ANGULAR RESOLUTION OF 2-DIMENSIONAL, HEXAGONALLY SAMPLED INTERFEROMETRIC RADIOMETERS

A theoretical analysis of the angular resolution of two-dimensional in terferometric radiometers for Earth observation from low-orbit satelli tes and its degradation due to spatial decorrelation effects is presen ted. The analysis extends basic effects known in the context of radio astronomy (application with narrow field of view, very few baselines) and in one dimension (ESTAR L band, few baselines interferometric radi ometer) to the wide-field-of-view, many-baseline, high-resolution two- dimensional system required by Earth observation applications and comp utes beam width, encircled energy (or main beam efficiency), and side lobe level as a function of windowing (apodization) to allow for an op timum angular versus radiometric resolution trade-off. It is found tha t the extension of the Barlett window (which has a poor performance in one dimensional signal processing) to two dimensions produces high-qu ality results, comparable or better than those of Gaussian and Blackma nn windows. Theory is extended to hexagonally sampled systems based on a Delta or Y-shaped instrument, with hexagonal- and star-shaped suppo rt regions in the visibility space, respectively. The superior perform ance of the latter over the former for the same number of antennas and correlators is quantified and details of the angular resolution of on e instrument of this kind, MIRAS, under development by the European Sp ace Agency, are presented. For this radiometer Gaussian or Barlett win dows should be used for good radiometric sensitivity or spatial resolu tion, respectively. In both cases the effects of decorrelation within the small alias-free field of view are negligible. It is also found th at the impact of hardware imperfections which exist within the strict requirements of the specifications have a negligible effect on the ang ular resolution. Finally, experimental angular resolution results with a laboratory breadboard in a focused near-field setup are presented a nd compared to the theoretical predictions.