Based on the simulated signal, a numerical simulation method of synthetic aperture radar (SAR) imaging for time-varying sea surfaces is proposed, which is helpful to study the SAR imaging mechanism of time-varying sea surfaces so as to better extract ocean wave parameters from SAR images. Not only are the modulation of ocean waves, speckle noise, and temporal decorrelation of the small-scale waves considered, but the velocity bunching (VB) effect caused by the motion of large-scale waves is also effectively added to the simulation of the SAR echo signal. To verify the reliability of the simulation method, the simulated SAR images using the parameters of the RADARSAT-2 SAR, the corresponding wind wave information measured by an in-situ buoy, and the reanalysis wave spectra have been compared with the actual RADARSAT-2 SAR images. The comparisons demonstrate that the characteristics of simulated SAR images, such as the intensity distribution and the image spectra, are consistent with those of actual RADARSAT-2 SAR images. Based on the numerical simulation method proposed by us, SAR images of ocean waves for different marine environments and radar platform parameters are simulated. The imaging results indicate that the texture feature of the wind waves would be severely damaged due to the VB effect, while the texture of swells in the simulated SAR images may not be damaged or even becomes clearer. From the simulated SAR image spectrum, it can be found that the azimuth wavenumber is cut off when the VB effect is considered in the simulation process, and the azimuth cut-off wavelength increases with the range-to-velocity ratio.

To assist in the detection of ship targets in complex sea conditions, a numerical simulation method is proposed to obtain synthetic aperture radar (SAR) images of time-varying ocean ship wakes under various radar, ship, and sea surface parameters. This method addresses the limitations of recent simulations, which failed to simultaneously incorporate different types of time-varying ship wakes, simulate based on the echo data, and discuss the velocity bunching (VB) effect on the image results. To address these issues, firstly, the time-varying wave height and velocity fields of the sea surface, Kelvin wakes, and turbulence wakes are simulated using the linear filtering method, classic fluid dynamics models, and attenuation function method, respectively. Secondly, raw data of the ocean ship wakes are obtained by calculating the backscattering fields using geophysical model functions (GMFs), as well as by determining the changing slant range varying with the elevation and velocity fields. Thirdly, by applying the Range-Doppler algorithm (RDA) for pulse compression and range cell migration correction (RCMC) on the echo data, SAR images with and without the VB effect are generated. Our simulation also accounts for the influence of speckle noise. The SAR imaging results indicate that whether the VB effect is considered or not, the radar electromagnetic wavebands, polarization modes, wind speeds, and the relative wind directions have distinct impacts on the SAR image intensity, and the texture and morphology of ship wakes vary significantly with the wind speeds, ship speeds, and the relative radar looking directions. When considering the VB effect, the azimuthal offset and blur in the images caused by the more intense wave motion also increase with the wave speeds.

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.

Systematic attitude errors (the roll, pitch, and yaw errors) are important sources of altimetric error in interferometric radar altimeters (IRAs). 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 article 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 Doppler characteristics of sea surface echoes reflect the time-varying characteristics of the sea surface and can be used to retrieve ocean dynamic parameters and detect targets. On airborne, spaceborne and shipborne radar platforms, radar moves along with the platforms while illuminating the sea surface. In this case, the area of the sea surface illuminated by radar beam changes rapidly with the motion, and the coherence of the backscattered echoes at different times decreases significantly. Therefore, the Doppler characteristics of the echoes would also be affected by the radar motion. At present, the computational requirements needed to simulate the Doppler spectrum of the microwave scattering field from the sea surface based on numerical methods are huge. To overcome this problem, a new method based on the sub-scattering surface elements has been proposed to simulate the Doppler spectrum of sea echoes acquired by a moving microwave radar. A comparison with the results evaluated by the SSA demonstrate the availability and superiority of the new method proposed by us. The influences induced by radar motion, radar beamwidth, incident angle, and thermal noise on the Doppler characteristics are all considered in this new method. The simulated results demonstrate that the spectrum bandwidth of sea surface echoes acquired by radar on the dive staring motion platform becomes somewhat narrower.

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.