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The optimization of normalized radar cross-section (NRCS) sampling by a scatterometer allows an increase in the accuracy of the wind retrieval over the water surface and a decrease in the time of the measurement. Here, we investigate the possibility of improving wind vector measurement with an airborne rotating-beam scatterometer mounted under the fuselage. For this purpose, we investigated NRCS sampling at various incidence angles, and the possibility of using NRCS samples obtained during simultaneous measurement at different incidence angles to perform wind retrieval. The proposed wind algorithms are based on a geophysical model function (GMF). Sea wind retrieval was carried out using Monte Carlo simulations with consideration of a single incidence angle or combinations of several incidence angles. The incidence angles of interest were 30°, 35°, 40°, 45°, 50°, 55°, and 60°. The simulation showed that the wind speed error decreased with an increase in the incidence angle, and the wind direction error tended to decrease with an increase in the incidence angle. The single incidence angle case is characterized by higher maximum wind retrieval errors but allows for a higher maximum altitude of the wind retrieval method’s applicability to be achieved. The use of several neighboring incidence angles allows a better wind vector retrieval accuracy to be achieved. The combinations of three and four incidence angles provided the lowest maximum wind speed and direction errors in the range of the incidence angles from 45° to 60° but, unfortunately, provide the lowest maximum altitude of applicability of the wind retrieval method. At the same time, the combination of two incidence angles is characterized by slightly higher maximum wind retrieval errors than in the cases of three and four incidence angles, but they are lower than in the case of the single incidence angle. Moreover, the two incidence angles’ combination is a simpler way to decrease the wind retrieval errors, especially for measurement near an incidence angle of 30°, providing nearly the highest maximum altitude of the wind retrieval method applicability. The results obtained can be used to enhance existing airborne radars and in the development of new remote sensing systems.

In this paper, a novel automated coastline extraction method from SAR (Synthetic Aperture Radar) data is presented. The method is designed to exploit radar backscatter coefficients (σ0) from multip...

Following the mission science plan of EPS/Metop-SG C-band scatterometer for 2023&ndash, 2044, we consider the potential application of the sea ice/water discrimination method based on the minimum statistical distance of the measured normalized radar cross sections (NRCS) to the geophysical model functions (GMF) of the sea ice and water, respectively. The application of the method is considered for the classical spacecraft scatterometer geometry with three fixed fan-beam antennas and the hypothetical prospective scatterometer geometry with the five fixed fan-beam antennas. Joint vertical (VV) and horizontal (HH) transmit and receive polarization are considered for the spaceborne scatterometer geometries. We show explicitly that the hypothetical five fixed fan-beam antenna geometry combined with the dual VV and HH polarization for all antennas provides better estimates of the sea wind speed and direction as well as sea ice/water discrimination during single spacecraft pass. The sea ice/water discrimination algorithms developed for each scatterometer geometry and dual VV/HH polarization are presented. The obtained results can be used to optimize the design of new spaceborne scatterometers and will be beneficial to the forthcoming satellite missions.

Radiometric calibration adjusts the measured pixel intensity to a physical property, the radar cross section (RCS). After calibration, this relationship is defined over the entire backscatter range: from low image power (near noise) up to high reflections (below saturation). Based on a proper radiometric calibration, the measured radar backscatter for the Sentinel-1 synthetic aperture radar (SAR) satellite constellation is validated over a wide backscatter range using different target types. Therefore, the RCS derived from point targets and radar brightness from distributed targets are compared between Sentinel-1A (S-1A) and Sentinel-1B (S-1B) acquisitions over the same observation area for regions where a stable target backscatter is expected for a certain period of time. Low differences (in the order of 0.3 dB) are found between S-1A and S-1B for high and medium backscatter derived from point targets or rainforest regions, but higher differences for low backscattering regions like ice areas and lakes. For comparing radar brightness containing low backscatter targets, an accurate derived noise level has to be taken into account. In addition to the measured lower noise equivalent beta zero (NEBZ) level, higher transmit power was detected for S-1B compared to S-1A. The higher antenna gain of S-1B leads finally to a higher sensitivity for low backscattering areas of S-1B compared to S-1A and explains the found differences.

We consider sea ice and water microwave backscatter features at the C-band with vertical transmit and receive polarization and present a method for sea ice/water discrimination using a multiple fixed fan-beam satellite scatterometer. The method is based on the criterion of the minimum statistical distance of measured backscatter values to the sea ice and water (CMOD7) geophysical model functions. Implementation of the method is considered both for a typical three fan-beam geometry as well as for a potential five fan-beam geometry of a satellite scatterometer. By using computer simulations, we show explicitly that the number of looks at the same cell from different azimuthal directions needs to be increased to provide better (unambiguous) retrieval of the wind vector and sea ice/water discrimination. The algorithms for sea ice/water discrimination are described, and the results obtained are also discussed along with recommendations for the number of different azimuthal looks (beams) at the same cell from the point of view of sea ice/water discrimination as well as unambiguous wind direction retrieval during the satellite&rsquo, s single pass.

Altimeter radar backscatter intensity, in terms of the normalized radar cross section (NRCS), is known to be modulated by surface wind forcing and the state of wind-sea development. Based on a data...

Extension of the existing airborne radars’ applicability is a perspective approach to the remote sensing of the environment. Here we investigate the capability of the rotating-beam radar installed over the fuselage for the sea surface wind measurement based on the comparison of the backscatter with the respective geophysical model function (GMF). We also consider the robustness of the proposed approach to the partial shading of the underlying water surface by the aircraft nose, tail, and wings. The wind retrieval algorithms have been developed and evaluated using Monte-Carlo simulations. We find our results promising both for the development of new remote sensing systems as well as the functional enhancement of existing airborne radars.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2019-09-27, 資料番号: PA2010006000

It is demonstrated that the synthetic aperture radar altimeter (SRAL) on board of the Sentinel-3A can detect short-period internal solitary waves (ISWs) with scales of the order of a kilometer. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter ( σ 0 ) and sea level anomaly (SLA). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track direction resulting from the sharpened SRAL footprint. The SRAL footprint is sufficiently small (300 m in the along-track direction) to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers along-track. Furthermore, it was possible to calculate the mean square slope ( s 2 ¯ ) for the dual-band (Ku and C bands) altimeter of Sentinel-3, which made the ISW signatures unambiguously identified because of the large s 2 ¯ variations in exact synergy with ocean and land color instrument (OLCI) images. Hence, the detection method is validated in cloud-free sun glint OLCI images. It is shown that both σ 0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL. The detection method that is used relies on the parameter s 2 ¯ which is calculated from σ 0 . This is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). An algorithm is developed to be used in any ocean region. Wavelets were applied for a first analysis of the s 2 ¯ variations because ISWs can be readily identified in high-frequency signals. Other geophysical parameters such as SLA were used to exclude phenomena that are unlikely to be ISWs.

For most land and ice surfaces, the measured radar backscatter at the spatial resolution of a wind scatterometer is insensitive to the azimuth angle. However, for regions of East Antarctica, the backscatter strongly depends on the azimuth angle. This relationship between backscatter and azimuth angle is often modeled with a Fourier series. Although previous work has separately examined the data from QuikSCAT, a Ku-band scatterometer, and the Advanced Scatterometer (ASCAT), a C-band scatterometer, this letter compares the two on the same high-resolution grid. We find that, although QuikSCAT has superior azimuth angle coverage (due to its measurement geometry) compared to ASCAT, both are suitable to estimate the radar backscatter azimuth angle modulation of East Antarctica. The ASCAT data exhibit a much larger azimuth modulation than the QuikSCAT data. This is attributed to the different wavelengths of the microwave signal: At C-band (5.7 cm), East Antarctica has features that show radar backscatter to be more dependent on azimuth angle than at Ku-band (2.2 cm). This letter also examines the ASCAT and QuikSCAT azimuth modulation over previously identified regions of wind glaze. Although azimuth modulation is expected to be minimal over wind glaze, we find the wind-glaze regions to contain more structure than previously suggested.

We use repeat synthetic aperture radar (SAR) observations and complementary altimetry passes acquired by the Cassini spacecraft to study the scattering properties of Titan’s empty lake basins. The best-fit coefficients from fitting SAR data to a quasi-specular plus diffuse backscatter model suggest that the bright basin floors have a higher dielectric constant, but similar facet-scale rms surface facet slopes, to surrounding terrain. Waveform analysis of altimetry returns reveals that nadir backscatter returns from basin floors are greater than nadir backscatter returns from basin surroundings and have narrower pulse widths. This suggests that floor deposits are structurally distinct from their surroundings, consistent with the interpretation that some of these basins may be filled with evaporitic and/or sedimentary deposits. Basin floor deposits also express a larger diffuse component to their backscatter, which is likely due to variations in subsurface structure or an increase in roughness at the wavelength scale (Hayes, A.G. et al. [2008]. Geophys. Res. Lett. 35, 9). We generate a high-resolution altimetry radargram of the T30 altimetry pass over an empty lake basin, with which we place geometric constraints on the basin’s slopes, rim heights, and depth. Finally, the importance of these backscatter observations and geometric measurements for basin formation mechanisms is briefly discussed.

Recently, one of the authors (A. V. G.) developed a theory of low-frequency plasma irregularities which are created as a consequence of neutral turbulence in the D and lower E regions. In the following this theory will be applied to coherent backscatter experiments with radars in a frequency range between 5 and 150 MHz. We discuss the dependence of the backscatter cross-section on ionospheric as well as on turbulence parameters. The backscatter increases strongly with decreasing radar frequency. Above 15 MHz the effects discussed here can probably only be detected by very powerful radars with large antenna arrays.

It is well known that internal waves (IWs) of tidal frequency (i.e., internal tides) are successfully detected in sea surface height (SSH) by satellite altimetry. Shorter period internal solitary waves (ISWs), whose periods (and spatial scales) are an order of magnitude smaller than tidal internal waves, have been generally assumed too small to be detected with conventional altimeters. This is because conventional (pulse-limited) radar altimeter footprints are somewhat larger than or of similar size, at best, as the typical wavelengths of the ISWs. Here we demonstrate that the synthetic aperture radar altimeter (SRAL) on board the Sentinel-3A can detect short-period ISWs. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter (sigma0), sea level anomaly (SLA), and significant wave height (SWH). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track sharpened SRAL footprint. The SRAL footprint is sufficiently small to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers. The ISW signatures are unambiguously identified in the SRAL because of the exact synergy with OLCI (Ocean Land Colour Imager) images, which in cloud-free conditions allow clear identification of the ISWs in the sunglint OLCI images. We show that both sigma0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL, which is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). Several case studies of ISW signatures are interpreted in light of our knowledge of radar backscatter in the internal wave field. An analysis is presented for the tropical Atlantic Ocean off the Amazon shelf to infer the frequency of the phenomena, being consistent with previous satellite observations in the study region.

Radar backscatter measurements under various road conditions are presented in this paper. With the aid of a millimeter-wave automotive radar, backscatter measurements of an asphalt road surface were obtained for a stationary and moving radar platform and under dry, rainy, snowy, and slushy conditions. The effect of the conditions on the radar's backscatter are examined, and the feasibility of a road-condition identification scheme is demonstrated.

A Frequency-Modulated Continuous Wave radar signal model is presented. The model demonstrates the underlying principle for the simultaneous measurement of range and velocity typically employed in automotive applications. Model validation against radar measurements are presented. The model is used to demonstrate phenomena such as the influence of target velocity on radar backscatter and the range-velocity uncertainty principle.

In this study, the radar backscatter features of the shallow water topography of Subei bank in the Southern Yellow Sea are investigated using ENVISAT (Environmental Satellite) ASAR (advanced synthetic aperture radar) images. Different bathymetric features are found on SAR imagery, which correspond to sea surface imprints of tidal channels or sand ridges, respectively. Preliminary analysis demonstrates that the ocean current and surface wind, as well as the wave breaking, play a significant role in the SAR imaging of shallow water topography in this region.

The paper reports on an extensive set of outdoor measurements aimed at characterizing the polarimetric radar backscatter response of vehicles at 222 GHz. This effort supports the future application of high frequency radars operating around 230 GHz in autonomous vehicle navigation and collision avoidance. It is observed that radar backscatter is primarily from the outer surfaces of the vehicle and that few strong scattering centers dominate the radar return. Derived radar statistics from reported data may be used by researchers in optimizing detection.

Modulation model of radar backscatters is an important topic in the remote sensing of oceanic internal wave by synthetic aperture radar (SAR). Previous studies related with the modulation models were analyzed mainly based on the hypothesis that ocean surface waves are Gaussian distributed. However, this is not always true for the complicated ocean environment. Research has showed that the measurements are usually larger than the values predicted by modulation models for the high frequency radars (X-band and above). In this paper, a new modulation model was proposed which takes the third-order statistics of the ocean surface into account. It takes the situation into consideration that the surface waves are Non-Gaussian distributed under some conditions. The model can explain the discrepancy between the measurements and the values calculated by the traditional models in theory. Furthermore, it can accurately predict the modulation for the higher frequency band. The model was verified by the experimental measurements recorded in a wind wave tank. Further discussion was made about applicability of this model that it performs better in the prediction of radar backscatter modulation compared with the traditional modulation model for the high frequency band radar or under lager wind speeds.

Using the quantitative geophysical model function (GMF) between the radar backscatter coefficient and the sea surface wind speed, wind direction, radar parameters and environmental parameters, the wind vector can be retrieved from backscattering measurement. In this paper, Extreme learning machine (ELM) approach is used to develop a unified GMF respectively using the simulated training data-set generated by the empirical GMF CMOD5.N and the wind data gained from the ASCAT. Analysis indicates that the method based on extreme learning machine showing a good inversion result compared with CMOD5.N with fast training and high accuracy. The new method provides a novel feasible way for future surface wind field inversion.

The contribution of electromagnetic wave scattering on density irregularities in the volume component of radar backscatter was analyzed for a thick snow pack containing internal hoar/ice layers. To evaluate the efiect of this scattering, Density Deviation Factor (DDF), a statistical parameter, was introduced into the backscattering coe-cient using the \slice" approach. DDF is proportional to the intensity of the density ∞uctuation and inverse to the mean density. The inverse dependence of backscatter with accumulation rate was discussed based on the DDF parameterization of snow inhomogeneities.

P-band radar remote sensing applied during the Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) mission has shown great potential for estimation of root zone soil moisture. When retrieving the soil moisture profile (SMP) from P-band radar observations, a mathematical function describing the vertical moisture distribution is required. Because only a limited number of observations are available, the number of free parameters of the mathematical model must not exceed the number of observed data. For this reason, an empirical quadratic function (second order polynomial) is currently applied in the AirMOSS inversion algorithm to retrieve the SMP. The three free parameters of the polynomial are retrieved for each AirMOSS pixel using three backscatter observations (i.e., one frequency at three polarizations of Horizontal-Horizontal, Vertical-Vertical and Horizontal-Vertical). In this paper, a more realistic, physically-based SMP model containing three free parameters is derived, based on a solution to Richards’ equation for unsaturated flow in soils. Evaluation of the new SMP model based on both numerical simulations and measured data revealed that it exhibits greater flexibility for fitting measured and simulated SMPs than the currently applied polynomial. It is also demonstrated that the new SMP model can be reduced to a second order polynomial at the expense of fitting accuracy.

This paper evaluates the rain effects on Ku-band radar backscatter at low incidence angles. The data used consisted of the sea surface backscatter and averaged rain rates from Tropical Rainfall Mapping Mission precipitation radar (TRMM PR) measurements and the collocated 10-m height numerical prediction wind speeds from the European Centre for Medium-Range Weather Forecasts (ECMWF). The wind-induced backscatter was estimated by the Ku-band low incidence backscatter model (KuLMOD) and possible bias due to different wind speed inputs was considered. The rain effect was analysed by comparing the TRMM PR-measured surface backscatter for the rain-affected sea surface with the collocated wind-induced backscatter. We found that the surface backscatter decreases with increases in the averaged rain rate. The rain-induced backscatter was clearly dependent of the wind speed and was slightly dependent of the incidence angle. Results show that it is necessary to develop a wind and rain backscatter model instead of single wind backscatter model.

The water cloud model is used to account for the effect of vegetation water content on radar backscatter data. The model generally comprises two parameters that characterize the vegetated terrain, A and B, and two bare soil parameters, C and D. In the present study, parameters A and B were estimated using a genetic algorithm (GA) optimization technique and compared with estimates obtained by the sequential unconstrained minimization technique (SUMT) from measured backscatter data. The parameter estimation was formulated as a least squares optimization problem by minimizing the deviations between the backscatter coefficients retrieved from the ENVISAT ASAR image and those predicted by the water cloud model. The bias induced by three different objective functions was statistically analysed by generating synthetic backscatter data. It was observed that, when the backscatter coefficient data contain no errors, the objective functions do not induce any bias in the parameter estimation and the true par...

Synthetic aperture radar (SAR) imagery from the sea can contain ships and their ambiguities. The ambiguities are visually identifiable due to their high intensities in the low radar backscatter background of sea environments and can be mistaken as ships, resulting in false alarms in ship detection. Analysing polarimetric characteristics of ships and ambiguities, we found that (a) backscattering from a ship consisted of a mixture of single-bounced, double-bounced and depolarized or diffused scattering types due to its complex physical structure; (b) that only a strong single- or double-bounce scatterer produced ambiguities in azimuth that look like relatively strong double- or single-bounce scatterers, respectively; and (c) that eigenvalues corresponding to the single- or double-bounce scattering mechanisms of the ambiguities were high but the eigenvalue corresponding to the depolarized scattering mechanisms of the ambiguities was low. With these findings, we proposed a ship detection method that applies t...

In the past, different soil moisture estimation models were developed using remotely sensed data. Some of these models are based on optical images (i.e., optical models), some are based on synthetic aperture radar (SAR) images (i.e., SAR models), and a few models were developed based on integration of optical and SAR images (i.e., hybrid models). In this study, these three different types of soil moisture estimation models are compared. Results show that generally the SAR models are more accurate than the optical models, and by using the hybrid models the accuracies improve.Using a calibrated Dubois model, a root mean square surface roughness parameter was estimated for all the in situ data. This parameter was then considered in the SAR models to correct the surface roughness effects on radar backscatter coefficients. Consideration of this parameter improved the SAR models.Vegetation water content (VWC) is an important parameter in the SAR models to correct vegetation effects on radar backscattering coeff...

We show the results of a statistical study on the effects in the high-latitude ionosphere of abrupt variations of solar wind dynamic pressure, using Super Dual Auroral Radar Network (SuperDARN) data in both hemispheres. We find that, during periods of quiet ionospheric conditions, the amount of radar backscatter increases when a variation in the dynamic pressure occurs, both positive (increase of the pressure) and negative (decrease of the pressure). We also investigate the behaviour of the Cross-Polar Cap Potential (CPCP) during pressure variations and show preliminary results.

The radar backscatter from various earth surfaces is sensitive to the frequency of the incident wave. This study examined the radar sensitivities for surface parameters such as soil moisture content and surface roughness of both bare and vegetated surfaces at X-band. Because L-band frequencies are often used for sensing the surface parameters, the sensitivities of X-band are also compared with those of the L-band. The sensitivities of the X-band radar backscatter were examined with respect to soil moisture content and surface roughness of rough bare soil surfaces. These sensitivities were also examined using the same parameters for vegetated surfaces for various vegetation densities and incidence angles. Use of the X-band radar for soil moisture detection was as effective as L-band radar for bare soil surfaces. For vegetated surfaces, the soil moisture could be detected using an X-band radar at lower incidence angles, where the upper limit of the incidence angles was dependent on vegetation density.

Synthetic aperture radar (SAR) data at X-, C-, and L-bands have been investigated to determine the relationship between backscatter and forest burn severity over three sites in Spain. The dependence of SAR backscatter on local incidence angle and environmental conditions has been analyzed. At HH and VV polarizations, the backscatter increased with burn severity for X- and C-bands, whereas it decreased for L-band. Cross-polarized (HV) backscatter decreased with burn severity for all frequencies. Determination coefficients were used to quantify the relationship between radar backscatter and burn severity for given intervals of local incidence angle. For X- and C-band copolarized data, higher determination coefficients were observed for slopes oriented toward the sensors, whereas for cross-polarized data, the determination coefficients were higher for slopes oriented away from the sensor. At L-band, the association strength of cross-polarized data to burn severity was high for all local incidence angles. C- and L-band cross-polarized backscatter showed better potential for burn severity estimation in the Mediterranean environment when the local incidence angle is accounted for. The small dynamic range observed for X-band data could hinder its use in forests affected by fires.

A new algorithm is proposed to estimate significant wave height from QuikSCAT scatterometer data. The results show that the relationship between wave parameters and the radar backscattering cross section is similar to that between wind and the radar backscattering cross section. Therefore, the relationship between significant wave height and the radar backscattering cross section is established with a neural network algorithm. If the average wave period is less than or equal to 7 s, the root mean square errors of the significant wave height retrieved from QuikSCAT data are 0.58 m for HH polarization (HH-pol) and 0.60 m for VV polarization (VV-pol). If the average wave period is greater than 7 s, the root mean square errors of the significant wave height retrieved from QuikSCAT data are 0.83 m (HH-pol) and 1.10 m (VV-pol), respectively.

The high spatio-temporal variability of soil moisture is the result of atmospheric forcing and redistribution processes related to terrain, soil, and vegetation characteristics. Despite this high variability, many field studies have shown that in the temporal domain soil moisture measured at specific locations is correlated to the mean soil moisture content over an area. Since the measurements taken by Synthetic Aperture Radar (SAR) instruments are very sensitive to soil moisture it is hypothesized that the temporally stable soil moisture patterns are reflected in the radar backscatter measurements. To verify this hypothesis 73 Wide Swath (WS) images have been acquired by the ENVISAT Advanced Synthetic Aperture Radar (ASAR) over the REMEDHUS soil moisture network located in the Duero basin, Spain. It is found that a time-invariant linear relationship is well suited for relating local scale (pixel) and regional scale (50 km) backscatter. The observed linear model coefficients can be estimated by considering the scattering properties of the terrain and vegetation and the soil moisture scaling properties. For both linear model coefficients, the relative error between observed and modelled values is less than 5 % and the coefficient of determination (R-2) is 86 %. The results are of relevance for interpreting and downscaling coarse resolution soil moisture data retrieved from active (METOP ASCAT) and passive (SMOS, AMSR-E) instruments.

Combination of laser and radar aboard an aircraft is used to directly measure long gravity wave surface tilting simultaneously with nadir-viewing microwave backscatter from the sea surface. The presented dataset is extensive, encompassing varied wind conditions over coastal and open-ocean wave regimes. Laser-derived slope statistics and Ka-band (36 GHz) radar backscatter are detailed separately to document their respective variations versus near-surface wind speed. The slope statistics, measured for λ > 1–2 m, show good agreement with Cox and Munk's oil-slickened sea measurements. A notable exception is elevated distribution peakedness and an observed wind dependence in this likely proxy for nonlinear wave–wave interactions. Aircraft Ka-band radar data nearly mimic Ku-band satellite altimeter observations in their mean wind dependence. The present calibrated radar data, along with relevant observational and theoretical studies, suggest a large (−5 dB) bias in previous Ka-band results. Next, wave-diverse inland, coastal, and open-ocean observations are contrasted to show wind-independent long-wave slope variance changes of a factor of 2–3, always increasing as one heads to sea. Combined long-wave and radar data demonstrate that this long-wave tilt field variability is largely responsible for radar backscatter variations observed at a given wind speed, particularly at wind speeds below 5–7 m s−1. Results are consistent with, and provide quantititative support for, recent satellite altimeter studies eliciting signatures of long-wave impacts resident in the radar backscatter. Under a quasi-optical scattering assumption, the results illustrate long-wave control on the variance of the total mean square slope parameter due to changes in the directional long-wave spectrum, with high-wavenumbers being relatively unaffected in a mean sense. However, further analysis suggests that for winds above 7 m s−1 the high-wavenumber subrange also varies with change in the longer wave field slope and/or energy, the short gravity wave roughness being measurably greater for smoother seas.

Synthetic Aperture Radar (SAR) imaging of ocean surface features is studied. The simulation of the turbulent and vortical features generated by a moving ship and SAR imaging of these wakes is carried out. The turbulent wake damping the ocean surface capillary waves may be partially responsible for the suppression of surface waves near the ship track. The vortex pair generating a change in the lateral flow field behind the ship may be partially responsible for an enhancement of the waves near the edges of the smooth area. These hydrodynamic phenomena as well as the changes of radar backscatter generated by turbulence and vortex are simulated. An SAR imaging model is then used on such ocean surface features to provide SAR images. Comparison of two ships’ simulated SAR images shows that the wake features are different for various ship parameters.

In this study an established microwave backscatter model is used to predict the radar backscatter behavior of rice and to understand the interaction between backscatter and the rice canopy during its growth cycle. The emphasis of this study is to understand the effect of physical plant parameters on the backscatter signatures as a function of polarization and how these signatures vary during a complete growth cycle of rice. Inputs to the backscatter model included the physical parameters of rice as obtained through field measurements. These measurements were acquired within a few days of multiple RADARSAT acquisitions of the Zhaoqing test site in southern China. RADARSAT observations and the modeling results were then compared and analyzed. The results show that the interaction mechanisms change during the rice growth cycle and polarimetric and (or) multi-polarization measurements at C band will contribute to rice monitoring programs.

Radar backscatter knowledge is a valuable input for many remote sensing applications, which are based on synthetic aperture radar (SAR) systems. The amount of spaceborne data acquired within the TanDEM-X mission allows for the characterization of X-band backscatter on a global scale. The objective of this paper is to present a method for the characterization of radar backscatter behavior using a global statistical approach. The worldwide data set of images acquired within the TanDEM-X mission is taken into account, having the chance to exploit the unique high-quality topography information associated to it. The input measurements are differently assessed, by using a quality-based approach. A series of models can be derived, focusing on the backscatter dependence on polarization, incidence angle, ground classification, and seasonal time. The developed approach for the algorithm's verification is presented as well, together with some preliminary results obtained from TanDEM-X mission data. The generation of up-to-date backscatter models for X-band will provide a useful database for the development of a large number of scientific applications and for the optimization of future radar systems.

International audience; The goal of this study is to discuss the effect of multi-frequency radar configurations on the relationship between surface soil moisture and the nature of radar backscatter over bare soils. In an attempt to answer this question, the Advanced Integral Equation Model (AIEM) was used to simulate backscatter from soil surfaces with various moisture vertical profiles, for three frequency bands: L, C and X. In these computations, we investigated the influence of the vertical heterogeneity of soil moisture on the characteristics of the backscattered signals. The effect of radar frequency is distinctly demonstrated. A database produced from Envisat ASAR and TerraSAR-X data, acquired over bare soils with in situ measurements of moisture content and ground surface roughness, was used to validate the usefulness of taking the soil moisture heterogeneity into account in the backscattering model.

Thermodielectric interactions between free and bound soil water at different soil textures and frequencies affect radar backscatter. Simulations with a semi-empirical backscatter model show a potential for temperature corrections in inference of water content. A new thermodielectric response backscatter difference index was derived for remote mapping of surface soil texture.

The technology of using spaceborne synthetic aperture radar (SAR) systems for soil moisture estimation has been refined over the last few years. The potential of microwave sensors to estimate soil moisture is well known, and its continuous monitoring on temporal and spatial bases has been realized recently. Several techniques have been developed for retrieving the surface parameters and soil moisture from the radar backscatter. In order to reduce the confounding effects of surface roughness on soil moisture inversion, the application of multifrequency SAR systems have shown promise. The shuttle imaging radar mission C (SIR-C) had an on board SAR system operating at L-, C-, and X-bands for high-resolution imaging of the Earth's surface. Data from SIR-C SAR have been investigated for soil moisture estimation and comparison with in situ data. The models used for soil moisture inversion, viz., (1) the linear regression, (2) the linear statistical inversion, and (3) the neural network models, are presented, and the results of soil moisture estimation using these models are compared. The resulting estimation of soil moisture using the above models is more accurate for the surface soil moisture than subsurface soil moisture estimation, as expected. In general, these models estimate soil moisture within a root mean squared (RMS) error of 3–5%.

Radar imaging is a valuable tool for the monitoring and management of tropical ecosystems. One of the obstacles to its successful usage is the variation of the radar backscatter with incidence angle caused by the side-looking mode of the sensor. Available research has modelled the effect of changes in incidence angle for soil, grass and scattering mechanisms in tree-covered areas. The effect varies depending on the scattering mechanism. To model the radar backscatter behaviour of a natural landscape, therefore, implies precise knowledge of the land-cover composition. A method for estimating and correcting the effect of changes in look angle on backscatter data is consequently proposed; it requires little field knowledge and encompasses the effects on most land-cover types in the study area. The method is based on the assumption that each line in azimuth direction contains a similar composition in regard to land-cover types. The backscatter frequency distribution of each azimuth line can then be employed to model and correct for the effect of variation in incidence angle. The results are evaluated using an existing land-cover classification to extract mean backscatter values for individual land-cover classes before and after the correction procedure. The correction procedure is shown to successfully adjust backscatter intensities to a nominal incidence angle for the vegetation communities in a coastal tropical savanna landscape in Australia's Northern Territory.

The use of spaceborne synthetic aperture radar (SAR) systems to estimate stem volume and biomass in boreal forests has shown some promising results, but with saturation of the radar backscatter at ...

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SYNERGIE [Axe_IRSTEA]TETIS-ATTOS; International audience; The aim of this letter is to discuss the influence of radar frequency on the relationship between surface soil moisture and the nature of radar backscatter over bare soils. In an attempt to address this issue, the advanced integral equation model was used to simulate backscatter from soil surfaces with various moisture vertical profiles, for three frequency bands, namely, L, C, and X. In these computations, we investigated the influence of the vertical heterogeneity of soil moisture on the characteristics of the backscattered signals. The influence of radar frequency is clearly demonstrated. A database produced from Envisat ASAR and TerraSAR-X data, which was acquired over bare soils with in situ measurements of moisture content and ground surface roughness, was used to validate the utility of taking the soil moisture heterogeneity into account in the backscatter model.

C-band radar backscatter generally seems to be inversely related to accumulation rate across Greenland's dry snow zone. A coupled snow metamorphosis-backscatter model was constructed in order to investigate the observation. Accumulation rate and mean annual temperature were the coupled-model inputs. In a local area where accumulation rate is known very accurately the model predicts the backscatter level along several transects within the calibration uncertainties of the ERS-1 SAR data. The analysis was applied to the remainder of the dry snow zone. The model showed that both accumulation rate and temperature can modulate surface backscatter across Greenland's dry snow zone with accumulation rate being the primary influence.

Accepted: 1998-03-04, 資料番号: SA1001238000

In this paper we demonstrate SAR system capability for detecting and characterizing marine surface slicks. During an aircraft measurement campaign over the Gulf of Genoa (Italy), a multi-frequency SAR system, operating in P-, L- and C-bands, explored a sea area heavily covered by slicks. At the same time in situ measurements were performed with an interferential microwave probe, installed on board a small boat, capable of measuring high resolution sea spectra up to frequencies of capillary waves. By plotting SAR pixel intensity versus sea wave Bragg frequency we obtained wide portions of the sea spectrum region affected by the surface film damping. Spectra derived from SAR imagery and from gauge data present comparable slopes and furthermore the ratio between clean to slicked water spectrum obtained with the two techniques were surprisingly similar. This demonstrates the multi-frequency SAR systems ability to detect and characterize sea surface films assuming the Bragg mechanism in the radar backscatter. ...

A full understanding of radar backscatter from urban areas is necessary in order to develop a robust methodology for monitoring and classifying urban characteristics using remotely sensed Synthetic Aperture Radar images.This paper examines the dominant backscattering mechanisms such as single bounce from roofs, double bounce from wall-ground structures and possibly triple bounce from wall-wall-ground structures, and their relative contributions to the backscatter. With the use of quad-polarized image data such as those acquired by the NASA/JPL AirSAR system, the backscatter can be decomposed into components caused by different backscattering mechanisms, offering a promise for urban monitoring and classification.

ERS-1 Windscatterometer measurements over Siberia were investigated to determine the potential of coarse resolution radar data for land cover mapping and monitoring. Two indices have been introduced: the slope index calculates the incidence angle dependence of the radar measurements, the radar backscatter index (RBSI) combines slope index and radar backscatter intensity. Using the RBSI, good correlations to canopy density were found and the freeze/thaw-transition can be determined. Promising correspondence was found with five-day interval time series of normalized radar cross sections and crop yield estimates.

This study investigates the improvement in above ground biomass estimates when using a synergistic model based on lidar derived forest structural information (i.e., canopy cover percentage) and radar backscatter. The results were cross-compared with a radar only model. A two-layered radar backscatter model was also tested. The results showed that lidar-based structural information has the potential to increase the accuracy of biomass estimation by up to 20% depending on polarization and acquisition date. A smaller improvement was observed when using a modeled estimate of the forest canopy cover as would be the case of a future lidar/radar joint space-borne mission. The two-layered vegetation backscatter model did not improve the biomass estimation accuracy with errors being higher when compared to a single-layer vegetation model.

The principles of coherent backscatter from irregularities created by plasma instabilities in the ionospheric E-region are described in this paper. It is intended as an introduction for readers who are not familiar with this subject and not as a review article. The general principles of radar backscatter are outlined as well as the nature of plasma instabilities that cause the irregularities. In subsequent sections the occurrence of the irregularities at different geographical latitudes is addressed and the spectrum of the backscatter is discussed. Finally, some applications, particularly of auroral radars, are briefly mentioned.

Complex dielectric constants determined by inversion of the polarized returns of radar images acquired in wet conditions clearly delineate the distribution of saline soils in the Tragowel Plains Irrigation Area of Victoria, Australia. There is good agreement between the areas delineated as having anomalous dielectric constants by the radar backscatter inversion techniques with saline areas as defined by geophysics and as inferred from dielectric constants determined in the field. The radar-determined complex dielectric constants are significantly smaller than might be expected from the known moisture contents of the soils at the time of image acquisition. The magnitudes of P-band radar-delermined dielectric constants most closely approach those expected from field determinations although the distribution of L-band-determined dielectric constants give the best discrimination between saline and non-saline areas as seen at the surface. Modification of the inversion algorithm to allow for the effects...

A VHF ocean radar system was deployed for one month in a salt marsh in coastal Georgia, USA. In this environment, radar backscatter from the marsh grass is highly attenuated and scatter from water in the narrow creeks has a broad band of Doppler shift due to the horizontal variation in water speed across the channel. By assuming a monotonic velocity profile from the edge to the middle of the channel, a theoretical relationship is derived to link the broadening of Bragg energy in the echo spectrum to varying water speed. A methodology is developed to derive the cross-creek profile of velocity variation in the main channel of the salt marsh.

Polar Mesospheric Summer Echoes (PMSE) have become a well known characteristic of radar backscatter from the summer mesopause regions in polar regions. The phenomenon has been observed at frequencies from 50 MHz to almost 1 GHz. In this paper we attempt to identify a space-time manifold for the occurrence of PMSE. The resulting manifold, defined only by temperature, agrees well with observations and furthermore indicates when and where we might expect to observe PMSE in the future.