This paper investigates the performance of multiple approaches (including Maximum Likelihood Estimation) for determining the Doppler frequency of radar returns from clutter scatterers. Fields backscattered from clutter are modeled as a complex stationary random process with a specified temporal correlation function received in the presence of additive thermal noise at a specified clutter-to-noise ratio (CNR). The MLE implementation is formulated, and its results are then compared through a simulation study to those obtained from traditional pulse-pair processing (PPP) and other FFT-based methods in terms of error statistics and computational complexity. The methods are also compared for measurements of sea clutter from an X-band radar system. The results of the study show that the MLE approach has modest advantages over other methods in certain conditions, but that the performance gains obtained may often be insufficient to justify the extra computational cost or a-priori knowledge requirements of the method.

This paper examines the impact of ocean surface swell waves on near-coastal L-band high-resolution synthetic aperture radar (SAR) data collected using the National Aeronautics and Space Administration’s (NASA) Soil Moisture Active/Passive (SMAP) radar at 40° incidence angle. The two-scale model and a more efficient off-nadir approximation of the second-order small-slope-approximation are used for co- and cross-polarized backscatter normalized radar cross-section (NRCS) predictions of the ocean surface, respectively. Backscatter NRCS predictions are modeled using a combined wind and swell model where wind-driven surface roughness is characterized using the Durden–Vesecky directional spectrum, while swell effects are represented through their contribution to the long wave slope variance (mean-square slopes, or MSS). The swell-only MSS is numerically computed based on a model defined using the JONSWAP spectrum with parameters calculated using the National Data Buoy Center and Wave Watch III data. The backscatter NRCS model is further refined to include fetch-limited and low-wind corrections. The results show an improved agreement between modeled and observed HH-polarized backscatter NRCS when swell effects are included and indicate a relatively larger swell impact on L-band compared to higher radar frequencies. Preliminary investigations into the potential swell retrieval capabilities in the form of excess MSS are encouraging, however further refinements are required to make broadly applicable conclusions.

A fully polarimetric bistatic formulation of the average radar cross-section (RCS) of a target above a rough surface is presented. A four path model approximation of interaction between the surface and the target is used in order to decouple scattering from the target and surface. Surface scattering is described using the small-slope approximation. The ensemble average over surface realizations of the radar cross-section is performed analytically for paths that undergo single scattering from both the target and surface. This approach eliminates the need for Monte Carlo simulations over rough surface realizations and the need to mesh large surfaces in computing the average target plus surface RCS. Predictions from the formulation are shown for plate and cylinder target geometries for a variety of rough surface properties. Predictions are also compared with other results from the literature and found to be in good agreement (within 1.5dB RMS error for the results shown).