Transionospheric Synthetic Aperture Radar Observation: A comprehensive review

The ionosphere is the Earth's upper atmosphere with sufficiently charged particles that influence the electromagnetic wave propagation, thereby impeding spaceborne synthetic aperture radar (SAR) observations. High precision imaging and the measurements of geographic and geophysical processes using interferometric SAR (InSAR) and polarimetric SAR (Pol-SAR) are hampered. Most susceptible to ionospheric effects are the low-frequency (L- and P-band) SAR systems, such as the L-band ALOS PALSAR, ALOS-2 PALSAR-2, SAOCOM-1A, LuTan-1, and Lutan-4 that have already been launched, and the projected L-band Tandem-L and P-band BIOMASS. This study presents a detailed and comprehensive review of the research on ionospheric effects on SAR, InSAR, and Pol-SAR, their correction approaches, and reverse guidance for ionospheric sounding. The involved effects are classified into two families. The first is the effects of group delay, phase advance, dispersion, and Faraday rotation imposed by the deterministic background ionosphere with large spatial scales and mild variations in space and time. The second is the effects of phase and amplitude scintillations imposed by ionospheric irregularities (or turbulence) with smaller scales and statistical characteristics. The prospect is raised at the end of the paper, and the ionosphere will continue to be a research hotspot in the remote sensing community due to the development of low-frequency SAR satellites. In addition, applying spacebonre SAR to ionospheric sounding is a promising issue for space weather and geospace physics due to high spatio-temporal resolution and abundant parameter information.