1,453 research outputs found
Challanges of the TanDEM-X Commissioning Phase
The paper describes the execution of the TanDEM-X commissioning phase. It provides the challanges and constraints of the different tasks and their impact on each other during the commissioning phase. In addition, the paper gives information about the tasks to be performed
A Polarimetric Phased Array Antenna for E-SAR in L-Band
The design of a new L-band antenna for DLRâs airborne synthetic aperture radar system (E-/F-SAR) is presented. Beside operations in L-band new components for a system upgrade were developed. These components are intro-duced. Special interest of this paper is the development of the dual-polarized L-Band antenna with enhanced bandwidth of 150MHz. The antenna feed network is equipped with 2bit hybrid phase shifters to steer the beam between 25° and 40° in elevation. For mounting the antenna at the fuselage of a Dornier Do 228-212 aircraft a rack is described to house several antenna configuration
Interplay between Secondary and Tertiary Structure Formation in Protein Folding Cooperativity
Protein folding cooperativity is defined by the nature of the finite-size
thermodynamic transition exhibited upon folding: two-state transitions show a
free energy barrier between the folded and unfolded ensembles, while downhill
folding is barrierless. A microcanonical analysis, where the energy is the
natural variable, has shown better suited to unambiguously characterize the
nature of the transition compared to its canonical counterpart. Replica
exchange molecular dynamics simulations of a high resolution coarse-grained
model allow for the accurate evaluation of the density of states, in order to
extract precise thermodynamic information, and measure its impact on structural
features. The method is applied to three helical peptides: a short helix shows
sharp features of a two-state folder, while a longer helix and a three-helix
bundle exhibit downhill and two-state transitions, respectively. Extending the
results of lattice simulations and theoretical models, we find that it is the
interplay between secondary structure and the loss of non-native tertiary
contacts which determines the nature of the transition.Comment: 3 pages, 3 figure
Development of the TanDEM-X Calibration Concept: Analysis of Systematic Errors
The TanDEM-X mission, result of the partnership
between the German Aerospace Center (DLR) and Astrium
GmbH, opens a new era in spaceborne radar remote sensing. The first bistatic satellite synthetic aperture radar mission is formed by flying the TanDEM-X and TerraSAR-X in a closely controlled helix formation. The primary mission goal is the derivation of a high-precision global digital elevation model (DEM) according to High-Resolution Terrain Information (HRTI) level 3 accuracy.
The finite precision of the baseline knowledge and uncompensated radar instrument drifts introduce errors that may compromise the height accuracy requirements. By means of a DEM calibration, which uses absolute height references, and the information provided by adjacent interferogram overlaps, these height errors can be minimized. This paper summarizes the exhaustive studies of the nature of the residual-error sources that have been carried out during the development of the DEM calibration concept.
Models for these errors are set up and simulations of the resulting DEM height error for different scenarios provide the basis for the development of a successful DEM calibration strategy for the TanDEM-X mission
Trotter product formulae for -automorphisms of quantum lattice systems
We consider the dynamics of an infinite quantum lattice
system that is generated by a local interaction. If the interaction decomposes
into a finite number of terms that are themselves local interactions, we show
that can be efficiently approximated by a product of
automorphisms, each of them being an alternating product generated by the
individual terms. For any integer , we construct a product formula (in the
spirit of Trotter) such that the approximation error scales as . Our
bounds hold in norm, pointwise for algebra elements that are sufficiently well
approximated by finite volume observables.Comment: Published in Annales Henri Poincar\'
Control of protein and lipid composition of photoreceptor outer segmentsâImplications for retinal disease
Vision is arguably our most important sense, and its loss brings substantial limitations to daily life for affected individuals. Light is perceived in retinal photoreceptors (PRs), which are highly specialized neurons subdivided into several compartments with distinct functions. The outer segments (OSs) of photoreceptors represent highly specialized primary ciliary compartments hosting the phototransduction cascade, which transforms incoming light into a neuronal signal. Retinal disease can result from various pathomechanisms originating in distinct subcompartments of the PR cell, or in the retinal pigment epithelium which supports the PRs. Dysfunction of primary cilia causes human disorders known as âciliopathiesâ, in which retinal disease is a common feature.
This chapter focuses on PR OSs, discussing the mechanisms controlling their complex structure and composition. A sequence of tightly regulated sorting and trafficking events, both upstream of and within this ciliary compartment, ensures the establishment and maintenance of the adequate proteome and lipidome required for signaling in response to light. We discuss in particular our current understanding of the role of ciliopathy proteins involved in multi-protein complexes at the ciliary transition zone (CC2D2A) or BBSome (BBS1) and how their dysfunction causes retinal disease. While the loss of CC2D2A prevents the fusion of vesicles and delivery of the photopigment rhodopsin to the ciliary base, leading to early OS ultrastructural defects, BBS1 deficiency results in precocious accumulation of cholesterol in mutant OSs and decreased visual function preceding morphological changes. These distinct pathomechanisms underscore the central role of ciliary proteins involved in multiple processes controlling OS protein and lipid composition
Structural Basis of Folding Cooperativity in Model Proteins: Insights from a Microcanonical Perspective
Two-state cooperativity is an important characteristic in protein folding. It
is defined by a depletion of states lying energetically between folded and
unfolded conformations. While there are different ways to test for two-state
cooperativity, most of them probe indirect proxies of this depletion. Yet,
generalized-ensemble computer simulations allow to unambiguously identify this
transition by a microcanonical analysis on the basis of the density of states.
Here we perform a detailed characterization of several helical peptides using
coarse-grained simulations. The level of resolution of the coarse-grained model
allows to study realistic structures ranging from small alpha-helices to a de
novo three-helix bundle - without biasing the force field toward the native
state of the protein. Linking thermodynamic and structural features shows that
while short alpha-helices exhibit two-state cooperativity, the type of
transition changes for longer chain lengths because the chain forms multiple
helix nucleation sites, stabilizing a significant population of intermediate
states. The helix bundle exhibits the signs of two-state cooperativity owing to
favorable helix-helix interactions, as predicted from theoretical models. The
detailed analysis of secondary and tertiary structure formation fits well into
the framework of several folding mechanisms and confirms features observed so
far only in lattice models.Comment: 28 pages, 7 figure
Multistatic SAR Imaging: First Results of a Four Phase Center Experiment with TerraSAR-X and TanDEM-X
Multichannel synthetic aperture radar (SAR) imaging offers the possibility to overcome the pulse repetition frequency (PRF) constraints inherent to single-channel SAR systems. The multichannel approach enables the acquisition of wide swathes with high azimuth resolution. Using a constellation or swarm of small satellites, a cost efficient, faulttolerant
system can be envisaged. This paper describes the first results of a multistatic four phase center experiment conducted with TerraSAR-X and TanDEM-X. The experiment is intended to increase the know-how and demonstrate the capabilities for the design of future SAR systems. Key challenges are addressed, the experimental acquisition is described and an evaluation approach is presented. Finally, first results focusing on the azimuth ambiguity performance are shown
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