Antenna Array Design in Aperture Synthesis Radiometers 169 Antenna Array Design in Aperture Synthesis Radiometers

During the past few decades, there has been growing interest in the use of microwave and millimeter wave radiometers for remote sensing of the Earth. Due to the need of large antennas and scanning mechanism, the conventional real aperture radiometer becomes infeasible for high spatial resolution application. Interferometric aperture synthesis was suggested as an alternative to real aperture radiometry for earth observation [Ruf et al., 1988]. Aperture synthesis radiometers (ASR) can synthesize a large aperture by sparsely arranging a number of small aperture antennas to achieve high spatial resolution without requiring very large and massive mechanical scanning antenna. The fundamental theory behind aperture synthesis technique is the same as the one used for decades in radio astronomy [Thompson et al., 2001], in which the product of pairs of small antennas and signal processing is used in place of a single large aperture. In aperture synthesis, the coherent product (correlation) of the signal from pairs of antennas is measured at different antenna-pair spacings (also called baselines). The product at each baseline yields a sample point in the Fourier transform of the brightness temperature map of the scene, and the scene itself is reconstructed by inverting the sampled transform. This chapter addresses the subject of antenna array design in ASR, which plays an important role in radiometric imaging of ASR. The chapter is organized as follows. In section 2, the basic principle of synthetic aperture radiometers is briefly formulated. In section 3, the topology optimization of the antenna array is concerned, aiming at minimum redundancy arrays (MRAs) for high spatial resolution. For one-dimensional case, different optimization methods for finding out minimum redundancy linear arrays (MRLAs) such as numerical algorithms and combinatorial methods are summarized, including their advantages and disadvantages. We also propose an effective restricted search method by exploiting the general structure of MRLAs. For two-dimensional case, different antenna array configurations as well as their spatial sampling patterns are compared, including rectangular sampling arrays, hexagonal sampling arrays, and nonuniform sampling arrays. Some original work on the design of thinned circular arrays is also described. In section 4, a novel antenna array for our HUST-ASR prototype is presented, which is a sparse antenna array with an offset parabolic cylinder reflector at millimeter wave band. 9