Conference Agenda

With BIOMASS, it was desired to exploit the capability of low-frequency

Synthetic Aperture Radar (SAR) to penetrate into volumetric targets such as

dense forest, snow and ice. With its long wavelength (approx 69 cm) and quad-pol

operation, it has proven to, for example, detect and separate targets below tree canopies (e.g., the ground and tree trunks) while maintaining the interferometric

coherence, facilitating the implementation of interferometric techniques such as

SAR tomography.

One of the main challenges associated with low-frequency radar is the distortion introduced by the ionosphere, which is seen in both single acquisitions and

interferometric pairs. The effects can be grouped mainly into two categories:

Faraday rotation affecting the polarimetry, and phase errors that introduce

defocusing of single images, and azimuth shifts (misregistration) and residual phase screens between interferometric pairs. Both types of errors are proportional to the ionospheric free electron content and the wavelength. Small-scale ionospheric irregularities introduce high-frequency Faraday rotation and phase

errors, usually referred to as scintillation. It has been proven that it is possible,

for example, to use the polarimetry to estimate Faraday rotation and correct

for phase errors [1] (this approach has proven to be effective at high latitudes).

However, the Faraday rotation sensitivity to small irregularities decreases

rapidly towards mid- and low-latitudes. Then, the problem can be addressed

by examining the phase signatures directly. Techniques such as autofocus help

correct the impact in single images. In interferometric pairs or stacks, the

differential ionospheric phase screen can be resolved by exploiting the induced

coregistration errors in azimuth. The two mentioned approaches are based on

cross-correlation between image pairs, for which the contrast and content of the

scenes highly compromise the performance.

In this work, we exploit the fact that the phase error is common to all

polarimetric channels to add redundancy to the observations. This allows us

to reduce the uncertainty in retrieving the ionospheric distortion map when

a least-mean squares inversion scheme is implemented. Similarly, we also use

the multiple passes in the interferometric stacks to attempt to improve the

performance of both the single-pass and interferometric corrections, as it was

proposed in [2]. Besides regaining coherence and reducing interferometric phase

errors, accurate scintillation maps will be valuable for studies that require high-

resolution ionospheric imaging.

References

[1] Kim, J. S., Papathanassiou, K. P., Scheiber, R., and Quegan, S. (2015).

Correcting distortion of polarimetric SAR data induced by ionospheric scin-

tillation, IEEE Transactions on Geoscience and Remote Sensing, 53(12),

6319-6335.

[2] F. Betancourt-Payan, M. Rodriguez-Cassola, P. Prats-Iraola and G. Krieger,

Towards an Interferometric Autofocus for the Estimation of Ionospheric Sig-

natures in Biomass, EUSAR 2024; 15th European Conference on Synthetic

Aperture Radar, Munich, Germany, 2024, pp. 1227-1231