Impact of Freezing on ALOS PALSAR Interferometric Coherence in Central Siberia
Thiel, Christian; Schmullius, Christiane
Friedrich-Schiller-University Jena, GERMANY

This work investigates the impact of freezing on the properties of the magnitude of interferometric coherence in Central Siberia, Russia. In this respect the consequent implications regarding forestry applications is addressed. In total, 87 acquisitions are employed and approximately 300 coherence images are delineated. The temporal baselines range from 46 days to 2.5 years. The random volume over ground (RVoG) model is applied to support the interpretation of the observations. The major outcomes are as follows. At frozen conditions, the experimental data shows no proof that the perpendicular baseline impacts the coherence over dense forest, while at unfrozen conditions an impact was detected. Furthermore, compared to unfrozen conditions, during freezing we observe increased coherence over open areas, decreased coherence over dense forest, decreased spread of coherence, and improved correlation between coherence and growing stock volume (GSV). While at frozen conditions an average coefficient of determination R² of 0.76 was detected, we observe at unfrozen conditions an R² of 0.30 for baselines < 1 km and 0.44 for baselines > 2 km. Moreover, during freezing a descent of the scattering phase centre is identified. Its height above ground corresponds to 40% of the phase centre height during unfrozen conditions. The RVoG model is parameterized accounting for both, unfrozen and frozen conditions. While the model was capable to simulate the general coherence characteristics at unfrozen conditions, the observations featuring frozen conditions cannot be fully explained by the model. According to the model results, freezing causes a reduction of the attenuation by factor 0.5. All in all, the observations can be rather explained by temporal decorrelation than by volume decorrelation.

Results of the study at hand show a distinct difference in scattering mechanism between frozen conditions in winter and unfrozen condition in summer. Due to freezing, a considerable drop of the dielectric constant of the boles causes a reduction of extinction. This in turn results in a deeper probing of the canopy, and thus increased volume decorrelation. The deeper penetration at frozen conditions could be demonstrated by means of the phase analysis. At unfrozen conditions, the interferometer was found to be almost insensitive for volume, when the baseline is smaller than 1,000 m. With increasing baseline, increasing sensitivity for GSV was observed. However, those images appear to be very noisy. Further evidence for differing scattering processes is provided by the entire decorrelation over forest, when the data was acquired at dissimilar conditions. The extension of the temporal baseline to about one year again allows for equal conditions (e.g. two times frozen conditions). Even when featuring a longer temporal baseline, decorrelation is increased against uneven conditions.

In summary, ALOS PALSAR data permits forest cover and to some extent GSV mapping in Siberia. Winter FBS co-herence, acquired at frozen conditions, exhibits the greatest potential.