Abstract
This paper describes a way to synthesize a larger coherent aperture from smaller apertures combined with motion, when only intensities are measured. It relies on collecting intensity patterns in two planes for each aperture, for example, the aperture plane and an image plane, and using a phase-retrieval algorithm to reconstruct the optical field in the aperture plane. As the sensor moves forward, a larger two-dimensional aperture is synthesized, allowing a much finer resolution image to be reconstructed. An algorithm to correct for the relative pointing (tip and tilt phases) and piston errors between different apertures and at different times is needed to phase up the synthetic aperture. Results of simulations, including the effects of speckle, are shown, and practical considerations are evaluated.
Highlights
To achieve fine resolution imagery at a given long distance and in a given wavelength band, one needs a collection system having a large enough effective aperture
Known as spatial heterodyne, can achieve fine resolution in angle–angle space by a string of apertures in the cross-track direction combined with aperture synthesis in the along-track direction; it can employ narrow laser bandwidths but must still interfere the return field from the object with a local oscillator (LO), requiring stable LO distribution from a master laser to all the telescopes
In Ref. 12, we showed that the same algorithm worked for the large 72-aperture synthetic aperture shown in the bottom of Fig. 1, for the higher signal-to-noise ratios (SNRs) (Npps 1⁄4 100), but the pupilphasing algorithm was not adequate for low SNRs
Summary
To achieve fine resolution imagery at a given long distance and in a given wavelength band, one needs a collection system (telescopes) having a large enough effective aperture. As an alternative to building and deploying larger single-aperture systems (which become increasingly bulky, heavy, and costly), one can perform aperture synthesis. This can be done either passively (using reflected sunlight) as in Michelson stellar interferometry or actively as in coherent laser illumination with phase-sensitive detection. Fienup: Direct-detection synthetic-aperture coherent imaging by phase retrieval coverage rate, collecting multiple images, is shown to be proportional to the laser power available and inversely proportional to the number of speckle realizations averaged to get one image.
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