In recent years, Earth observation by means of aperture synthesis radiometry has received special attention by some space agencies as a possible solution to achieve high radiometric accuracy and spatial resolution at low microwave frequencies (L band), where the apparent brightness temperature is much more sensitive to soil moisture and sea surface salinity. This paper presents the characterization and calibration procedure, as well as some synthetic images measured with an X band experimental Y-shaped Synthetic Aperture Interferometric Radiometer prototype developed at the Polytechnic University of Catalonia. The instrument is composed of a single pair of antennas that can be moved along the arms of an Y structure to synthesize a set of baselines. An experimental procedure is proposed to evaluate and then calibrate offset, inphase, quadrature, and amplitude errors generated by receivers and correlators.¿ 1997 American Geophysical Union