Aerospace and Electronic Systems Magazine August 2016 - 44

News & Information Technology advances often take the form of a series of S-curves [5]. SAR and ISAR imaging has reached the stage where extremely high resolutions can now be obtained, both from airborne and satellite platforms, and techniques such as radar polarimetry, interferometry, and coherent change detection (CCD) all provide information that can be used to help classify and identify targets. High resolution Synthetic Aperture Radar (SAR) and its coherence products have been found to be of great utility in both identifying targets and detecting changes that occur on the ground. Detectable changes of interest include vehicle tracks, water flow, and small scale subsidence. The coherent change detection procedure involves performing repeat pass radar collections to form the coherence product, where ground disturbances such as tire tracks can induce detectable incoherence. Currently, SAR imagery of between 10 cm and 30 cm resolution is considered to be high resolution, allowing some success in target identification and in the detection of subtle changes on the ground. It is interesting to consider what the physical limits may be on resolution and coherence. If such radar modes were available, how could we best profit from them? The CCD SAR image on the top right is a spectacular example of what is now possible. This is a Ka-band image in which dark areas indicate a low degree of coherence between the two images - in other words, areas that have changed. The inset area at the bottom right has been expanded and foreshortened, showing two sheep and their tracks. Key to this technology, though, is to understand the physics of the interaction of the radar signal with the target, including the effects of multipath reflections, target motion and vibration. This may allow the important information to be extracted and interpreted in the right way. Radar Phenomenology may be defined as the study of the physical processes of propagation and scattering. It might be imagined that these are by now adequately understood, since they have been studied since the earliest days of radar, and the physics has not changed. This is not the case, however, both because of advances in radar technologies, and because of the application of radars to sensing in ever more complex environments. In the former category we can note the development of signal sources with extremely low phase noise, the advances in processing power, the evolution of sophisticated signal processing 44 Ultra-high resolution Coherent Change Detection SAR image from the Thales BRIGHT SPARK Synthetic Aperture Radar (SAR) sensor. The inset shows two sheep, and their tracks in a field. BRIGHT SPARK is a Dstl Experimental SAR system designed, built and operated by THALES UK. It is a Ka-band sensor providing unprecedented resolution and was used as a technical demonstration of the 'art of the possible'. techniques, the availability of GPS and geographical information systems, access to real-time high-bandwidth communication services, the remarkable miniaturisation of components and circuits, and the invention of new materials with unprecedented mechanical and thermal behaviour, and novel electromagnetic properties with startling implications. The second category embraces such applications as throughthe-wall radar imaging, formation of images through turbulence in the atmosphere and the ionosphere, surveillance in the heavily built-up urban environment, mounting of radars on constellations of airborne or spaceborne platforms. We should also recognise the challenges posed by camouflage, concealment, and deception. To address such missions, and to take full advantage of the enormous improvements in radar technologies, the phenomenological models we employ to describe propagation and scattering must be of commensurate fidelity. In practice this means that the structural and dynamic properties of the propagation media must be characterised in great detail wherever they impact on either the radar signal or the behaviour of the target object. Transformation of signal properties by mechanisms such as dispersion, polarisation transformation, and decoherence cannot be ignored. Equally, the electromagnetic properties of the candidate targets - which may include nonlinearity, inhomogeneity, anisotropy, and so on - must be accounted for, along with any multiple scattering processes. Further, we must not overlook the physical impact of the target on its environment and the associated perturbations to the scattered field. These phenomenological considerations impact on radar design, choice of frequency and waveform, optimum siting, network design, IEEE A&E SYSTEMS MAGAZINE AUGUST 2016

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016