Electromagnetic ray-tracing for the investigation of multipath and vibration signatures in radar imagery

Synthetic Aperture Radar (SAR) imagery has been used extensively within UK Defence and Intelligence for many years. Despite this, the exploitation of SAR imagery is still challenging to the inexperienced imagery analyst as the non-literal image provided for exploitation requires careful consideration of the imaging geometry, the target being imaged and the physics of radar interactions with objects. It is therefore not surprising to note that in 2017 the most useful tool available to a radar imagery analyst is a contextual optical image of the same area. This body of work presents a way to address this by adopting recent advances in radar signal processing and computational geometry to develop a SAR simulator called SARCASTIC (SAR Ray-Caster for the Intelligence Community) that can rapidly render a scene with the precise collection geometry of an image being exploited. The work provides a detailed derivation of the simulator from first principals. It is then validated against a range of real-world SAR collection systems. The work shows that such a simulator can provide an analyst with the necessary tools to extract intelligence from a collection that is unavailable to a conventional imaging system. The thesis then describes a new technique that allows a vibrating target to be detected within a SAR collection. The simulator is used to predict a unique scattering signature - described as a one-sided paired echo. Finally an experiment is described that was performed by Cranfield University to specifications determined by SARCASTIC which show that the unique radar signature can actually occur within a SAR collection

Imaging SAR phenomenology of concealed vibrating targets

This paper describes the novel imaging of SAR phenomena produced from vibrating targets with multipath effects. It has been established, through numerical SAR experiments, that different physical mechanisms interact to produce new artefacts. The computations demonstrated that the edges of a dielectric medium can act as a source for multipath effects to emanate from, leading to the hypothesis that SAR artefacts can arise from through-wall SAR imagery. This deduction and mechanism of origin were validated through several experimental measurements, undertaken at Cranfield University’s Antennas and Ground-based SAR laboratory, yielding results that closely match those predicted

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

ICEYE Microsatellite SAR Constellation Status Update:Evaluation of First Commercial Imaging Modes

The ICEYE constellation features the first operational mi-crosatellite based X-band SAR sensors suitable for all-weather day-and-night Earth Observation. In this paper we report on the status of the ICEYE Constellation and describe the characteristics of the first operational imaging modes

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

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

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 feature's 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

A new resonance phenomenon observed in UWB 14-50GHz SAR and its application to the retrieval of dielectric properties of scene features

We describe the observation of a previously unreported resonance phenomenon in UWB SAR imaging, and its application in a novel scheme for the retrieval of dielectric properties of scene features. The resonance was observed in a 14-50GHz laboratory study of very high-resolution SAR imagery. It presented as a periodic fluctuation in the backscatter of a scene feature in response to changes in the radar measurement frequency. Complimentary static reflectivity measurements of a range of dielectric materials showed the phenomenon could be understood as a material behaving as a resonant microwave cavity. Modeling simulations were able to accurately reproduce the observed nulling characteristics for visually homogeneous materials (acrylic, MDF, plasterboard), and which dis-played a regular frequency spacing between nulls. Those with inhomogeneous structures (chipboard, plywood) showed much more irregular nulling patterns. The spacing of the backscatter nulls is set by the product of the permittivity and thickness of the material. By exploiting diversity in viewing geometry, the two terms can be separated and measured. The scheme offers a valuable new opportunity in SAR surveillance and monitoring of man-made structure

Synthetic Aperture Radar for Through-Wall Detection and Imaging of Complex Vibrating Targets

<div>Poster presented at the 2017 Defence and Security Doctoral Symposium.</div><div><br></div><div>Low frequency Synthetic Aperture Radar (SAR) is a proven solution for generating through-wall images, although obtaining a clear picture of the structure and contents of buildings has proven to be difficult. Whilst the effects created by moving objects within a SAR image have been investigated before, the complex effects moving objects have upon a SAR image when located behind a wall, are less known. This is especially the case when the motion of the object being imaged is of vibration and when multipath effects influence the artefacts produced within the SAR image. This occurs when the transmitted SAR microwaves reflect from other objects within the imaged scene, before being detected by the receive antenna. </div><div><br></div><div>The research presented, investigates these complex SAR phenomena with multistatic radar geometries, using the Cranfield University Antennas and Ground Based SAR (AGBSAR) laboratory. This research is conducted in support of the Dstl Remote Intelligence of Building Interiors (RIBI) programme, and addresses the problem of the detection of running machinery within buildings, amongst other challenging scenarios.</div><div><br></div