Seasonality of Alos Palsar Interferometric Coherence and Interferometric Phase in Central Siberia and Its Implication on Forest Parameter Retrieval

The boreal zone is characterised by unique environmental conditions, which need to be considered throughout the SAR data exploration. During winter the trees are frozen and thus almost transparent for the incoming radar wave. The backscatter from the trees is significantly reduced, as is the contrast between forest and non-forest areas. However, the environmental conditions are very stable. Due to the very low temperatures the very dry snow hardly impacts the scattering. As the soil is also frozen, changes in soil moisture do not appear. With regards to coherence these circumstances lead to very low temporal decorrelation. Even temporal baselines of 44/46 days (JERS-1/ALOS PALSAR) do not necessarily lead to problems caused by temporal decorrelation. From a number of studies it became evident that in particular coherence images acquired during winter do have great potential for forest biomass estimation. Furthermore it was recognised, that the thawing season is the most unsuitable time. At midsummer the major sources of temporal decorrelation are changing soil moisture, movement of the trees due to wind, and precipitation (interception water). Thus, the repeat pass coherence for forest is assumed being in general much smaller compared to mid-winter. However, not much is known about mid-summer coherence in the boreal zone. Though, first investigations surprisingly reveal an overall high coherence in summer, even for dense forest. This fact implies the feasibility of POLINSAR methods for forest height estimation. This paper investigates multi-seasonal ALOS PALSAR interferometric coherence images for forestry applications in the boreal zone. Coherence is estimated using PALSAR FBS and FBD data. The test sites are located in Russia (Central Siberia). First results substantiate the great potential of midwinter coherence. With regards to midsummer coherence almost no correlation with regards to stem volume was evident. However, the interferometric phase proved being sensitive to tree height.