Exploring the potential of high temporal resolution X-band SAR time series for various permafrost applications with ground truth observations in the Lena River Delta, Siberia.

Permafrost is a subsurface phenomenon that cannot be directly monitored with satellite remote sensing. A variety of indirect approaches are currently being developed which aim to measure permafrost-related processes and environmental variables. Results of these studies aid the planning of future satellite missions which will allow large-scale permafrost monitoring. This thesis contributes to this ongoing effort by assessing the potential of repeat-pass TerraSAR-X (TSX) time series for permafrost-related applications. For the first time, multi-year Synthetic Aperture Radar (SAR) data with high temporal (11 days) and spatial (3 m) resolution was analysed for a region characterized by continuous permafrost in the Siberian Arctic. Extensive in situ data was collected during three summer and winter expeditions to validate and interpret remote sensing results. Three case studies were carried out: (i) the detection of land surface changes (e.g. ground freezing and thawing, surface wetness variations, snow cover onset and melt); (ii) monitoring bedfast lake ice and ice phenology (freeze-up, melt onset, break-up); and (iii) differential SAR interferometry (DInSAR) for thaw subsidence monitoring. For the first two case studies, time series of both backscatter intensity and 11-day interferometric coherence (i.e. a measure of phase stability between two SAR images) were investigated. Backscatter intensity was generally shown to be insensitive to the land surface changes but responded to events that occurred at the time of TSX acquisition (rain, snow shower, melt/freeze crust on snow). Interferometric coherence decreased dramatically across the entire image upon snow cover onset and melt, permitting the possible use of coherence for the monitoring of these events. Backscatter intensity was found to be an excellent tool for the detection and monitoring of bedfast lake ice due in part to improved temporal resolution compared to previously used SAR systems. Ice phenology was mostly well tracked with backscatter intensity. Interferometric coherence was found to be sensitive to the lake ice grounding and to the onset of surface melt on the lakes with bedfast ice. The investigation of coherence was a useful preparative step for the following DInSAR analysis. For the third case study, coherent 11-day and 22-day interferograms were available only for one summer of the two-year TSX time series. The cumulative DInSAR displacement strongly underestimated the subsidence observed on the ground. In situ observations revealed high variability of subsidence, which likely caused errors in phase unwrapping. Conventional DInSAR processing might therefore not be suitable for the accurate representation of permafrost thaw subsidence. This study highlights the importance of field measurements for the quantification of thaw subsidence with DInSAR, which were mostly omitted in the previous studies. All in all, this thesis shows the limitations and potential of TSX time series to spatially and temporally monitor permafrost. It thus provides an important contribution to the methodological development of a long-term permafrost monitoring scheme.