Statistical investigations of the synthetic aperture radar images

The focused and non-focused image-forming methods for airborne synthetic aperture radar (SAR) are investigated. The more precise receiving signal model which includes not only traditional additive noise but the multiplicative noise which is the consequence of the aircraft's velocity and height fluctuations is developed. Results of numerical calculations of the root-mean-square (rms) errors of the set of principal image parameters as functions of the additive noise variance and multiplicative noise standard deviation for several weighting windows and the histograms of the distributions of some image parameters are presented.     

A model comparison study to the imaging of submarine reefs with synthetic aperture radar

Signatures of submarine reefs near Heligoland in the North Sea were observed in airborne radar images recorded at L-, C- and X-bands on 14 November 1990 during rather high wind speed of 9 ms^-1. Predictions from various models of the imaging mechanism were compared to these observations. One of the models is the so-called weak hydrodynamic interaction theory (WHIT) model. It is fully two-dimensional in position as well as wavenumber space, so any surface current variation can be handled. Also more sophisticated scattering models than first-order Bragg scattering can be included. The model contains a number of parameterizations for the roughness length, the equilibrium wave height spectrum and the relaxation rate as well as different forms for the local relaxation source term. In the model intercomparison, the WHIT model performed not very well. It is shown here that this is due to the choice of the radial relaxation rate. In a sensitivity analysis it is shown that also the form of the relaxation source term is important. A linear source term may lead to unrealistically high positive hydrodynamic modulations (up to 50 dB) at some positions over the reefs for waves with a wavelength of about 0.6 m. Such effects do not occur in quadratic or cubic source terms, which are therefore to be preferred. The parameterizations chosen for the roughness length and the angular relaxation rate have little influence on the model results. Also shoaling may be neglected. A scattering model based on first iteration of the Stratton-Chu equation gives results similar to that of an improved two-scale model. When compared to the observations, good agreement is obtained at L-band, but at C- and X-bands the model underestimates the modulations. A number of possible causes is discussed, but additional data are needed to settle this question.     

Review Article. The imaging of waves by satelliteborne synthetic aperture radar: the effects of sea-surface motion

The effects of sea-surface velocities in the imaging of waves by synthetic aperture radar (SAR) are considered using the ‘facet’ concept of the backscattering process. It is shown that if the sea wave spectrum is divided at the nominal limit of resolution of the SAR the effect of the long and short wavelength parts can be considered separately, the former being treated by numerical simulation and the latter by statistical methods. It is found that the motions due to: the short wavelengths produce an azimuthal smearing which can be represented by a Gaussian low-pass filter acting on the azimuthal component of wavenumber in the image. The cut-off wavelength is typically some hundreds of metres in moderate winds. Images obtained with the SEASAT SAR frequently show such an effect.     

Synthetic aperture radar imaging of ocean surface waves: multi-look and single-look systems

A single two-dimensional expression is given for the (speckle-averaged) image associated with a model of ocean swell which may be applied to single-look and multi-look synthetic aperture radar (SAR) systems (albeit with different arguments). That expression accommodates real and artificial cross-section modulation mechanisms, shows rescaling effects (i.e. rotation of the image of a wavefield). suggests the use of processor refocusing to maximize image contrast, accounts for scene coherence, and indicates a variation of image contrast with range (assuming homogeneous swell). Most of the above have been observed and discussed previously in the literature. However, in this paper it is shown that all the effects are derived (simultaneously). by a relatively simple, rigorous and self-consistent analysis: it is not always necessary to treat the SAR imaging of ocean waves in a piece-wise manner. The analysis differs from that most frequently found in the literature. A space-time variant point spread function is not presumed; and some of the conclusions, in particular with regard to the optimum image processing strategy, differ with those following the use of the more conventional forms of analysis. Throughout, care is taken to minimize, but also to make explicit, the underlying assumptions in the treatment.     

Multi-look versus single-look processing of synthetic aperture radar images with respect to ocean wave spectra estimation

Abstract

Full-bandwidth C-band synthetic aperture radar (SAR) data are compared with 7-look and 3-look data. The peak-to-background ratio of the image intensity power spectrum describing the wave detectability is found to be on average 8-9dB higher for the 7-look data and 2-5dB higher for the 3-look data than the single-look data. This is mainly due to the decrease in the speckle noise level when going from single-look to multi-look processing. In addition, look-sum processing is evaluated against spectral-sum processing for various temporal look separations. A significant improvement in image spectral peak contrast is observed for the spectral-sum data versus the look-sum data, with increasing temporal separations between the looks. No such improvement is observed in the corresponding image spectral noise contrast parameter. These observations are in agreement with the spatial misregistration inherent in look-sum data. Finally, the acceleration contribution to the observed aximuth smearing in the spectra is found to be negligible compared with the velocity smearing contribution.     

Spectral signal to clutter and thermal noise properties of ocean wave imaging synthetic aperture radars

Abstract

The high wavenumber detection cut-off is determined above which the spectrum of ocean waves imaged by a synthetic aperture radar (SAR) is lost in the background noise spectrum consisting of the clutter noise associated with the Rayleigh statistics of the backscattering surface and the thermal noise originating in the SAR system itself. For given power, the maximum detection cut-off wavenumber is attained if the SAR resolution is chosen such that the clutter and noise spectra are equal at the cut-off wavenumber. Assuming a constant modulation transfer function relating the image modulation and wave slope spectra, the cut-off wavenumber is in this case proportional to (ρ_aρ_g)^?1/2, where ρ_a and ρ_g represent the full bandwidth (single look) azimuthal and ground range resolutions, respectively. The same proportionality holds (but with a cut-off wavenumber increased by a factor √2) for a clutter limited cut-off, the normal operating condition of an SAR. To first order, incoherent multilook averaging has no influence on the signal-to-background detection cut-off wavenumber, provided the reduced Nyquist cut-off wavenumber resulting from the reduced multilook spatial resolution remains greater than the signal-to-background cut-off wavenumber. Estimates of the detection cut-off wave-numbers are given for the Seasat SAR and the SAR proposed for the European Remote Sensing Satellite ERS-1.     

A simple algorithm for the direct extraction of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar

Abstract

A simple method is derived for 1the determination of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar (SAR) without explicitly producing an image. The algorithm is similar in structure to a two-dimensional Fourier transformation, but the transformed function also depends explicitly on the wavenumber components. The function consists of the quadratic product of the complex return signal amplitude and the time-delayed complex conjugate return signal, the time delays with respect to range and azimuth time being proportional to the corresponding wavenumber components in these directions. The algorithm appears to be sufficiently simple to be implemented in real time aboard a satellite. Because of the considerable data compression achieved in reducing the original image to its statistical variance spectrum, this opens the possibility of obtaining global surface-wave spectral data from satellites without the excessive costs of sophisticated telemetry and a ground station network required for real-time line-of-sight transmission of the unprocessed signal data. The contracted signal-image-Fourier-transform (SIFT) algorithm may be interpreted as the application of the SAR as a continuous, two-dimensional Δκ (dual frequency) scatterometer. The difference frequencies arise through the multiplication of the chirped return signal with the time-lagged (i.e. frequency shifted) complex return signal. Since the return signal is chirped with respect to both range time and azimuth time, the introduction of two time lags corresponds to a two-dimensional Δκ modulation. The standard dual frequency scatterometer yields a modulation wave only in the radar propagation direction, but the basic principle of producing a beat wave by multiplying two chirped signals which are displaced in time relative to each other can be applied also to the azimuthal Doppler chirp of a broad beam dual frequency scatterometer. The generalized two-dimensional Δκ-scatterometer obtained in this manner differs from a SAR-SIFT processor only in the manner in which the spectrum of difference frequencies is generated: in a Δκ-scatterometer, the duration of the difference frequency sweep is normally large compared with the signal travel time, whereas in a SAR, the entire spectrum of difference frequencies is generated within a single pulse in a time shorter than the average signal travel time. The relative operational advantages or disadvantages of the two methods of obtaining microwave surface image spectra will need to be clarified in a more detailed technical analysis.     

Interferometric SAR imagery of a monochromatic ocean wave in the presence of the real aperture radar modulation

Abstract

The model which allows one to simulate the Synthetic Aperture Radar (SAR) and the along track Interferometric SAR (INSAR) imagery of a monochromatic ocean wave is presented. The model accounts for the effects of the real aperture radar (RAR) modulation of the radar backscatter cross-section. The comparison of the SAR and the INSAR imagery of the wave system under consideration is carried out and the performance of the two imaging systems is studied under various operational conditions. The results of the study indicate that the interferometric SAR is less sensitive to the RAR modulation as compared lo the regular SAR. This fact further augments the assertion that INSAR has considerable advantages over SAR in its potential of providing quantitative information about the ocean wave lengths, directions and amplitudes     

Pre-survey suitability evaluation of the differential synthetic aperture radar interferometry method for landslide monitoring

The active remote-sensing technique differential radar interferometry (D-InSAR) is a powerful method for detection and deformation monitoring of landslides. But the radar-specific imaging geometry causes specific spatial distortions in radar images (as e.g. the layover and shadowing effect), which have a negative impact on the suitability of these images for D-InSAR applications. To address this issue, we present a geographical information system (GIS) procedure to accurately predict the areas in which layover and shadowing will occur, before the area of interest is recorded by radar. Additionally, the percentage of measurability of movement of a potential landslide can be ascertained. In the third part of the GIS procedure, the main types of land cover are classified in regard to their influence on applicability of the D-InSAR technique, depending on the characteristics of the sensor used. The results of the analyses are objective pre-survey estimation of the potential applicability of the D-InSAR technique for landslide monitoring prior to the costly investment of a radar survey.     

Observation of atmospheric effects on repeat-pass interferometric synthetic aperture radar over the Kuwait desert

Twenty-eight advanced synthetic aperture radar (ASAR) scenes from the Environmental Satellite (ENVISAT) are analysed to select suitable pairs for generating a digital elevation model (DEM) and displacement maps. For this purpose, the repeat-pass interferometric synthetic aperture radar (InSAR) technique is implemented using GAMMA interferometric modules. The perpendicular component of baseline (B┴) is taken as the criteria for selecting the pairs: 0 < B┴ <100 m for displacement maps and 200 < B┴ < 400 m for the DEM. Though there are many pairs satisfying the above criteria, only four case studies are presented here to illustrate the effects of atmosphere on the DEM and displacement maps over the Kuwait desert climate. In each case study, two examples are selected: one where the atmosphere is a serious problem and another example the atmosphere has no significant problem. The DEM of the Shuttle Radar Topographic Mission (SRTM) is taken as a reference for root mean square (RMS) error estimation in the DEM. The RMS error varies from as low as 2 m to as high as 40 m. Some DEMs showed fringe-like structures resembling precipitable water vapour (PWV) fields. Similarly, the measured displacement values were found to vary randomly from place to place and time to time. The displacement maps showed vertical structures similar to PWV. The DEM was corrected for PWV. The results are encouraging. From this study, it is clear that, even for desert areas, there is a need to look into the effects of PWV on the DEM and displacement maps before the results are used.     

Estimation bias of ocean current measured by along-track interferometric synthetic aperture radar and its compensation methods

In previous works on current measured by along-track interferometric synthetic aperture radar (ATISAR), the decorrelation function of an ocean-surface backscattering signal was usually assumed to be a real Gaussian function, i.e. the phase term was omitted. In this study, it is proved that the omission of the phase term included in the decorrelation function results in a significant estimation bias that can be modelled by a higher-order function of time lag and that the coefficients of this higher-order function can be expressed as a series of higher-order Doppler spectral moments. This model is validated by the scatterometer data obtained from an experimental wind-wave tank. The estimation bias especially needs to be considered for the ATISAR system with a long time lag. Simulation results show that if the time lag is equal to the coherence time, the estimation bias of the current can reach about 0.2 m s^?1, which is not insignificant in high-precision current-retrieving applications. However, because most real-life ATISAR systems, including TerraSAR-X, operate with time lags significantly shorter than the expected coherence time, the estimation biases in these systems are relatively small or even negligible. Finally, four possible compensation methods for the estimation bias are proposed and discussed.     

Mapping hydrologically sensitive areas on the Boreal Plain: a multitemporal analysis of ERS synthetic aperture radar data

Characterizing the spatial and temporal dynamics of hydrologically sensitive areas (HSAs) is vital to the effective management of the boreal forest. HSAs are defined as saturated or inundated areas that, if disturbed, might result in a significant change in the movement of water, nutrients and biota within landscapes. This study presents a remote sensing technique that uses archived European Remote Sensing Satellite (ERS)‐1 and ERS‐2 synthetic aperture radar (SAR) images to monitor HSAs in the Willow River watershed (1030 km²) on the western Boreal Plain of Canada. ERS images were used to generate a probability of HSA occurrence map for a 10‐year period (1991–2000). This map revealed the complexity of HSAs on the western Boreal Plain, where some areas remained consistently dry or wet whereas others were dynamic, transitioning from dry to wet and vice versa. A probability map of HSA occurrence provides spatial and temporal information previously unavailable for this region that may expand our understanding of the hydrological behaviour of drainage basins and serve as a planning tool for land management decisions.     

Analysing changes of the Poyang Lake water area using Sentinel-1 synthetic aperture radar imagery

Poyang Lake is the largest freshwater lake in China. Monitoring changes of its water area is essential for the conservation of important wetlands and ecological resources, and plays an important role for sustainable water use and management. Landsat and Moderate-Resolution Imaging Spectroradiometer (MODIS) sensor images are widely used for mapping waterbodies, because of their sensitivity for spectral reflectance of water. However, studies using Landsat images have limited their investigations of changes of Poyang Lake to dry season due to the impairment by cloud cover. Further limited by the rather long 16 day revisit cycle, most existing studies build on the vague assumption that Poyang Lake undergoes only relatively slow changes during this season. MODIS, in contrast, provides a very short revisit period, but has been proven not to be able to assess the water area of Poyang Lake accurately due to low spatial resolution. Therefore, the contribution of this study is to investigate recent Poyang Lake water area changes both during high- and low-water period with unforeseen temporal and spatial resolution using Sentinel-1 synthetic aperture radar (SAR) imagery. More specifically, we aim at investigating Poyang Lake’s recent water area changes in intra-month scales. During the observation period from October 2014 to March 2016, October 2014 was the month with the largest max/min water coverage ratio. Water coverage of winter in 2014 and 2015 was completely different, as a severe drought happened in 2014 and an unusual winter flood happened in 2015. Thus, this study demonstrates the potential of using Sentinel-1 SAR data to reveal intra-month variations, benefiting from the sensor’s regular observation capabilities independent of weather conditions. It is shown that Sentinel-1 SAR data, with rapid availability and free-of-charge distribution policy, as well as relatively high spatial and temporal resolutions, is becoming an indispensable data source for a detailed monitoring of important inland waterbodies and wetlands.     

Ocean sand ridge signatures in the Bohai Sea observed by satellite ocean color and synthetic aperture radar measurements

Satellite measurements from Synthetic Aperture Radar (SAR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua platform are used to study the ocean sand ridges in the eastern Bohai Sea in China. Even though the imaging mechanisms for SAR and MODIS-Aqua remote sensing are different, the sand ridges are shown to have exactly the same patterns in images from both sensors. Therefore, the location, extension and coverage of the ocean sand ridges can be detected and cross-examined by both SAR and MODIS-Aqua observations. Satellite images show quite different sand ridge distribution pattern from the published bathymetry map (based on in situ data) that shows six sand ridges in the area. 10 finger-shaped sand ridges are identified from satellite observations. The tidal-current/sand-ridge interaction driven physical and optical changes are assessed and evaluated. The existence of sand ridges causes enhanced water diffuse attenuation coefficient K_d(490) and elevated normalized water-leaving radiance at the red and near-infrared (NIR) wavelengths. The sea surface over the sand ridges experiences significant seasonal variability of water turbidity and shows remarkable differences from nearby ocean regions. During winter, K_d(490) values are about 2-3 m^− 1 over the ridges, while the maximum K_d(490) in the neighboring oceans is approximately 1.5 m^− 1. In summer, the enhancement of the sea surface turbidity is less significant than that which occurs in winter.     

Large discharges from ships in the Black Sea studied by synthetic aperture radar with the support of automated identification systems

During routine oil spill monitoring in the eastern Black Sea in 2011 and 2012 by RDC ScanEx, unprecedentedly large patches of oil of shipping origin were detected on synthetic aperture radar (SAR) images. The oil spills were detected mostly in the Russian sector of the sea or on its borders. The presence of large oil spills was confirmed using Terra and Aqua optical images. Using automated identification systems data, the vessel candidates were identified, and two of them were inspected. This paper presents for the first time the results of monitoring that show that the central part of the Black Sea is used for tank-washing operations and illegal dumping of oily products. The legality of producing these spills in the framework of the International Convention for the Prevention of Pollution from Ships (MARPOL) is also discussed. These and other similar events registered during the monitoring require the attention of environmental and nature conservation organizations. Summary maps of large oil spills detected in the Eastern Black Sea in 2011 and 2012 are presented and discussed.     

A technique for the identification and analysis of icebergs in synthetic aperture radar images of Antarctica

This paper describes an image analysis technique developed to identify icebergs depicted in synthetic aperture radar images of Antarctica and to determine the outlines of these icebergs. The technique uses a pixel bonding process to delineate the edges of the icebergs. It then separates them from the background water and sea ice by an edge-guided image segmentation process. Characteristics such as centroid position and iceberg area were calculated for each iceberg segment and placed in a file for input to appropriate statistical data analysis software. The technique has been tested on three ERS-1 SAR sub-images in which it succeeded in identifying virtually all segments containing icebergs of size six pixels or larger. The images were first passed through an averaging filter to reduce speckle. This process produced a pixel size of 100m x 100m. As implemented, the technique overestimates iceberg areas by about 20% on average and the detection rate falls off rapidly for icebergs less than six pixels in size. Performance in these areas is expected to improve when additional stages, based on a more detailed analysis of pixel intensity, are implemented.     

Revealing the geology of the Great Nicobar Island,Indian Ocean,by the interpretation of airborne synthetic aperture radar images

A number of new geological structures have been revealed in the Great Nicobar Island, Indian Ocean, from the analysis of airborne synthetic aperture radar (SAR) data. The advantages of SAR images for mapping geological structures over other images for the Great Nicobar Island, the southern most island of Andaman-Nicobar arc, have been highlighted. A visual analysis of lineaments in the southern part of the Great Nicobar Island using SAR and Landsat TM colour composites, TM bands 2, 3, and 4, and TM bands 4, 2 and SAR, reveals a wealth of structural information not shown on previous maps. Other identified features include lithological units derived from landforms, northwest-southeast trending faults, faults trending towards the mouth of Galathea river, the graben valley, and many other new faults. The geological features reported in this work were verified using limited ground checking.     

Objective analysis of a two dimensional scalar field by successive corrections using a personal computer

The use of a successive correction method for interpolating data from irregularity spaced points to a regularly spaced grid has long been used in meteorological applications. This procedure was automated using a personal computer and a demonstration program was written based on the Cressman method. The program also produce isopleth analysis for standard meteorological scalar fields such as temperature and height at constant pressure levels.     

Monitoring snowpack evolution using interferometric synthetic aperture radar on the North Slope of Alaska,USA

This research investigates the use of Interferometric Synthetic Aperture Radar (InSAR) to generate a time-series of snow water equivalent (SWE) for dry snow within the Kuparuk watershed, North Slope, Alaska, during the winter of 1993/1994. Maps depicting relative change in phase and the theoretical relative change in SWE between satellite acquisitions are created for 3-day periods at the end of March 1994 using both ascending and descending ERS-1 overpasses. The theoretical coefficient relating relative change in phase and relative change in SWE for C-band is found to be at least twice as large as what is expected when using a simple single-layer snow model for this study area and time period. Without any direct measurements of SWE on the ground, station measurements of snow depth and hourly wind are linked to each 3-day relative change in phase map. Along with a qualitative assessment, quantitative measures of the rate and magnitude of phase change around these stations are directly compared to the hourly wind data for a given 3-day period. InSAR-derived maps acquired around a measured precipitation event show a considerable relationship to the predominant direction of strong winds over each 3-day period while maps acquired around no measureable precipitation depict much less correlation between phase change and predominant direction of strong winds. Despite limited ground measurements to infer snowpack conditions, these results show continued promise for the InSAR technique to measure changes in snowpack conditions (e.g. SWE) at much higher resolutions than manual sampling methods or passive microwave remote sensing. The extension of this technique to current L-band InSAR satellite platforms is also discussed.     

Discrimination of sea ice in the Labrador marginal ice zone from synthetic aperture radar image texture

This study addresses the modelling of synthetic aperture radar (SAR) image texture for sea ice scenes in the Labrador marginal ice zone (MIZ). The image texture of distributed scatterers contains a substantial component relating to the imaging system as well as information about the scene itself. Theory shows that the image autocorrelation function (ACF) may be analysed to separate system contributions from scene contributions under certain conditions. The main theses of the study are: (i) SAR intensity images of sea ice are spatially nonGaussian; and (ii) the predominant types and forms of MIZ sea ice may be discriminated based upon ACF model parameters. Experimental results indicate that the model provides an excellent fit to the measured ACFs. The image texture was found to be a strong function of the form of the sea ice as well as its type. For a given type, the various forms could be discriminated through a single SAR channel. For full discrimination of all types and forms observed, a two-channel combination was necessary: XHV CHH or XHV CHV.