Daniel Andre

Multipath simulation and removal from SAR Imagery

Current SAR imaging techniques assume that radar pulses are reflected from a scene by a single bounce event (reflection from a sphere), or multiple bounces producing a fixed phase-centre (a trihedral). However, scattering is often more complex; e.g. the pulse may reflect off the ground before interacting with a vehicle, leading to additional bright returns in the image which are not located at the position of either bounce. In this paper we use simulation to assess the affect of multipath on vehicle signatures and develop techniques for the identification and removal of multipath returns from SAR imagery.

An analysis of 3D SAR from single pass nonlinear radar platform trajectories

An analysis of 3-D SAR image formation under the challenging condition of single pass sampling in the elevation dimension is presented. The analysis is operationally relevant as it is often not possible for a radar platform to collect radar data at sufficient grazing angles to satisfy the Nyquist sampling criterion. It is found that these sampling issues can partly be overcome through the use of non-linear radar platform trajectories. In conventional 2-D SAR imaging this approach can be viewed as detrimental, as the image depth of focus is reduced, however for 3-D imaging a reduced depth of focus has been found to be advantageous. The approach however, comes at the cost of resultant unusual image point spread functions, with coarser resolution in the vertical dimension. It is possible to obtain a wide range of point spread functions as a function of collection parameters including range, the form of the non-linear radar platform trajectories and centre frequency. This work explores this parameter space to find advantageous radar collection geometries. The image point spread functions are difficult to characterise analytically and so a numerical approach is undertaken.

Spatially variant incoherence trimming for improved SAR CCD

Conventional synthetic aperture radar (SAR) Coherent Change Detection (CCD) has been found to be of great utility in detecting changes that occur on the ground. The CCD procedure involves performing repeat pass radar collections to form a coherence product, where ground disturbances can induce detectable incoherence. However there is always a difference in the radar collection geometry which can lead to incoherent energy noise entering the CCD. When sensing flat terrain in a far-field regime, the incoherence due to collection geometry difference can be removed through a conventional global Fourier image support trimming process. However, it has been found that when the terrain is either in a near-field regime or contains non-flat topography, the optimal trimming process is substantially more involved, so much so that a new per-pixel SAR back-projection imaging algorithm has been developed. The new algorithm removes incoherent energy from the SAR CCD collection pair on a per-pixel basis according to the local radar geometry and topography, leaving a higher coherence CCD product. In order to validate the approach, change detection measurements were conducted with GB-SAR, a ground-based indoor radar measurement facility.

Very high resolution Coherent Change Detection

Synthetic Aperture Radar (SAR) Coherent Change Detection (CCD) has been found to be of great utility in detecting changes that occur on the ground. Detectable changes of interest include vehicle tracks, water flow, and small scale subsidence. The CCD procedure involves performing repeat pass radar collections to form a coherence product, where ground disturbances can induce detectable incoherence. Currently, SAR imagery of between 10cm and 30cm resolution is considered to be a high resolution, allowing the detection of subtle changes on the ground, however it is of interest to examine CCD images resulting from very high resolution SAR down to 1cm resolution, which in principle could be collected through airborne or spaceborne radar platforms. To perform this study, laboratory data was generated with a ground-based SAR system.

Sidelobe rotation and apodization

Sidelobes in imagery from radar and other sensors can be troublesome because they obscure useful information within the image. Much effort has been made in the radar community to control sidelobes and many window weighting and apodization schemes are in existence [F. Harris, (1978), W.G. Carrara, (1995)]. We present here a novel algorithm, firstly to rotate sidelobes to uncover image information. The approach has been further developed into an apodization scheme. The approach has been applied in several forms, one of which outputs imagery of a resolution higher than that resulting from the standard window weightings, such as the Hamming. The algorithms have been applied to real and simulated data at both image processing and post-processing stages. Here we show results from three case studies, including from a ground-vehicle target. We show that scattering centers are associated with vehicle features, implying that they are not image artifacts.

Target decomposition through polarimetric, disjoint Doppler and frequency band (I)SAR images

A fully polarimetric (I)SAR image target decomposition process has been developed to aid automatic target recognition (ATR) for disjoint Doppler and frequency bands. In this paper the principle of target decomposition into scattering primitives is first demonstrated for a flat plate and then for a more complex target.

Synthetic aperture radar for lane boundary detection in driver assistance systems

In this paper we investigate the feasibility for using a Synthetic Aperture Radar (SAR) to detect radar scatterers in support of advanced driver assistance systems. Specifically, we consider the detection of radar scatterers physically embedded into lane and carriageway boundaries similar to way optical retroreflectors (cats eyes) are used in present infrastructure. We use simulations to generate high resolution SAR images for detecting and localizing radar scatterers. The simulated results presented here highlight the feasibility of the technique and provide a platform for further investigation. This paper facilitates the realization of the role of modified infrastructure for improving the sensing capability of highly assisted and autonomous vehicles.

Hyperspectral 10–50GHz SAR imaging of building materials

A laboratory-based study was carried out to assess the performance and unique intelligence capabilities of an extreme wideband 10-50GHz SAR. Very high resolution range-profile measurements were obtained of samples used in building construction. Periodic features in reflectivity with frequency were interpreted as a resonance between the front and back face reflections of a sample. The features characteristics were in good agreement with the idea of a sample as a resonant microwave cavity. The resonance behaviour was preserved in SAR imaging of the samples, examined by sub-band analysis. Selecting a composite Perspex-MDF target, there was good agreement between simulation and measurement.

Target detection in SAR imagery by diffraction patterning

We report on an investigation into the detection of power and telephone cables in SAR imagery by the presence of diffraction patterning. Laboratory SAR imaging measurements on metal pipes suspended above a gravel surface produced downrange fringe patterns in both backscatter and interferometric phase. Modelling was carried out using an incremental diffraction algorithm, and the positioning and modulation characteristics of the fringes could be understood by the relative positioning of the targets above the gravel and the imaging geometries. Whereas previous studies have relied upon the direct return from the cables, this study has shown their presence might still be inferred from the persistent presence of ground fringes even when a direct return is absent. The effect could find an application in collision avoidance of power and telephone cables by low-flying aircraft, as well as in surveillance and monitoring.

Detection of aerial features by ground diffraction patterning in SAR imagery

We report on an investigation into the secondary detection of objects in SAR imagery by the presence of diffraction patterning. Laboratory SAR imaging measurements were carried on metal pipes suspended above a gravel surface. Clear fringe patterns were seen across the imaged gravel in both backscatter and interferometric phase. Modelling was carried out using an incremental diffraction algorithm. The positioning and modulation characteristics of the fringes could be understood by the relative positioning of the targets above the gravel and the imaging geometries. Even if the pipes or wires are not visible in the imagery from the direct return, their presence might be inferred from the persistent presence of ground fringes.