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.     

Sea state in the Baltic Sea from space-borne high-resolution synthetic aperture radar imagery

In this work, remote sensing synthetic aperture radar (SAR) data from X-band TerraSAR-X and TanDEM-X (TS-X and TD-X) satellites have been used to adopt the algorithms for estimating sea state parameters in the specific condition of the Baltic Sea with archipelago islands and where short steep sea state dominates. Since the moving targets can be defocused and shifted in SAR images, sea state consisting of short windsea waves with strong local orbital velocities and wave breaking needs additional effort for accurate estimation of the total significant wave height that consists of swell and windsea parts. The XWAVE_C algorithm, developed for the North Sea, where the long swell waves coming from the Atlantic Ocean are present during storms, was further enhanced for the short steep windsea which dominates under ordinary storm conditions in the Baltics. For the empirical XWAVE_C model function, based on the spectral analysis of subscenes as well as on local wind information, an additional term was incorporated for assessment the minimal windsea significant wave height by applying JONSWAP wave spectra. A term to compensate spectral distortions triggered by windsea waves moving in SAR flight direction has also been introduced. In total, 95 TS-X/TD-X StripMap scenes between 2012 and 2017 were acquired in Eastern Baltic Sea, processed and analysed. The wave height results from SAR images were compared with collocated in situ data from 11 available buoys. The analysed data include both high and low windsea conditions. The comparison of SAR-derived wave heights with measured wave heights shows high agreement with a correlation coefficient r of 0.88. The wind speed, estimated from SAR images, was compared to measurements from 14 collocated in situ stations, yielding a high agreement with an r value of 0.90. This article is focused on the algorithm developments; however, it is also the first study of sea state retrieval in the Baltic Sea using high-resolution satellite-based techniques. The results show the local variability in the wave fields connected to atmospheric features. The observed local wave height can increase by 1–2 m in kilometre-size cells that are accompanied by wind gusts. The developed algorithms are installed in the German Aerospace Center’s (DLR) ground station Neustrelitz and can also be used in near-real-time.     

Multi-look processing of synthetic aperture radar data from dynamic ocean surfaces

The basic theory is described of multi-look processing of synthetic aperture radar (SAR) data from dynamic ocean surfaces. The principal effects inherent to multi-look SAR on the impulse response function are first illustrated using a moving point target model. The theory is then extended to the multi-look imagery of diffusely scattering ocean surface waves. Finally, the use of a sliding synthetic aperture is suggested to investigate the short life-times of small scale surface waves that are considered to be the predominant scatterers on the ocean surface.     

An assessment of a FMICW ground-wave radar system for ocean wave studies

In February 1982 the first successful trial of a frequency modulated, interrupted, continuous wave (FMICW), monostatic ground-wave radar for sea backscatter studies was carried out. Since then data have been collected in a number of different meteorological conditions over a range of HF frequencies. The data are presented and compared with simulated Doppler spectra obtained by evaluating the integral equation, which relates the sea backscatter spectrum to the ocean wave directional spectrum (Barrick 1972). The dynamic range performance of the system is as good as or better than has been achieved by pulse radars currently in operation. Good agreement with the theoretical formulation is demonstrated     

A standard index of spatial resolution for distributed targets in synthetic aperture radar imagery

In this article we outline the need for a consistent method of quoting synthetic aperture radar (SAR) resolution given the influence of speckle upon SAR images. Standard measures of resolution depend upon the separability of point targets; however, this is not a useful analogy in the context of SAR. We contend that quoting resolution for a 3–4-look product may be unrealistic given the influence of speckle. Our approach considers the separability of targets that differ in intensity by a known contrast ratio, with a ratio of 2, that is, 3 dB difference, used as the threshold value. It is demonstrated that 12 looks represents a more realistic estimate of the capabilities of the system and should be used to quote an equivalent spatial resolution (ESR) when describing potential instrument performance.     

Adaptive non-local level-set model for despeckling and deblurring of synthetic aperture radar imagery

ABSTRACT

In this article, we modify Mumford–Shah level-set model to handle speckles and blur in synthetic aperture radar (SAR) imagery. The proposed model is formulated using a non-local regularization framework. Hence, the model duly cares about local gradient oscillations (corresponding to the fine details/textures) during the evolution process. It is assumed that the speckle intensity is gamma distributed, while designing a maximum a posteriori estimator of the functional. The parameters of the gamma distribution (i.e. scale and shape) are estimated using a maximum likelihood estimator. The regularization parameter of the model is evaluated adaptively using these (estimated) parameters at each iteration. The split-Bregman iterative scheme is employed to improve the convergence rate of the model. The proposed and the state-of-the-art despeckling models are experimentally verified and compared using a large number of speckled and blurred SAR images. Statistical quantifiers are used to numerically evaluate the performance of various models under consideration.     

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.     

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.     

Wind-directional effects on the hydrodynamic modulation of microwave radar images of ocean waves

Abstract

A numerical simulation has been constructed of the hydrodynamic modulation transfer function (HMTF) which defines the spatial and spectral properties of swell waves as observed by an imaging radar. While the simulation is based on the Alpers and Hasselmann (1978) two-scale modulation transfer model, it explicitly takes account of directional properties of the short-wave spectrum and does not necessarily assume an isotropic wind-wave spectrum. Several different spreading functions are modelled and it is demonstrated that an anisotropic wind wave field significantly distorts the HMTF. Depending on the wind direction relative to the radar azimuth, the effect can be to shift the direction and the wave number of the peak of the image spectrum relative to the true swell spectrum. The effect of fetch limitation on the wind-wave spectrum is also examined based on the JONSWAP spectrum, but the consequences of this for the HMTF imaging mechanism are found to be minimal.     

Comparison of a new radar ocean imaging model with SARSEX internal wave image data

Abstract

Good agreement is demonstrated between both L-band and X-band SARSEX internal wave image data and the predictions of a new radar ocean imaging model that incorporates Bragg, specular, and composite scattering effects. It then follows that a two-step hydrodynamic modulation process, as hypothesized by Hughes and Gower, Thompson and Gasparovic, and Watson, does not appear lo be required to explain why the SARSEX L-band and X-band internal wave image modulations are comparable.     

Estimation of sea state from Sentinel-1 Synthetic aperture radar imagery for maritime situation awareness

A new empirical algorithm CWAVE_S1-IW for estimation of significant wave height H_s including swell and wind sea from C-band satellite-borne Synthetic Aperture Radar (SAR) data has been developed for Sentinel-1 (S1) Interferometric Wide Swatrh Mode (IW) imagery. The algorithm was implemented into the Sea Sate Processor (SSP) for fully automatic processing for near real time (NRT) services and allow the estimation of wave fields of thousands of kilometres in the flight direction and 250 km swath from S1 IW scenes consisting of a sequence of individual images.

The priority of CWAVE_S1-IW development was an automatic, fast and robust raster processing independent of wave patterns, applicable even when only clutter is visible in the SAR images. The algorithm is based on the spectral analysis of subscenes in wavenumber space. The empirical function allows direct H_s estimation from image spectra without first converting them into wave spectra and uses integrated image spectra parameters as well as estimated local wind information. A texture analysis based on Grey Level Co-occurrence Matrices (GLCM) is also applied. In this way, also the parameters of short waves can be estimated, which are not visible in S1 IW images and are only represented by clutter.

The algorithm was tuned worldwide using in-situ collocated measurements of 92 buoys with more than 2500 acquisitions. The validated SSP allows automatic processing of worldwide S1 IW images in VV or HH polarization, including Atlantic storms, cyclones, and huge storms in the Gulf of Alaska with a root-mean-square (RMSE) error of 80 cm for H_s. For the closed seas like the North Sea, Baltic Seas and Black Sea the accuracy is higher with an RMSE = 55 cm. The algorithm is integrated into a demonstration service, used for further validation at the DLR ground station in Neustrelitz. The NRT processing has been tested by supporting a research ship cruise in the Antarctic Sea.     

Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract

The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the different facet velocities within a SAR resolution cell. The imaging properties in this regime are largely governed by two non-dimensional parameters, the velocity bunching and velocity smearing parameter. The nonlinear imaging distortions are strongest for broad spectra (windseas) and are significantly weaker for narrow-band swell. In the linear and weakly nonlinear imaging regime, the superposition of the hydrodynamic and tilt cross-section modulation and the velocity bunching transfer function normally produces a rotation of the spectral peak towards the azimuthal direction. The interference characteristics of these different modulation mechanisms depends on the wave propagation direction and can lead to a significant distortion of the image. This is often seen in large differences in the image modulation depths of waves propagating parallel and anti-parallel to the flight direction.     

Observations of wind and ocean wave fields using ERS Synthetic Aperture Radar imagery

The aim of the research reported here is to evaluate Synthetic Aperture Radar (SAR) capability to estimate the wind vector and associated directional wave spectrum. Two ERS–2 SAR images of the Mediterranean Sea, one over the Sicily Channel and one over the Ligurian Sea, were selected as case studies. Wind speed was estimated using SAR calibrated backscatter response, in conjunction with empirically derived ERS scatterometer models such as CMOD4 and CMOD–IFREMER. The predictions of these models were then compared with the actual sea surface wave spectra either provided by in situ measurements or resulting from the inversion of the SAR image spectrum. SAR-detected effects of both wind and wave features, induced either by atmospheric boundary layer instability or by land shadowing, were also used as reliable indicators of wind direction.     

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.     

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.     

Cover: Detection of oil spills near offshore installations using synthetic aperture radar (SAR)

The number of sunspots is characterized by a long-term temporal variation, reaching its maximum or its minimum approximately every 11 years (the solar cycle). This variation, in turn, has an effect in terms of variation in the global climate. Since 1979, the use of satellite-borne radiometers has enabled accurate measurements of total solar irradiance (TSI). For instance, the sunspot numbers that are scaled to correspond to Nimbus-7 TSI observations for 1979–1993 show little long-term trend. However, while the observations of different extremes of the solar cycle, which are available from 1749, seem irregular in time and magnitude and difficult to quantify, they appear to have a strong correlation between them when they are sorted pairwise according to their size rather than sequentially in time. A similar relation holds among the solar cycle lengths (periods) and the solar cycle extremes, which, in parallel, obey a linear relation that is reminiscent of the Gutenberg–Richter seismic law. This can be used for a probabilistic approach to forecast solar parameters that are connected to global climate.

Based on the reasonable assumption that the basic parameters like extremes and length of the 11-year solar cycle are associated with the energy oscillating between the dipolar and quadrupolar phases of the cycle, it is concluded that these parameters obey a power-law distribution similar to that of the Gutenberg–Richter seismic law.

The question of whether solar activity is deterministically chaotic is also investigated by exploring the behaviour of the main characteristics of the 11-year solar cycle. This is done by constructing return maps of solar cycle strength and duration, which seem to take the familiar up–down U shape, implying both non-linearity and re-injection. The results suggest that there might exist a coupling between two or three different non-linear deterministic dynamical systems on the Sun, depending on which variable of the solar cycle is being considered.     

地形坡度对INSAR系统参数设计的影响

利用干涉合成孔径雷达(INSAR)可以获取数字高程图(DEM)，为了得到高精度的DEM需要对干涉系统参数进行优化设计。通过对INSAR系统单视和多视处理时的测高误差分析，在考虑地形坡度的影响下，利用相同的准则对系统参数——天线视角、信号带宽和工作基线进行了优化设计，得出了最优参数随地形变化的规律性，并证明了单视和多视处理情况下优化设计的一致性，为干涉系统的合理设计和性能分析提供了理论指导。     

The computation of ocean wave heights from GEOS-3 satellite radar altimeter data

The GEOS-3 satellite, carrying a short pulse radar altimeter, was launched into orbit round the earth in April 1975. The altimeter was designed to provide an accurate measurement of the distance of the satellite above the earth, and also to record the shape of the radar return pulse as a measure of the mean roughness of the earth's surface. The satellite is intended to operate over the oceans, where surface height changes show variations in the earth's gravitational field, and roughness changes are due to waves.This paper is concerned with methods of determining waveheights from the shape of the radar return pulse and the corrections that have to be taken into account. The effects of timing variations on the shape of the average return pulse shape are discussed in detail. Accurate calibration of the sampling gates that measure this shape is found to be particularly critical.The waveheights deduced are compared with ground truth derived from ship reports on waveheights in the N.E. Pacific Ocean and routine measurements made at Ocean Weather Station PAPA. It is found that with suitable calibration and adjustments, the satellite measurements agree with surface observations to about 0.5 meters in $\text{H}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$.     

Measurement of ocean surface slopes and wave spectra using polarimetric SAR image data

Methods have been investigated which use fully polarimetric synthetic aperture radar (SAR) image data to measure ocean slopes and wave spectra. Independent techniques have been developed to measure wave slopes in the SAR azimuth and range directions. The azimuth slope technique, in particular, is a more direct measurement than conventional, intensity based, backscatter cross-section measurements.In the azimuth direction, wave-induced perturbations of the polarimetric orientation angle are used to sense the wave slopes. In the range direction, a new technique involving the alpha parameter from the Cloude-Pottier H-A-? (Entropy, Anisotropy, and (averaged) Alpha) polarimetric scattering decomposition theorem is used to measure slopes. Both measurement types are sensitive to ocean wave slopes and are directional. Taken together, they form a means of using polarimetric SAR (POLSAR) image data to make complete measurements of either ocean wave slopes, or directional wave spectra.These measurements must still contend with fundamental nonlinearities in the SAR image processing (i.e., azimuth direction “velocity bunching”) that are due to wave velocity and acceleration effects.NASA/JPL/AIRSAR L-, and P-band data from California coastal waters were used in the studies. Wave parameters measured using the new methods are compared with those developed using both conventional SAR intensity based methods, and with in situ NOAA National Data Center buoy measurement products.     

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.