Abstract
Data from TanDEM-X in single-pass and bistatic interferometric mode together with the interferometric water cloud model (IWCM) can provide estimates of forest height and stem volume (or the related above-ground biomass) of boreal forests with high accuracy. We summarize results from two boreal test sites using two approaches, i.e., 1) based on model calibration using reference insitu stands, and 2) based on minimization of a cost function. Both approaches are based on inversion of IWCM, which models the complex coherence and backscattering coefficient of a homogeneous forest layer, which includes gaps where free-space wave propagation is assumed. A digital terrain model of the ground is also needed. IWCM is used to estimate forest height or stem volume, since the two variables are assumed to be related through an allometric equation. A relationship between the fractional area of gaps, the area-fill, and stem volume is also required to enable model inversion. The accuracy of the stem volume estimate in the two sites varies between 16% and 21% for height of ambiguity <100 m. The results clearly show the importance of using summer-time acquisitions. Based on the TanDEM-X results at X-band, C-band data from the ERS-1/ERS-2 tandem mission are revisited to investigate the potential of a future bistatic C-band interferometric mission. Out of nine ERS-1/ERS-2 pairs, only one pair was found to be acquired at summer temperatures, without precipitation and with high coherence. A simulated bistatic phase height is shown to give approximately the same sensitivity to stem volume as TanDEM-X.
Highlights
B IOMASS is a key variable in climate models and of high importance for management of natural resources and economic values related to forestry
Some properties of IWCM This paper is focused on IWCM observations from singlebaseline bistatic VV-polarization TanDEM-X and lidar-based DTM. It includes IWCM analysis based on C-band ERS data and discusses properties of a possible future bistatic Cband mission
Stem volume or AGB has been in focus in this paper due to its importance as an Essential Climate Variable, but estimation of height, density, growth and management actions using IWCM have been described and summarized from [8, 9, 20, 21, 28, 29]
Summary
B IOMASS is a key variable in climate models and of high importance for management of natural resources and economic values related to forestry. On the other hand satellites sensors such as ICEsat/GLAS [2] is used to characterize height and density of forests by means of lidar Another promising technique is single pass SAR interferometry using X-band, made possible with the TanDEM-X satellites available since 2010 [3, 4]. IWCM has been applied to a number of boreal/hemi-boreal field sites with good results for ERS-1/2 when the temporal ground decorrelation is low, and more recently to bistatic TanDEM-X observations together with a DTM. The goal so far has been to apply the model to areas for which accurate field conditions are available, and in this way determine the potential of the model to produce highly accurate stem volume or AGB and other forest properties such as height and vegetation density. In order to investigate the potential of a bistatic C-band mission, a set of ERS-1/ERS-2 tandem data are revisited and conclusions drawn
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