The aim of this study is to assess the performance of single-pass X-band bistatic SAR interferometric forest height estimation of boreal and temperate deciduous forests under variable seasonal conditions. For this, twelve acquisitions of single- and dual-polarized TanDEM-X coherence images over 118 forest stands were analyzed and compared against LiDAR forest height maps. Strong correlations were found between interferometric coherence magnitude and LiDAR derived forest stand height for pine forests (r2=0.94) and spruce forest (r2=0.87) as well as for deciduous trees (r2=0.94) during leaf-off conditions with temperatures below 0°C. It was found that coherence magnitude based forest height estimation is influenced by leaf-on and leaf-off conditions as well as daily temperature fluctuations, height of ambiguity and effective baseline. These factors alter the correlation and should be taken into account for accurate coherence-based height retrieval. Despite the influence of the mentioned factors, generally a strong relationship in regression analysis between X-band SAR coherence and LiDAR derived forest stand height can be found. Moreover, a simple semi empirical model, derived from Random Volume over Ground model, is presented. The model takes into account all imaging geometry dependent parameters and allows to derive tree height estimate without a priori knowledge. Our results show that X-band SAR interferometry can be used to estimate forest canopy height for boreal and deciduous forests in both summer and winter, but the conditions should be stable.
Thermal imagers are often used under ambient conditions, which differ significantly from the calibration conditions. In
this paper a method for characterization of thermal imagers under various ambient conditions is described in the ambient
temperature range from -10 °C to +23 °C. A flat-plate blackbody source attached to a climatic chamber has been used to
simulate the measurement conditions corresponding to the use of the imagers in thermography of buildings. The lower
temperature limit has been selected based on typical field measurement conditions in the Nordic regions while the upper
limit is a typical laboratory temperature during the calibration of the instruments. Correction factors of more than 1 K
relative to the calibration at the laboratory conditions have been observed at lower temperatures with a high-quality
imager under test. Analysis of the measurement results with corresponding uncertainty estimation is described. The
expanded uncertainty (k = 2) of the correction factor has been estimated to be 0.4 K.
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