In this study, four models describing the interferometric coherence of the forest vegetation layer are proposed and compared with the TanDEM-X data. Our focus is on developing tools for hemiboreal forest height estimation from single-pol interferometric SAR measurements, suitable for wide area forest mapping with limited a priori information. The multi-temporal set of 19 TanDEM-X interferometric pairs and the 90th percentile forest height maps are derived from Airborne LiDAR Scanning (ALS), covering an area of 2211 ha of forests over Estonia. Three semi-empirical models along with the Random Volume over Ground (RVoG) model are examined for applicable parameter ranges and model performance under various conditions for over 3000 forest stands. This study shows that all four models performed well in describing the relationship between forest height and interferometric coherence. Use of an advanced model with multiple parameters is not always justified when modeling the volume decorrelation in the boreal and hemiboreal forests. The proposed set of semi-empirical models, show higher robustness compared to a more advanced RVoG model under a range of seasonal and environmental conditions during data acquisition. We also examine the dynamic range of parameters that different models can take and propose optimal conditions for forest stand height inversion for operationally-feasible scenarios.

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum $\varDelta $Δ V capability of $600\text{ ms}^{-1}$600 ms − 1 . Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

This paper demonstrates the use of X-band bistatic synthetic aperture radar (SAR) interferometric coherence for retrieving tree height of coniferous and deciduous forests during leaf-off season in early spring and leaf-on period in summer. TerraSAR-X add-on for Digital Elevation Measurements (TanDEM-X) HH and VV polarization channel coherence images were studied for over 249 ha of forests in Estonia and compared against light detection and ranging (LiDAR) and forest registry field inventory data. Strong correlation was found between interferometric coherence magnitude and LiDAR measured average tree height, especially for winter period. The regression models show the strongest correlation between pine stand heights and single-polarization interferometric coherence, where the correlation coefficients (r ² ) range between 0.75 and 0.97. The highest correlation for mixed deciduous tree stands was found during leaf-off period with r ² ranging from 0.87 to 0.94, whereas leaf-on period resulted in r ² from 0.58 to 0.75. Strong correlations were also found for spruce trees with r ² between 0.54 and 0.83. Moreover, a simple semiempirical model based on random volume over ground model framework was constructed to describe the relation between the forest height and interferometric coherence. Also, the seasonal variability of the correlation was studied. Our results demonstrate that under Northern-European conditions, seasonal changes have a significant effect for deciduous trees as standard deviations dropped from 1.34-1.78 m during leaf-off conditions to 2.22-3.16 m for leaf-on conditions. Thus, height estimation of deciduous stands requires leaf-off conditions for accurate coherence-based height retrieval. Correlation coefficients for pine stands were unvarying across different weather conditions and least affected by the season. The observed strong sensitivity of interferometric coherence to forest height makes it feasible for estimating canopy height for boreal and deciduous forests in both summer and winter conditions. The estimation algorithm works best for coniferous forests.

In this study we compare semi-empirical interferometric coherence models, proposed in [1], for tree height estimation from TanDEM-X coherence scenes. The models are derived from Random Volume over Ground model, by applying simplifications and introducing empirical parameters at different complexity levels so that the models can be adapted to available ancillary data. Several different TandDEM-X interferometric scenes from Estonia are used to test the model performance in various conditions. All the results are compared with highly accurate canopy height models measured using airborne laser scanning. We demonstrate that models which are very simple to invert, produce accurate tree height estimates when the conditions are most favorable. Best results can be seen for winter images for frozen and dry snow conditions. Simple parametric sinc model can produce accurate tree height maps over large areas with pixel-wise deviation only few meters.