Ocean waves can either be the target of interferometric radar altimeter (IRA) observations or may affect observations of mesoscale and submesoscale ocean dynamic phenomena. Due to the low incidence angle of the IRA, in addition to the azimuth shift caused by the motion of the ocean waves, the range shift caused by the height of the ocean wave is also significant. In this article, based on previous research, we propose a new nonlinear integral transform relating ocean wave spectra to phase image spectra of an IRA at a low incidence angle that includes the effect of range shift. This new nonlinear integral transformation can be replaced by a quasilinear transformation when the nonlinear variation is weak or the wavelength of the ocean wave is large. We analyze the effect of swell and wind waves on the phase spectra under different conditions based on the new nonlinear integral transform through simulation. The height of the ocean wave and the incidence angle of the radar affect the range shift, leading to a range nonlinear effect in the phase spectra. The orbital velocity of the ocean wave affects the azimuth shift, leading to an azimuth nonlinear effect in the phase spectra. The wavelength and propagation direction of the ocean wave also induces a distortion in the phase spectra. Overall, the nonlinear effect has a small effect on the swell and a large effect on the wind wave.

Rainfall over the ocean strongly influences microwave remote sensing by altering electromagnetic scattering mechanisms, complicating radar backscatter interpretation and its relationship to surface wind. However, in-depth understanding of these effects under different surface and frequency conditions remains limited. In this study, for the first time, coordinated multi-frequency radar observations at C-, X-, and Ku-bands were carried out to investigate scattering from (i) airborne raindrops, (ii) rain perturbed windless water surface, and (iii) wind-driven sea surface agitated by rain. Field measurements combined radar backscatter intensity with Doppler spectral signatures to elucidate the underlying scattering mechanisms. Synchronized radar and optical imaging enabled joint characterization of scattering signatures and geometric features of wind waves and rain-induced ring waves. Results reveal pronounced variations in both airborne raindrop scattering and surface backscattering under different rainfall rates, frequency bands and polarizations. Under low sea-state conditions, Doppler spectra exhibit a distinct dual-peak structure attributable to ring wave backscattering, whereas at higher sea states this feature is masked by the dominant spectral energy of wind waves. Using rain-induced ring wave theory and the small-slope approximation (SSA), Doppler spectra due to ring waves were simulated and validated against experimental measurements. This study clarifies the mechanisms of rain-induced scattering and demonstrate the dominant role of ring waves in shaping the observed Doppler spectral signatures.

It is of great theoretical and practical significance to study the multidimensional statistical characteristics of sea clutter at different range resolutions, such as optimizing clutter modeling and suppression, improving radar system design and remote sensing data interpretation ability. The analysis of the effect of range resolution based on different sea clutter datasets poses a significant challenge in the drawing of reliable conclusions, owing to the variability in radar parameters and sea state conditions. In this paper, a new method is proposed to obtain sea clutter data with different range resolutions based on high resolution radar data by reducing the sampling time of the original intermediate frequency (IF) signal. Base on this method, the statistical characteristics of sea clutter at different range resolutions are analyzed for the first time, including the amplitude distribution, coherence length of sea texture, phase difference distribution, Doppler spectrum, and wave radial velocity of sea clutter at three range resolutions: high (0.75m), moderate (7.5m), and low (25m), while all other environmental parameters and radar parameters remain constant. The results clearly show the effect of range resolution on sea clutter properties and can be used to guide development of clutter models.

The Doppler characteristics of the backscattering field of sea surface are related to the motion of sea waves, which can be used for ocean dynamic parameter inversion and marine target detection. In order to study the Doppler characteristics of multiband sea surface echo at low grazing angles, a series of joint observation experiments of L-, S-, and X-band sea surface echo were carried out in Lingshan island located at Qingdao, China. The experimental results show that the Doppler shift and spectrum bandwidth all increased with the frequency of the incident electromagnetic wave. When radar looks along the up-wave direction, the Doppler shift of sea echo of HH-Pol is found to be greater than that of VV-Pol. For Doppler spectrum bandwidth, we also find that HH-Pol is slightly larger than VV-Pol. Moreover, the comparison of the results under different sea states shows that the Doppler shift and spectrum bandwidth all increase with significant wave height (SWH). In order to better understand the Doppler characteristics of sea echo at low grazing angles, a multiband theoretical Doppler model of sea echo at low grazing angles is established, taking into account tilt modulation, hydrodynamic modulation and the shadow effect of sea waves, and wave breaking under moderate-to-high sea states. The comparison results show that the simulated results are more consistent with the measured data after considering the influence of the breaking wave. The experimental and simulation results can provide a reference for the subsequent study of Doppler characteristics of multiband sea surface at low grazing angles.

Wind speed and wind fetch are two critical factors influencing the development of wind waves. However, due to the challenges in acquiring accurate wind fetch, most existing empirical models for retrieving significant height of wind waves (SHww) primarily consider wind speed while neglecting the influence of wind fetch, resulting in relatively low inversion accuracy. Given the strong correlation between wind wave mean period and wind fetch, this study indirectly accounted for the impact of wind fetch on wind wave development by incorporating wind wave mean period, and developed twoSHwwinversion models based on the Elfouhaily spectrum and theSHww, wind speeds and wind wave mean periods provided by ECMWF, which are defined as Model 1 (M1) and Model 2 (M2), respectively. The two developedSHwwinversion models were evaluated using multiple datasets, including sea surface data provided by ECMWF and NDBC, GNSS buoy measurements, wind speeds provided by meteorological station, and airborne SAR imagery. The results demonstrate that both models achieve high inversion accuracy forSHwwin most scenarios, with M2 exhibiting particularly robust stability, and the correlation coefficients between theSHwwretrieved by M2 and the referenceSHwware greater than 0.95, theRMSEandMAEare approximately 0.19m and 0.13m, respectively. Furthermore, when developing theSHwwinversion model, this study enhanced the inversion accuracy by incorporating the wind wave mean period to indirectly account for wind fetch effects on wind wave development, thereby providing a novel research direction for obtaining high-precisionSHww.

Accurate mapping of oil concentrations is essential for effective response to oil spill emergencies. The complexity of microwave scattering over oil-contaminated sea surfaces poses substantial challenges for synthetic aperture radar (SAR) applications, primarily due to the limited understanding of non-Bragg scattering mechanisms. This knowledge gap restricts the development of robust retrieval algorithms for quantifying oil spill concentrations. To address this issue, a novel retrieval approach is proposed based on the co-polarization difference (PD), which is independent of non-Bragg scattering. The influence of oil on the sea surface is attributed to two dominant factors: suppression of short gravity-capillary waves and reduction in the effective dielectric constant. By analyzing SAR imagery of oil spills with varying concentrations, it is found that the damping effect of oil spills on small-scale waves can be predicted using the Marangoni damping model. Once the contribution of wave suppression to PD reduction is isolated, the residual PD variation is attributed to changes in the dielectric constant. Oil concentration is then retrieved by comparing the PD of each pixel within the contaminated area to a theoretical PD lookup table. The proposed method is validated using simulated SAR datasets representing different oil concentrations and subsequently applied to SAR data acquired during the Deepwater Horizon oil spill in the Gulf of Mexico.

Ocean wave information is very important for the safety of seaplanes. Generally, wave height inversion from X-band coherent radar images requires high spatial resolution. However, for airborne radar platforms, higher platform altitude often results in lower radar azimuth resolution, even exceeding the scale of the swell. It is well known that the correlation coefficient of coherent radar sea echoes is related to the sea state. Therefore, in this letter, a new method based on correlation coefficient was proposed to invert significant wave height (SWH) from the sea echoes acquired by the airborne coherent X-band radar. First, based on the parameters of aircraft and radar, the influence of system noise, spatial mismatch, angle variation, and sea surface motion on sea surface echoes correlation were discussed and the theoretical formulas were derived. Then, the empirical relationship between the correlation coefficient and the SWH was established based on the shore-based radar data. Finally, based on the empirical relationship between the correlation coefficient and the SWH, the SWH can be estimated by airborne coherent X-band radar images. This method was verified by an airborne X-band coherent radar experiment in the South China Sea. In comparison to the European Centre for Medium-Range Weather Forecasts (ECWMF), the root-mean-square error (RMSE) of the estimated SWH is 0.15 m with a bias of 0.12 m. The results indicate that this method can be used to estimate the SWH from airborne coherent X-band sea surface images.

The assessment of the microwave backscattering characteristics from breaking waves is crucial for ocean remote sensing and ocean target recognition. Nevertheless, due to the tough experimental environment and the complex geometric profile of breaking waves, research on the microwave scattering characteristics of breaking waves is still preliminary. In this study, the echoes from breaking waves near shore were measured by a C-band radar, which was conducted at incident angles from 20° to 60° with both horizontal polarization (HH-Pol) and vertical polarization (VV-Pol), to reveal the properties of the normalized radar cross section (NRCS) and the Doppler spectrum associated with wave breaking. The results show that the radar NRCS increase obviously when water waves are breaking. And the HH-VV polarization ratio of the scattering field from breaking waves is close to 1. On the other hand, the frequency shift and the band width of the Doppler spectrum of the scattered fields from breaking waves are both increasing. And the Doppler spectrum may have multiple spectral peaks due to the stronger scattering intensity and higher velocity of the breaking waves. In this work, an empirical model for the NRCS backscattered from breaking waves is proposed within the incidence angle range from 20° to 60° along different observation directions. The experimental results can provide reference for the subsequent research on microwave scattering characteristics of breaking waves.

Systematic attitude errors (the roll, pitch, and yaw errors) are important sources of altimetric error in interferometric radar altimeters (IRA). So far, the effect of systematic attitude errors on the measurement of IRA has been investigated based on the interferometric geometry, in which the impact of the attitude errors on the IRA image quality and the coupling between each attitude error are not considered. In this work, the influence of attitude errors on the measurement accuracy of IRA is reanalyzed according to the IRA imaging principle and the interferometry theory. The theoretical formulas derived in this paper demonstrate that the roll is the most important factor affecting the measurement accuracy of IRA. Although the altimetric error directly introduced by yaw and pitch is smaller, the yaw and pitch would cause the altimetric error to be offset along the azimuth direction, and the offset grows approximately linearly across the range direction in the IRA images. Meanwhile, the attitude jitter would also introduce additional high-frequency altimetric errors, although these high-frequency altimetric errors can be removed through a Gaussian low-pass filter with the cut-off frequency determined by wavelet analysis. To verify the validity of the theoretical formulas and exhibit the impact of diverse attitude errors, a full-link simulation method is proposed, which encompasses the entire process from IRA observation to imaging and interference processing. Furthermore, the theoretical results are compared with the results of the full-link simulation and the experimental data acquired by an airborne IRA.

The azimuth cutoff of interferometric radar altimeter (IRA) image, which is acquired at small incidence angles, is mainly determined by the vertical component of the orbital velocity of ocean waves and is almost independent of the wave direction. Using this property, an inversion method for wind waves and wind speed has been proposed based on the azimuth cutoff of IRA image in combination with the Elfouhaily wind wave spectrum, which can effectively solve the problem of small-scale wind wave parameters loss caused by velocity bunching. In the present work, the wind speed and the significant height of wind waves (SH$_{\mathrm {ww}}$) have been retrieved from five pairs of airborne IRA images acquired in offshore areas. The results show that the differences between SHww retrieved from the five pairs of IRA images used in this article by the new method and the reference SHww are acceptable in ocean wave inversion. However, if the wind fetch is small and the wind direction is inconsistent with wave propagation direction, there is a significant difference between the retrieved wind speed and the reference wind speed when using the new method to retrieve wind speed. Moreover, the results also show that for the sea area with infinite wind fetch, the inversion accuracy of wind speed and SHww determined by the accuracy of the retrieved radar radial significant orbital velocity of wind waves (SV$_{\mathrm {ww}}$). However, for the sea area with finite wind fetch, the inversion accuracy would also be affected by wind fetch.

Wide swath near-nadir interferometric altimetry is a newly developed technology for sea surface height (SSH) measurement. However, the absence of actual measurement data makes this novel SSH mapping technique difficult to verify and apply for wide swath interferometric altimeters. To verify the designed performance of the scheduled wide swath single-pass interferometric altimeter in the "Guanlan Mission", an airborne campaign was carried out off the coast of Rizhao, China on November 16, 2020. An airborne dual-frequency interferometric radar altimeter system (ADIRAS) with a single-pass mode was utilized for SSH measurement as the first flight. Two pioneering and fundamental works have been conducted: an intensive altimetry error analysis according to the ADIRAS parameter settings along the incident direction, an effective SSH reconstruction approach based on a multichannel likelihood (ML) function, and detailed validation procedures through airborne campaigns illustrated in this study. The results indicated that the difference between the wave-induced sea surface elevation (WSSE) variances derived by the ML approach and GNSS buoy was 2 cm2, which was smaller than the results of the single band on Ku (11 cm2) and Ka (6 cm2). Moreover, the estimated Significant Waves Height (SWH) bias of joint bands was 10 cm, which was also superior to that of Ku (39 cm) and Ka (24 cm). Both simulated data and real airborne dual-frequency InSAR data were employed in this study for cross-validation of the proposed method. This approach represents an effective technique for SSH reconstruction of future spaceborne/airborne interferometric altimeters.

It is still a challenge to get relatively accurate wind speed under extreme wind conditions. In this study, thirteen RADARSAT-2 (RS-2) ScanSAR wide mode dual-polarization data were selected to establish a new hurricane wind speed model. Combined with European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) data and Stepped-Frequency Microwave Radiometer (SFMR) data, the wind speed retrieval model, which has been corrected by an incidence angle function, was proposed through strip segmentation. Compared with the ECWMF wind speed below 22 m/s, the bias of 2.056 m/s RMSE and 1.579 m/s were obtained. Compared with the SFMR measured wind speed above 22 m/s, the RMSE was 3.229 m/s and the bias was 2.489 m/s. Compared with Shen model and Horstmann model, the accuracy of wind velocity inversion is improved to a certain extent, especially under high wind speed conditions.

Interferometric radar altimeter (IRA) is a new ocean remote sensing sensor. It can be used to retrieve sea surface height (SSH) by means of cross-track interferometry. Compared with the traditional cross-track interferometric synthetic aperture radar (XT-InSAR), IRA works at very small incidence angles for higher altimetry sensitivity. In this case, multiple discontinuous surface scatterers at sea surface would be cut into a same range pixel which leads to severe layover. This layover induced by ocean waves will reduce the correlation between the master-slave images acquired by IRA and increase random interferometric phase noise. At present, how to quantitatively analyze the impact of the ocean wave layover on the decorrelation of IRA images is still a problem that needs in-depth discussion. In this letter, theoretical analysis of the effect of ocean waves on the decorrelation of IRA images has been carried out when the ocean waves layover is considered. And the theoretical results are also compared with the airborne IRA data. It is found that the layover of ocean waves has significant influence on the decorrelation between the master-slave IRA images, especially at very low incidence angles.

The interferometric radar altimeter (IRA) is one of the main payloads of the “Guanlan” ocean science satellite proposed by the National Laboratory for Marine Science and Technology of China. To evaluate the effectiveness and accuracy of the IRA in retrieving the ocean dynamic parameters, such as sea surface height (SSH), ocean wave spectrum, and wind speed, two airborne IRA experiments were carried out at Qingdao Xiaomaidao (XMD) sea area on March 31, 2019, and Rizhao sea area on November 16, 2020. In the present work, wave-induced sea surface elevation (SSE) and its spectrum have been retrieved based on the interferograms acquired by the airborne IRA. To suppress the random phase noise, a mean filtering algorithm has been used in the multilook process of calculating the complex IRA images. The results show that the size of the filter window has a significant effect on the retrieved SSE. If the size of the filter window along THE range direction is too large, the flat earth would cause the spectral density of the retrieved ocean wave to be higher. In addition, the comparisons of the retrieved spectra with the buoy measurements demonstrate that the swell can be well-retrieved by IRA images at low sea-state conditions with significant wave height (SWH) less than 0.7 m. However, for wind wave, because of the effect of the velocity bunching along the azimuth direction, the wind wave spectrum can be extracted only when it propagates approximately along the ground-range direction of the IRA images.

Retrieving the oil-water mixture ratio (M, the proportion of oil in an oil-water mixture) of the marine oil spill is of vital importance for the emergency treatment of oil pollution, which can be achieved by a copolarization ratio (PR) method. In the framework of the PR method, it is assumed that Bragg scattering dominates at moderate incidence angles, and the relative contribution of non-Bragg scattering to total scattering (RCNT) is typically neglected. However, in this work, it is found that the RCNT cannot be neglected for L-band, especially for HH polarization at higher wind speeds (the RCNT is approximately 0.6~0.8 for wind speeds of 10~12 m/s). The significant impact of non-Bragg scattering results in the PR method not being accurate enough for retrieving M. After separately investigating the influences of the damping effect and effective dielectric constant reduction on the RCNT, a novel approach is proposed for more accurately estimating the M of marine oil spills by eliminating the impact of non-Bragg scattering. The proposed method was applied to UAVSAR images collected during the Deepwater Horizon (DWH) oil spill accident. The inversion results of our approach show that the M of most oil spill areas ranges from approximately 0.3 to 0.6, which matches the reality better than that of a traditional PR method (ranging from approximately 0.6 to 0.8).

The tilt modulation is one of the key factors affecting sea wave inversion accuracy based on some microwave sensors such as SAR, microwave spectrometer, and so on. Up to now, the traditional tilt modulation-transfer functions (MTFs) based on the Bragg scattering model and Phillips spectrum have been used in the sea wave spectrum retrieval algorithm for microwave sensors operating at medium incident angles. The traditional tilt MTFs are only related to radar polarization and incident angle, but not affected by wind speed and wind direction. However, the values of the tilt MTFs evaluated on the basis of the C-band empirical geophysical model function (GMF) CMOD5.na demonstrate that wind speed and wind direction also have significant effects on the tilt MTFs. In addition, the comparisons show that the values of the traditional tilt MTFs for both HH (horizontal) and VV (vertical) polarization cases are obviously smaller, which indicates that the traditional tilt MTFs would underestimate the influence of the local incident angle change on the scattering fields. On the other hand, the difference between the horizontal and the vertical polarization tilt MTFs, however, is overestimated by the traditional tilt MTFs. Finally, by fitting the data evaluated based on the C-band empirical GMF, we propose new tilt MTFs, which can overcome the shortcomings of the traditional tilt MTFs.

In this article, a theoretical study is carried out to seek an approach to remotely measure the thickness of the oil film. It is known that it is difficult to probe the oil thickness using a microwave radar due to the fact the dielectric constant of oil is very small compared with seawater for microwave frequencies. While in the range of terahertz (THz) electromagnetic (EM) wave frequency, the dielectric constants of oil and seawater are comparable, resulting in the THz EM fields scattered from the air-oil interface and those from the oil-water interface can produce constructive and destructive interference with the variation in the oil film thickness. Based on this principle, the sensitivity of the THz EM wave to the oil thickness is investigated in this work. First, the oil-seawater two-layer medium is equivalent to a single-layer medium by an equivalent dielectric constant model. Then, the THz EM scattering from the oil-covered sea surface is simulated using the first-order small slope approximation method (SSA-1) and the obtained equivalent dielectric constant. Meanwhile, the influences caused by the damping effect of the oil film, the reduction for friction velocity, and the change in equivalent permittivity on the normalized radar cross section (NRCS) are analyzed. The numerical results show that the NRCSs of THz frequencies are more sensitive to the change in oil thickness than that of microwave frequencies. This property makes the THz EM wave have the potential to probe the thickness of the oil film.

This article is devoted to investigating the microwave scattering mechanisms of oil-free and oil-covered sea surfaces for an incidence angle smaller than 30° in a backscattering configuration. The Elfouhaily spectrum is used to simulate an oil-free sea surface, whereas the Elfouhaily spectrum combined with the Jenkins damping model is applied to the simulation of an oil-covered sea surface. Then, the Kirchhoff approximation-stationary phase approximation (KA-SP) and the first order of small-slope approximation (SSA-1) are employed to simulate the scattering coefficients induced by specular scattering and total scattering, respectively. Importantly, a new parameter defined as specular scattering to total scattering ratio (STR) is proposed in this article, which can be used to measure the ratio of specular backscattered power to total backscattered power. The dependencies of the scattering coefficient and the STR on incidence angles, wind speeds, wind directions, oil thicknesses, and so on, are investigated. This article provides new insights for a better understanding of the evolution of microwave scattering mechanisms from oil-free and oil-covered sea surfaces in the transition region of incidence angles (from about 15° to 30°).

The two-scale model (TSM) has been frequently used in the study of EM (electromagnetic) scattering from rough surface due to its simple and practical merit. However, for microwave scattering from sea surface, it cannot provide accurate predictions for hh (horizontal) polarization. To overcome this problem, an improved version of the TSM (ITSM) which can be better used for predicting microwave scattering from sea surface is proposed in this paper. In the ITSM, we propose to use two cutoff parameters to separate sea surface roughness. For kwcs, the KA-SP (Kirchhoff approximation-stationary phase approximation) rather than KA-GO (Kirchhoff approximation-geometric optics approximation) is employed to simulate the specular scattering component. For kw>kcb, the SPM modulated by tilts of large-scale waves is employed to simulate the Bragg scattering component. The values of kcsand kcbare chosen according to the validity conditions of the KA and the SPM. The numerical comparisons illustrate that the ITSM performs better than the TSM and the SSA-1, especially in the prediction of hh polarized scattering coefficient.

Aiming at the observation of sub-mesoscale ocean dynamics, Pilot National Laboratory for Marine Science and Technology (Qingdao) proposed the GuanLan marine satellite program. The new interferometric imaging radar altimeter (InIRA) sensor is used to realize high accuracy observation of marine dynamics through Ku band and Ka band, which makes up for the deficiency of traditional altimeter in observation of mesoscale and sub-mesoscale marine dynamics. This paper mainly discusses the influence of random errors on the measurement results of InIRA. In the discussion, the random error caused by significant wave height (SWH) of global sea surface at different times, radar frequency, incident angle and correlation coefficient are considered and discussed respectively. The random errors caused by different factors are analyzed quantitatively.

In view of the influences of spatial resolution, time sampling length and SWH (significant wave height), a spectral width model is established. The theoretical results based on the model demonstrate the spectral width becomes wider with the resolution unit and time sampling length, but eventually tends to be constant. Moreover, when the incident angle is larger, the azimuth angle has a greater influence on the spectrum width. Finally, the theoretical results are compared with the estimated results based on measured data required by CSIR-X band radar. The comparisons prove the reliability of the theoretical results. The results obtained in this paper have certain reference value for the retrieval of the SWH.

It's well known that wind speed anomaly (WSA) are positively correlated and very nearly collocated spatially with the sea-air temperature difference anomaly (SATDA). The coupling relationship between SATDA and WSA has been discussed using the satellite data firstly. A phenomenon that the coupling coefficients, which can represent the degree of the response of WSA to SATDA, have a regular seasonal variation has been revealed in this paper: a higher value in summer and a lower value in winter. According to the atmospheric stability theory, the neutral wind speed may depend upon both background mean wind speed (MWS) and mean sea-air temperature difference (MSATD), these two are also the significant characteristic parameters in different seasons and different regions. In order to explain the seasonal variation of the coefficients, we analyze the influences of these two parameters on the coupling coefficients in 4 sea regions where the air-sea interaction is intense, respectively. It turns out a strong negative correlation with SATD in unstable layer which is the main reason for the seasonal variation, and a weak positive correlation with WS in unstable layer.

In this paper, the microwave (5GHz) and terahertz (1THz) EM (electromagnetic) wave scattering from sea surface covered with oil are studied in theory. The sea surface covered with oil can be regarded as a two-layered structure, the upper layer is oil and the lower layer is seawater. For microwave scattering, it is known that the oil covered on sea surface mainly influence the sea surface from two aspects: the damping effect for small scale wave and the reduction for friction velocity. In this work, the modification for Fresnel scattering coefficient due to the presentation of oil film is also taken into account. The numerical simulations show that the NRCS (normalized radar cross section) of microwave is not sensitive to the thickness of oil film. On the contrary, the NRCS of terahertz wave is more sensitive to the thickness of oil film, which provides us more information to probe oil slicks on sea surface with terahertz EM waves.

C band microwave radars have been widely used in ocean observations, e.g. oil spill monitoring, ship detection, wind speed retrieval, etc. All these applications rely on the measured normalized radar cross section (NRCS) of sea surface. Therefore, it is necessary to develop accurate models to predict the radar cross sections of sea surface. In this paper, based on the six commonly used sea spectra models, i.e. Elfouhaily, Hwang, Romeiser, Apel, Fung and Pierson spectra, the normalized radar cross sections are calculated by Small Perturbation Method (SPM), Two Scale Model (TSM) and the first order Small Slope Approximation (SSA-1), respectively. Meanwhile, to better evaluate the accuracy of different sea spectra, comparisons between numerical calculations and the empirical CMOD5 model are made for various incident angles, wind speeds, and wind directions. The comparisons show that the normalized radar cross sections calculated based on Romeiser spectrum agree better with CMOD5 in some specific cases. The works presented in this paper will be helpful for applications of C band microwave radars in ocean observations.

Based on the complex Wishart distribution, statistical analysis of sea clutter acquired by quad-pol coherent X-band marine radar is presented. In this work, the probability density distribution functions (PDFs) of the intensity as well as the phase, the real and the imaginary parts of the elements in the polarimetric covariance matrix have been derived, and the theoretical models are expressed in closed forms. In order to validate our theoretical results, the PDFs are compared with the experimental data collected by the McMaster IPIX Radar. If we remove the low intensity echoes which are mainly dominated by the noise of radar system at the shadow regions of large-scale waves, the comparisons demonstrate that our models are in good agreement with the experimental data. On the other hand, we also find that the PDFs will be slightly affected by the inhomogeneity of sea clutter. The theoretical results obtained in this work are promising for better understanding the statistical properties of the sea clutter acquired by quad-pol coherent marine radar at low grazing angles.

The surface equivalent neutral wind field (ENWF) response to the sea surface temperature (SST) is investigated utilizing the monthly average ENWF acquired by the Quick Scatterometer (QuickSCAT) and the monthly average SST by the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) from June 2002 to November 2009 in four typical sea areas, i.e. the Kuroshio Extension, the Gulf Stream, the Brazil Malvinas and the Agulhas return current regions. And the coupling coefficients between the wind speed abnormity (WSA) and the sea surface temperature abnormity (SSTA) have been analyzed. The results show that the coupling coefficients have obvious regional and seasonal variations in the four typical sea areas. In order to reveal the reasons for these phenomena, the dependences of the coupling coefficient on the environment parameters such as the SST, the ENWF and the air temperature (AT) above sea surface have been discussed in detail. We find that the coupling coefficients have positive relationships with the SST and AT, and have negative relationships with the wind speed and sea-air temperature difference (SST-AT). However, the rules of the coupling coefficients with the environmental parameters in the four typical sea areas are not consistent with each other, except the parameter SST-AT. Thus, the results indicate that the SST-AT is probable the dominated factor resulting in the variations of the coupling coefficients.

Compared with single-polarization synthetic aperture radar (SAR), full-polarization SAR can not only collect the echo intensity of backscattered from the sea surface, but also can collect the phase information. For oil spill detection, polarimetric SAR has more advantages. Features extraction is a crucial step for oil spill discrimination. In this paper, eight kinds of polarimetric features, such as the polarimetric entropy, average scatter angle and so on, have been investigated to analyze the capability of the polarimetric features to observe an oil spill at C-band and L-band, and to identify the most powerful multi-polarimetric SAR descriptors for oil spill detection. And a systematic comparison of the eight multi-polarization features is provided. Here, the analysis is conducted on the quad-pol SAR images acquired by the SIR-C/X-SAR. The experiment results show that the pedestal height and the parameter P perform better than the other polarimetric features at C-band.

The influences of atmospheric stability and wind fetch on microwave scattering from sea surfaces are presented. The equivalent neutral wind speed (ENWS) and the friction velocity (FV) above sea surface are evaluated by the similarity theory of Monin and Obukhov in combination with the Charnock relation. The numerical simulations show that atmospheric stability and wind fetch would make effects on ENWS and, therefore, further on the scattering coefficients (NRCS). And the NRCS decreases with increasing air-sea temperature difference (ΔT=Ta-Ts) because the ENWS is lower than actual wind speed in stable atmospheric condition, and vice versa. Moreover, the ENWS/FV and the NRCS decrease with increasing wind fetch under unstable atmospheric condition. However, in stable atmospheres, the ENWS/FV and the NRCS are almost insensitive to wind fetch. In this paper, the response of NRCS to ΔT is also discussed. Compared with FV, the variations of the angle spread function with ΔT can be neglected because of its minor contribution.

Results of microwave radar Doppler spectra from 1-D nonlinear ocean surface at moderate- and low-grazing angles are calculated by the composite surface scattering model. For the large-scale undulating surface description, the narrow-band Lagrange model is used, which takes into account the vertical and horizontal skewnesses. Moreover, the shadow and the curvature effects of large-scale waves on the Doppler spectra are also considered in our calculations. Comparisons of computed curves with experimentally measured Doppler spectra at different incidence angles and at various wind speeds show that the simulated results can fit the measured data well at moderate incident angles. From the simulations, we also find that the hydrodynamic modulation and the horizontal skewness of the large-scale waves can induce remarkable influence on Doppler shift. In addition, when the shadow and the curvature effects of large-scale waves are considered in the calculations, the Doppler shifts grow more quickly and the spectral widths become narrower at low-grazing angles, and this is consistent with the numerical results given by Toporkov in the nonlinear surface case. The conclusions obtained in this work seem promising for better understanding the properties of time-dependent radar echoes from oceanic surfaces.

In the general framework of the second-order small-slope approximation (SSA) (SSA-II), Doppler shifts of backscattered fields from time-varying sea surfaces are predicted at small and moderate incidence angles. The composite sea surface is modeled as a superposition of large-scale gravity waves and small-scale ripples. Here, the elevation of the large-scale surface component, on which each small facet travels along a closed orbit under the condition of the first-order approximation, is described by the coordinates of the small facets. The predicted Doppler shifts are compared with the results obtained by the small perturbation method (SPM), the geometrical optics method, the first-order SSA (SSA-I), and the two-scale scattering method (TSM). We can find that the predicted Doppler shifts for SPM and SSA-I in copolarized configuration are insensitive to the polarization state. However, the results obtained by SSA-II are consistent with those obtained by TSM and yield significant differences between HH and VV polarizations. Spectrum bandwidth is mainly induced by sea-surface orbit motions. In this paper, the formula of the spectrum bandwidth for the scattered echoes from the composite sea surface is also derived on the basis of TSM. The predicted bandwidths are compared with the Monte Carlo simulated results and the measured data. Furthermore, the dependences of the Doppler shift and the bandwidth on the parameters, such as the polarization, wind speed, radar frequency, etc., are also discussed.

Adaptive filter implementations require real-time conversion of coefficients to canonical signed digit (CSD) or similar representations to benefit from multiplierless techniques for implementing filters. Multiplierless approaches are used to reduce the hardware and increase the throughput. This paper introduces a novel hardware implementation method that converts two's complement numbers to their CSD representations using a fixed number of shift and logic operations. As a result, we can greatly reduce the power consumption and area requirements for hardware implementation of DSP algorithms in which coefficients are not known a priori. Because all CSD digits are produced simultaneously, the conversion speed and thus the throughput are improved when compared to overlap-and-scan techniques such as Booth's recoding.

Implementation of digital signal processing (DSP) algorithms in hardware, such as field programmable gate arrays (FPGAs), requires a large number of multiplications. In this paper, we introduce a novel multiplier structure that converts from 2's complement to canonical signed digit (CSD) representation in real time. The proposed algorithm increases the number of zero partial products (which can be simplified by shift operations) to approximately 66.7% compared with 50% for modified Booth's recoding. Also, the proposed hardware structure reduces the non-zero partial products to a minimum, and consequently the number of arithmetic operations in the carry-save structure is reduced. So, our proposed hardware decreases both the time required for multiplication and the power consumption of the multiplier. Furthermore, because the proposed structure uses real time CSD recoding, and does not require a fixed value for the multiplier input to be known a priori, the proposed multiplier can be used to implement digital filters with non-fixed filter coefficients, such as adaptive filters.

Adaptive filtering is a widely used technique in active noise control (ANC). In order to make the adaptive filter in an FXLMS (filtered-x LMS) ANC system stable, the reference signal must pass through an estimation filter whose phase response is within /spl plusmn/90/spl deg/ of the phase of the secondary path. In this paper, we study the quantization effects on the filter phase response and the relationship between the phase response and the location of the zeros and poles. In addition, we propose a filter structure and nonuniform quantization method in which we quantize the filter coefficients so they each contain a small number of nonzero bits, based on the distance of the zeros/poles to the unit circle, to guarantee that the /spl plusmn/90/spl deg/ allowable phase deviation is met - greatly reducing the implementation cost. We combine these ideas with that of multiplierless implementations of adaptive FIR filters to realize an efficient active noise control using field programmable gate arrays or other digital hardware.