Sub-Nyquist Synthetic Aperture Radar Imaging and Ground Moving Target Indication
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Abstract
Synthetic Aperture Radar (SAR) is an all-weather, day or night, imaging modality that can be utilized to produce high-resolution images of the reflectivity of the ground. To achieve high azimuth resolution in SAR, the scatterers in the scene of interest should be observed over a long synthetic aperture. This will generate a wide Doppler bandwidth which should be sampled using a High Pulse Repetition Frequency (HPRF). An HPRF will limit the maximum range swath width that can be covered. Therefore, wide-swath SAR imaging requires using a Low PRF (LPRF). Simultaneous High-Resolution Wide-Swath (HRWS) SAR imaging can be achieved by using a more complex SAR system that utilizes multiple antenna phase centers and multiple channels. Alternatively, if the scene of interest contains a small number of bright scatterers in a dark background which, for instance, is the case when imaging ships in a calm sea background, HRWS SAR imaging can be achieved by sampling the synthetic aperture using multiple sub-Nyquist PRFs. Such PRFs are carefully chosen to allow for the reconstruction of an alias-free image from highly aliased sub-Nyquist images. The use of the sub-Nyquist PRFs further allows for a reduction in the amount of data to be stored and communicated.
One of the main limitations of the sub-Nyquist SAR imaging techniques recently proposed in the literature is the appearance of residual ambiguities (ghost images) in the reconstructed SAR image. This effect is the result of unresolved aliases and it is more pronounced for scenes with a large number of scatterers. In Chapter 3, Multichannel Coprime SAR (MC-CopSAR) is proposed. MC-CopSAR utilizes an additional along-track receive phase center to suppress the ghost images and preserve true scatterers. Simulation results show that the improvement in image reconstruction provided by MC-CopSAR is superior.
To allow for wide range swath coverage, sub-Nyquist HRWS imaging techniques use multiple semi-orthogonal waveforms or multiple carrier frequencies. In Chapter 4, Staggered Coprime Pulse Repetition Frequencies SAR (SCopSAR) is proposed. In SCopSAR, the reference PRF is chosen higher than the Doppler Nyquist rate. Moreover, the two sub-Nyquist PRFs used to sample the synthetic aperture are related to the Nyquist PRF by two sub-sampling factors. SCopSAR trades half the azimuth resolution for an extension in the range swath by a number of times that equals the smaller sub-sampling factor. SCopSAR requires only a single waveform such as the conventional Linear Frequency-Modulated (LFM) chirp, a single carrier frequency, and a single channel. Moreover, SCopSAR allows for a reduction in the amount of data to be stored and communicated. Compared to current HRWS SAR imaging techniques, SCopSAR simplifies the system requirements greatly. Simulations and real ERS-2 satellite raw data are used to validate the theoretical findings presented in Chapter 4.
In Chapter 5, a full sub-Nyquist SAR based Ground Moving Target Indication (GMTI) solution is provided. Such a solution is dubbed CopGMTI and it extends the maximum range swath width in which a moving target can be unambiguously detected and has its radial velocity estimated. CopGMTI further allows for a reduction in data storage and communication. The solution provided in Chapter 5 tackles the problem of alias suppression, channel calibration, and phase centers separation estimation using sub-Nyquist azimuth samples. The theoretical results presented in Chapter 5 are validated using the airborne multichannel Air Force Research Laboratory (AFRL) Gotcha SAR GMTI data set.