Investigation of the Impact of Freezing on the Scattering Phase Center in Sibe-Ria using ALOS PALSAR FBS and FBD Data
Thiel, Christian; Schmullius, Christiane
Friedrich-Schiller-University Jena, GERMANY
Introduction and Summary
In a natural environment, freezing results in a decreased dielectric constant of both, vegetation and surface. The drop of the permittivity results in reduced attenuation, and thus leads to deeper penetration of the EM wave into media, such as forest canopy (Way et al. 1990, Kwok et al. 1994). When talking about forest, the dielectric constant of the bole is of major interest. Within a bole, the highest dielectric constant can be found in the xylem, which is located approximately 3 cm underneath the bole's surface. The dielectric constant of the remaining parts of the bole is much lower (by an order of magnitude). As a result of freezing, (Way et al. 1994) reported a drop of the xylem’s permittivity from 32 to 4 for white spruce, and from 25 to 6 for black spruce. Similar results were reported by (Dobson et al. 1990, Way et al. 1990). The reduced attenuation and the deeper penetration cause a decent of the scattering phase center (SPC). Information on the location of the SPC would allow inferring crucial parameters for coherence modeling such as penetration length and extinction. However, extensive studies reporting on the location of the SPC, in particular at frozen conditions, are not published.
The benefit of synthetic aperture radar (SAR) data for forestry applications has been demonstrated by numerous studies. Many authors employ the magnitude of repeat pass coherence as estimator for forest biomass. The rationale behind is that increasing growing stock volume (GSV) results in decreasing coherence caused by volume and temporal decorrelation. In the boreal zone, the pronounced seasonality needs to be considered throughout the SAR data exploration (Koskinen et al. 2001, Pulliainen et al. 2003). During winter, the trees are typically frozen. The general effect of freezing on coherence over forested areas was investigated by a number of studies e.g. (Hagberg et al. 1995, Koskinen et al. 2001, Santoro et al. 2002, Askne et al. 2003, Eriksson et al. 2004). At frozen conditions, coherence is less affected by temporal decorrelation. Also, the contrast between forested and non-forested areas is increased against unfrozen conditions. Moreover, the correlation between GSV and coherence was reported to improve (Koskinen et al. 2001, Eriksson et al. 2003). These findings were not yet discussed being the result of deeper penetration, and thus increased volume decorrelation. Indeed, deeper probing increases the sensitivity for the volume and can shift the saturation level to higher GSV values.
In this conference contribution, we present our observations for 11 local sites in Central Siberia. This study investigates the impact of freezing on the location of the SPC in forest. In total, 87 PALSAR acquisitions are available. From these acquisitions 36 interferograms are computed, of which 16 interferograms cover frozen and 20 interferograms cover unfrozen conditions. The temporal baseline is 46 days in all cases. The location of the SPC is delineated by measuring the phase shift at forest edges. The phase shift is scaled to meters to be independent of the perpendicular baseline and to be comparable with forest height derived from ministerial forest inventory data. The comparison of forest height and SPC is accomplished for 56 forest stands. Additionally, a much higher number of forest edges is investigated. However, for these 320 samples no forest height information is available. Still, the phase shift difference between unfrozen and frozen conditions is analyzed. The observations demonstrate that in a moderate to dense and unfrozen Siberian forest with an average height of 23 m, the electromagnetic wave penetrates approximately two thirds of the forest stand height. At frozen conditions, the penetra-tion is roughly twice as high. This outcome can be a valuable input for coherence simulations in this region.
This work has been undertaken [in part] within the framework of the JAXA Kyoto & Carbon Initiative. ALOS PALSAR data have been provided by JAXA EORC.
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