Practical Consideration on Ultrawideband Synthetic Aperture Radar Data Processing

Sammanfattning: The practical considerations in ultrawideband (UWB) synthetic aperture radar (SAR) data processing in general and UWB SAR imaging in particular are clarified and presented in detail in this thesis. They are imaging algorithm, impulse response function in SAR imaging (IRF-SAR), apodization, RF interference (RFI) and SAR image quality measurement. Different algorithms in both time- and frequency domain and their suitability to process UWB SAR data are investigated and evaluated. The necessary modifications for these algorithms are proposed to fulfill the requirements of UWB SAR data processing. The time-domain imaging algorithms are highly recommended for UWB SAR data processing due to their characteristics such as integrated motion error compensation, unlimited scene size and local processing. A new IRF-SAR, which is a function of fractional bandwidth and antenna beamwidth, is derived. The function allows us to investigate different UWB SAR systems. Such investigations are not facilitated by currently used IRF-SAR, Sinc functions. The derived IRF-SAR is totally valid to investigate narrowband (NB) SAR systems. A discussion about the apodization techniques and possibilities to apply to UWB SAR data processing is given in this thesis. Handling orthogonal and non-orthogonal sidelobe in UWB SAR imaging is shown to be challenging with the currently used apodization approaches. The linear apodization approaches always result in the loss in resolutions while the phase information can be destroyed by the nonlinear apodization approaches. A new approach to suppress RFI in UWB SAR signal, which is easy to be disturbed by RFI sources, is suggested. The advantages of the approach compared to the others can be found in adaptive and real time processing characteristics. A new definition of SAR image quality measurement is also presented in this thesis. The complicated behavior of IRF-SAR over fractional bandwidth and antenna beamwidth results in the unsuitability of the currently used definition for UWB SAR image quality measurements. The unsuitability is mainly caused by the inappropriate delimitation of mainlobe and sidelobe areas, the fixed broadening factors and the fixed spreading factor of the orthogonal and non-orthogonal sidelobes. Based on these practical considerations, the thesis also presents some possibilities to propose a definition of UWB SAR which is still not available. The beginning investigated results show that these possibilities comply with the UWB definition proposed by Federal Communications Commission (FCC) in 2002.