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Vorträge im Institutsseminar 2018
Gabriel Brito "Advanced techniques for Persistent Scatter Interferometry"
Freitag, 26. Oktober 2018
10.30 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Several processing approaches have been developed for the extraction of displacement information from interferometric data stacks. These techniques mainly rely on the use of PSs and commonly assume that the evolution of the deformation over time is smooth. Usually, the linear deformation and the DEM error are estimated first using, e.g., the periodogram. In TAXI, this estimation is performed using a grid-search approach, which can be computationally heavy for scenes with a dense distribution of PSs. In this work, we investigate the use the DIRECT algorithm as an alternative to considerably reduce the computational effort of this estimation step without a significant loss of quality. Another topic addressed in this work is the estimation of the non-linear deformation. Given the assumption of the smoothness of time series, a low-pass filter is commonly employed to separate the atmosphere from the non-linear deformation. However, this filtering can lead to sub-optimal monitoring of time series containing discontinuities, which can occur, for example, in the event of earthquakes. To mitigate this effect, we examine the use of total variation filters of first and second orders. The performance of the strategies investigated in this work is demonstrated using a newly developed TAXI module for data-stacks simulation as well as real interferometric stacks.
Felipe Celso Reis Pinheiro "Investigation of Alternatives for the Calibration of Reflector-Antenna SAR Systems with Multiple Azimuth Channels"
Dienstag, 23. Oktober 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Reflector antennas with a multichannel feed are a well-known trend for the implementation of new High-Resolution Wide-Swath (HRWS) imaging methods. Advantages include a larger antenna aperture, which provides improved gain. In combination with SCORE, this enables the system to image a wider swath. For instance in the Tandem-L mission, these factors contribute to enable the system to map the entire Earth’s surface in a shorter time. The use of multiple azimuth channels is a promising extension of such systems. Most applications as well as azimuth beamforming techniques are known to require precise pattern knowledge. During the flight path of a SAR system, phase and magnitude imbalances on the azimuth patterns arise, due e.g. to temperature and gravity variations influencing the feed and reflector geometry. These influence the backscattered signals received by the channels. The objective of the Along-Track Calibration methods is to suppress these imbalances, yielding a better performance for applications and improved image quality after beamforming. Many Along-Track Calibration methods have well-known behavior and proven efficiency for Planar Antenna-Based SAR systems. However, in the rather new reflector case, a gap remains to be filled regarding calibration techniques, as known calibration techniques cannot be directly applied due to different channel properties (including channel correlation). The work analyses some sources of phase and magnitude imbalances on Digital-Array-Fed Reflectors with multiple channels in azimuth. Alternatives to overcome this challenge are investigated, by exploring analogies between Planar and Reflector-Based systems. Specifically, an approach to extend the Digital Channel Balancing technique to reflector systems, suitable for correction of residual calibration errors, is analyzed in detail.
Miriam Schönfeldt "Monitoring the Dynamics of an Alpine Glacier with TanDEM-X Dual-Pol SAR Data"
Montag, 22. Oktober 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Snow and ice masses form a major part of the Earth’s cryosphere. As the primary source of fresh water within our ecological environment, snow and ice play an essential role in the climate system with severe impact also on the society and the economy. In the last decades, space-borne radar remote sensing has led to significant developments in the field of glaciology, due to the large spatial coverage with a relative short revisit time and a high spatial resolution. Thanks to the penetration capability of microwaves into dry snow and ice, conventional SAR is sensitive to surface features as well as to the subsurface structure of glaciers and ice sheets. In additions, polarimetric SAR (PolSAR) allows to gain information about different scattering mechanisms as well as geometric and dielectric properties of the scatterers. This study investigates the potential of dual-polarimetric TanDEM-X time series data to monitor the temporal surface dynamics of the Aletsch glacier, in the Swiss Alps. In a first step, the sensitivity of a set of PolSAR indicators is assessed with respect to the different ice zones and to the seasonal and multiannual dynamics of the glacier. The second part of the study is focused on the analysis of polarimetric phase differences for the detection of snowfall events, which represent a primary contribution to the glacier mass balance. For this, a propagation model is employed which links phase differences to snow microstructure. The results of this study further confirm the uniqueness of polarimetric SAR for glaciological studies, as well as the need of further efforts to access its underexploited potential.
Paola Rizzoli "Radar Backscatter Investigation and Modeling Based on Global TanDEM-X Mission Data"
Montag, 01. Oktober 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Radar backscatter represents the portion of a transmitted electromagnetic signal that is redirected back toward the antenna from a target on ground. Its properties change depending on the radar wave frequency and polarization, acquisition geometry, ground cover type, and soil conditions. Backscatter information is of paramount importance for the design of SAR missions and is widely used for the development of scientific models in the fields of, e.g., the biosphere and cryosphere. The main goal of this work is to exploit the global TanDEM-X SAR data set to model radar backscatter at X band, considering different acquisition parameters and land cover types and to provide then the scientific community with an up-to-date set of backscatter models at a global scale. A novel approach for statistically model the backscatter information, which takes into account the quality of the input measurements, has been developed. The results are weighted polynomial models for different land cover types, taken from the ESA GlobCover map. The attention is then focused on the Greenland Ice Sheet, which is characterized by the presence of different kinds of snow cover, from dry to wet snow. Here, the insufficient level of detail that is provided by the GlobCover map over Greenland (one single class for the entire ice cap) does not allow for a reliable modeling of backscatter. This obstacle set the motivation for developing a new approach for analyzing the information content of interferometric TanDEM-X data, aimed at locating different snow facies by means of the c-means fuzzy clustering algorithm. A set of four different snow facies has been derived, and their properties interpreted with the help of external reference data. The obtained map has then been used to generate an incidence angle dependent backscatter model for each snow facies, separately, by using a modified version of the developed algorithm for the generation of global backscatter models.
Luca Dell’Amore "Assessment of Image Quality of Waveform-Encoded Synthetic Aperture Radar Using Real Data"
Montag, 24. September 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Synthetic aperture radar (SAR) remote sensing is very attractive for the systematic observation of dynamic processes on the Earth’s surface since it allows high resolution imaging independently of weather conditions and sunlight illumination. Waveform-encoded SAR is a novel SAR concept based on pulse-to-pulse variation of the transmitted waveform that allows focusing the nadir echo and the range ambiguities and suppressing them through multi-focus post-processing. However, the assessment of the ambiguity suppression performance for such a system is not trivial, as the processing involves a (non-linear) thresholding and blanking approach. This work proposes a novel methodology, which exploits real TerraSAR-X data to accurately simulate the effect of the range ambiguity on the useful signal and allows for a quantitative assessment of the image quality of a waveform-encoded SAR. The analysis considers different waveform variation schemes (e.g., up- and down-chirps, cyclically-shifted chirps) and a contrast-minimization technique for threshold selection, as well as the best achievable performance (i.e., optimum threshold). The results of this work further highlight the potentialities of the waveform-encoded SAR concept and also allow accounting for its ambiguity suppression capability in the design of a SAR system.
Nicola Gollin "Predictive Quantization for Staggered Synthetic Aperture Radar Systems"
Freitag, 21. September 2018
10.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
For present and future spaceborne SAR missions, an increasing amount of onboard data is going to be required, due to the employment of large bandwidths, multiple polarizations, and large swath widths, which lead to hard requirements in terms of onboard memory and downlink capacity. In this context, SAR raw data quantization represents an essential aspect, since it affects both, the amount of data to be stored and transmitted to the ground, and the quality of the resulting SAR products. In this master thesis, a data reduction approach based on predictive quantization is investigated in the context of Tandem-L, a DLR proposal for a highly innovative L-band radar satellite mission to monitor the dynamic processes of the Earth. Tandem-L employs staggered PRI, a novel acquisition mode which allows for a swath width up to 350 km and an azimuth resolution in the order of 10 m, resulting in a required data volume of about 8 Terabyte per day. In this case, a certain azimuth oversampling is mandatory in order to properly reconstruct the data in presence of the gaps introduced by the staggered SAR mode. The proposed technique takes advantage of the time variant autocorrelation properties of the non-uniform azimuth raw data stream in order to reduce the amount of data through a novel quantization method, named Predictive-Block Adaptive Quantization. Different prediction orders are investigated by considering the trade-off between achievable performance and complexity. Simulations for different target scenarios show that a data reduction of about 15-20% can be achieved with the proposed technique with a modest increase of the system complexity. Moreover, having a-priori information on the position of the gaps, a technique for their reconstruction based on dynamic bit allocation has been successfully implemented, showing no significant loss of information.
Tobias Rommel "Development, Implementation, and Analysis of a Multiple-Input Multiple-Output Concept for Spaceborne High-Resolution Wide-Swath Synthetic Aperture Radar"
Dienstag, 21. August 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Since the development of the first Synthetic Aperture Radar (SAR), radar has become an important sensor for imaging applications. Manifold fields of application ranging from climate change research, over change-detection, and 4-D mapping up to earthquake and flood monitoring are covered by SAR. Especially high-resolution, day- and night-, as well as weather-independent imaging capabilities led to the success of recent spaceborne SAR missions. However, due to a high demand for global SAR data sets, state-of-the-art sensors reach their limits in terms of resolution, swath-width, repeat cycle, and flexibility. To solve SAR-inherent limitations, in this dissertation, a new generation of SAR sensors with multiple transmit and multiple receive channels (MIMO) and digital beam-forming (DBF) capabilities is proposed. Apart from an increased flexibility, this enables a large variety of new operation- and acquisition-modes for high-resolution wide-swath SAR imaging. To separate the individual channels in the post-processing, established processing techniques can not be applied, since SAR is highly sensitive to interferences. Hence, the Space-Time-Frequency Adaptive Processing (STFAP) algorithm is derived, which allows channel separation without any interferences. Basically, STFAP forms frequency- and time-variant antenna beams, which follow the echo signals of each transmitted waveform individually. However, this method demands waveforms of certain structures, which are described and suitable waveform types are compared in detail. STFAP and other DBF-SAR techniques need very sharp antenna beams in elevation. In presence of unknown topography, when the assumed geometrical model of the Earth surface is complex, antenna pointing might be mismatched with the surface geometry. To handle this issue, it is suggested to apply an additional algorithm prior to DBF, which determines the angle of the incident echo signal on the antenna array. The algorithm, which is proposed in this dissertation, is based on the matrix pencil method and allows a highly accurate and signal-adaptive DBF in real-time. For an experimental verification of new imaging modes and STFAP, a groundbased MIMO-SAR demonstrator in X-band has been developed. The dissertation concludes with its description and promising experimental results, which proof that the suggested concepts have the potential to enhance the capabilities of state-of-the-art SAR sensors significantly.
Andreas Benedikter "Analysis of imaging characteristics of small ice-covered planetary bodies (Enceladus)"
Donnerstag, 19. Juli 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
The presentation is about the imaging characteristics of a SAR-Mission for the Saturn moon Enceladus. Enceladus is a ice-covered moon on which life is expected in the ocean underneath the ice. The presentation will cover the development of an orbit propagator, a mission analysis (Coverage, Resolution, Sensitivity analysis) and a point target simulation for a target located in the ice.
Sindhu Deerka Dhanabal "Efficient On-board Quantization for Interferometric SAR Systems"
Dienstag, 08. Mai 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
For present and future spaceborne SAR missions, an increasing amount of onboard data is going to be demanded, due to the employment of large bandwidths, high pulse repetition frequencies, and multiple polarizations, which lead to hard requirements in terms of onboard memory and downlink capacity. In this context, SAR raw data quantization represents an essential aspect since the data rate employed for the digitization of the recorded radar signals affects both the amount of data to be stored and transmitted to the ground and the quality of the resulting SAR products. The impact of quantization is often considered as an additive white Gaussian noise source. In reality, however, quantization effects are strongly dependent on the specific SAR scene characteristics, and have to be treated as a signal-dependent and highly nonlinear error source. The need to better understand the effects of raw data quantization in interferometric SAR performance motivates this thesis work. For this purpose, a standard block adaptive quantizer (BAQ) is considered as a compression scheme, and simulations as well as investigations on experimental data acquired by the TanDEM-X bistatic SAR mission are conducted. From these, a statistical characterization (look-up tables) of the resulting interferometric phase errors is derived. Furthermore, a novel quantization method, named Performance Optimized Block Adaptive Quantization (PO-BAQ), is introduced, which allows for an optimization of the resource allocation and, at the same time, to control the interferometric performance degradation, by exploiting a priori knowledge about the SAR backscattered information. Application examples of the proposed PO-BAQ method are finally shown in the context of real mission scenarios.
Nina Caldarella "Joint Retrieval of Winds and total Surface Current Vectors using experimental Bidirectional ATI TanDEM-X Data"
Freitag, 06. April 2018
10.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
This presentation discusses the method and quality of Total Surface Current Vector (TSCV) retrieval using an experimental Bidirectional Along-Track Interferometric (ATI) Synthetic Aperture Radar acquisition mode with TanDEM-X. The measurement concept relies on the ATI phase, which provides an estimate of the first moment of the Doppler spectrum associated to the surface motion. Observing with two beams squinted as far as 15.1 degree apart in azimuth on ground, allows the surface motion to be observed in line of sight of the beams. Projection to the ocean surface gives a velocity field. This surface motion consists of a NRCS weighted average of velocities of sea-state dependent biases such as short wind generated waves, long swell waves and underlying currents. Assuming the surface motion is dominated by wind generated waves and underlying currents, the method attempts to solve for Total Surface Current Velocity simultaneously with the surface wind vector by coupling geophysical model functions (GMF) for returned Doppler Centroid (DC) and NRCS from an ocean surface shaped by wind. For NRCS the empirical GMF XMOD2 for X-band radar is used, based on the same regression algorithm as the widely used CMOD5 in scatterometry. For DC a GMF based on statistics of the sea surface and the Kirchhoff Approximation developed by IFREMER is used. A cost function of the wind vector is defined as the squared difference between NRCS observations and values of the GMF in both beams. Minimization of the cost function solves for the wind vector and evaluating the GMF for DC with the wind vector results in a component of surface motion caused by wind generated waves. Wind wave induced surface motion and TSCV can then be separated. Performed analysis of the amplitude ratio of the beams shows that there is an observable difference in amplitude of 0.4 dB, therefore, possibly providing 4 observables to solve for the wind vector and TSCV. Conducted simulations show that the approach gives an estimation of wind speed magnitude with an error standard deviation of 2.5 m/s for noise levels in Beta Naught up to 3 dB. Estimation of direction of the wind vector however is very sensitive to noise. In most cases wind generated waves dominate the observations. The assumption that direction of observed surface motion equals direction of wind then applies. In cases of very strong currents and/or weak winds the proposed approach is not suitable and a solution space for wind direction needs to be constrained using external meteo data and the difference in NRCS.
Budhaditya Pyne "Efficient Antenna Pattern Computation Tool for Reflectors and Planar Arrays with Surface Deformations using FFT"
Montag, 26. März 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
This research aims at efficiently computing antenna patterns for reflectors and planar arrays using FFTs, while taking into account possible surface deformations in the radiators. As a case study, the 1D and 2D antenna patterns are computed for Tandem-L (reflector) and MicroX-SAR (planar array) using the developed Pattern-Tool in Python, and the simulation results are compared with commercially available tools: GRASP and Ansys HFSS, along with measurement results (for MicroX-SAR). Comparisons showed that the simulation results are accurate within +-0.1dB and the computation time is only a few seconds compared to several hours for GRASP and HFSS.
Tobias Bollian "A Dedicated Sniffing Beam for In-Beam RFI Mitigation"
Mittwoch, 14. März 2018
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR) systems as more services share the frequency spectrum in an increasing effort to make our world more wireless. New advanced SAR instruments, such as DLR’s Tandem-L mission, employ a multichannel structure capable of Digital Beamforming (DBF). Radars with DBF are capable of notching the antenna pattern in specific directions. This can be utilized to suppress out-of-beam RFI, however it is too computational expensive to perform on-board. On the other hand, an on-ground processing requires the transmission of all channels to the ground, which would result in an unreasonable data volume. To overcome this dilemma, this talk investigates the formation of a dedicated sniffing beam for on-ground RFI mitigation. The benefits of DBF for RFI mitigation can still be applied at the cost of only being able to remove one dominant interferer at each frequency. Further, the extracted out-of-beam RFI signal can be used to estimate in-beam interference, which overcomes a fundamental limitation of DBF.
Tobias Bollian "RFI Mitigation with Digital Beamforming for Synthetic Aperture Radar"
Donnerstag, 18. Januar 2018
11.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR) systems as scientific motivations push the radars to lower frequencies and as more services share the frequency spectrum in an increasing effort to make our world more wireless. New advanced SAR instruments, such as NASA’s EcoSAR and DBSAR and DLR’s Tandem-L mission, employ a multichannel structure capable of Digital Beamforming (DBF). Radars with DBF are capable of notching the antenna pattern in specific directions. This can be utilized to suppress RFI, but the number of notches is limited by the number of active channels. This presentation explores different methods to adaptively change the notching in range time and frequency. The performance is evaluated with simulated data. Afterwards, the filtering is applied to NASA EcoSAR single-pass interferometric data.
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