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.

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.

Sea clutter modelling and parameter estimation/prediction are important basis of system design, performance assessment, and target detection of maritime radars. Besides parameter estimation from measured data, parameter prediction from radar parameters, sea state, and the viewing geometry of the radar is an alternative way. In this letter, a dual-polarimetric full-recorded sea clutter database measured by an island-based X-band experimental radar on the Yellow sea of China is introduced. On this database, the model selection reveals that the compound-Gaussian model with inverse Gaussian (CGIG) texture is suited for HH high-resolution sea clutter data, the generalized Pareto (GP) intensity distributions are for HH moderate-resolution data, and the K amplitude distributions are for high and moderate-resolution VV polarized data. Further, three five-parametric empirical formulae are constructed to predict the shape parameter of the CGIG, GP, and K distributions of sea clutter from the area of spatial resolution cell, the grazing angle, the significant wave height, the average wave period, and the wave direction relative to the sight line of radar. The new empirical formulae attain more accurate prediction than existing empirical formula on the database.

As a basic characteristic of sea clutter, the reflectivity of sea surface depends on many factors. Various universal empirical models of low precision have been developed to predict the reflectivity of sea surface. In this letter, a method is proposed to train specific predictors by big data learning, where the universal empirical models are embedded to the architecture of the generalized regression neural network (GRNN) to enhance the learning ability and efficiency. On the sea clutter database measured by an island-based UHF-band radar in the offshore waters of the Yellow sea of China at low grazing angle, the GRNN-based predictors of different structures are compared with other predictors. The results on the database show that the GRNN-based predictors behave better at learning efficiency, prediction precision, and robustness.

Radar external calibration is an important measurement in determining the radar cross sections of unknown targets and the reflectance coefficients of the sea surface. This measurement is often made in an anechoic chamber by standard programs. Sometimes, this measurement must be made in outdoor conditions, and the ambient environment severely affects the calibration precision. Thus, the scheme of the measurement must be specially designed. In this article, a calibration conducted at the sea near Lingshan Island is reported, and a full program of an external calibration on the sea surface and relevant analysis are presented for a P-band island-based sea clutter measurement radar. Data analysis shows that the multipath effect from the sea surface is a major factor to degrade the precision of the calibration. Measurement data over 4 days and the multipath reflectance model of the sea surface are combined to estimate the power increment from the multipath effect. Radar system constant is estimated based on the estimated power increments at different sites, where the viewing geometries of the radar are different. The results show that the proposed external calibration method on the sea surface attains a satisfactory precision.