# Ten Years of Experience with Scientific TerraSAR-X Data Utilization
## 4 Discussion  
This paper assesses the utilization of the TerraSAR-X mission by the international scientific user community with respect to research themes. Furthermore it identifies key applications and preferred imaging modes and settings. The study is mainly based upon 1636 scientific TerraSAR-X data proposals. This number corresponds to the experience from other missions [15]. Here, more than 1000 projects were initiated based on European remote sensing satellite (ERS) data. In response to the Envisat prelaunch AO, 577 scientific proposals requested access to ASAR data. While researchers from 40 countries participated in the Envisat-AO in 2001, the TerraSAR-X user community increased to 57 countries in 2017. This indicates the growing interest in SAR data in general and is a sign of the positive development of the EO market that all SAR missions jointly serve. During its 17 years of operation, RADARSAT-1 provided images to governmental, commercial, and scientific users in 60 countries worldwide [5]. The analysis of the proposals and publications reveals that high-resolution X-band data are requested from all fields of geo-research. It even shows a largely homogenous distribution over the different spheres. Science projects supported by ALOS-2 are grouped into biosphere, geosphere, cryosphere, marine and disaster [9]. Similar categories were defined for RADARSAT-1 and RADARSAT-2 [18]. The potential of Sentinel-1 for scientific ocean, cryosphere, and land applications is summarized in [7]. All three publications describe the required capabilities, established by intensive studies and expert consultations before the launch. The approach of this TerraSAR-X study is to analyze the entirety of individual proposals and related publications and thus directly the utilization by the scientific user community when the mission is in the operational phase. The thematic fields of this study (see Figures 4-8) correspond very well to those of other SAR missions, although the fields of research (Figure 3) slightly differ from the ones in [7; 9; 18]. This confirms the general applicability and appreciation of SAR data for these applications. Every SAR mission has its assets and advantages. They depend on the technical layout like wave length and spatial resolution, but also on the mission operation design. The features of TerraSAR-X are the high spatial resolution, the short wave-length, a high orbit repetition rate of 11 days with a short revisit time of less than three days and the multimode capability. The high spatial resolution is the main asset of TerraSAR-X as it enables investigations in all fields of research at a high level of detail. The evaluation of surface displacements is the prime application of a high-resolution SAR system like TerraSAR-X and is the principal topic of 63 percent of all proposals. The preferred method is SAR interferometry and its different variants, with feature tracking techniques being employed as well. SAR interferometry benefits from the higher number and density of long-term stable scatterers,but also from the high repeat cycle of 11 days. Feature tracking can be applied to data sets with a longer temporal baseline, but strongly depends on the identification of terrain features and structures to be tracked. The majority of proposals request single polarized data. The investigation and utilization of multipolarized SAR data is of interest for nine percent of the proposals. TerraSAR-X can only be ordered in dual-polarization mode which is sufficient for applications like snow and ice mapping. However, the development and application of polarimetric decomposition techniques is limited. Nevertheless, the demand for dual-polarized TerraSAR-X data has increased since 2014. TerraSAR-X is designed to provide users with the highest level of flexibility for selecting suitable resolution, coverage, and polarization for their research. As a consequence, the data are mostly acquired for a specific need and not necessarily suitable for multi-use purposes.