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Vorträge im Institutsseminar 2020
Islam Mansour „Analysis of Multi-Frequency Polarimetric SAR Data over Permafrost Regions”
Dienstag. 15. Dezember 2020 um 14.00 h
Dieser Vortrag findet virtuell statt!
Abstract:
Permafrost thawing is a major consequence of global warming and has a profound impact on both global and regional scale. For instance, it is estimated that frozen soil in permafrost regions contains about 1.600 billion tons of carbon, twice as much as present in the atmosphere. By thawing, carbon as well as other greenhouse gases (e.g. methane and carbon dioxide), which are trapped in the permafrost, are released into the atmosphere accelerating in turn global warming. Consequently, permafrost has become a main concern for the scientific community in the last years. Radar remote sensing offers the possibility to monitor larger areas in a systematic way, overcoming the typical limitations of in-situ techniques, in terms of sparsity of the measurements and difficulties to access remote permafrost areas. This thesis work presents an investigation based on multi-frequency (L- and S-band) polarimetric SAR data acquired in the frame of the PermASAR18 and PermASAR19 campaigns, over the Herschel Island, in the Canadian Arctic, by the DLR’s F-SAR airborne system. The objective of the study is to assess the sensitivity of polarimetric SAR (PolSAR) measurements to permafrost characteristics, such as soil type and conditions, vegetation type and topography. In particular, the availability of multi-temporal data (August 2018 and February 2019) allows to explore the dependency of the SAR measurements on changing environmental conditions from summer to winter. For the analysis, a set of polarimetric descriptors is considered to derive a physical interpretation with the support of available reference data and existing literature. Preliminary results seem to indicate that summer measurements are mainly influenced by the spatial distribution of different vegetation types. On the other hand, an overall lower polarimetric sensitivity is found for the winter data, when vegetation is absent and the dominant scattering contribution is generated at the frozen soil surface. Details of the analysis are discussed and an interpretation of the results is proposed with the support of external reference data. Finally, some ideas and suggestions for future work are presented.
Christian Huber „Development of Radar Travel Time Transmission Tomography Approaches for Subsurface Ice Exploration”
Freitag, 4. Dezember 2020 um 14.00 h
Dieser Vortrag findet virtuell statt!
Abstract:
In the Frame of DLR’s Enceladus Explorer Initiative (EnEx), the combination of a lander, equipped with an ice-penetrating probe, and a radar orbiter for imaging and characterization of the ice crust and localization of the probe is suggested for a future mission concept. Low-frequency imaging of the subsurface ice and localization of the probe within the ice crust proves challenges, as the radar signal propagation strongly depends on the permittivity of the penetrated medium, requiring precise approximation of the permittivity distribution. Besides, the permittivity can be directly associated with important geophysical parameters such as density, temperature and composition of the ice crust. We present an inversion procedure to estimate the probe location and the permittivity distribution of subsurface ice by exploiting the transmission line between the orbital radar and a transponder integrated in the ice-penetrating probe. The travel time of the signal between the different radar and transponder positions can be utilized by inversion methods known from seismic tomography to approximate the permittivity. The inversion method splits into three parts: the parametrization of the region of interest, computing travel times with a given model, and minimizing the squared error between the measured and computed travel times in an iterative manner. The region of interest is parameterized as homogeneous permittivity blocks on a Cartesian grid. Computing the travel times using this parameterized model is performed using the Fast Marching Method, a finite difference implementation of the Eikonal equation (high-frequency approximation of the wave equation). The squared error is minimized using numerical optimization methods, such as steepest descent and Newton methods. This process is repeated till the method converges or a satisfying result is obtained. The performance of the proposed approach is evaluated on exemplary test scenarios in preparation for a field test on an alpine glacier with DLR’s airborne sensor F-SAR and a transponder positioned in boreholes in the ice.
Tobias Karrer „Development and test of a sensor combination using microwave radar and microwave radiometer”
Dienstag. 17. November 2020 um 14.00 h
Dieser Vortrag findet virtuell statt!
Abstract:
Heutzutage ist sowohl die Radartechnik als auch die Radiometrie ein bewährtes Messverfahren im Mikrowellenbereich für zivile und sicherheitsrelevante Anwendungen. Die Beleuchtungs- und Abbildungsprinzipien der beiden Verfahren sowie die zugehörigen Streumechanismen von Objekten liefern oft verschiedene und komplementäre Informationen einer Szene, weshalb eine geeignete Kombination der beiden Messtechniken erweiterte Objektinformationen liefern kann. Um eine möglichst zeitgleiche und geometrisch ähnliche Abbildung von Szenen zu ermöglichen, sollten Radar und Radiometer in einem aufeinander abgestimmten kombinierten Sensorsystem zusammengeführt werden. Im Seminar werden die Entwicklung und der Aufbau eines derartigen Kombinationssystem für einen Frequenzbereich von 1 – 8 GHz vorgestellt. Abschließend wird die Funktionalität anhand erster Messungen diskutiert.
Andreas Benedikter „SAR Autofocus Approach for Penetration and Permittivity Estimation from Single-Pass Acquisitions in the Cryosphere”
Dienstag, 27. Oktober 2020 um 14.00 h
Dieser Vortrag findet virtuell statt!
https://zoom.us/j/94208199170?pwd=Y0tFMUkyMVZ5SVZBeVplUmJEMHBzZz09
Abstract:
The intrinsic difficulty in the physical interpretation of low-frequency SAR imagery of semi-transparent media, such as ice sheets, is the position ambiguity of the scattering structures within the glacial volume. Commonly tackled by applying interferometric and tomographic techniques, their spaceborne implementation exhibits by orders higher complexity compared to missions relying on single-pass SAR acquisitions, making them cost expensive or, in the context of planetary missions, even impractical due to limited navigation capability. Besides, even these sophisticated techniques are commonly biased due to inaccurate permittivity estimates, leading to geometric distortions up to several meters. We present an inversion procedure to estimate volume parameters of ice sheets, namely, the depth of the scattering scene within the glacial volume and the dielectric permittivity of the ice, based on single-pass single-polarization SAR acquisitions. The information is inherent in the processed SAR data as phase errors on the azimuth signals resulting from uncompensated non-linear propagation of the radar echoes through ice. We suggest a local map-drift autofocus approach to quantify and spatially resolve the phase errors and inversion models to relate them to the sought volume parameters. Testing proposed technique using P-band SAR data acquired by DLR's airborne sensor F-SAR during the ARCTIC15 campaign in Greenland shows promising results and good agreement with tomographic products of the analyzed test site.
Joao Turchetti Ribeiro „Hybrid Mode Design and Performance Prediction for Experimental SAR Acquisitions using TerraSAR-X”
Mittwoch, 21. Oktober 2020 um 14.00 h
Dieser Vortrag findet virtuell statt!
https://zoom.us/j/94208199170?pwd=Y0tFMUkyMVZ5SVZBeVplUmJEMHBzZz09
Abstract:
Stripmap and spotlight have been the standard and most used acquisition modes in SAR remote sensing. Each mode has its own characteristics and is used according to customers' requirements. However, there are situations in which both modes are necessary, namely scenarios in which not only a broader view but also a high resolution image of a small region is required. In these situations, the main solution to acquire both images is to wait until the next satellite pass over the given region, which may take up to 11 days (for TerraSAR-X). The objective of this work is to design and validate a hybrid mode in which both acquisition modes are acquired simultaneously. In order to make this possible, a very high pulse repetition frequency (PRF) must be used. Therefore, the work focuses on timing constraints and image performance prediction. Finally, experiments have been carried out to validate and evaluate the hybrid mode performance.
Fabian Glatz „Assessment of Nadir Echo Suppression in Waveform-Encoded Synthetic Aperture Radar (SAR) Using Real TerraSAR-X Data”
Dienstag, 29. September 2020 um 13.00 h
Dieser Vortrag findet virtuell statt!
https://zoom.us/j/94208199170?pwd=Y0tFMUkyMVZ5SVZBeVplUmJEMHBzZz09
Abstract:
Waveform-encoded synthetic aperture radar (SAR) is a novel SAR concept based on the pulse-to-pulse variation of the transmitted waveforms that allows ignoring to account for the nadir echo exclusion zones in the pulse repetition frequency (PRF) selection and remove the nadir echo through dual-focus post-processing. This work assess the performance of the technique using real SAR data and a realistic nadir echo model and shows how the nadir echo characteristics, the system parameters, and the processing scheme affect the resulting image quality. In particular, as the waveform variation determines a smearing of the nadir echo that assumes noise-like features, the residual nadir-echo disturbance can be quantified in terms of an equivalent additional noise level.
Sergio Alejandro Serafin Garcia „Parameter Selection Criteria for TomoSAR Focusing”
Donnerstag, 30. Juli 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
The synthetic aperture radar (SAR) tomography (TomoSAR) inverse problem is commonly tackled in the context of the direction-of-arrival estimation theory. The latter allows achieving super-resolution, along with ambiguity levels reduction, thanks to the use of parametric focusing methods, as multiple signal classification (MUSIC), and statistical regularization techniques, like the maximum-likelihood inspired adaptive robust iterative approach (MARIA). Nevertheless, in order to correctly suit the considered signal model, MUSIC and most regularization approaches require an appropriate setting of the involved parameters. In both cases, the accuracy of the retrieved solutions depends on the right selection of the assigned values. Thus, with the aim of dealing with such an issue, this seminar addresses several parameter selection strategies, adapted specifically to the TomoSAR scenario. Parametric techniques as MUSIC solve the TomoSAR problem in a different manner as the regularization methods do, hence, each approach demands different methodologies for the proper estimation of their parameters. Consequently, we refer to the Kullback-Leibler information criterion for the model order selection of parametric techniques as MUSIC, whereas we rather explore the Morozov’s discrepancy principle, the L-Curve, the Stein’s unbiased risk estimate and the generalized cross-validation, to choose the regularization parameters. The capabilities of the addressed strategies are first analyzed through simulations, later on, after the incorporation of these criteria, a comparison between MUSIC and MARIA is made via experimental results, utilizing real data acquired from an urban area.
Koenraad Mouthaan „Modeling of reflector based antennas and digital beamforming using Keysight's Advanced Design System (ADS)”
Dienstag, 28. Juli 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
With the ever increasing demands on new SAR systems, accurate modeling becomes more important in the early stages of the design of the systems. For example, in reflector based systems the mutual coupling within the feed array and with the reflector, as well as non-ideal frequency dependent behaviors, impact the performance of the system. Having an accurate model available supports the work of system architects and system designers. Keysight's Advanced Design System (ADS) is a software tool for the design and analysis of microwave and high speed components, circuits, and (sub-)systems. ADS supports various types of analysis such as DC, AC, small-signal scattering parameters, large-signal simulation, and time-domain. This presentation explores the use of ADS for the modeling of reflector based antennas and digital beamforming. The modeling allows the incorporation of non-ideal behaviors through simulated data, obtained from electromagnetic simulators, or measured data. The presentation starts with a short introduction of ADS and the motivation, followed by a brief introduction of scattering parameters. Subsequently the modeling of antennas, using electromagnetic data generated by ANSYS HFSS, is demonstrated. Several short case studies, including a planar array and a reflector based antenna, illustrate the modeling approach. Finally, conclusions and recommendations are presented.
Paul Kroll „Internal Instrument Calibration – Mathematical Models and Simulations“
Dienstag, 14. Juli 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
Two key requirements for a modern, space-borne Synthetic Aperture Radar are a high azimuth resolution and a wide swath. This can be achieved by the use of a SAR instruments with multiple digital channels. Differences between the channels reduce the performance due to the degradation of the antenna pattern. To compensate the differences the system has to be calibrated. To describe the behavior of such a system, mathematical models were developed, based on realistic hardware behavior. These models are used to represent the signals, the instrument drift, the temperature distribution, error sources and other physical effects. Further, a simulation tool, to estimate the errors, residual errors and aid in the system design, was implemented. The tool is adaptable to any multi-channel system and can be used for different calibration methods (e.g. single/multiple calibration signals, chirp signals, etc.). An internal calibration concept/methodology is suggested which allows for a simultaneous calibration during SAR transmit and receive operation. For the receive calibration, a single tone calibration signal, the CalTone, is sequentially coupled to the echo signal path. The transmit calibration is done by first characterizing the receiver (radio frequency unit and digital beam-forming unit) with a CalTone and then couple a part of the transmitted signal to the receiver. To account for frequency dependencies the frequency of the calibration signal can be varied. This presentation will show the above mentioned models, concepts and simulation results.
Chang Hyun Choi "Improving Forest Biomass Estimation using TanDEM-X Horizontal Structure"
Dienstag, 07. Juli 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract.
Forest biomass is a key element towards a better quantification of the global carbon cycle. One way to estimate biomass is to use allometric relationships to (top) height. However, the height-to-biomass allometry varies across stands with different structural characteristics. Thus, the integration of forest structure information representing (horizontal) density generalizes the height-to-biomass allometry towards an increased estimation accuracy. In this respect, interferometric synthetic aperture radar (InSAR) configurations can provide forest height and structure measurements, and enable the application of the height-to-biomass allometry over large areas with high spatial resolution and repeatedly in time. In particular, the bistatic DLR TanDEM-X mission makes available for the first time interferometric measurements at X-band without temporal decorrelation globally. The objective of this work is to investigate the derivation and use of structure information derived from TanDEM-X InSAR measurements in order to improve the estimation performance of the height-to-biomass allometry. In particular, a horizontal structure metric is applied to the X-band phase center heights with respect to the ground topography. This structure metric is shown to be a proxy for forest density, and to be effective in improving the accuracy of biomass estimates from TanDEM-X heights at 1 ha resolution. However, the application of such a framework requires the availability of an external ground topography to deal with the lack of X-band penetration in denser stands. Thus, possibilities to retrieve both height and horizontal structure without relying on an external topography are discussed, and the robustness of height-to-biomass allometry is evaluated. Experimental results are presented in three tropical forest sites in Gabon, where TanDEM-X and NASA’s LVIS (Land, Vegetation, and Ice Sensor) lidar acquisitions were carried out in 2016 in the frame of the AfriSAR campaign. A continuous biomass map derived from the LVIS data is used as a reference.
Roman Guliaev "Vertical reflectivity parameterization for forest height estimation"
Dienstag, 30. Juni 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
Model-based forest height inversion from Pol-InSAR data relies on the parameterization (i.e. modeling) of the vertical radar reflectivity function (i.e. the vertical cross-section through the 3D radar reflectivity) in terms of a set of parameters and using then interferometric (complex) coherence measurements at different polarisations and/or spatial baselines to estimate these parameters. The main limitation of this approach is imposed by the dimensionality of the observation space that does not allow a high dimensional and, thus, a realistic and/or flexible parameterization of the vertical radar reflectivity function in the context of a balanced inversion problem. In this seminar the use of additional knowledge / measurements available (in form of Lidar waveforms, forest inventory plots or tomographic SAR measurements) towards an adaptive and flexible parameterization of the vertical radar reflectivity function for an enhanced forest height inversion performance is discussed. Two such cases are discussed: 1) The use of Lidar Waveforms for the inversion of single polarimetric TanDEM-X coherence data. A common profile is derived from a set of LiDAR samples. The insufficient penetration of X-band in dense forest regions limits the performance and leads to additional performance trade-offs. 2) The use of TomoSAR profiles for the inversion of BIOMASS quad-pol repeat-pass data. P-band Capon tomographic profiles derived from a tomographic acquisition are used to estimate the forest height with an improved performance.
Philipp Posovszky "Improving Data Locality in Distributed Processing of Multi-Channel Remote Sensing Data with Potentially Large Stencils"
Dienstag, 23. Juni 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
Distributing a multi-channel remote sensing data processing with potentially large stencils is a difficult challenge. The goal of this master thesis was to evaluate and investigate the performance impacts of such a processing on a distributed system and if it is possible to improve the total execution time by exploiting data locality or memory alignments. The thesis also gives a brief overview of the actual state of the art in remote sensing distributed data processing and points out why distributed computing will become more important for it in the future. For the experimental part of this thesis an application to process huge arrays on a distributed system was implemented with DASH, a C++ Template Library for Distributed Data Structures with Support for Hierarchical Locality for High Performance Computing and Data-Driven Science. On the basis of the first results an optimization model was developed which has the goal to reduce network traffic while initializing a distributed data structure and executing computations on it with potentially large stencils. Furthermore, a software to estimate the memory layouts with the least network communication cost for a given multi-channel remote sensing data processing workflow was implemented. The results of this optimization were executed and evaluated afterwards. The results show that it is possible to improve the initialization speed of a large image by considering the brick locality by 25%. The optimization model also generate valid decisions for the initialization of the PGAS memory layouts. However, for a real implementation the optimization model has to be modified to reflect implementation-dependent sources of overhead. This thesis presented some approaches towards solving challenges of the distributed computing world that can be used for real-world remote sensing imaging applications and contributed towards solving the challenges of the modern Big Data world for future scientific data exploitation.
Ezgi Özis "Semi-analytical methods in the design of transparent metasheets with applications to microwave radomes"
Dienstag, 9. Juni 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
The aim of this study is the control of microwave transmission through metasheets realized as periodic planar array of circular ring inclusions. A semi-analytical method is developed that has an empirical expression with free parameters is proposed to describe the transmission behavior of the metasheet by using the results of full-wave simulations at discrete values of the geometrical parameters of the rings. A metasheet (metalens) compensating the phase distortion of a hemispheric radome for a fixed frequency in the Ka-band is designed by using developed semi-analytical method. The electric fields transmitted through the finite electrically large metasheet are calculated by Physical Optics method. The results of calculations and measurements are compared.
Anna-Maria Büchner „Presentation of Measurement and Simulation Results Regarding the Mutual Coupling of Dual-Polarized L-Band Horn Antennas for SAR System Calibration”
Dienstag, 26. Mai 2020 um 14.00 h
Dieser Vortrag wird virtuell stattfinden! - Die Log-in Daten werden zeitnah bekanntgegeben.
Abstract:
For upcoming missions in L-band the Calibration group is developing a new generation of the so called “Kalibri” transponders. Transponders are used as active reference targets to calibrate synthetic aperture radar (SAR) satellites during their commissioning phase. By utilizing active reference targets, it is possible to provide an accurate, stable and high radar cross section (RCS). One important goal in the “Kalibri – Next Generation” project is the achievement of a compact L-band transponder design. To make this possible, compact dual-polarized L-band horn antennas have been developed and in each transponder two of these antennas are to be used. One for the reception and one for the retransmission of the SAR signal which takes place simultaneously. To provide an accurate RCS, the decoupling between both antennas should be as high as possible, while still allowing a compact transponder design. In the context of a master’s thesis, the mutual coupling between these two antennas was quantified and investigated. Based on the obtained results additional measures to further improve the decoupling were implemented and tested. The method as well as the final results will be presented during the seminar.
Ryo Natsuaki "Investigation for RFI detection and ambiguity suppression for multi-channel and multi-waveform SAR"
Dienstag, 10. März 2020
14.00 h Großer Besprechungsraum HR - 078, Gebäude 102
Abstract:
Recent synthetic aperture radar (SAR) has applied multi-channel and/or multi-waveform technologies. Multi-channel SAR can observe wider swath than traditional single-channel ones while multi-waveform SAR can suppress range ambiguities effectively. These advanced technologies are necessary for the future high-resolution and wide-swath SAR. In the context of radio frequency interference (RFI) suppression, multi-channel SAR can take advantage of the merit that multiple receivers record the signals simultaneously. On the other hand, such a multi-channel and multi-waveform system raises additional azimuth ambiguities caused by the phase distortions originated from, for example, intrapulse effect. In this research, a coherence-based analyses are applied for both RFI detection & suppression and ambiguity suppression. That is, we assume that RFI signals have greater similarity than the summation of radar echoes among multiple receivers while the additional azimuth ambiguities make Doppler-shifted ambiguous signals which are similar to the non-ambiguous signals. In the presentation, we discuss the effectiveness of the proposal with supportive experimental results.
Jan Krecke "Design of SmallSat SAR for Dedicated New Zealand Applications"
Donnerstag, 27. Februar 2020
14.00 h Großer Besprechungsraum HR - 078, Gebäude 102
Abstract:
Spaceborne Synthetic Aperture Radar (SAR) is a proven technology for remote sensing of the surface of the Earth and is used for a variety of applications. However, conventional SAR satellites are large, heavy and expensive, preventing the formation of large constellations of SAR satellites, which are desirable to reduce the time between acquisitions of one particular area on the ground. To reduce the costs typically associated with spaceborne SAR platforms, it is proposed to design a SAR system for use on a small satellite platform. This small-satellite SAR system is designed for a specific scenario: detection of fishing vessels in the New Zealand Exclusive Economic Zone (EEZ). It is shown that the performance limitations of small-satellite SAR systems—mainly high ambiguity levels and low radiometric sensitivity—can be tolerated for the targeted application. The sparse nature of ships on the ocean can be exploited to resolve ambiguities, and to distinguish targets from noisy background. The work consists of two main parts. First, a simulation of ship targets on the ocean surface was set up. The purpose of this simulation is to determine for different resolutions the Signal-to-Noise Ratio (SNR) necessary for successful ship detection. In a second step, these requirements were translated into different design examples. The proposed SmallSat SAR system is developed within the framework of a recently initiated collaboration between the German Aerospace Center (DLR), the New Zealand Space Agency (NZSA) and the University of Auckland.
Prof. Lorenzo Bruzzone "Challenges of Remote Sensing Data Analysis in the Artificial Intelligence Era"
Freitag, 7. Februar 2020
10.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
This talk addresses the status and the challenges of remote sensing data analysis in the artificial intelligence era, where deep learning is becoming the dominating processing paradigm. Despite the large success of deep learning in computer vision, where deep architectures provided groundbreaking results in solving operational problems related to classification and semantic segmentation of big data, the use of deep learning in operational remote sensing still requires addressing many crucial challenges. On the one hand, there is the need to define architectures that can properly model the properties of different kinds of remote sensing data (e.g., multispectral, hyperspectral, SAR, LiDAR). On the other hand, there is the intrinsic limitation on the available reliable labeled data required for a robust training of the deep architectures in a scenario where different applications often require the recognition different target classes according to different legends. In the talk, these challenges are presented and critically analyzed, identifying high level strategies for addressing them. This involves the analysis of learning components, learning paradigms, available data sources for the definition of large training sets, integration with physical models as well as exploitation of advanced methodologies related to active learning and domain adaptation. This will be discussed presenting hierarchical strategies for the definition of a common approach to the development of sensor specific and application specific processing architectures.
Eduardo Rodrigues Silva Filho "GNSS-based Position and Baseline Determination and Clock Synchronization of Multistatic SAR"
Donnerstag, 30. Januar 2020
14.00 h Großer Besprechungsraum HR, Gebäude 102
Abstract:
Multistatic constellations can offer various advantages for SAR remote sensing. These concepts are challenging to implement for a series of technical difficulties. The lack of synchronization, caused by the operation of transmitter and receiver with different master clocks, poses one of the fundamental operational problems, contaminating the phase signatures of the radar imaging and challenging its differential ranging accuracy. In addition, baseline accuracy of a few millimeters must achieved, preferably using data obtained from low-cost GNSS receivers. In this work, we evaluate a synchronization method based on GNSS navigation data and Precise-Orbit Determination. The method consists in using in each satellite the same oscillator for the master clock of the GNSS receiver and of the SAR payload, so that the relative time estimation obtained in the precise orbit determination can be used to synchronize the radar data in the post-processing. The simulations suggest the proposed approach is capable of delivering reliable estimates of phase errors in the absence of strong baseline velocity deviations and if multipath and other systematic errors are successfully suppressed or calibrated. In addition, we evaluate different configurations in an attempt to improve the individual baselines estimates by combining GNSS data from several satellites flying in close formation. In our preliminary studies we conclude that the individual baseline can potentially be improved by using intersatellite links and by implementing a consistency check by comparing the height biases between DEMs generated from different pairs of satellite.
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