HRWS: The upcoming German X-Band Spaceborne SAR Mission

This paper presents the High-Resolution Wide-Swath (HRWS) mission as the next German civilian spaceborne Synthetic Aperture Radar (SAR) system for earth observation in X-Band. Following the successful path of the TerraSAR-X and TanDEM-X missions, HRWS is designed to guarantee the X-band data and service continuity for institutional, scientific and commercial users well beyond the year 2030. HRWS is a very innovative multistatic SAR mission exploiting the formation flight of one active main satellite and three smaller passive companion satellites. With HRWS the novel MirrorSAR concept will be implemented in space for the first time. The HRWS mission has been approved for realization in December 2020 and, as for the current planning, the launch is expected in the time frame of 2026/2027. This paper presents an overview of the outstanding capabilities and new technologies of HRWS, including F-SCAN, digital beamforming, hybrid agility, and single-pass dual-baseline interferometry, as well as the various fields of applications

Quantification of a full year water balance of a thermokarst lake in East Siberia based on field measurements

Thermokarst lakes and basins are major components of the ice-rich permafrost landscapes in East Siberian coastal regions. One of the major control factors of thermokarst lake development is the local water balance. Variations in environmental and climate conditions due to climate change might have severe impacts on the water balance. Higher evapotranspiration and an increased active layer thickness could enhance the water flow and thus favor the thermal degradation of the tundra landscape. In this study we quantified precipitation, evapotranspiration, runoff and storage of a thermokarst lake on Kurungnakh island. The island is located in the central part of the Lena River delta, northern Siberia and underlain by continuous, ice-rich permafrost to about 400-600m depth. The investigated lake has a surface area of approximately 1.2 km² with a maximum depth of about 8 m and a volume of about 4x106 m3. Field measurements of the water balance components were conducted in the period from August 2014 to end of July 2015. Precipitation was recorded by an automatic rain gauge, at a nearby site on Kurungnakh Island. The outflow of the lake was determined with an automatic sensor on a RBC-flume. The evaporation of the thermokarst lake was calculated by using water temperature of the lake, climate data from weather stations on Kurungnakh Island and the neighboring Samoylov Island. The lake water storage was measured using an automated water level sensor. A previous study (Niemann, 2014) investigated only the summer balance (August 2013) of the lake and showed that evaporation dominated the water balance during this time period. Here we analyzed the seasonal and annual water balance components (precipitation, evaporation, runoff, change in storage) of the lake and the contribution of snow cover to the water storage

TerraSAR-X Time Series Fill a Gap in Spaceborne Snowmelt Monitoring of Small Arctic Catchments—A Case Study on Qikiqtaruk(Herschel Island), Canada

The timing of snowmelt is an important turning point in the seasonal cycle of small Arctic catchments. The TerraSAR-X (TSX) satellite mission is a synthetic aperture radar system (SAR) with high potential to measure the high spatiotemporal variability of snow cover extent (SCE) and fractional snow cover (FSC) on the small catchment scale. We investigate the performance of multi-polarized and multi-pass TSX X-Band SAR data in monitoring SCE and FSC in small Arctic tundra catchments of Qikiqtaruk (Herschel Island) off the Yukon Coast in the Western Canadian Arctic. We applied a threshold based segmentation on ratio images between TSX images with wet snow and a dry snow reference, and tested the performance of two different thresholds. We quantitatively compared TSX- and Landsat 8-derived SCE maps using confusion matrices and analyzed the spatiotemporal dynamics of snowmelt from 2015 to 2017 using TSX, Landsat 8 and in situ time lapse data. Our data showed that the quality of SCE maps from TSX X-Band data is strongly influenced by polarization and to a lesser degree by incidence angle. VH polarized TSX data performed best in deriving SCE when compared to Landsat 8. TSX derived SCE maps from VH polarization detected late lying snow patches that were not detected by Landsat 8. Results of a local assessment of TSX FSC against the in situ data showed that TSX FSC accurately captured the temporal dynamics of different snow melt regimes that were related to topographic characteristics of the studied catchments. Both in situ and TSX FSC showed a longer snowmelt period in a catchment with higher contributions of steep valleys and a shorter snowmelt period in a catchment with higher contributions of upland terrain. Landsat 8 had fundamental data gaps during the snowmelt period in all 3 years due to cloud cover. The results also revealed that by choosing a positive threshold of 1 dB, detection of ice layers due to diurnal temperature variations resulted in a more accurate estimation of snow cover than a negative threshold that detects wet snow alone. We find that TSX X-Band data in VH polarization performs at a comparable quality to Landsat 8 in deriving SCE maps when a positive threshold is used. We conclude that TSX data polarization can be used to accurately monitor snowmelt events at high temporal and spatial resolution, overcoming limitations of Landsat 8, which due to cloud related data gaps generally only indicated the onset and end of snowmelt

Intra-annual dynamics of ice-rich riverbank erosion in the central Lena Delta

Arctic warming accelerates the rapid degradation of ice- and organic-rich permafrost landscapes through thermokarst and thermal-erosion. These processes lead to the retreat of ice-rich coasts, riverbanks, lake shorelines, to surface subsidence and gullying. The subsequent reactivation of ancient carbon previously stored in the eroded ice- and organic-rich sediments could have tremendous impact on the carbon cycle from regional to global scale. Yet, information at high temporal and spatial resolution is often lacking to describe the rates and the timing of permafrost degradation. Synthetic aperture radar (SAR), which operates independently of atmospheric distortions, is particularly valuable to alleviate these issues because of its potential for high temporal resolution monitoring in a region where cloud cover often limits the use of optical satellite imagery. In this study, we used SAR data to investigate the spatiotemporal dynamic of a rapidly degrading ice- and organic-rich up to 50-m-high and 2000-m long riverbank in the central Lena Delta. Our main objectives were to 1) assess the applicability of synthetic aperture radar (SAR) satellite data for high-temporal resolution monitoring of rapidly eroding riverbanks and 2) to identify the seasonal timing of ice-rich permafrost riverbank erosion. We analyzed a unique time-series of high-spatial and temporal SAR images from the German TerraSAR-X (TSX) satellite, operating in X-band wavelength, as well as very high resolution optical satellite imagery and in-situ time-lapse data. We processed 77 HH- polarized SAR backscatter images with acquisition dates between August 2012 and October 2015. The imagery was first pre-processed using the Sentinel-1 toolbox from the European Space Agency. We then applied a thresholding to better identify the transition line from undisturbed tundra surface to the actively eroding cliff we refer to as cliff top line. We then calculated cliff top retreat rates and finally compared these with environmental baseline data to identify the main driving factors of riverbank retreat. Visual interpretation of the TSX time-series showed that the cliff of the riverbank is only visible in the months June to October. Annual erosion rates were in the same range when comparing the optical reference with the SAR datasets. The in-situ time-lapse data for the summer of 2015 showed similar results for the intra-annual erosion compared to the SAR derived results. Based on the SAR dataset we detected mostly constant erosion rates at our test site throughout the thawing period for the years 2013, 2014 and 2015. Our results show that the cliff-top at the test site retreats constantly over the thawing season rather than event driven (i.e. through the spring peak discharge only). The studied cliff top is protected from spring flood events by sandbanks in front of the riverbank. However, runoff caused by permafrost thaw, precipitation and flooding will degrade the protecting sand banks and consequently will lead to a reconnection of the cliff system to the Lena River System, even when water level is lower towards the end of the thawing season. We conclude that x-band backscatter time-series are valuable for monitoring rapid permafrost degradation with high spatial and temporal resolution. Our results indicate that cliff top erosion of ice-rich riverbanks takes place constantly over the thawing period and is not event driven

Electrodiagnostic subtyping in Guillain–Barré syndrome patients in the International Guillain–Barré Outcome Study

Background and purpose: Various electrodiagnostic criteria have been developed in Guillain–Barré syndrome (GBS). Their performance in a broad representation of GBS patients has not been evaluated. Motor conduction data from the International GBS Outcome Study (IGOS) cohort were used to compare two widely used criterion sets and relate these to diagnostic amyotrophic lateral sclerosis criteria. Methods: From the first 1500 patients in IGOS, nerve conduction studies from 1137 (75.8%) were available for the current study. These patients were classified according to nerve conduction studies criteria proposed by Hadden and Rajabally. Results: Of the 1137 studies, 68.3% (N = 777) were classified identically according to criteria by Hadden and Rajabally: 111 (9.8%) axonal, 366 (32.2%) demyelinating, 195 (17.2%) equivocal, 35 (3.1%) inexcitable and 70 (6.2%) normal. Thus, 360 studies (31.7%) were classified differently. The areas of differences were as follows: 155 studies (13.6%) classified as demyelinating by Hadden and axonal by Rajabally; 122 studies (10.7%) classified as demyelinating by Hadden and equivocal by Rajabally; and 75 studies (6.6%) classified as equivocal by Hadden and axonal by Rajabally. Due to more strictly defined cutoffs fewer patients fulfilled demyelinating criteria by Rajabally than by Hadden, making more patients eligible for axonal or equivocal classification by Rajabally. In 234 (68.6%) axonal studies by Rajabally the revised El Escorial (amyotrophic lateral sclerosis) criteria were fulfilled; in axonal cases by Hadden this was 1.8%. Conclusions and discussion: This study shows that electrodiagnosis in GBS is dependent on the criterion set utilized, both of which are based on expert opinion. Reappraisal of electrodiagnostic subtyping in GBS is warranted

Exploring the seasonality of rapid Arctic changes from space - Monitoring of permafrost disturbance, snow cover and vegetation in trundra environments with TerraSAR-X

Arctic warming has implications for the functioning of terrestrial Arctic ecosystems, global climate and socioeconomic systems of northern communities. A research gap exists in high spatial resolution monitoring and understanding of the seasonality of permafrost degradation, spring snowmelt and vegetation phenology. This thesis explores the diversity and utility of dense TerraSAR-X (TSX) X-Band time series for monitoring ice-rich riverbank erosion, snowmelt, and phenology of Arctic vegetation at long-term study sites in the central Lena Delta, Russia and on Qikiqtaruk (Herschel Island), Canada. In the thesis the following three research questions are addressed: • Is TSX time series capable of monitoring the dynamics of rapid permafrost degradation in ice-rich permafrost on an intra-seasonal scale and can these datasets in combination with climate data identify the climatic drivers of permafrost degradation? • Can multi-pass and multi-polarized TSX time series adequately monitor seasonal snow cover and snowmelt in small Arctic catchments and how does it perform compared to optical satellite data and field-based measurements? • Do TSX time series reflect the phenology of Arctic vegetation and how does the recorded signal compare to in-situ greenness data from RGB time-lapse camera data and vegetation height from field surveys? To answer the research questions three years of TSX backscatter data from 2013 to 2015 for the Lena Delta study site and from 2015 to 2017 for the Qikiqtaruk study site were used in quantitative and qualitative analysis complimentary with optical satellite data and in-situ time-lapse imagery. The dynamics of intra-seasonal ice-rich riverbank erosion in the central Lena Delta, Russia were quantified using TSX backscatter data at 2.4 m spatial resolution in HH polarization and validated with 0.5 m spatial resolution optical satellite data and field-based time-lapse camera data. Cliff top lines were automatically extracted from TSX intensity images using threshold-based segmentation and vectorization and combined in a geoinformation system with manually digitized cliff top lines from the optical satellite data and rates of erosion extracted from time-lapse cameras. The results suggest that the cliff top eroded at a constant rate throughout the entire erosional season. Linear mixed models confirmed that erosion was coupled with air temperature and precipitation at an annual scale, seasonal fluctuations did not influence 22-day erosion rates. The results highlight the potential of HH polarized X-Band backscatter data for high temporal resolution monitoring of rapid permafrost degradation. The distinct signature of wet snow in backscatter intensity images of TSX data was exploited to generate wet snow cover extent (SCE) maps on Qikiqtaruk at high temporal resolution. TSX SCE showed high similarity to Landsat 8-derived SCE when using cross-polarized VH data. Fractional snow cover (FSC) time series were extracted from TSX and optical SCE and compared to FSC estimations from in-situ time-lapse imagery. The TSX products showed strong agreement with the in-situ data and significantly improved the temporal resolution compared to the Landsat 8 time series. The final combined FSC time series revealed two topography-dependent snowmelt patterns that corresponded to in-situ measurements. Additionally TSX was able to detect snow patches longer in the season than Landsat 8, underlining the advantage of TSX for detection of old snow. The TSX-derived snow information provided valuable insights into snowmelt dynamics on Qikiqtaruk previously not available. The sensitivity of TSX to vegetation structure associated with phenological changes was explored on Qikiqtaruk. Backscatter and coherence time series were compared to greenness data extracted from in-situ digital time-lapse cameras and detailed vegetation parameters on 30 areas of interest. Supporting previous results, vegetation height corresponded to backscatter intensity in co-polarized HH/VV at an incidence angle of 31°. The dry, tall shrub dominated ecological class showed increasing backscatter with increasing greenness when using the cross polarized VH/HH channel at 32° incidence angle. This is likely driven by volume scattering of emerging and expanding leaves. Ecological classes with more prostrate vegetation and higher bare ground contributions showed decreasing backscatter trends over the growing season in the co-polarized VV/HH channels likely a result of surface drying instead of a vegetation structure signal. The results from shrub dominated areas are promising and provide a complementary data source for high temporal monitoring of vegetation phenology. Overall this thesis demonstrates that dense time series of TSX with optical remote sensing and in-situ time-lapse data are complementary and can be used to monitor rapid and seasonal processes in Arctic landscapes at high spatial and temporal resolution

Characterizing thermo-erosional landforms in Siberian ice-rich permafrost. Morphometric investigations in the Lena Delta using high-resolution satellite imagery and digital elevation models

Rapid warming of the Arctic promotes widespread degradation of permafrost and affects the stability of arctic ecosystems. Thermokarst and thermal erosion are two major processes of permafrost degradation. The spatial extent of thermo-erosional processes and related landforms (e.g. gullies and valleys) and their impact on the widespread degradation of permafrost remains not well quantified. Addressing this research gap, this study is using a combination of field data, high-resolution satellite data and photogrammetically derived digital elevation models (DEMs) to conduct a detailed inventory and spatial analysis of thermo-erosional landforms in order to understand their dynamics as well as their relevance for permafrost degradation. The study area in the central Lena Delta is mainly composed of ice- and organic-rich and syngenetically frozen deposits of Late Pleistocene age called Ice Complex. Besides polygonal tundra, thermokarst lakes, wide thermokarst depressions and undisturbed Ice Complex surfaces the study area features a broad variety of thermo-erosional valleys. During an expedition in summer 2013 RTK GPS measurements of 11 ground control points and 28 transversal and 4 longitudinal profiles were conducted in three key sites. Geometric data fusion of GeoEye-1, RapidEye and ALOS PRISM datasets was performed and several high-resolution DEMs were generated. The DEMs were evaluated for absolute height and slope against the field dataset and the thermo-erosional stream network was mapped and morphometric analysis of the identified features was performed. The highest DEM accuracy was achieved when using a combination of six stereopairs from the years 2006 and 2009. The DEM derived transversal and longitudinal profiles reflect the actual shape and morphometry of the valleys. The identified stream network shows levels of high organization within thermokarst depressions and is poorly developed on the Ice Complex surface. The stream orientation tends to follow the height gradient of the study area towards north¬west, while streams outside of thermokarst depression show two main directions that could reflect the polygonal network in the ground. Accompanying permafrost warming thermokarst and thermo-erosional activity will further promote permafrost degradation. In this context thermo-erosional stream networks will expand within the Ice Complex and act as a major agent for the transport of remobilized fossil organic carbon to the Laptev Sea system

Analyzing intra-seasonal dynamics of ice-rich permafrost degradation in the Lena Delta using TerraSAR-X backscatter time-series

Arctic warming is leading to substantial changes of Arctic environments, such as the rapid degradation of ice- and organic-rich permafrost coasts and riverbanks. Reactivation of these ancient carbon pools and the release of carbon to the atmosphere could further accelerate climate warming. Short and long term annual retreat rates of permafrost coasts and riverbanks are mostly based on optical aerial and satellite imagery. However, in the Arctic cloud coverage often limits the use of optical remote sensing. Synthetic aperture radar (SAR) systems operate unaffected by atmospheric distortions. SAR data with high temporal resolution imagery can be used to detect seasonal variations of coastal retreat. The TerraSAR-X (TSX) satellite of the German Space Agency (DLR) is a X-band active microwave system that provides high-spatial (2 m ground resolution) and temporal resolution (11 day repeat period). We used a TSX backscatter time-series from the years 2012, 2013, 2014 and 2015 to analyze rapidly eroding cliff tops along an ice- and organic-rich permafrost riverbank within the central Lena Delta. The images were analyzed using a threshold approach. The clearly visible transition line between undisturbed tundra surface and actively eroding cliff was subsequently mapped for every image. Very high resolution optical satellite images acquired in August 2010 and August 2014 were used to validate the TSX results. In spring 2015 we conducted a GPS survey and installed a time-lapse camera as well as wooden poles with 50cm distance perpendicular to a rapidly eroding cliff top sequence. Time-lapse images were acquired from late June to late August. The TSX extracted annual retreat rates are in the same range as the ones from the optical reference dataset. The intra-seasonal cliff top retreat lines from 2014 showed equal rates of 2 to 3 m per month. The time-lapse field data at the same place showed similar rates in summer 2015. TSX backscatter time-series show a high potential for monitoring rapid permafrost degradation with high spatial and temporal resolution. The results are valuable for the understanding of degradation process dynamics within a summer season. In the second part of the project we will focus on near to surface soil moisture and freeze and thaw dynamics on the watershed scale on Herschel Island, Yukon territory. In summer 2015 we installed four automated stations that measure near to surface soil moisture and temperature within a watershed on Herschel Island

Characterizing thermo-erosional landforms in Siberian ice-rich permafrost - Morphometric investigations using high resolution satellite imagery and digital elevation models

The spatial extent of thermoerosional processes and related landforms (e.g. gullies, valleys) and their impact on the widespread degradation of permafrost is still not well quantified. Remote sensing data and digital elevation models (DEMs) are widely used to derive and analyse geomorphometric relief characteristics in order to detect periglacial landscape dynamics such as thermal erosion. However, geometric correction of remote sensing data and generation of DEMs in polar lowlands is challenging due to the low relief of arctic tundra landscapes and often scarce reference data. Therefore, high-resolution DEMs of high quality are hardly available, especially for Siberian study regions. Addressing these difficulties, this study is using a multi-sensor and multi-temporal data approach for a detailed inventory and spatial analysis of thermo-erosional landforms on Kurungnakh Island in the central Lena Delta, which is composed of ice-rich permafrost. Two DEMs were generated from ALOS PRISM stereo-datasets acquired in 2006 and 2009 for detection of short-time volumetric change. Mapping of thermo-erosional landforms was then performed using a time-series of orthorectified GeoEye-1 and RapidEye datasets. We present a validation of the generated DEMs based on extensive ground measurements on Kurungnakh Island, performed during an expedition in July 2013. Longitudinal and transversal profiles of thermo-erosional valleys were measured at three key sites, each representing different valley types. Profiles were extracted from the DEMs and compared to profiles measured in the field in order to provide error ranges for DEM-based valley profile analyses. Our datasets provide a new level of accuracy and a basis for further studies on Kurungnakh Island