92)

  SIZEX 92 was a sea ice validation experiment for ERS-1 SAR, where in situ observations of different ice types were made using an icestrengthened vessel (R/V Polarsyssel), helicopter and aircraft.

Second year ice

  Ice which has survived only one summers' melt. Because it is thicker and less dense than first year ice, it stands higher in the water. In contrast to multiyear ice, second year ice during the summer melt shows a regular pattern of numerous small puddles. Bare patches and puddles are usually greenish blue.

Sensitivity Time Control (STC)

  Pre-programmed change in radar gain during the time that reflections are received after each pulse to offset unwanted changes in average signal 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.

Shear zone

  Current front with sharp gradient.

Shore lead

  A lead between pack ice and the shore or between pack ice and an ice shelf or a glacier.

Shuga

  An accumulation of spongy white ice lumps a few centimeters across, formed from grease ice or slush and sometimes from anchor ice rising to the surface.

Shuttle Imaging Radar

  see SIR-A, SIR-B and SIR-C/X-SAR

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 sidelobes 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 meters squared). See also radar cross section.

Sigma nought ()

  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 meter. 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, and 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.

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 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).

Slicks

  Dark patches on the surface of the sea observed in SAR images, caused by a thin layer of oil or other natural substances (surfactants) that damps the short surface waves.SLRSide-Looking Radar

Slush

  Snow which is saturated and mixed with water on land or ice surfaces, or forms as a viscous mass floating in water after a heavy snowfall.

Snow ice

  The equigranular ice that is produced when slush freezes completely.

Special Sensor Microwave Imager (SSM/I)

  Passive microwave radiometer, flown on DMSP satellites. It detects thermal energy emitted by the Earth's atmosphere in the microwave portion of the spectrum. SSM/I data is also used to determine ocean surface wind speed, ice coverage and age, areas and intensity of precipitation, cloud water content, and land surface moisture.

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.

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 ()

  Approximately 300,000,000 meters 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.

Squall lines

  Features in SAR images caused by sudden wind gusts.

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 .

Stratification

  Vertical density structure of the atmosphere or ocean, caused by different air or water masses.Support dataIncludes all data other than the actual satellite acquired raw data, the product data, mission plans, and schedules (e.g., look-up tables, and geophysical calibration data).

Surfactants

  Chemical or biological substances that form a film at the sea surface.SwathWidth of the imaged scene in the range dimension, measured either in ground range or in slant range.

Swell

  Ocean waves that have travelled away from the area in which they are generated; they are of relatively large wavelength and period, and are regular in character.

Synoptic wind

  The wind field in a region at a specified time.

Synthetic Aperture Radar (SAR)

  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.

TDRSS

  Tracking and Data Relay Satellite System

TM

  Thematic Mapper

TOP

  Sea Surface Topography (ERS RA product)

TOS

  Tracking Data and Orbit Processing system (D-PAF)

TSS

  Tromsø Satellite Station

Target

  Term used to describe the area from which the radar signal is scattered back to the receiver. It arose originally because radars where used against specific objects such as aircraft and ships, but it is also convenient when radars are beamed at a mere area on the ground.

Telemetry

  Electromagnetically transmitted data stream of measured values not including command, computer memory transfer, audio or video signals. The information content of the signal can be analogue or digital format.

Temperature front

  A boundary between two water or air masses characterised by a difference in temperature.

Temperature inversion

  A layer in a large body of water in which temperature increases with depht.

Texture

  Second order spatial average of brightness. Scene texture is the spatial variation of the average reflectivity. Image texture consists of scene texture multiplied by speckle.

Tilt modulation

  A mechanism at the sea surface responsible for the varying backscatter of SAR microwaves.

Time-Bandwidth Product (TBP)

  Parameter 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.

Track

  One orbit within a cycle. Equivalent to path.

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.

Troposphere

  The atmosphere layer between the Earth's surface and about 10 km of altitude.

UI16

  SAR fast delivery image 16 bits (ERS SAR product)

UILR

  SAR Low Resolution Image

UK-PAF

  United Kingdom Processing and Archiving Facility (see also DRA)

URA

  Altimeter FD product (ERS RA product)

UTC

  Universal Time Coordinated

UTM

  Universal Tranverse Mercator projection

UWA

  SAR Wave Mode FD Product (ERS SAR product)

UWI

  Wind Scatterometer FD Product (ERS Wind Scatterometer product)

Upwelling

  The process by which water rises from a deeper to a shallower depht, usually as a result of divergence of offshore currents.

User identifier

  A unique identification code provided to the user by the EECF.

VH

  Vertically Polarized Transmitted wave, Horizontally Polarized Received wave

VIS

  Visible

VMP

  Verification Mode SAR Processor

VV

  Vertically polarized transmitted wave, Vertically polarized received wave

Velocity bunching

  A mechanism at the surface of the sea responsible for the varying backscatter of SAR microwaves.

Viscosity

  The resistance that a gaseous or liquid system offers to flow when it is subjected to a shear stress. Also known as flow resistance.

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.

WGS84

  World Geodetic System from 1984

WMO

  World Meteorological Organisation

WRS

  World Reference System

WSC

  Wind Scatterometer

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.

Wave current interaction

  When ocean currents modify the waves (change surface wavelenght, steepness, direction of propagation).

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.

Waveguide

  A layer in atmosphere or ocean where waves can be trapped and propagate horizontally.

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 meters).

Wavenumber ()

  By convention, the ratio , where is the wavelength.

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.

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.

Wind front

  A boundary between areas with different wind speed and direction. The front usually separates two air masses with different temperature.

Wind shear

  The local variation of the wind vector or any of its components in a given direction.

Wind wave

  A wave resulting from the action of wind on a water surface.

X-SAR

  see SIR-C

X-band

  Microwave band with wavelengths at or near 3 cm.

Young ice

  Ice in the transition stage between nilas and first-year ice, 10-30 cm thick. May be subdivided into grey ice and grey-white ice. The expression young ice is also commonly used in a more general way to indicate the complete range of ice thickness between 0 and 30 cm (as in "the formation and growth of young ice"). Usually these difference in meaning are clear from the context of the discussion.

bpi

  bits per inch

dB

  see deciBel

deciBel (dB)

  Measurement of signal strength, properly applied to a ratio of powers. For a signal power compared by ratio to a reference power , the formal definition is . 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.

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