Calibration, Performance, and Imaging Tests of a Fully Digital Synthetic Aperture Interferometer Radiometer

This work presents the calibration, characterization, and imaging tests of the Passive Advanced Unit-Synthetic Aperture instrument (PAU-SA). PAU-SA is a fully digital Y-shaped two-dimensional synthetic aperture interferometric radiometer operating at the Global Positioning System (GPS) L1 band (f
_L1
= 1.57542 GHz ), conceived as a test-bed for potential technological improvements in future instruments. Calibration of instrumental offsets is performed by looking to a microwave absorber and the “cold” sky. Since the instrument operates in the GPS L1 band, GPS satellites are imaged therefore, a new way to compute the Flat Target Response (FTR) has been devised. Internal phase/amplitude calibration is performed using a new technique that consists of injecting pseudo-random noise signals. Different paths from the input switch to the antennas are calibrated by means of an external beacon. A near-field to far-field transformation is applied to compensate for differences in the propagation paths. Finally, absolute amplitude calibration is achieved by imaging the GPS satellites constellation when pointing to the zenith. Evaluation of the images' quality in terms of angular resolution, radiometric resolution and precision, show the goodness of the techniques applied to compensate for instrumental errors, the imaging capabilities of the instrument, and demonstrate their applicability in future missions.