Conference Agenda

We investigated P-band SAR interferometry using early interferometric acquisitions of the BIOMASS mission obtained through our Open Cal/Val project. In an initial assessment of the data, we checked the orbit data, noise equivalent sigma zero, spatial resolution, range and azimuth spectra, the presence of radio frequency interference (RFI) and the co-registration accuracy of the STA product. Special care was then given to the identification and mitigation of ionospheric effects. Both split-beam interferograms and azimuth offset refinement fields determined in the co-registration of interferometric image pairs clearly indicated the presence of ionospheric effects. Gradients in the ionospheric path delay cause azimuth positional offsets. Not considering these in the co-registration results in a significant reduction of the interferometric coherence. In our processing we considered the orbit and topography based geometry as well as an offset field refinement determined using cross-correlation based matching techniques. For the correction of the ionospheric path delay phase, we investigated using a split-spectrum technique [1]. Alternatively, the low frequency part of the ionospheric, tropospheric and residual orbital phase was estimated using spatial filtering techniques.

Both the backscatter and the coherence very clearly separate forest and open water areas in the Amazonas area. For short spatial (< 50m) and temporal (3 days) intervals the coherence levels over the tropical forest were generally very high. Over open water very low coherence levels were observed. This is expected, but it also serves as a confirmation of a “clean signal” without significant azimuth ambiguities. In some of the forest areas we observed reduced coherence levels, possibly related to precipitation. This reduction was the smallest at HH polarization, larger at VV polarization and even larger at cross polarization. Our interpretation is that the precipitation slightly changed the direct backscattering as well as the propagation parameters in the vegetation cover. Localized backscatter and phase variations observed might relate to changes in the water level below the canopy.

Hopefully, we will soon be able to also investigate pairs with longer spatial baselines to investigate the dependence of the coherence on the baseline and forest parameters. Furthermore, we look forward to study the P-band coherence in other geographic and thematic areas.

We will present the processing methods applied and discuss the results obtained.

[1] U. Wegmüller, C. Werner, O. Frey, and C. Magnard, “Estimation and Compensation of the Ionospheric Path Delay Phase in PALSAR-3 and NISAR-L Interferograms,” Atmosphere, vol. 15, no. 6, p. 632, May 2024, doi: 10.3390/atmos15060632.