Advanced High Resolution Methods for Radar Imaging and Micro-Doppler Signature Extraction

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Imaging radars are airborne or spaceborne radars which generate a reflectivity map of an illuminated area through transmission and reception of electromagnetic energy. Among many types of microwave sensors, special attention has been paid in the past to Synthetic Aperture Radar (SAR) because of its high spatial resolution, day or night all weather operational capabilities. With its fine two-dimensional resolution capability SAR has evolved to satisfy a variety of applications for both civilian and military users. These applications centre on target imaging and terrain mapping. A target is a specific object of interest that the radar illuminates. The typical target is man-made and consists of multiple scattering centres. An imaging radar system must distinguish between single and multiple scatters located in close proximity. Resolution is, nominally, the minimum distance needed between adjacent scatters to separate them in the image. Fine resolution provides the capability to image a complex object or scene as a number of separate scattering centres. This type of image provides detailed information to detect, characterize, and identify specific objects of interest. Because of the importance of object identification in military applications, much development effort has been directed at improving radar resolution. Military SAR applications include intelligence gathering, battlefield reconnaissance, and weapons guidance. Civilian applications include topographic mapping, oil spill monitoring, sea ice characterization and tracking, agricultural classification and assessment, lands use monitoring, and planetary or celestial investigations. Normally imaging radars provide a two-dimensional representation of a scatterer in the illuminated volume with no resolution or positioning of scatterer in the third dimension. Generally, we speak of monstatic (the transmitter and receiver are co-located) radar resolution in the range and cross-range or azimuth directions. Bistatic radars, where the transmitter and receiver are positioned in different physical positions have several operational advantages. In particular such bistatic systems help to increased receiver survivability while minimising receiver cost. Furthermore when one or both of the platforms are manoeuvring in an non linear planar path allows the resolution to be computed in the 3rd dimension thus facilitating the acquisition of target height information as well range and cross range resolution.When a radar interrogates a moving target it is traditional to exploit the target's Doppler for identification and characterisation. If the target possesses additional rotational, vibration or other internal motions then these induce additional spectral components separate from the main Doppler. These are termed microdoppler components and reside as additional sidebands around the main Doppler. A human walking or running will exhibit microdopplers due to swinging arms and leg movements. A military tank will exhibit microdopplers due to the wheel tracks while a helicopter and engine target will exhibit key microdoppler components. The use of time frequency signal representation such as the short time Fourier transform and wavelet analysis has been used to examine these microdopplers in the past. Good quality microdoppler signatures are important in new automatic target identification and recognition systems. Quality is directly related to the extracted microdoppler resolution extracted.The aim of this work is to explore new signal processing techniques which can be used to improve the resolution of the imaging radars algorithms and microdoppler signature extraction. The work will derive new mathematical relationships for bistatic spotlight SAR image formation and microdoppler signature extraction based on the Fractional Fourier transform and empirical mode decomposition. An FrFT compute engine will be realised and the algorithms will be tested on simulated and real data.