Advantages and Drawbacks of Near-field Characterization of Large Aperture Synthesis Radiometers

Accurate characterization of an aperture synthesis radiometer requires making measurement of its performances inside an anechoic chamber. However, due to size limitations, the sources used in the process have to be located at distances too short to be able to apply the far-field condition of the instrument. Although the fundamental equation of aperture synthesis radiometry derived for far-field fails in this case, it is still possible to measure most of the critical parameters such as spatial resolution, sensitivity or impulse response using point sources inside an anechoic chamber. Nevertheless, it is not possible to retrieve images of complex shapes, so near field characterization must be complemented with open air measurements of complex images. Near-field techniques have been used to characterize a reduced model of MIRAS and are being considered also for further characterizations of the whole instrument during the phases C/D of the SMOS mission.