UAVSAR Observations of InSAR Polarimetric Phase Diversity: Implications for NISAR Ionospheric Phase Estimation. Issue 4 (2nd April 2021) - Brancato, V.
Abstract: The NASA‐ISRO Synthetic Aperture Radar (NISAR) is a repeat‐pass radar mission that will acquire fully polarimetric SAR data in an innovative acquisition mode known as quasi‐quad pol (QQP). It consists of simultaneously operating a HH/HV and a VH/VV dual‐pol modes in the lowest and upper parts of the transmitted frequency spectrum. Compared to more traditional acquisition modes, the ionospheric phase estimation algorithms for QQP repeat‐pass data will likely exploit the difference of two sub‐band SAR interferograms acquired not only at distinct center frequencies but also at different polarizations (e.g., HH/VV), making the ionospheric phase estimation vulnerable to possible bias caused by polarimetric interferometric scattering phase components. Using UAVSAR airborne data, we show that temporal variations of the interferometric scattering phase may introduce spurious polarimetric‐dependent phase terms which may bias QQP ionospheric phase estimates. The magnitude of this bias depends on the type of observed land cover. For bare soil and forested areas, we detect HH‐VV interferometric phase discrepancies up to 30° over 12 days, corresponding to a 10 cm bias at L‐band. Over comparable time intervals, changes in vegetation vitality introduce HH‐VV interferometric phase inconsistencies beyond 90° for vertically oriented agricultural fields. We simulate the QQP ionospheric phase screen over the San Andreas Fault, USA, a region characterized by a mixture of vegetated and bare soil surfaces. Based on the results from the UAVSAR data analysis, we recommend using the same polarization on the main and side‐bands of the NISAR operational science modes (e.g., single‐pol or dual‐pol) to avoid potential biases in the ionospheric phase estimates. Key Points: The L‐band NASA‐ISRO NISAR mission will acquire fully polarimetric radar data at different center frequencies (i.e., quasi‐quad mode, QQP) In QQP mode, operational algorithms will use the difference of co‐polarized sub‐band interferograms to estimate the ionospheric phase delay Polarimetric‐dependent changes of the scattering phase may introduce spurious terms which may bias QQP ionospheric phase estimates at L‐band