Luke Rosenberg

Application of the K+Rayleigh distribution to high grazing angle sea-clutter

The probability distribution of the radar backscatter is commonly used to determine the threshold for separating targets from clutter. Analysis of sea-clutter data collected at high grazing angles, between 15° and 45°, by the Defence Science Technology Organisation (DSTO) Ingara fully polarimetric X-band radar has been used extensively to test distribution models given a large number of samples. The focus of this paper is to determine the most appropriate sea-clutter model for high grazing angle sea-clutter given the smaller number of samples in a typical target detection scenario. For this purpose, a recently proposed K+Rayleigh distribution is introduced to account for the extra Rayleigh scattering observed in the radar backscatter.

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.

Modelling X-band sea clutter at moderate grazing angles.

This document reports on work undertaken at DSTO towards modelling the mean ocean backscatter coefficient at low to medium grazing angles for X-band radar. The particular range of angles used lies within the so-called plateau region where Bragg scattering dominates. The motivation for the work is to consider future maritime radar surveillance from high altitude airborne platforms. The requirement is for a model which takes account of radar polarisation, imaging geometry and ocean surface conditions. In order to assess modelling performance, a comprehensive set of ocean backscatter data was collected using Ingara (DSTOpsilas airborne multi-mode radar system.) Several candidate backscatter models were assessed against this data set and found to be unsuitable. Consequently, a new empirical model was developed which provides a significantly better fit to the measured data than the existing models. This paper reports on the measured data, the creation of the new model and the comparison against the existing models.

Characterising the Doppler spectra of high grazing angle sea clutter

This paper characterises the Doppler spectra of radar sea clutter using data collected by the Ingara radar at grazing angles in the range 29° to 38°. It is shown that recently proposed modelling and simulation methods for lower grazing angles are still applicable for this data. The modelling method is also extended to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or downwind.

Characterization of High Grazing Angle X-Band Sea-Clutter Doppler Spectra

Collection of radar sea-clutter is typically performed from the top of a cliff looking out to sea, as it is relatively simple and inexpensive. This constrains the radar look direction with respect to the wind and limits the grazing angle. To improve our understanding at high grazing angles in the range 15° to 45°, the Defence Science and Technology Organisations Ingara airborne X-band fully polarimetric radar has been used to collect 12 days worth of sea-clutter data. It has previously been shown that Walkers mean Doppler spectrum model is not suitable at these grazing angles; hence, a new two-component model is proposed that captures both the slow Bragg component and the fast non-Bragg component of the radar backscatter. A temporal decorrelation model is then presented that can be used to provide realistic performance prediction modeling.

Simulation of coherent sea clutter with inverse gamma texture

A method for simulating coherent sea clutter has been adapted for Pareto distributed clutter by using an inverse gamma distribution for the local clutter intensity. The model is applied to the range variation of Doppler spectra from Pareto distributed clutter collected at grazing angles of 31°-37° with the DSTO Ingara radar. The model produces Doppler spectra with a Gaussian shape that vary with range in a similar way to those obtained from the data. The simulations contain clutter spikes that replicate the properties of real sea spikes in respect of their intensity statistics, Doppler spectra and position near the wave peaks.

The NRL Multi Aperture SAR system

This paper describes the Naval Research Laboratory Multi Aperture Synthetic Aperture Radar (NRL MSAR) and presents initial results from the first field deployment of this system. The NRL MSAR is an airborne test bed designed to investigate remote sensing and surveillance applications that exploit multiple along-track phase centers, in particular, applications that require measurement of scene motion. The system operates at X-band and supports 32 along-track phase centers through the use of two transmit horns and 16 receive antennas. As illustrated in this paper, SAR images generated with these phase centers can be coherently combined to directly measure scene motion using the Velocity SAR (VSAR) algorithm. In September 2014, this unique radar was deployed for the first time on an airborne platform, a Saab 340 aircraft. This paper presents a description of the system, initial images from the September 2014 tests, and the results of initial coherent analyses to produce estimates of scene and target motion. These images were collected over an ocean inlet and contain a variety of moving backscatter sources, including automobiles, ships, shoaling ocean waves, and tidal currents.

Constrained Fast-Time STAP for Interference Suppression in Multichannel SAR

Forming a synthetic aperture radar (SAR) image while suppressing an airborne broadband jammer can potentially destroy large regions of the image. In addition to this, multipath reflections from the ground known as hot clutter or terrain scattered interference (TSI) will add a nonstationary interference component to the image. A number of constrained fast-time space time adaptive processing (STAP) techniques are presented which exploit adaptive beamforming to suppress the interference with minimal distortion to the final image.

The effect of temporal correlation with K and KK-distributed sea-clutter

Sea-clutter has been studied for many years over a range of sea conditions, polarisations and geometries. The Defence Science and Technology Organisations (DSTO) Ingara airborne X-band fully polarimetric radar has been used to collect data in the medium grazing angle region to evaluate models of the observed sea phenomenology and determine their impact on target detection. The focus of this work is the temporal correlation that may be present in either the target and / or clutter components of the received pulses. If this correlation is not accounted for in a radar model, the required signal to interference ratio (SIR) for a given probability of detection (Pd) will be incorrect by several dB, resulting in over-estimated performance. This paper describes a model for the temporal correlation of medium grazing angle X-band sea-clutter and quantifies the detection performance for Rayleigh fluctuating targets with both K and KK distributed sea-clutter.

Maritime Signature Correction With the NRL Multichannel SAR

This paper describes the Naval Research Laboratory Multichannel Synthetic Aperture Radar (NRL MSAR) and presents initial results from the first field deployment of this system. The NRL MSAR is an airborne test bed designed to investigate remote sensing and surveillance applications that exploit multiple along-track phase centers, particularly applications that require measurement of scene motion. The system operates at X-band and supports 32 along-track phase centers through the use of two transmit horns and 16 receive antennas. As illustrated in this paper, SAR images generated with these phase centers can be coherently combined to directly measure scene motion using the velocity SAR algorithm, and these measurements can then be used to correct the image distortion that the motion causes. In September 2014, this unique radar was deployed for the first time on an airborne platform. This paper presents a description of the system and results from its inaugural deployment, including the correction of distorted maritime signatures. These data were collected over an ocean inlet and contain a variety of moving backscatter sources, including automobiles, ships, shoaling ocean waves, and tidal currents.