Investigating the impact of freezing on the ALOS PALSAR InSAR phase over Siberian forests

Abstract

It is a well-known problem in studies over boreal forests that freezing has a great impact on the properties of radar data such as backscatter, interferometric coherence and polarimetric signatures. In this study, we investigate the impact of freezing on the interferometric synthetic aperture radar (InSAR) phase in the forests of Central Siberia, Russia. The analysis was carried out for 11 local test sites, covering a total area of 382 km2. Between 2006 and 2009, 87 Advanced Land Observing Satellite Phased-Array L-Band Synthetic Aperture Radar (ALOS PALSAR) scenes were acquired covering the 11 test sites. From these acquisitions, 36 interferograms were computed, all of which feature a temporal baseline of 46 days. Sixteen interferograms cover frozen conditions and 20 cover unfrozen conditions. The impact of forest on the InSAR phase was delineated by measuring the phase shift at forest edges. We compare the phase shift with forest heights taken from ministerial forest inventory data for 56 forest stands featuring a high forest density and an average height of 23 m. Further, 320 forest edges were investigated, however, with no forest inventory information available. For these 320 samples, only the phase shift difference between unfrozen and frozen conditions was analysed. The results demonstrate that under unfrozen conditions, the InSAR phase height is located at approximately 8 m above the ground and under frozen conditions, it is located at about 4 m above the ground. This information is crucial for accurate coherence simulations in Central Siberia.