Phased array and spaced antenna radar techniques developed and employed at the Jicamarca Radio Observatory for ionospheric research are reviewed. The ionospheric scatterers studied by Jicamarca are not spatially homogeneous and not beam filling in some cases, and distinguishing the bearing of the radar backscatter is essential for correctly interpreting the radar data as well as for discovery research. Both incoherent and coherent scatter from thermal and nonthermal plasma density irregularities, respectively, are strong functions of magnetic aspect angle. Radar interferometry can be used to discriminate the arrival angle or brightness distribution of the backscatter and, in the case of coherent scatter from field-aligned irregularities, determine its aspect sensitivity. Using multiple spaced receivers, the entire brightness distribution can be estimated from the measured visibility using aperture synthesis imaging techniques. This is the most incisive remote sensing tool available for studying ionospheric plasma irregularities and the instabilities that produce them. Fi-nally, a novel technique for antenna compression introduced at Jicamarca can be used to synthesize very broad radiation patterns that nevertheless make use of the entire Jicamarca phased array for transmission.
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