Support: US Air Force Office of Scientific Research (AFOSR)

Period of Performance: January 1st, 2021 — December 31st, 2023

Budget: $900,000

Summary: Earth’s ionosphere causes distortions of spaceborne SAR images. They depend on the state of the ionosphere and parameters of the sensor. Previously, we have analyzed a host of related physical phenomena proposed strategies for recovering the relevant ionospheric quantities, and used them to compensate for the distortions. The current effort will address two uniquely difficult scenarios not covered by our earlier investigations. Turbulent ionosphere presents a major challenge because image distortions acquire a random component. The performance in azimuth of low-frequency systems (P-band) can be particularly vulnerable to random fluctuations of the propagation phase. However, we have found this effect mathematically similar to that of geometric uncertainties. Based on this finding, we will develop a mitigation approach for turbulent distortions that will exploit the ideas of autofocus. Both traditional 1D techniques that apply to range compressed data and modern 2D ones that apply to raw data will be quantitatively analyzed and compared to build the optimal distortion compensation methodology. Another SAR product susceptible to ionospheric distortions is interferograms. They may remain adversely affected even for the radars that operate on higher frequencies, L-band and above, where plain SAR images are less of a concern. Indeed, even small variations of the propagation phase due to the ionosphere could be intolerable for SAR interferometry, which requires sub-wavelength precision. The propagation of SAR signals through the ionosphere is accompanied by Faraday rotation (FR). This phenomenon can be used to reconstruct the ionospheric quantities needed for correcting the distortions. Yet its role may also be detrimental, because the variation of FR angle across the radar bandwidth contributes to the cross-talk between the polarimetric channels. We will thoroughly analyze this effect for polarimetric SAR interferometry, identify the regimes and conditions under which a mitigation of ionospheric distortions may be required, and develop approaches for correction.