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ASA_XCH_AX: ASAR External characterization data
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ASA_WS__BP: ASAR Wide Swath Browse Image
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ASA_WV__0P: ASAR Wave Mode Level 0
ASA_WS__0P: ASAR Wide Swath Mode Level 0
ASA_MS__0P: ASAR Level 0 Module Stepping Mode
ASA_IM__0P: ASAR Image Mode Level 0
ASA_GM__0P: ASAR Global Monitoring Mode Level 0
ASA_EC__0P: ASAR Level 0 External Characterization
ASA_APV_0P: ASAR Alternating Polarization Level 0 (Cross polar V)
ASA_APH_0P: ASAR Alternating Polarization Level 0 (Cross polar H)
ASA_APC_0P: ASAR Alternating Polarization Level 0 (Copolar)
Level 1 Products
ASA_IMS_1P: ASAR Image Mode Single Look Complex
ASA_IMP_1P: ASAR Image Mode Precision Image
ASA_IMM_1P: ASAR Image Mode Medium Resolution Image
ASA_IMG_1P: ASAR Image Mode Ellipsoid Geocoded Image
ASA_GM1_1P: ASAR Global Monitoring Mode Image
ASA_APS_1P: ASAR Alternating Polarization Mode Single Look Complex
ASA_APP_1P: ASAR Alternating Polarization Mode Precision Image
ASA_APM_1P: ASAR Alternating Polarization Medium Resolution Image product
ASA_WSS_1P: Wide Swath Mode SLC Image
ASA_WVS_1P: ASAR Wave Mode Imagette Cross Spectra
ASA_WSM_1P: ASAR Wide Swath Medium Resolution Image
ASA_APG_1P: ASAR Alternating Polarization Ellipsoid Geocoded Image
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ASA_WVW_2P: ASAR Wave Mode Wave Spectra
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5.2 RADAR and SAR Glossary

Table 5.2
Across-track An across-track sensor is one that uses a mirror system that moves from side to side in the range to obtain remote sensing data. ( See also "Imaging Geometry" in the Geometry glossary )
Active Microwave Instrument (AMI) Ordinarily referred to as a radar, this is the term commonly used to describe the radar instrument onboard the ERS satellites. (See Microwave)
Active Remote Sensing System A system that provides its own source of energy and illumination (i.e. radar system). A remote sensing system that transmits its own electromagnetic emanations at an object(s) and then records the energy reflected or refracted back to the sensor.
Advanced Synthetic Aperture Radar (ASAR)

This the newest SAR instrument from ESA. The Advanced Synthetic Aperture Radar ( ASAR ) instrument onboard the ENVISAT satellite extends the mission of the Active Microwave Instrument ( AMI ) Synthetic Aperture Radar ( SAR ) instruments flown on the European Remote Sensing ( ERS ) Satellites ERS-1 and ERS-2. ASAR uses an active phased-array antenna, with incidence angles between 15 and 45 degrees. Applications for this sensor will include the study of ocean waves, sea ice extent and motion, and land surface studies such as deforestation and desertification, to name a few.

( see also chapter 1 "ASAR User Guide"(Chapter 1. ) ).

Along-track An along-track sensor is made up of a linear detector array of CCDs (Charge Coupled Device) that obtains data in the platform's direction of motion (azimuth or along-track dimension). The sensor's instantaneous field of view extends the length of the swath width. The dimension parallel to the path of the platform carrying the sensor. The along-track dimension is the imaging direction of the sensor that is parallel to the direction in which the satellite or aircraft is moving . For side-looking radars (SLARs), this dimension is sometimes called the cross range. The typical two-dimensional remotely sensed image is created by the movement of the platform in the along-track direction, while the sensor scans or aims at the orthogonal direction. ( See also "Imaging Geometry" in the Geometry glossary )
Alternating Polarisation (AP) These products are similar to Image Mode products but include a second image acquired using a second polarisation combination. The raw data is acquired in bursts of alternating polarisation. See Products glossary AP_MODE.
Amplitude Measure of the strength of a signal, and in particular the strength or height of an electromagnetic wave (units of voltage). The amplitude may imply a complex signal, including both magnitude and the phase.
Antenna Part of the radar system, which transmits and/or receives electromagnetic energy. (See Radar Antenna )
Antenna Array An arrangement of several individual antennas so spaced and phased that their individual contributions add in the preferred direction and cancel in other directions. SAR systems, employ a short physical antenna, but through modified data recording and processing techniques, they synthesise the effect of a very long antenna. The result of this mode of operation is a very narrow effective antenna beamwidth, even at far ranges, without requiring a physically long antenna or a short operating wavelength. For example, in a SAR system, a 2m antenna can be made effectively 600 m long.
Array Antenna See Antenna Array
Attenuation Decrease in the strength of a signal. The decrease in the strength of a signal, is 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.
Azimuth Ambiguity A form of ghosting that occurs when the sampling of returned signals is too slow.
Azimuth Bandpass Filtering Bandpass filtering selects a certain band of frequency components in the signal. Azimuth bandpass filtering refers to filtering in the azimuth direction of the two-dimensional SAR signal. The location of signal energy in the azimuth frequency domain depends on the antenna pointing angle (Doppler centroid 2.6.1.2.2. ), so bandpass filtering is necessary to maximize the signal energy in the processed image.
Azimuth Beam Inversion

The azimuth Impulse Response Function (IRF) has an amplitude which is modulated by the position of the echo in the azimuth beam pattern as the instrument passes the target. This modulation is removed by modifying the reference function by the inverse of the azimuth beam pattern (expressed as a function of Doppler frequency).

This equalises the echo data across the Doppler bandwidth but exaggerates the noise - thereby degrading the noise figure by the so-called Beamshaping loss.

Azimuth Compression

In the SAR signal domain, the raw data is spread out in the range and azimuth directions and must be coherently compressed to realise the full-resolution potential of the instrument. Azimuth compression consists of coherently correlating the received signal with the azimuth replica function. (See Azimuth Beam Inversion). The appropriate Hamming weighting is applied also to the reference function. Subsequent correlation has the effect of modulating both signal and noise by similar amounts and hence the signal-to-noise ratio is unchanged by this process

Azimuth Descalloping Radiometric correction (descalloping) is performed with a vector multiply in the azimuth direction. See Descalloping in products glossary.
Azimuth Frequency See Doppler Frequency
Azimuth Time The time along the flight path.
Backscatter Backscatter is the portion of the outgoing radar signal that the target redirects directly back towards the radar antenna. Backscattering is the process by which backscatter is formed. The scattering cross section in the direction toward the radar is called the backscattering cross section; the usual notation is the symbol sigma . It is a measure of the reflective strength of a radar target. The normalised measure of the radar return from a distributed target is called the backscatter coefficient, or sigma nought , and is defined as per unit area on the ground. If the signal formed by backscatter is undesired, it is called clutter. Other portions of the incident radar energy may be reflected and scattered away from the radar or absorbed.
Band A selection of wavelengths or range of radar frequencies.
Bandwidth A measure of the span of frequencies available in the signal or passed by the band limiting stages of the system. Bandwidth is a fundamental parameter of any imaging system and determines the ultimate resolution available.
Beam A focused pulse of energy. The antenna beam of a side-looking radar (SLAR) is directed perpendicular to the flight path and illuminates a swath parallel to the platform ground track. Due to the motion of the satellite, each target element is illuminated by the beam for a period of time, known as the integration time. (See also chapter 1 "Principles of Measurement" 1.1.3. )
Beam Mode The SAR operating configuration defined by the swath width and resolution.
Beam Position The area within the total possible swath that is actually illuminated while being governed by the characteristics of a specific beam mode.
Beta Nought (ß°A radar brightness coefficient. The reflectivity per unit area in slant range which is dimensionless.
Bragg Scattering

Bragg Scattering is the enhanced backscatter due to coherent combinations 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.

image

Bragg Scattering
( See also )
Brightness Property of an image in which the strength of the radar reflectivity is expressed as being proportional to a digital number (digital image file) or to a grey scale (photographic image), which for a photographic positive shows bright as white. The attribute of visual perception in accordance with which an area appears to emit more or less light. Brightness may be a result of variations in tone, texture, or in the case of radar imagery, radar artefacts. The topography and surface roughness of the terrain will affect the image brightness. Where the local incidence angle is large, the image will be dark. Conversely, the image will be brighter where the local incidence angle is small.
Cardinal Effect A tendency of a radar to produce very strong echoes from a city street pattern or other linear feature oriented perpendicular to the radar beam.
C-Band A nominal frequency range, from 8 to 4 Ghz (3.75 to 7.5 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. C-band has been the frequency of choice for several experimental aircraft SAR systems as well as a series of single-band satellite SAR systems, including the European ERS-1, ERS-2 and Envisat SAR systems and Canada'"s RADARSAT SAR. The corresponding wavelength for these systems is on the order of 5.6 cm, which has been found useful in sea ice surveillance as well as in other applications. Imaging radars equipped with C-band are generally not hindered by atmospheric effects and are capable of 'seeing' through tropical clouds and rain showers. Its penetration capability with regard to vegetation canopies or soils is limited and is restricted to the top layers. C-band is also used in range instrumentation radars
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.

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Chirp Frequency (1)
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Chirp Frequency (2)
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Chirp Phase
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The Real Part of Chirp
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The Imaginary Part of Chirp
Chirp Compression

The echo signal is correlated with a suitable reference function. This correlation is performed in the frequency domain after suitable Fast Fourrier Transform from the time domain.

The reference function of interest should represent the chirp signal which illuminates the target.
This chirp varies from one swath to another due to the phase and amplitude settings introduced to produce beam steering. The beam steering (and shaping if necessary) is achieved by changing the excitations - phase and/or amplitude - to the many phase centres of the planar array antenna. The reference function should reflect these differences.

In ASAR the phase of the excitation to any one row is constant along its length. Thus any row can be considered to be homogeneous. The characteristics of such a row are determined by exciting just that row - and hence deriving a reference function for it. A total reference function can then be derived by combining 32 such reference functions - using the commanded excitation phases.

Circular Polarisation A polarisation state in which the two perpendicular components of the electric field have equal magnitudes and a 90-degree phase difference. In this case, the tip of the electric field vector traces a circle on a plane that is perpendicular to the wave propagation direction.
Circularly Polarised Antenna An antenna that is designed to radiate a left-hand or right-hand circularly polarised electromagnetic wave in its far field.
Clutter Unwanted echoes in a radar return. See also Backscatter.
Coherence Coherence is the fixed relationship between waves in a beam of electromagnetic (EM) radiation. Two wave trains of EM radiation are coherent when they are in phase. That is, they vibrate in unison. In terms of the application to things like radar, the term coherence is also used to describe systems that preserve the phase of the received signal.
Coherence in an Interferogram

In an interferogram, coherence is a measure of correlation . It ranges from 0.0, where there is no useful information in the interferogram; to 1.0, where there is no noise in the interferogram (a perfect interferogram). Both extremes are rarely seen-- most images lie somewhere in between.

Coherence is affected by, (in approximate order):

  • Local slope (steep slopes lead to low coherence)
  • Properties of the surface being imaged (vegetated or moving surfaces have low coherence).
  • Time lag between the passes in an interferogram (long lags lead to low coherence)
  • The baseline (large baselines lead to low coherence)
  • Technical details of the generation of the interferogram (poor co-registration or resampling leads to low coherence)

Coherence can serve as a measure of the quality of an interferogram; tell you more about the surface type (vegetated vs. rock); or tell you when a tiny, otherwise invisible change has occured in the image, and it is only visible in the phase image of an interferogram.

High coherence makes for attractive, not-noisy interferograms. Here is a coherent phase image of a mountain.

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Coherent Phase Image of a Mountain

Low coherence makes unattractive, noisy interferograms, as shown below. Often, these interferograms are difficult to phase unwrap.

image

Incoherent Phase Image of a Mountain

Coherence is the magnitude of an interferogram's pixels, divided by the product of the magnitudes of the original image's pixels. It is usually calculated on a small window of pixels at a time, from the complex interferogram and images.

Interferometric SAR (InSAR) techniques make use of the coherence of the radar signal and the fact that the signal phase is equal to twice the path length between the sensor and the earth's surface. The phase difference between measurements generated from two SAR images with the sensor separated by a baseline, allows measurement of the slant range difference to fractions of a radar wavelength. The slant range difference can be geometrically related to the terrain height. By subtracting two signals, you generate an interference pattern. By subtracting the phases of two co-registered SAR images, you generate an interferogram.

There are two basic SAR interferometry methods. In the first, two antennas are placed on the same platform and simultaneously acquire images of the scene from two different angles. The relative phase difference may then be used to construct a Digital Elevation Model (DEM).

In the second, a pair of images from the same sensor are taken at different times. This is now a well-proven concept and has been demonstrated with several spaceborne SAR sensors, including RADARSAT. For this repeat-pass interferometry, the scenes are acquired at different times, so there is a time difference as well as viewing geometry to consider. The passes must have rather similar geometry in order to allow extraction of the relative phase difference. This usually requires that the satellite be on an exact repeat orbit.

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

For radar systems, a complex number 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 real in-phase component (I) and the imaginary quadrature component (Q). 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.

image

Complex Number
( See also ).
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 Co-ordinates Principle, for Synthetic Aperture Radar (SAR) 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 (Radar) 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.
Co-polarisation Maxima The antenna polarisation state for which maximum backscattered power is received from a particular target. For co-polarisation, the transmit and receive antennas are the same.
Co-polarisation Nulls The antenna polarisation state for which zero backscattered power is received from a particular target. For co-polarisation, the transmit and receive antennas are the same. Co-polarisation nulls may not correspond to the maximum cross polarisation received power.
Co-polarisation Signature The received signature when the transmit and receive antennas have the same polarisation properties.
Corner Reflector A combination of two or more intersecting specular surfaces that combine to enhance the signal reflected in the direction of the radar. The strongest reflection is obtained for materials having a high conductivity (i.e. ships, bridges).
Cross Polarisation Maxima The antenna polarisation state for which maximum cross-polarised backscattered power is received from a particular target. Co-polarisation nulls may not correspond to the maximum cross polarisation received power.
Cross Polarisation Nulls The antenna polarisation state for which zero cross-polarised backscattered power is received from a particular target. For co-polarisation, the transmit and receive antennas are the same. Note that for cross polarisation nulls the co-polarisation power is maximum.
Cross Polarisation Signature The received signature when the transmit and receive antennas have orthogonal polarisations.
Cross-polarised Waves ( or Orthogonal Waves ) Each wave in a pair of cross-polarised waves are completely polarised. However, an antenna optimised to receive the co-polarisation maximum of one wave will receive no power from the other wave. Note that, in general, an arbitrary wave may be treated as the sum of two cross-polarised waves.
Degree of Polarisation An electromagnetic wave can have a polarised and a nonpolarised component. The degree of polarisation is given by the ratio of the power in the polarised part of an electromagnetic wave to the total power in the electromagnetic wave
Depolarisation The polarisation state of an electromagnetic wave can change when the wave scatters from a target. Depolarisation is a measure of the change in the degree of polarisation of a partially polarised wave upon scattering. For example, a target may scatter a wave with a greater degree of polarisation than the incident wave, in which case the depolarisation is negative. Depolarisation is also used to indicate spatial or temporal variation of the degree of polarisation for a completely polarised wave
Detection ( Radar ) 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.
Dipole Sheet Transform Also called radon transform. A three dimensional transform used in scattering of radar signals, optical data processing, and the like.
Doppler Frequency The Doppler frequency depends on the component of satellite velocity in the line-of-sight direction to the target. This direction changes with each satellite position along the flight path, so the Doppler frequency varies with azimuth time. For this reason, azimuth frequency is often referred to as Doppler frequency.
Doppler Radar A radar system which differentiates between fixed and moving targets by detecting the change in frequency of the reflected wave caused by the doppler effects. The system can also measure target velocity with high accuracy.
Dynamic Range A description of the variety of signal amplitudes available in a system. 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.
Electromagnetic Displacement Image distortion in the range-direction caused Infra Red, microwave, and radio waves. By terrain features in the scene being above, or below, the reference elevation contour and, in fact, being closer to, or farther from, the radar than their planimetric position. The effect may be used to create radar stereo images.
Electromagnetic Spectrum The ordered array of known electromagnetic energy extending from the shortest rays, through gamma rays, X-rays, Ultra Violet, visible, Infra Red, microwave, and radio waves.( see Scientifc Background 1.1.2.1. )
Elliptical Polarisation A polarisation state in which the two perpendicular components of the electric field have unequal magnitudes and a non-zero phase difference. In this case, the tip of the electric field vector traces an ellipse on a plane that is transverse to the wave propagation direction.
Elliptically Polarised Antenna An antenna that radiates elliptically-polarised electromagnetic waves in its far field
European Remote Sensing (ERS) Satellite series (ERS-1 and -2) launched by the European Space Agency (ESA) in July 1991 and April 1995. One instrument (AMI) includes a C-band SAR, VV polarisation, and 23° incidence angle, and 30-metre resolution. Predecessors to the ENVISAT satellite.
Frequency Number of oscillations per unit time or number of wavelengths that pass a point per unit time. Rate of oscillation of a wave. In remote sensing, this term is most often used with radar. The frequency bands used by radar (radar frequency bands) were first designated by letters for military secrecy. In the microwave region, frequencies are on the order of 1 GHz (Gigahertz) to 100 GHz. ("Giga" implies multiplication by a factor of a billion). 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. * In the microwave region, frequencies are on the order of 0.3 GHz-300 GHz, having wavelengths of 1mm - 1 m respectively.
Frequency Assignment Frequency at which a sensor (especially radar) operates.
Frequency Modulation A technique in which the frequency of a signal is changed about a fundamental or carrier frequency.
FM signals are used in many applications in remote sensing and in the radio broadcasting community. In SAR, the outgoing chirp signal has a linear FM of several megahertz. GPS signals are broadcast in FM.
Frequency Rate The frequency rate is the coefficient of quadratic phase variation in the linear FM signal
Horizontal Polarisation Linear polarisation with the lone electric vector oriented in the horizontal direction in antenna co-ordinates.
Horizontal Transmit - Horizontal Receive Polarisation (HH) A mode of radar polarisation where the microwaves of the electric field are oriented in the horizontal plane for both signal transmission and reception by means of a radar antenna.
Horizontal Transmit - Vertical Receive Polarisation (HV) A mode of radar polarisation where the microwaves of the electric field are oriented in the horizontal plane for signal transmission, and where the vertically polarised electric field of the backscattered energy is received by the radar antenna.
Imaging Radar Most imaging radars produce two-dimensional images. The two dimensions are called range, and azimuth.
Impulse Response

Also known as the point spread function, impulse response is the two-dimensional brightness pattern in an image (after processing) corresponding to the signal reflected by an object whose sigma falls within the dynamic range of the system, and for which the width of the imaged pattern is determined by the radar and processor rather than by the size of the object. A trihedral corner reflector is the most commonly used object for generating an impulse response in a test image. A good impulse response has a relatively large value for the pixel that maps the point scatterer location, and very small values for all surrounding pixels. The impulse response is a basic building block in describing a given radar's imaging performance, since an image is built up from the linear combination of impulse responses from all individual scatterers illuminated by the radar. The impulse response width (IRW, or resolution) of the central peak is the most important characteristic of the impulse response, together with the shape of the impulse distribution, both close to and remote from its centre.

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Impulse Response to a Point Target
Incoherent Property of a signal or data set in which the phases of the constituents are not statistically correlated, or systematically related in any fashion. Also known as non-coherent. ( See Coherence )
In-Phase (I) The real component of the signal that has the same phase as the complex reference frequency. In-phase is represented by the constant I ( see Complex Number and )
In-Phase/Quadrature Channels (I/Q) In-phase component of a ( See Complex Number and )
Integrated Side Lobe Ratio (ISLR) Given a single point scatterer on the ground, the processed SAR image consists of a narrow, strong peak at the location of the scatterer (the mainlobe), surrounded by smaller peaks called sidelobes. The sum of energy in the sidelobes, divided by the sum of energy in the mainlobe, is the Integrated Side Lobe Ratio (ISLR).
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.
Interferometric Synthetic Aperture Radar (InSAR)

SAR interferometry is a technique involving phase measurements from successive satellite SAR images to infer differential range and range changes for the purpose of detecting very subtle changes on, or of, the earth'"s surface with unprecedented scale, accuracy and reliability.
SAR interferometry has been demonstrated successfully in a number of applications, including topographic mapping, measurement of terrain displacement as a result of earthquakes, and measurement of flow rates of glaciers or large ice sheets. The term InSAR, is most commonly associated with repeat-pass interferometry, as discussed in the section entitled "Interferometrey" 1.1.5.4. in the User Guide. In contrast, D-InSAR is used to described differential interferometery.

( see also the section entiled "Processing Algorithms For AP SLC" 2.6.2.1.1.2.1.4. in chapter 2).

Interferometry A technique that uses the measured differences in the phase of the return signal between two satellite passes to detect slight changes on the Earth's surface. ( see Interferometre ) The combination of two radar measurements of the same point on the ground, taken at the same time, but from slightly different angles, to produce stereo images. Using the cosine rule from trigonometry to calculate the distance between the radar and the Earth's surface, these measurements can produce very accurate height maps, or maps of height changes. Mapping height changes provides information on earthquake damage, volcanic activity, landslides, and glacier movement. ( see also the section entitled "Interferometrey" in the User Guide 1.1.5.4. )
Inverse Synthetic Aperture Radar (ISAR) A SAR system that makes use of the motion of the target to synthesise a large aperture antenna.
Linear Frequency Modulation (FM) A linear FM signal has a quadratic phase variation with time, so the instantaneous frequency varies linearly with time.
Linear Polarisation A polarisation state in which one of the perpendicular components of the electric field has zero magnitude. In this case, the polarisation ellipse collapses to a straight line; the tip of the electric field vector traces a straight line on a plane that is transverse to the wave propagation direction.
Linear Range Cell Migration Correction (RCMC) A shift and interpolation operation is performed in the range direction to correct for linear Range Cell Migration (RCM).
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. Linearity, over the dynamic range of the system, is an essential attribute of most measurement devices such as an imaging radar.
Look Direction The radar look direction defines the angle in the horizontal plane in which the radar antenna is pointing when transmitting a pulse and receiving the return signal from the ground or from an object. The look direction is an angular measurement (in degrees) and is usually made with respect to true North. In side-looking imaging radar (SLAR), the look direction is often orthogonal (normal) to the flight trajectory (azimuth) of the platform carrying the radar and is thus synonymous with the range direction. The radar look direction is an important parameter when analysing features with a preferred orientation, for example fracture patterns in rock formations, regular street patterns, or ocean waves, as these may be enhanced through choice of appropriate radar illumination direction. ( See also "Imaging Geometry" in the geometry glossary ).
Looks Radar terminology refers to individual looks as groups of signal samples in a SAR processor that splits the full synthetic aperture into several sub-apertures, each representing an independent look of the identical scene. The resulting image formed by incoherent summing of these looks is characterised by reduced speckle and degraded spatial resolution. The SAR signal processor can use the full synthetic aperture and the complete signal data history in order to produce the highest possible resolution, albeit very speckled, single-look complex (SLC) SAR image product. 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. ( See also ).
Matched Filtering

The matched filter can be thought of as a filter with a different time delay for each frequency component of the signal passing through the filter. That is, the different frequency components of the linear FM signal are each delayed so that they all arrive at the same time at the output of the filter. This way, all the signal energy is gathered into a narrow peak in the compressed pulse. ( see also "ASAR Level 1B Algorithm Physical Justification" - Pulse Compression 2.6.1.1.3. )

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Real Part of Point Target Signal
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Real Part of Matched Filter
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Compressed Target Signal
Microwave A very short electromagnetic wave. The portion of the electromagnetic spectrum lying between the far infrared (IR) and the conventional radio frequency portion. While not bounded by definition, it is commonly regarded as extending from 1 mm to 1 m in wavelength (300 GHz to 0.3 GHz frequency). Passive systems operating at these wavelengths sometimes are called microwave systems. Active systems are called radar, although the literal definition of radar requires a distance measuring capability not always included in active systems.
Motion Compensation Adjustment of a sensing system and/or the recorded data to remove effects of 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.
Multifrequency Radar Broadband systems that transmit pulses in a range of frequencies and wavelengths.
Multipolarisation Radar A radar capable of simultaneously and coherently acquiring several independent complex polarisation measurements for every pixel in the image.
Night Sensing The collection of non-photographic data during the period of time between sunset and sunrise.
Noise Equivalent Sigma Nought A measure of the sensitivity of a given SAR. It describes the strength of the (additive) system noise in terms of the equivalent (average) power in the image domain that would result from an idealised distributed scatterer of the stated reflectivity. Smaller noise equivalent sigma nought values are better. Within physical limitations, smaller may be achieved by increasing the power of the radar transmitter, or by decreasing the noise figure of the electronics.
Noise Figure Factor that describes the noise level in a radar receiver relative to the that in a theoretically perfect receiver. The noise figure, which is always larger than one, is typically two or more, and is usually expressed in decibels.
Optical Correlator A device that enhances weak signals in noise by performing an optical operation approximating the computation of a correlation function. A radar optical correlator uses the original synthetic aperture radar signal film recording of doppler phase histories to make the radar image by methods that are similar to those used in optical Fourier transformation.
Passive System Systems that sense naturally available energy.
P-Band A frequency range from 0.999 to 0.2998 GHz (30 to 100 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. P-band is an experimental SAR frequency that has only been used to-date for research and development purposes. It is part of the NASA JPL AIRSAR multi-frequency (C-, L- & P-band) SAR system designed for Earth observation experiments. P-band is not hindered by atmospheric effects and is capable of seeing through heavy rain showers. P-band SAR penetration capabilities are very significant with regard to vegetation canopies, glacier or sea ice, and soil. Its vegetation canopy imaging capability is considered a key element in estimating vegetation biomass by means of remote sensing.
Phase See Radar Phase
Phase Preserving When the phase at the peak is correct, the processing algorithm is referred to as phase preserving, regardless of the phase variation across the impulse response.
Phase Unwrap In SAR interferometry (InSAR) , the phase delay of the carrier signal at a certain point in the interferogram is a function of the terrain height at that point. However, the phase of the carrier signal can only be measured to within one cycle, or 360 degrees. Phase unwrapping refers to converting the measured phase to the absolute phase, by adding the appropriate number of cycles, or multiple of 360 degrees, to the measured phase.
Polarimetric Active Radar Calibrator (PARC) Device used to receive and retransmit radar pulses. These devices usually consist of a polarisation sensitive receive and transmit antenna and a stable amplifier which boosts the signal level so that the device being calibrated receives a high signal of a given polarisation.
Polarimetric Radar A radar which permits measurement of the full polarisation signature of every resolution element.
Polarisation The process of confining the vibrations of the magnetic, or electric field, vector of light or other radiation to one plane. Orientation of the plane of the electric field relative to the Earth's surface. See also Radar Polarisation in Geometry glossary.
Polarisation Beamwidth For a pair of antennas in a transmit-receive configuration, the polarisation beamwidth is defined as the angle between the two directions from the transmitter at which the polarisation efficiency is one-half of its maximum value.
Polarisation Efficiency For an electromagnetic wave that is incident upon a receiving antenna, the polarisation efficiency is defined as the ratio of power actually received in an impedance-matched load, to the maximum power that could be received if the antenna had optimum polarisation for the received wave.
Polarisation Ellipse For an elliptically polarised wave, the tip of the electric field vector traces an ellipse on a plane that is transverse to the wave propagation direction. This polarisation ellipse describes the polarisation properties of the electromagnetic waves, including the ratio of the perpendicular electric field components and their relative phases.
Polarisation Loss The inverse of polarisation efficiency.
Pulse A short burst of electromagnetic radiation transmitted by the radar. Also described as a 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 Compression In collecting the SAR data, a long-duration linear FM pulse is transmitted. This allows the pulse energy to be transmitted with a lower peak power. The linear FM pulse has the property that, when filtered with a matched filter, the result is a narrow pulse in which all the pulse energy has been collected to the peak value. Thus, when a matched filter is applied to the received echo, it is as if a narrow pulse were transmitted, with is corresponding range resolution and signal-to-noise ratio (SNR). ( see also "ASAR Level 1B Algorithm Physical Justification" - Pulse Compression 2.6.1.1.3. )
Pulse Repetition Frequency (PRF) Rate of recurrence of the pulses transmitted by a radar.
Quadrature (Q) Channel The signal component that is 90° out of phase with respect to the reference frequency. It is represented by the letter Q. It is the imaginary part, which indicates the magnitude of the signal, of the complex number. See Complex Number.
Quadrature Polarisation Radar (Quad Pol Radar) A Radar system designed to simultaneously collect imaging data of a scene in two orthogonal polarisation states on transmit and the same two polarisation states on receive. From such a data set a complete scattering matrix of the reflectivity of the scene may be synthesised, leading to the concept of polarisation signature.
Radar See "Radio Detection and Ranging"
Radar Angles A radar echo from a region where there are no visible targets; may be caused by insects, birds, or refractive index variations in the atmosphere.
Radar Antenna The radar antenna is a structure for transmitting and receiving radiated energy; it is an important subsystem that defines, to a great extent, a radar's operational capabilities and cost. In radar remote sensing the main function of the antenna is to concentrate a radiated microwave energy into a beam of required shape, referred to as the antenna pattern, to transmit it into the desired direction (look direction), and to receive the returned energy from surfaces or objects. Radar remote sensing antennas provide scene illumination ( See Active Remote Sensing System ). The main parameters of radar antennas are operating frequency band, antenna pattern shape (directivity), power (or antenna-) gain, beam-width, side-lobe level, polarisation, and power handling capability. Moving antennas can be used to form a synthetic aperture, where the physical antenna is small compared to the synthesised antenna, and has a sufficiently wide radiation pattern to illuminate the observed surface over a significant period of platform motion.
Radar Beam The vertical fan-shaped beam of electromagnetic energy produced by the radar transmitter.
Radar Cross Section (RCS) Measure of radar reflectivity. The Radar Cross Section (RCS) is 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.
Radar Height Indicator (RHI) Also known as Range Height Indicator.
Radar 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.
Radar Phase Phase is a property of a periodic phenomenon, for example a wave, referring to its starting point or advancement (fraction) relative to an arbitrary origin. In radar remote sensing, the concept of phase is usually applied to the oscillation of electromagnetic waves. When viewed as a cyclical phenomenon, like wave motion or the crankshaft motion of a bicycle pedal, phase can be expressed in degrees. One-quarter cycle represents a phase rotation of 90 degrees; completion of one complete cycle corresponds to a phase rotation of 360 degrees. Waves are considered in-phase, if their origins of phase 0 degrees are perfectly aligned; out-of-phase conditions are met when phase 0 and 180 degrees are aligned. Precise knowledge of phase properties in radar signal data is a key element in interferometric as well as in polarimetric SAR.
Radar Transmission Energy sent by the radar, normally in the form of a sequence of pulses, to illuminate a scene of interest.
Radar Wave 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 It is expressed in units of watt-seconds = joules = j
RADARSAT-1 RADARSAT-1 is an advanced Earth observation satellite project developed by Canada to monitor environmental change and to support resource sustainability. It is Canada's first Earth observation satellite and the world's first operationally-oriented radar sensor. With a planned lifetime of five years, RADARSAT-1 is equipped with a Synthetic Aperture Radar (SAR). Launched in November 1995, this C-band SAR satellite includes a steerable beam, which offers a wide selection of image scales and resolutions. It operates at 5.3 GHz. RADARSAT-2 is scheduled for launch in late 2003.
Radio Altimetry The science and techniques involved in using a radar altimeter for precise measurements of distance between the sensor and the surface or feature. The basic concept of radar altimetry involves a short radar pulse transmitted toward the surface or feature and accurate measurement of the round trip time to the reflecting surface, which yields precise distance. The accuracy, which is generally on the order of several centimetres or tens of centimetres, depends mainly on the sharpness of the pulse, or signal bandwidth, and the footprint of the radar beam. Measurement techniques involve beam-limited and pulse-limited radar altimeters that provide two dimensional, high-resolution height profiles along the flight line. The synthetic aperture approach is also used by SAR altimeters to reduce the size of the footprint along the flight line. . Prominent geoscientific applications of radar altimetry include measurement of ocean surface dynamics as well as solid surface topographic mapping.
Radio Band The range of wavelengths or frequencies of electromagnetic radiation designated as radio waves; approximately 10(4) to 10(9) Hz in frequency.
Radio Detection And Ranging (RADAR ) A method, system or technique, including equipment components, for using beamed, reflected, and timed electromagnetic radiation to detect, locate, and (or) track objects, to measure altitude and to acquire a terrain image. The radio detection instrument consists of a transmitter that sends out high-frequency radio waves and a receiver that picks them up after they have been reflected by an object. 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. The advantage radar sensors have over other types of sensors, is that microwaves can penetrate clouds, most rain storms, and even dry snow. Therefore, for those parts of the world where cloud and rain present a problem in acquiring images (tropics, coastal/maritime regions), radar is highly beneficial. ( see also , chapter 4)
Radio Echo The signal reflected by a radar target, or the trace produced by this signal on the screen of the cathode-ray tube in a radar receiver.
Radio Frequency (RF) A frequency that falls within the radio band: 10(4) to 10(9) Hz.
Radiometer An instrument for quantitatively measuring the intensity of electromagnetic radiation in some band of wavelengths in any part of the electromagnetic spectrum. Usually used with a modifier, such as an infrared radiometer or a microwave radiometer.
Radiometric Compensation The sensitivity of the antenna is sensitive to angle and the radiometric compensation adjusts for variation in the received signal due to this effect. See Side Lobes.
Range Time The fast time within a received pulse, relative to the pulse transmission time
Real Aperture Radar (RAR) A radar system where the antenna beamwidth is controlled by the physical length of the antenna. Also known as brute force or noncoherent radar. A SLAR system in which azimuth resolution is determined by the physical length of the antenna and by the wavelength. The radar returns are recorded directly to produce images. The advantages of RAR is their simple design and data processing. However, its resolution is poor so RAR are limited to short range, low altitude missions, scanning short wavelengths. The use of the data is limited as shorter wavelengths experience a large amount of atmospheric effects, scattering and dispersion, for example. Because the missions are flown at low altitudes, the coverage is small. The resolution is limited by the length of the antenna. The antenna needs to be many times longer than the wavelength to produce narrow bandwidths. However, it is impractical to design an antenna long enough to produce high-resolution data. ( see also FAQS(Chapter 4. ) )
Remote Sensing (R/S) Group of techniques for collecting image or other forms of data about an object from measurements made at a distance from the object, and the processing and analysis of the data. ( see also , chapter 4).
Repeat Pass Interferometry Method based on two image acquisitions of the same scene from slightly displaced orbits of a satellite. Phase information of the two image data files are superimposed. The two phase values at each pixel are then subtracted, leading to an interferogram that records only the differences in phase between the two original images. Phase differences can be related to the altitude variation at each position in the swath and enable the production of a Digital Elevation Model (DEM). For optimum results, there should be no change in the backscatter to maintain coherence; vegetated sites are therefore a problem. For detection of feature movement (e.g. tracking glaciers) orbits should be as close as possible. Ground control points (GCPs) are required to accurately superimpose the two images. And knowledge of the sensor location is critical. With a good baseline and coherence, this technique can be better than stereo ( ~10 m vertical accuracy). ( see Interferometry )
Rotor-SAR (ROSAR) A synthetic aperture radar concept whereby antennas are mounted at the tips of the rotor blades (i.e. helicopter) illuminating a circular ring-shaped swath.
SAR Focusing In a long synthetic aperture (array), SAR focusing involves the removal and compensation of path length differences from the antenna to the target on the ground. The main advantage of a focused synthetic aperture is that it increases its array length over those radar signals that can be processed, and thus increases potential SAR resolution at any range. SAR focusing is a necessary process when the length of a synthetic array is a significant fraction of the range to ground being imaged, as the lines-of-sight (range) from a particular point on the ground to each individual element of the array differ in distance. These range differences, or path length differences, of the radar signals can affect image quality. In a focused SAR image these phase errors can be compensated for by applying a phase correction to the return signal at each synthetic aperture element. Focusing errors may be introduced by unknown or uncorrected platform motion. In an unfocused SAR image, the usable synthetic aperture length is quite limited.
S-Band A nominal frequency range from 4 to 2 GHz (7.5 to 15 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. S-band radars are used for medium-range meteorological applications, for example rainfall measurements, as well as airport surveillance and specialised tracking tasks.
Scanning Synthetic Aperture Radar (ScanSAR)

A having the capability to illuminate several subswaths by scanning its antenna off-nadir into different positions.

image

ASAR ScanSAR geometry
Sea Satellite (SEASAT) NASA ocean research satellite that was in operation July-September of 1978. Seasat was the first (civilian) satellite to carry a SAR. It operated at L-band, using horizontal polarisation at 22° incidence angle. Data from Seasat is still important for applications and processing technique development.
Sensitivity Time Control (STC) Pre-programmemed change in radar amplitude due to weaker backscatter from greater ranges and varying incidence angles across the imaged swath.
Short Pulse Radar Radar system capable of generating nanosecond pulses of energy at microwave frequencies.
Sidelobes

Non-zero levels in a distribution that are separated from the desired central response. Sidelobes arise naturally in antenna patterns, for example, although in general they are a nuisance, and must be suppressed as much as possible. Large side-lobes may lead to unwanted multiple images of a single feature.

image

( See also Beamwidth in the Geometry glossary )

Side-Looking Aperture Radar (SLAR) A high-resolution real aperture radar (RAR) having antennas aimed to the right or left of the flight path. Also referred to as side-looking radar. An all-weather, day/night remote sensor which is particularly effective in imaging large areas of terrain. It is an active sensor, as it generates its own energy which is transmitted and received to produce a photo-like picture of the ground. It provides high-resolution strip maps with photograph-like detail. ( see also FAQS(Chapter 4. ) )
Signal Noise Signal noise is any unwanted or contaminating signal competing with the desired signal. In a SAR, two common kinds of noise are additive (receiver) noise and signal dependent noise, usually either additive or multiplicative. The relative amount of additive noise is described by the signal-to-noise ratio (SNR). Signal dependent noises, such as azimuth ambiguities or quantization noise, arise from system imperfections, and are dependent on the strength of the signal itself. Speckle is sometimes considered to be a kind of signal dependent multiplicative noise in a SAR system.
Spectral Window A band of the electromagnetic spectrum which offers maximum transmission and minimal attenuation through a particular medium.
Spectrum See Electromagnetic Spectrum
Stereo Sensing Science and techniques used in obtaining stereo imagery (other than aerial photography).
Synthetic Aperture A synthetic aperture, or virtual antenna, consists of a long array of successive and coherent radar signals that are transmitted and received by a physically short (real) antenna as it moves along a predetermined flight or orbital path. The synthetic aperture is formed by pointing the real radar antenna of relatively small dimensions, which are restricted in size by the satellite platform, broadside to the direction of forward motion of that platform. The points at which successive pulses are transmitted can be thought of as the elements of a long synthetic array, which a signal processor will then use and process to generate a SAR image. This detailed array of radar signal data is the key to achieving high azimuth resolution. This long virtual antenna concept is the basis for synthetic aperture radar, or SAR. ( see also "Scientific Background" 1.1.2. )
Synthetic Aperture Radar (SAR) A synthetic aperture radar, or SAR, is a coherent radar system that generates high-resolution remote sensing imagery. Signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture to create an image. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance). High range resolution is achieved through pulse compression techniques. In order to map the ground surface the radar beam is directed to the side of the platform trajectory; with a sufficiently wide antenna beam width in the along-track direction, an identical target or area may be illuminated a number of times without a change in the antenna look angle. ( see also "Scientific Background" 1.1.2.3. )
Synthetic Aperture Time The time for a target to cross the azimuth antenna beam.
Unpolarised Wave A polarisation state in which the two perpendicular components of the electric field have equal magnitudes and a random relative phase difference.
Vertical Polarisation Linear polarisation with the lone electric vector oriented in the vertical direction in antenna co-ordinates.
Vertical Transmit-Horizontal Receive Polarissation (VH) A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for signal transmission, and where the horizontally polarised electric field of the backscattered energy is received by the radar antenna.
Vertical Transmit-Vertical Receive Polarisation ( VV ) A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for both signal transmission and reception by means of a radar antenna. In this case, the plane of the electric field of the microwave energy is designated by the letter V (vertical) for both transmit and receive events, i.e. VV; this transmit-receive polarity is also called like-polarised as opposed to cross-polarised (horizontal transmit - vertical receive, HV). The amount of radar backscatter received at a particular linear polarisation state from a particular ground surface or object depends, in part, on the scattering mechanism and depolarisation effects involved. The transmit-receive acronym is often used in conjunction with the frequency band (wavelength) designation of a particular radar system, e.g. C-VV for C-band. Several satellite SAR designs have used single-band, horizontally like-polarised systems, for example the European ERS-1 and ERS-2 (C-VV).
Wavelength In a periodic wave, the distance between two points of corresponding phase in consecutive cycles
Wavelet Filter for enhancing and compressing radar images. Wavelets are operators that act both in space (azimuth and range) and in frequency (signal content). The classification and identification of texture and features in images incorporates fractal and multi-fractal models that can parameterise geophysical processes. Wavelets may be tailored to retain desired signal behaviour or to preserve smoothness. They are also used to characterise complex signals.
X-Band A nominal frequency range from 12.5 to 8 GHz (2.4 to 3.75 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. X-band is a suitable frequency for several high-resolution radar applications and has often been used for both experimental and operational airborne SAR systems, designed for military as well as civilian remote sensing applications. The corresponding wavelength for these systems is on the order of 3 cm, which has been found useful for mapping and surveillance tasks.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry