The Doppler Spectra of Medium Grazing Angle Sea Clutter; Part 1: Characterisation

This paper is concerned with the characterisation of Doppler spectra from high range resolution X-band radar sea clutter observed from an airborne platform over the range of grazing angles, 15° to 45°. It is observed that when looking up or down wind there is a strong correlation between mean Doppler shift and local spectrum intensity. When combined with random fluctuations of spectrum width, these characteristics give the spectra a temporal and spatial variability. This behaviour has previously been observed in low grazing angle data and these results confirm the wider applicability of the models developed using that data. The modelling method is also extended here to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or down-wind

Modelación estadística de la textura del clutter marino en Matlab

Context: The statistical modeling of the interference signal known as sea clutter is achieved assuming the input results from the combination of two components: one for the speckle and another for the texture. The Gamma distribution is the more widely applied for the texture component. Nevertheless, several authors have defended the idea of using the Inverse Gamma and Inverse Gaussian instead.Method: In order to provide an easy access to the handling of the models and the execution of comparisons between them, the authors of the current paper simulated in MATLAB the main characteristics of these distributions. In addition, the Root-Gamma model was also included because it replaces the Gamma distribution when samples are processed in the amplitude domain. The applied method consisted in a deep bibliography review for finding the corresponding expressions for each simulated model; the method also included additional computational simulations that allowed to identify occasional errors that were committed by different authors when characterizing the models.Results: A small framework was created for stochastic simulation containing density and distribution functions, mechanisms for random variable generation, parameter estimation methods and statistical moment closed expressions, among others. Besides, complementary functions were prepared for guaranteeing the validation by comparison with results provided by third parties and through the interaction between the different components of the library.Conclusions: The created library enables the use of multiple distributions for the modeling of the electromagnetic echo received from the sea surface. This will certainly motivate the creation of new radar detectors adapted to heterogeneous conditions such as the ones existing in Cuban coastal regions, where one may find different depth levels, mangrove swamps, brackish water, islets, prominent aquatic vegetation, among others.Contexto: La modelación estadística de la señal interferente conocida como clutter marino se efectúa a través de dos componentes: uno de capilaridad y otro de textura. La distribución más utilizada para la textura es la gamma. No obstante, varios autores han defendido el uso alternativo de la inversa gamma y la inversa gaussiana.Método: Con el objetivo de brindar un acceso fácil a la manipulación de los modelos y a la realización de comparaciones entre ellos, los autores del presente artículo simularon en Matlab las características principales de estas tres distribuciones. Adicionalmente, se agregó la distribución raíz gamma que sustituye a la gamma cuando se trabaja con muestras de amplitud. El método aplicado consistió en la revisión bibliográfica para encontrar las expresiones de cada uno de los parámetros modelados, y la posterior simulación computacional que permitió detectar errores ocasionales que surgen al consultar diferentes estudios.Resultados: Se creó una pequeña librería de simulación estocástica que incluye funciones de densidad y distribución, generación de variables aleatorias, estimación de parámetros y cálculo de momentos estadísticos, entre otros. Además, se elaboraron funciones informáticas complementarias que permitieron la validación por comparación con resultados dados por terceros y mediante la interacción de los diferentes componentes de la librería.Conclusiones: La librería creada habilita el uso de múltiples distribuciones, para la modelación del eco electromagnético de la superficie marina. Esto permitirá generar nuevos detectores de radar que se adapten a condiciones heterogéneas como las encontradas en las costas cubanas, donde alternan distintos niveles de profundidad, manglares, aguas salobres, islotes, vegetación acuática prominente, entre otras

Application of the pareto plus noise distribution to medium grazing angle sea-clutter

Robust maritime surveillance with radar requires an accurate description of the backscatter from the sea. The probability distribution of the backscatter is commonly used to determine the threshold for separating targets from clutter. Analysis of data collected at medium grazing angles, between 15° and 45°, by the Defence Science Technology Organisation (DSTO) Ingara fully polarimetric X-band radar has shown that the Pareto distribution is extremely useful as it both captures the high-magnitude components of the sea-clutter and allows significantly simpler optimal and suboptimal detectors to be designed. To further enhance the usefulness of this distribution, this paper presents a multilook formulation which accounts for the thermal noise in the radar. A number of techniques for evaluating the distribution are then presented, including a numerical integration scheme and a number of approximations, which retain the original form of the Pareto distribution.Luke Rosenberg and Stephen Bocque

Modeling the amplitude distribution of radar sea clutter

Ship detection in the maritime domain is best performed with radar due to its ability to surveil wide areas and operate in almost any weather condition or time of day. Many common detection schemes require an accurate model of the amplitude distribution of radar echoes backscattered by the ocean surface. This paper presents a review of select amplitude distributions from the literature and their ability to represent data from several different radar systems operating from 1 GHz to 10 GHz. These include the K distribution, arguably the most popular model from the literature as well as the Pareto, K+Rayleigh, and the trimodal discrete (3MD) distributions. The models are evaluated with radar data collected from a ground-based bistatic radar system and two experimental airborne radars. These data sets cover a wide range of frequencies (L-, S-, and X-band), and different collection geometries and sea conditions. To guide the selection of the most appropriate model, two goodness of fit metrics are used, the Bhattacharyya distance which measures the overall distribution error and the threshold error which quantifies mismatch in the distribution tail. Together, they allow a quantitative evaluation of each distribution to accurately model radar sea clutter for the purpose of radar ship detection