Analysis and Design of a Portal for Ionospheric Data

Since 2004 DLR Neustrelitz operates the Space Weather Application Center – Ionosphere (SWACI). This Center is involved in several projects of the Space Situational Awareness (SSA) program of ESA and of EU FP7. It is largely based on services and tools of DLR’s Data and Information Management System (DIMS) and its long-standing User interface EoWEB. SWACI provides access to near real time products and ensures long-term preservation. The lecture will analyze the existing solution and various project-specific applications and will derive requirements for a recent user interface. In the second part the talk will draw a design of an user interface for the Ionospheric Monitoring and Prediction Center (IMPC), which continues utilizing existing services of DLR, offers OGC compliant interfaces and which is so generalized that future projects can be easily plugged in

A Human Motor Control Framework based on Muscle Synergies

In spite of the complexities of the human musculoskeletal system, the central nervous system has the ability to orchestrate difficult motor tasks. Many researchers have tried to understand how the human nervous system works. Yet, our knowledge about the integration of sensory information and motor control is incomplete. This thesis presents a mathematical motor control framework that is developed to give the scientific community a biologically-plausible feedback controller for fast and efficient control of musculoskeletal systems. This motor control framework can be applied to musculoskeletal systems of various complexities, which makes it a viable tool for many predictive musculoskeletal simulations, assistive device design and control, and general motor control studies. The most important feature of this real-time motor control framework is its emphasis on the intended task. In this framework, a task is distinguished by the kinematic variables that need to be controlled. For example, in a reaching task, the task variables are the position of the hand (individual joint angles are irrelevant to the reaching task). Consequently, the task space is defined as the subspace that is formed by all the controlled variables. This motor control framework employs a hierarchical structure to speed up the calculations while maintaining high control efficiency. In this framework, there is a high-level controller, which deals with path planning and error compensation in the task space. The output of this task space controller is the acceleration vector in the task space, which needs to be fulfilled by muscle activities. The fast and efficient transformation of the task space accelerations to muscle activities in real-time is a main contribution of this research. Instead of using optimization to solve for the muscle activations (the usual practice in the past), this acceleration-to-activation (A2A) mapping uses muscle synergies to keep the computations simple enough to be real-time implementable. This A2A mapping takes advantage of the known effect of muscle synergies in the task space, thereby reducing the optimization problem to a vector decomposition problem. To make the result of the A2A mapping more efficient, the novel concept of posture-dependent synergies is introduced. The validity of the assumptions and the performance of the motor control framework are assessed using experimental trials. The experimental results show that the motor control framework can reconstruct the measured muscle activities only using the task-related kinematic/dynamic information. The application of the motor control framework to feedback motion control of musculoskeletal systems is also presented in this thesis. The framework is applied to musculoskeletal systems of various complexities (up to four-degree-of-freedom systems with 15 muscles) to show its effectiveness and generalizability to different dimensions. The control of functional electrical stimulation (FES) is another important application of my motor control framework. In FES, the muscles are activated by external electrical pulses to generate force, and consequently motion in paralysed limbs. There exists no feedback FES controller of upper extremity movements in the literature. The proposed motor control model is the first feedback FES controller that can be used for the control of reaching movements to arbitrary targets. Experimental results show that the motor control model is fast enough and accurate enough to be used as a practical motion controller for FES systems. Using such a biologically-plausible motor control model, it is possible to control the motion of a patient's arm (for example a stroke survivor) in a natural way, to accelerate recovery and improve the patient's quality of life

High resolution water column phytoplankton composition across the Atlantic Ocean from ship-towed vertical undulating radiometry.

Different phytoplankton groups dominate ocean biomes and they drive differently the marine food web and the biogeochemical cycles. However, their distribution over most parts of the global ocean remains uncertain due to limitations in the sampling resolution of currently available in situ and satellite data. Information below surface waters are especially limited because satellite sensors only provide information on the first optical depth. We present measurements obtained during Polarstern cruise PS113 (May–June 2018) across the Atlantic Ocean from South America to Europe along numerous transects. We measured the hyperspectral underwater radiation field continuously over several hours from a vertical undulating platform towed behind the ship. Equivalent measurements were also taken at specific stations. The concentrations of phytoplankton pigments were determined on discrete water samples. Via diagnostic pigment analysis we derived the phytoplankton group chlorophyll a concentration (Chla) from this pigment data set. We obtained high resolution phytoplankton group Chla data from depth resolved apparent optical properties derived from the underwater radiation data by applying an empirical orthogonal function (EOF) analysis to the spectral data set and subsequently developing regression models using the pigment based phytoplankton group Chla and selected EOF modes. To our knowledge, this is the first data set with high horizontal coverage (50–150 km) and resolution (∼1 km) that is also resolved vertically for the Chla of major taxonomic phytoplankton groups. Subsampling with 500 permutations for cross validation verified the high robustness of our estimates to enable predictions of seven different phytoplankton groups’ Chla and of total Chla (R2 and median percent differences of the cross validation are within 0.45–0.68 and 29–53%, respectively). Our depth resolved phytoplankton groups’ Chla data reflect well the different biogeochemical provinces within the Atlantic Ocean transect and follow the distributions encountered by previous point observations. This verifies the high quality of our retrievals and provides the prospect to put similar radiometers on profiling floats or gliders which would enable the large-scale collection of vertically resolved phytoplankton data at much improved horizontal coverage relative to discrete sampling

Modeling of ionospheric scintillation

A signal, such as from a GNSS satellite or microwave sounding system, propagating in the randomly inhomogeneous ionosphere, experiences chaotic modulations of its amplitude and phase. This effect is known as scintillation. This article reviews basic theoretical concepts and simulation strategies for modeling the scintillation phenomenon. We focused our attention primarily on the methods connected with the random phase screen model. For a weak scattering regime on random ionospheric irregularities, a single phase screen model enables us to obtain the analytic expression for phase and intensity scintillation indices, as well as the statistical quantities characterizing the strength of scintillation-related fades and distortions. In the case of multiple scattering, the simulation with multiple phase screens becomes a handy tool for obtaining these indices. For both scattering regimes, the statistical properties of the ionospheric random medium play an important role in scintillation modeling and are discussed with an emphasis on related geometric aspects. As an illustration, the phase screen simulation approaches used in the global climatological scintillation model GISM is briefly discusse

Geometric enhancement for scintillation modeling

Continuous improvement of scintillation models is an important task required for adequate analysis and prediction of scintillation events caused by ionospheric irregularities. Some improvement can be achieved by an improved geometric description of small-scale perturbations in the ionosphere. Recently we revise the classical results of Ref. [1] obtained in the flat-earth approximation and generalized them for the case when the finite curvature of the earth has to be considered. Assuming that the earth is spherical, we obtained the analytic expressions for phase and intensity scintillation indices [2] in the approximation of a single thin phase changing screen. Figure 1 illustrates the difference between to mentioned geometries that becomes especially apparent at large zenith angles of the ground-based observer. The obtained results for spherical-earth geometry are divergence-free and represent the appropriate position of the enhancement maximum as a function of the dip angle for field-aligned ionospheric irregularities. Thus, the spherical-earth model is suitable for scintillation modeling and forecasting in such user cases as limb sounding, reflectometry, positioning at small elevation angles. Implementation of the proposed geometric considerations in the Global Ionospheric Scintillation Model is also briefly discussed. 1 C. L. Rino, "A power law phase screen model for ionospheric scintillation: 1. Weak scatter," Radio Sci., 14, November 1979, pp. 1135-1145, doi: 10.1029/RS014i006p01135 2 D.V. Vasylyev, Y. Bèniguel, M. Kriegel, V. Wilken, J. Berdermann, "Modeling ionospheric scintillation," JSWSC, 12, 22 June 2022 doi: 10.1051/swsc/202201

Anisotropic scintillation indices for low elevation angles

This contribution deals with the extension of the flat-earth model to a spherical one. The correlation properties of ionospheric electron density fluctuations responsible for scintillation occurrence are modeled conventionally as the ellipsoidal surfaces of constant value for the autocorrelation. The relative position of such ellipsoids and the radio-wave ray path modulates the scintillation strength and has purely geometrical origin. The information on communication link geometry is used for proper generation of phase screens used further for simulation of wave propagation through randomly inhomogeneous ionosphere. For clarity and simplicity we have used also the single phase screen model and derived the analytic formulas for amplitude and phase scintillation indices following the approach of C. Rino'79. We show that the accounting of the finiteness of earth-/ ionospheric-shell- curvature yields the non-divergent values for scintillation indices at low elevation angles. Additionally to this, the regions of geometric enhancement of scintillation at low elevations appear to be displaced from the corresponding regions predicted within the flat-earth approximation. The found discrepancy is important for proper determination of regions of high scintillation activity at high latitudes, e.g., as regions mapped on sky plots for a certain groundbased receiver. Incorporation of the proposed geometric model in the scintillation climatological models such as the GISM or the WBMOD will be consistent with their extension to low elevation angles and, hence, will be useful for some aforementioned user-cases. C. L. Rino, "A power law phase screen model for ionospheric scintillation: 1. Weak scatter," Radio Sci., 14, November 1979, pp. 1135-1145, doi: 10.1029/RS014i006p01135

Global Ionospheric Scintillation Model: current status and further development strategies

When a electromagnetic wave propagates through a random inhomogeneous medium, scattering by the refractive index inhomogeneities can lead to a wide variety of phenomena that have been the subject of extensive study and modelling. The Global Ionospheric Scintillation Model (GISM) is primarily intended to model the phenomena relevant for the GNSS applications and provides the amplitude and phase scintillation indices. Due to the three dimensional nature of the GISM model it is capable to describe a variety of communication geometries such as satellite-ground station or satellite-satellite communication link. Moreover, it can calculate the scintillation maps at specific altitude allowing to obtain the 3D picture of scintillation. Recently the GISM model has been handed over to the newly established DLR Institute of Solar-Terrestrial Physics. Since then the model underwent several modernization steps. For example, the programming paradigm has been changed to the object-oriented one in order to bring more flexibility into the code. In the present contribution we present the first results of our works and discuss strategies for further development, extension, and validation of the GISM

Geometric enhancement for scintillation modeling

Continuous improvement of scintillation models is an important task required for adequate analysis and prediction of scintillation events caused by ionospheric irregularities. Some improvement can be achieved by an improved geometric description of small-scale perturbations in the ionosphere. Recently we revise the classical results of Ref. [1] obtained in the flat-earth approximation and generalized them for the case when the finite curvature of the earth has to be considered. Assuming that the earth is spherical, we obtained the analytic expressions for phase and intensity scintillation indices [2] in the approximation of a single thin phase changing screen. The obtained results for spherical-earth geometry are divergence-free and represent the appropriate position of the enhancement maximum as a function of the dip angle for field-aligned ionospheric irregularities. Thus, the spherical-earth model is suitable for scintillation modeling and forecasting in such user cases as limb sounding, reflectometry, positioning at small elevation angles. Implementation of the proposed geometric considerations in the Global Ionospheric Scintillation Model is also briefly discussed 1 C. L. Rino, "A power law phase screen model for ionospheric scintillation: 1. Weak scatter," Radio Sci., 14, 1135 (1979) 2 D.V. Vasylyev, Y. Bèniguel, M. Kriegel, V. Wilken, J. Berdermann, "Modeling ionospheric scintillation," 12, 22, (2022

Ionospheric scintillation impact on the performance of communication satellites

Satellite-mediated radio communication links are impacted by ionospheric propagation effects. Communication outage can result from a cumulative effect of ionospheric absorption, Faraday rotation of wave polarization components, random fading of signal amplitude, and of the random phase modulation due to scattering on ionospheric irregularities. The latter two effects, referred to as the scintillation phenomena, are highly variable in spatial and temporal domains and also depend on space weather conditions. As we show in Ref. [1], the modeling of ionospheric scintillation can facilitate the planning and the optimization of CubeSat low budget scientific missions. Such LEO CubeSat-type satellites collect and process scientific data and transmit these via radio transceivers to ground stations. For the ground stations in regions with high scintillation activity, the estimates of communication outage correlate with enhanced amplitude scintillation indices. As illustrated exemplary for the low-latitude stations in Brazil we show that the design the parameters of transceiver antennas, such as power characteristics and gains, can be optimized in advance aiming to minimize the risk of scintillation impact. [1] A.A. Ferreira, R. A. Borges, L. R. Reis, C. Borries, D. Vasylyev, “Investigation of ionospheric effects in the planning of the AlfaCrux UHF satellite communication system,” IEEE Access, 10, 14, June 2022, pp. 65744–65759, doi:10.1109/ACCESS.2022.3183152