Nick J. S. Stacy

Comparison of Compact Polarimetric Synthetic Aperture Radar Modes

Compact polarimetry is a technique that allows construction of pseudo quad-pol information from dual-polarization synthetic aperture radar (SAR) systems. Compact polarimetry showed promise of being able to reduce the complexity, cost, mass, and data rate of a SAR system while attempting to maintain many capabilities of a fully polarimetric system. In this paper, we study different transmit/receive configurations to determine which polarimetric configurations allow for superior reconstruction of the fully polarimetric data. We discuss modifications of the original reconstruction algorithm proposed by Souyris , which show potential to better reconstruct fully polarimetric data.

Detecting scene changes using synthetic aperture Radar interferometry

In repeat-pass interferometric synthetic aperture radar (SAR), man-made scene disturbances are commonly detected by identifying changes in the mean backscatter power of the scene or by identifying regions of low coherence. Change statistics such as the sample mean backscatter-power ratio and the sample coherence, however, are susceptible to high false-alarm rates unless the change in the mean backscatter power is large or there is sufficient contrast in scene coherence between the changed and unchanged regions of the image pair. Furthermore, as the sample mean backscatter-power ratio and sample coherence measure different properties of a SAR image pair, both change statistics need to be considered to properly characterize scene changes. In this paper, models describing the changed and unchanged regions of a scene are postulated, and the detection problem is expressed in a Bayesian hypothesis-testing framework. Forming the log-likelihood ratio gives a single sufficient statistic, encoding changes in both the coherence and the mean backscatter power, for discriminating between the unchanged- and changed-scene models. The theoretical detection performance of the change statistic is derived and shows a significant improvement over both the sample mean backscatter-power ratio and sample coherence change statistics. Finally, the superior detection performance of the log-likelihood change statistic is demonstrated using experimental data collected using the Defence Science and Technology Organisations Ingara X-band airborne SAR

Polarimetric analysis of fine resolution X-band SAR sea clutter data

The DSTO Ingara multi-mode X-Band radar collected fine resolution fully polarimetric sea clutter data over a wide variety of sea states, incidence and azimuth angles. Analysis includes polarimetric decomposition and estimation of K distribution clutter statistics. The results show the backscatter is in the low entropy surface scatter zone associated with the Bragg scattering expected from the sea surface, and the K-distribution shape parameter υ has a variation in azimuth angle strongly coupled to the mean backscatter in the VV and HV polarizations.

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.

The Global Hawk UAV Australian deployment: imaging radar sensor modifications and employment for maritime surveillance

The Global Hawk system is a high altitude endurance unmanned aerial vehicle developed under the United States Air Force Advanced Concept Demonstrator program primarily as a reconnaissance system for use against fixed and mobile targets. The Global Hawk system deployed to Australia in April 2001 for six weeks and in this period conducted 11 missions with the focus on maritime surveillance. The Australian deployment was the culmination of two years collaboration between the United States and Australia that included modifications to the radar sensor, system control and exploitation to support a surveillance focus. This paper presents aspects of the Australian contribution to the Global Hawk deployment including the rationale behind the sensor modifications and employment that achieved a surveillance capability with a system primarily designed for land reconnaissance.

Scene coherency at X-band from repeat pass polarimetric interferometry

Polarimetric SAR interferometry (POLINSAR) has been successfully applied to L-band measurements over forests to resolve different scattering mechanisms within the scattering volume. Using a Random Volume over Ground (RVoG) model biophysical parameters including the vegetation layer height, extinction and the ground topography have been estimated. More recently interest has turned to examining the polarimetric dependence of the interferometric coherence of agricultural crops and the recovery of biophysical parameters. In this paper the polarimetric dependence of the interferometric coherence over a wheat crop is investigated using multi-baseline repeat pass X-band data. Analysis of the interferometric coherence as a function of baseline indicates that the HH channel contains a strong ground contribution while the VV return is predominantly volume scattering from the wheat canopy. Examination of the interferometric phase differences between the polarimetric channels indicates a height separation of the scattering mechanisms in each channel. An estimate of the crop height is obtained however, the effects of temporal decorrelation over the repeat pass interval introduce significant biases into the estimate.

Characterisation of low-PRF X-band sea-clutter doppler spectra

Understanding the characteristics of the sea is critical in forming a picture of the sea-clutter as seen by a radar. Current research at the DSTO is interested in the properties of sea-clutter at medium grazing angles, over all azimuth directions, with full polarisation and differing weather conditions. This paper builds on previous work which studied the Doppler spectrum over all azimuth directions using the three component Walker scattering model. An improved model is presented here which includes the aliasing present in low pulse repetition frequency (PRF) data collected using Ingara - the DSTOpsilas X-band synthetic aperture radar (SAR). This enables a large number of data sets to be analysed with differing grazing angles and weather conditions. The goal of this work is to characterise the sea-clutter and improve the fidelity of future analysis into the performance of medium grazing angle target detection algorithms in the maritime environment.

A change detection statistic for repeat pass interferometric SAR

In repeat pass synthetic aperture radar interferometry (In-SAR) the scene coherency can be used to detect subtle scene changes that may not be evident in the image magnitude data alone. The sensitivity of the coherency estimate for identifying man-made disturbances is dependent on the size and nature of these disturbances compared to the other sources of disturbance such as wind, system noise and processing aberrations that affect the scene images. In this paper the detection problem is formulated in a hypothesis testing framework and a new change statistic is proposed. Analytic expressions for the probability of detection and false alarm are derived which show a significant improvement over the commonly used coherence based change detection. Application of the new change statistic to experimental data obtained using the DSTO Ingara SAR demonstrates the improved detection performance.

Estimating X-band synthetic aperture radar detection of objects obscured by foliage using the DSTO Ingara airborne imaging radar

Estimating the Probability of Detection (P/sub d/) of synthetic aperture radar (SAR) imagery of foliage-obscured objects requires a knowledge of the object backscatter and the obscuration properties of the foliage. At X-band frequencies minimal penetration of the foliage occurs and as such the assumption that the object is visible only through gaps in the foliage can be made. Given this assumption this paper discusses the use of Hemispherical Canopy Photographs (HCPs) and SAR imagery of unobscured objects to estimate the P/sub d/ of obscured objects. These estimates are compared with P/sub d/s measured directly from imagery of obscured objects. The results of this comparison show good agreement between the estimated and measured P/sub d/ values, indicating that a knowledge of the particular object backscatter and the HCP of a particular obscuration scenario is sufficient to predict the P/sub d/ without the need to image the obscured object directly.

Image formation on undulating terrain using the upgraded Ingara L-band radar system

A beamforming approach to SAR image formation is shown to facilitate spatially variant aperture trimming which maximises the potential repeat-pass coherence when focusing onto undulating terrain. The method is demonstrated using the upgraded Australian airborne radar system, Ingara.