Sar

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Chirp Compression

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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.

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Across-track

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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 )

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Amplitude

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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.

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Attenuation

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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.

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Azimuth Ambiguity

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A form of ghosting that occurs when the sampling of returned signals is too slow.

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Azimuth Compression

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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

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Backscatter

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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.

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Bandwidth

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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.

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Beam

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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. )

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Brightness

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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.

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C-Band

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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

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Doppler Frequency

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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.

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Doppler Radar

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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.

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Electromagnetic Spectrum

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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. )

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Frequency

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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.

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Horizontal Transmit - Horizontal Receive Polarisation (HH)

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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.

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Horizontal Transmit - Vertical Receive Polarisation (HV)

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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.

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Impulse Response

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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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Interferometric Synthetic Aperture Radar (InSAR)

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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.

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Interferometry

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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. )

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Linear Frequency Modulation (FM)

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A linear FM signal has a quadratic phase variation with time, so the instantaneous frequency varies linearly with time.

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Look Direction

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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 ).

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P-Band

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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.

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Pulse

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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.

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Pulse Compression

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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. )

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Pulse Repetition Frequency (PRF)

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Rate of recurrence of the pulses transmitted by a radar.

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Radar Antenna

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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.

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Radar Beam

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The vertical fan-shaped beam of electromagnetic energy produced by the radar transmitter.

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Radar Parallax

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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.

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Radar Phase

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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.

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RADARSAT-1

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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.

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Radio Detection And Ranging (RADAR )

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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)

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Radio Echo

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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.

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Range Time

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The fast time within a received pulse, relative to the pulse transmission time

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Real Aperture Radar (RAR)

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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. ) )

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Repeat Pass Interferometry

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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 )

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SAR Focusing

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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.

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S-Band

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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.

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Side-Looking Aperture Radar (SLAR)

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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. ) )

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Signal Noise

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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.

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Synthetic Aperture

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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. )

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Synthetic Aperture Radar (SAR)

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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. )

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Vertical Polarisation

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Linear polarisation with the lone electric vector oriented in the vertical direction in antenna co-ordinates.

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Vertical Transmit-Horizontal Receive Polarissation (VH)

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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.

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Vertical Transmit-Vertical Receive Polarisation ( VV )

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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).

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Wavelength

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In a periodic wave, the distance between two points of corresponding phase in consecutive cycles

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X-Band

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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.