UAVSAR observations of InSAR polarimetric phase diversity: implications for NISAR ionospheric phase estimation

The NASA-ISRO Synthetic Aperture Radar (NISAR) is a repeat-pass radar mission that will 15 acquire fully-polarimetric SAR data in an innovative acquisition mode known as quasi-quad 16 pol (QQP). It consists of simultaneously operating a HH/HV and a VH/VV dual-pol modes 17 in the lowest and upper parts of the transmitted frequency spectrum. Compared to more 18 traditional acquisition modes, the ionospheric phase estimation algorithms for QQP repeat19 pass data will likely exploit the dierence of two sub-band SAR interferograms acquired 20 not only at distinct center frequencies but also at dierent polarizations (e.g HH/VV), 21 making the ionospheric phase estimation vulnerable to possible bias caused by polarimetric 22 interferometric scattering phase components. 23 Using UAVSAR airborne data, we show that temporal variations of the interferometric 24 scattering phase may introduce spurious polarimetric-dependent phase terms which may 25 bias QQP ionospheric phase estimates. The magnitude of this bias depends on the type 26 of observed land cover. For bare soil and forested areas, we detect HH-VV interferometric 27 phase discrepancies up to 30 degrees over 12 days, corresponding to a 10 cm bias at L28 band. Over comparable time intervals, changes in vegetation vitality introduce HH-VV 29 interferometric phase inconsistencies beyond 90 degrees for vertically-oriented agricultural 30 elds. We simulate the QQP ionospheric phase screen over the San Andreas Fault, USA, a 31 region characterized by a mixture of vegetated and bare soil surfaces. Based on the results 32 from the UAVSAR data analysis, we recommend using the same polarization on the main 33 and side-bands of the NISAR operational science modes (e.g. single-pol or dual-pol) to 34 avoid potential biases in the ionospheric phase estimates.