This project assembled a SAR algorithm analysis testbed and demonstrated the ability to focus moving targets possessing non-linear motion over a synthetic aperture collection. The testbed consisted of the ImSyn optoelectronic processor connected to a low end SGI workstation. A suite of software tools were developed to support focusing moving targets and to provide the necessary user interaction with the imaging process. We developed and tested moving target algorithms on a variety of simulated data sets and real SAR data. This report details the approach and algorithms that were employed to successfully process both SAR and ISAR data collections to add value to target discrimination and non-cooperative target identification.
Essex has developed the ImSyn processor, a sophisticated hybrid of optics and electronics. ImSyn calculates a discrete Fourier transform. The current production ImSyn is optimized for synthetic aperture radar processing, but has been used to process MRI, acoustic tomography, and synthetic aperture microscope data. The key feature of the production ImSyn is the ability to calculate images from non- rectilinearly gridded data. This data cannot be transformed with the FFT algorithm without interpolation or regridding. An alternative version of ImSyn is being developed for correlation applications. The correlator will be optimized for speed in performing rectilinear transforms.
A new photonics concept for true time delay beamforming and steering is described. To delay a signal an array of fibers, or other optical waveguides, containing cavities of different resonant frequencies are used to channelize the signal. Each spectral component of the signal is phase shifted by an amount proportional to the frequency of that spectral component and proportional to the time delay desired. These phase shifted spectral components are then summed to obtain the delayed signal. This new approach does not rely on switching between different lengths of delay lines. As a result, the pointing direction of an antenna array can be finely controlled over a continuum of angles, and the time to change direction can be on the order of 10 nanoseconds or faster. The concept has been refined, analyzed, and implementation issues addressed. The approach appears to be feasible. All the integral components with characteristics necessary to meet the requirements of an operational true time delay beamforming system are realizable. The time delay module as a channelizer has a number of alternate applications. For electronic warfare the channelizer provides the ability to analyze a wide bandwidth signal on the order of tens of Gigahertz at a resolution of 10 to 20 Megahertz. As a wavelength demultiplexer in optical communications, it would allow for a greatly increased density of channels within a given spectral range.
The ImSynTM Processor is an optoelectronic signal processor developed by Essex Corporation to accelerate coherent imaging processes. This paper focuses on the application of the ImSyn Processor to SAR imaging where severe range differential curvature is present. This occurs in SAR systems imaging large scenes with fine resolution, foliage penetrating (FOPEN) radar and ground penetrating radar. Application of the range migration algorithm removes the differential range curvature but results in a non- uniform or warped frequency space. The ImSyn processor operates directly on the frequency data permitting a discrete Fourier transform in warped frequency space without data interpolation. Both the range migration algorithm and the standard polar formatting algorithm benefit from the increased speed and resolution available from the ImSyn processor. A discussion of the ImSyn processor, the range migration algorithm and an example of a FOPEN image processed on our prototype system are presented.
Conventional true time delay beamforming and steering devices rely on switching between various lengths of delay line. Therefore only discrete delays are possible. Proposed is a new photonics concept for true time delay beamforming which provides a finely controlled continuum of delays with switching speeds on the order of 10's of nanoseconds or faster. The architecture uses an array of waveguide cavities with different resonate frequencies to channelize the signal. Each spectral component of the signal is phase shifted by an amount proportional to the frequency of that component and the desired time delay. These phase shifted spectral components are then summed to obtain the delayed signal. This paper provides an overview of the results of a Phase I SBIR contract where this concept has been refined and analyzed. The parameters for an operational system are determined and indication of the feasibility of this approach is given. Among the issues addressed are the requirements of the resonators and the methods necessary to implement fiber optic Bragg gratings as these resonators.
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