An empirical sea clutter model for low grazing angles

The most fundamental characteristic of sea clutter, as used in radar performance evaluation, is its apparent reflectivity defined as sigma 0 (m 2 /m 2 ). The word apparent is used here as a reminder that any measurement of sea clutter reflectivity inevitably includes the effects of propagation close to the sea surface. Sea clutter reflectivity depends on many factors including sea state, wind velocity, grazing angle, polarization, and radar frequency. A comprehensive tabulation of measurements from around 60 sources were included in the 1991 edition of Nathanson's book and this probably represents the most complete database of sea clutter reflectivity available. Also included in this book by Nathanson was a detailed description of an empirical sea clutter model proposed by Horst et. al., the so-called Georgia Technical Institute (GTI) model. This model has found widespread acceptance in the radar community although its technical basis may be somewhat vague. As pointed out by Nathanson, his tabulated measured sea clutter data does not agree too well with the GTI model, in particular at low sea states. While this difference qualitatively can be explained by measurement inaccuracies, unknown propagation conditions (such as ducting), and uncertainties in defining the underlying sea state, these discrepancies are at times quite large and may lead to overly optimistic radar performance predictions if the GTI sea clutter model is used. In this paper a new empirical model for sea clutter reflectivity at low grazing angles is developed. The model is defined as a function of radar frequency, polarization, sea-state, and grazing angle. The parameters of the empirical equation have been optimized such that the average absolute dB-deviation between the model and the experimental data tabulated by Nathanson is minimized for grazing angles up to 10 degrees. Subsequently we shall refer to this new model as the NRL sea clutter model.