Forest stem volume estimation using C-band interferometric SAR coherence data of the ERS-1 mission 3-days repeat-interval phase

Interferometric synthetic aperture radar (InSAR) coherence datasets from the 3-days phase of the European Remote Sensing Satellite (ERS-1) mission during the winter months of 1992 have been analyzed to assess the capability to retrieve forest stem volume in boreal forests. For three test sites in Sweden and Finland, coherence decreased for increasing repeat-pass interval from 3- to 12-days due to increasing temporal decorrelation. Overall strong decorrelation was observed for images acquired under unfrozen conditions or thawing conditions. Coherence was best preserved under stable frozen conditions and, in addition, short perpendicular component of the interferometric baseline (i.e., <300 m). Stem volume was estimated with the Interferometric Water Cloud Model (IWCM) and multi-temporal combination of single-image estimates. For the test site of Tuusula, Finland, data from eight 3-days image pairs yielded a relative RMSE of 28.2% for stem volumes ranging between 3 and 482 m3/ha. For the 6-days repeat-pass interval, the relative RMSE was only slightly worse (32.4%). For longer repeat-pass intervals, the relative RMSE was above 50%. This study suggests that C-band short-term coherence has potential to support retrieval of forest biomass but the prospects of Sentinel-1 coherence to estimate biomass appear to be limited to the 6-days coherence in areas characterized by stable frozen, or likewise stable dry, conditions.