N.J.S. Stacy

Ingara: an integrated airborne imaging radar system

The Ingara airborne imaging radar system is a technology demonstrator aimed at evaluating the application of multi-mode radar to broad area land surveillance. An integrated system has been developed that includes an X-Band radar with SAR and MTI modes, real-time signal processing system, air-to-ground data link, automated target detection algorithms and a simple image analysis geographical information system.

Ingara: the Australian Airborne Imaging Radar System

The Ingara Australian Airborne Imaging Radar System, formerly called AuSAR, is a low cost airborne imaging radar technology demonstrator currently under development at the Defence Science and Technology Organisation in Adelaide, Australia. The aims of Ingara are to evaluate airborne multimode radar technology, such as synthetic aperture radar and moving target indicator, for the Australian Defence Forces and to provide the scientific and remote sensing community access to an in-country imaging radar sensor for assessment. The Ingara system first flew in March 1993 and in June 1993 was commissioned in its base form. It currently undergoing further development and evaluation until the end of 1995.<<ETX>>

Polarimetric analysis of maritime SAR data collected with the DSTO ingara X-Band radar

Fine resolution spotlight synthetic aperture radar (SAR) imagery collected on a circular flight path is used to investigate the variation in ship detection performance with respect to three fundamental radar parameters: the transmit/receive polarisation combination, the incidence angle and the azimuth angle. The polarisation combinations examined are HH, HV, VV, RR and cross-slant 45deg. Three different incidence angles are considered - 50deg, 60deg and 70deg - corresponding to collection geometries for high altitude maritime surveillance systems. Performance is assessed using a simple target-to-background contrast measure. Results are compared with preliminary results from a four component decomposition of the Mueller matrix. While the latter and cross-slant 45deg show promise, in general HH is shown to have the best performance.

The DSTO Ingara airbone X-Band SAR polarimetric upgrade: first results

The Ingara airborne multi-mode X-Band imaging radar system developed by DSTO has been upgraded to a fully polarimetric collection capability with a 600 MHz bandwidth. The initial trial of the upgraded system in December 2002 included the collection of synthetic aperture radar data sets of a calibration scene including interferometric repeat passes. The application of calibration techniques to the data yields a measured channel cross talk less than -30 dB.

Space variant filtering of polar format spotlight SAR images for wavefront curvature correction and interferometric processing

In repeat pass SAR interferometry the scene coherence can be used to detect subtle scene changes that occur in the interval between image collections. In spotlight SAR mode the azimuth resolution of the baseline and repeat pass imagery may be improved by collecting an extended aperture therefore allowing finer resolution estimates of the scene coherence. The formation of the imagery however requires adequate compensation of the wavefront curvature effects which become increasing significant in finer resolution imagery. In N. E. Doren (1997) it is shown that for spotlight SAR imagery, formed using the Polar Format Algorithm (PFA), wavefront curvature effects give rise to residual phase errors that may be removed using an image domain filtering technique. The analysis however is based on the formation of a slant plane image obtained by projection of the scene reflectivity along circular arcs centered on the flight track. In this paper a full 3D imaging geometry is considered to derive an appropriate post PFA image domain filter to form a ground plane image. This allows multipass images to be formed in a common image plane for subsequent interferometric processing. The technique is used to generate a fine resolution coherence map using data collected in a repeat pass SAR experiment using the DSTO Ingara SAR.

Analysis of medium grazing angle X-band sea-clutter Doppler spectra

Doppler analysis of radar sea-clutter is typically performed from a static cliff top location looking out to sea. This constrains the grazing angle to low values and the radar look direction with respect to the wind. Current research at the DSTO is interested in the properties of sea-clutter at medium to high grazing angles, over all azimuth directions, full polarisation and different spatial resolutions. The sea clutter Doppler spectrum investigated in this paper was collected by an airborne platform and analysed using the three component Walker scattering model modified to include Doppler shifts from the moving platform. The modelling results enable the underlying Doppler spectrum and autocorrelation of the sea clutter to be estimated. This will enable future analysis into the performance of medium to high grazing angle target detection algorithms in the maritime environment.

Polarimetric Characteristics of X-Band SAR Sea Clutter

The polarimetric characteristics of X-Band sea clutter are investigated using data from the DSTO Ingara airborne radar operating in a circular spotlight geometry. An analysis of the Doppler spectrum as a function of azimuth geometry with respect to the wind direction shows the difference in the surface velocity between the HH and VV scattering components reaches maxima in the upwind and down wind directions and the cross polarized HV channel has very similar Doppler characteristics to the VV channel. The Cloude-Pottier polarimetric decomposition parameters are in agreement with predominantly Bragg signal surface scattering with an additional HH sea spike component in the upwind direction.

Range cell migration in the spotlight SAR polar format algorithm

Compensation of scatterer range cell migration is an important aspect of accurate spotlight SAR image formation. The polar format algorithm (PFA) is a method for processing fine resolution SAR radar data which utilizes a polar arrangement of the echo spectra in the backscatter frequency domain. It is shown that the PFA resampling from a polar to rectangular grid generates a linear phase function on the resampled data that after the Fourier transform to the image domain, fully compensates the linear component of the range cell migration. This new perspective of the PFA range cell migration correction is derived using signal phase history and tomographic approaches.

Change detection in repeat pass interferometric synthetic aperture radar

In this paper, a scene change statistic based on a log likelihood ratio has been considered for detecting man-made scene disturbances in repeat pass SAR interferometry. The change statistic identifies scene disturbances through changes in the scene coherence as well as changes in the scene backscatter coefficient. Theoretical analysis shows that the statistic gives significantly better detection performance than that of the commonly used sample coherence and backscatter coefficient ratio change statistics. The performance improvement has been experimentally verified using repeat pass SAR data collected with the DSTO Ingara SAR.

The Ingara Bistatic SAR Upgrade: First Results

An upgrade programme to add a bistatic collection capability to the Ingara airborne X-band fully-polarimetric imaging radar has recently been completed, and the first field trials of the new bistatic SAR system were conducted in December 2007. This involved a set of experimental SAR data collections in a bistatic geometry, in which the airborne radar was operated in a fine-resolution (600 MHz bandwidth) circular spotlight-SAR mode, in conjunction with a newly-developed fully-polarimetric stationary ground-based bistatic receiver. These data collections were largely successful, and produced a set of fully-polarimetric bistatic SAR data, collected over a wide range of bistatic angles. In this paper, we give an overview of the Ingara bistatic SAR system, describe the recent bistatic SAR experiment, discuss the techniques employed for processing the data, and present preliminary experimental results.