The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Amir Leshem - One of the best experts on this subject based on the ideXlab platform.
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Synthetic aperture Radio Telescopes
IEEE Signal Processing Magazine, 2010Co-Authors: Ronny Levanda, Amir LeshemAbstract:Next-generation Radio Telescopes will be much larger, more sensitive, have a much larger observation bandwidth, and will be capable of pointing multiple beams simultaneously. Obtaining the sensitivity, resolution, and dynamic range supported by the receivers requires the development of new signal processing techniques for array and atmospheric calibration as well as new imaging techniques that are both more accurate and computationally efficient since data volumes will be much larger. This article provides an overview of existing image formation techniques and outlines some of the directions needed for information extraction from future Radio Telescopes. We describe the imaging process from measurement equation until deconvolution, both as a Fourier inversion problem and as an array processing estimation problem. The latter formulation enables the development of more advanced techniques based on state-of-the-art array processing. We also demonstrate the techniques on simulated and measured Radio telescope data.
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Image formation in synthetic aperture Radio Telescopes
IEEE Signal Processing Magazine, 2010Co-Authors: Ronny Levanda, Amir LeshemAbstract:Next generation Radio Telescopes will be much larger, more sensitive, have much larger observation bandwidth and will be capable of pointing multiple beams simultaneously. Obtaining the sensitivity, resolution and dynamic range supported by the receivers requires the development of new signal processing techniques for array and atmospheric calibration as well as new imaging techniques that are both more accurate and computationally efficient since data volumes will be much larger. This paper provides a tutorial overview of existing image formation techniques and outlines some of the future directions needed for information extraction from future Radio Telescopes. We describe the imaging process from measurement equation until deconvolution, both as a Fourier inversion problem and as an array processing estimation problem. The latter formulation enables the development of more advanced techniques based on state of the art array processing. We demonstrate the techniques on simulated and measured Radio telescope data.
Ansgar Greiwe - One of the best experts on this subject based on the ideXlab platform.
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Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin Telescopes project
Journal of Geodesy, 2019Co-Authors: Michael Lösler, Cornelia Eschelbach, Rüdiger Haas, Ansgar GreiweAbstract:The receiving properties of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional Radio Telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the Radio Telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new Radio Telescopes, which are of a more compact and stiffer design and are able to move faster than conventional Radio Telescopes. These new Telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of Radio Telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin Telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about $$\pm \,1\,\hbox {mm}$$ ± 1 mm . To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about $$- \,0.5$$ - 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS Radio Telescopes. The uncertainty is almost $$200\,\upmu \hbox {m}$$ 200 μ m ( $$2\sigma $$ 2 σ ) and is derived by Monte Carlo simulations applied to the entire analysis process.
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Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin Telescopes project
Journal of Geodesy, 2019Co-Authors: Michael Lösler, Cornelia Eschelbach, Rüdiger Haas, Ansgar GreiweAbstract:The receiving properties of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional Radio Telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the Radio Telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new Radio Telescopes, which are of a more compact and stiffer design and are able to move faster than conventional Radio Telescopes. These new Telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of Radio Telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin Telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about $$\pm \,1\,\hbox {mm}$$ ± 1 mm . To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about $$- \,0.5$$ - 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS Radio Telescopes. The uncertainty is almost $$200\,\upmu \hbox {m}$$ 200 μ m ( $$2\sigma $$ 2 σ ) and is derived by Monte Carlo simulations applied to the entire analysis process.
A. Nothnagel - One of the best experts on this subject based on the ideXlab platform.
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A complete VLBI delay model for deforming Radio Telescopes: the Effelsberg case
Journal of Geodesy, 2014Co-Authors: T. Artz, A. Springer, A. NothnagelAbstract:Deformations of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) observations belong to the class of systematic error sources which require correction in data analysis. In this paper we present a model for all path length variations in the geometrical optics of Radio Telescopes which are due to gravitational deformation. The Effelsberg 100 m Radio telescope of the Max Planck Institute for Radio Astronomy, Bonn, Germany, has been surveyed by various terrestrial methods. Thus, all necessary information that is needed to model the path length variations is available. Additionally, a ray tracing program has been developed which uses as input the parameters of the measured deformations to produce an independent check of the theoretical model. In this program as well as in the theoretical model, the illumination function plays an important role because it serves as the weighting function for the individual path lengths depending on the distance from the optical axis. For the Effelsberg telescope, the biggest contribution to the total path length variations is the bending of the main beam located along the elevation axis which partly carries the weight of the paraboloid at its vertex. The difference in total path length is almost $$-$$ - 100 mm when comparing observations at 90 $$^\circ $$ ∘ and at 0 $$^\circ $$ ∘ elevation angle. The impact of the path length corrections is validated in a global VLBI analysis. The application of the correction model leads to a change in the vertical position of $$+120$$ + 120 mm. This is more than the maximum path length, but the effect can be explained by the shape of the correction function.
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A complete VLBI delay model for deforming Radio Telescopes: the Effelsberg case
Journal of Geodesy, 2014Co-Authors: T. Artz, A. Springer, A. NothnagelAbstract:Deformations of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) observations belong to the class of systematic error sources which require correction in data analysis. In this paper we present a model for all path length variations in the geometrical optics of Radio Telescopes which are due to gravitational deformation. The Effelsberg 100 m Radio telescope of the Max Planck Institute for Radio Astronomy, Bonn, Germany, has been surveyed by various terrestrial methods. Thus, all necessary information that is needed to model the path length variations is available. Additionally, a ray tracing program has been developed which uses as input the parameters of the measured deformations to produce an independent check of the theoretical model. In this program as well as in the theoretical model, the illumination function plays an important role because it serves as the weighting function for the individual path lengths depending on the distance from the optical axis. For the Effelsberg telescope, the biggest contribution to the total path length variations is the bending of the main beam located along the elevation axis which partly carries the weight of the paraboloid at its vertex. The difference in total path length is almost $$-$$ - 100 mm when comparing observations at 90 $$^\circ $$ ∘ and at 0 $$^\circ $$ ∘ elevation angle. The impact of the path length corrections is validated in a global VLBI analysis. The application of the correction model leads to a change in the vertical position of $$+120$$ + 120 mm. This is more than the maximum path length, but the effect can be explained by the shape of the correction function.
Ronny Levanda - One of the best experts on this subject based on the ideXlab platform.
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Synthetic aperture Radio Telescopes
IEEE Signal Processing Magazine, 2010Co-Authors: Ronny Levanda, Amir LeshemAbstract:Next-generation Radio Telescopes will be much larger, more sensitive, have a much larger observation bandwidth, and will be capable of pointing multiple beams simultaneously. Obtaining the sensitivity, resolution, and dynamic range supported by the receivers requires the development of new signal processing techniques for array and atmospheric calibration as well as new imaging techniques that are both more accurate and computationally efficient since data volumes will be much larger. This article provides an overview of existing image formation techniques and outlines some of the directions needed for information extraction from future Radio Telescopes. We describe the imaging process from measurement equation until deconvolution, both as a Fourier inversion problem and as an array processing estimation problem. The latter formulation enables the development of more advanced techniques based on state-of-the-art array processing. We also demonstrate the techniques on simulated and measured Radio telescope data.
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Image formation in synthetic aperture Radio Telescopes
IEEE Signal Processing Magazine, 2010Co-Authors: Ronny Levanda, Amir LeshemAbstract:Next generation Radio Telescopes will be much larger, more sensitive, have much larger observation bandwidth and will be capable of pointing multiple beams simultaneously. Obtaining the sensitivity, resolution and dynamic range supported by the receivers requires the development of new signal processing techniques for array and atmospheric calibration as well as new imaging techniques that are both more accurate and computationally efficient since data volumes will be much larger. This paper provides a tutorial overview of existing image formation techniques and outlines some of the future directions needed for information extraction from future Radio Telescopes. We describe the imaging process from measurement equation until deconvolution, both as a Fourier inversion problem and as an array processing estimation problem. The latter formulation enables the development of more advanced techniques based on state of the art array processing. We demonstrate the techniques on simulated and measured Radio telescope data.
Michael Lösler - One of the best experts on this subject based on the ideXlab platform.
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Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin Telescopes project
Journal of Geodesy, 2019Co-Authors: Michael Lösler, Cornelia Eschelbach, Rüdiger Haas, Ansgar GreiweAbstract:The receiving properties of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional Radio Telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the Radio Telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new Radio Telescopes, which are of a more compact and stiffer design and are able to move faster than conventional Radio Telescopes. These new Telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of Radio Telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin Telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about $$\pm \,1\,\hbox {mm}$$ ± 1 mm . To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about $$- \,0.5$$ - 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS Radio Telescopes. The uncertainty is almost $$200\,\upmu \hbox {m}$$ 200 μ m ( $$2\sigma $$ 2 σ ) and is derived by Monte Carlo simulations applied to the entire analysis process.
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Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin Telescopes project
Journal of Geodesy, 2019Co-Authors: Michael Lösler, Cornelia Eschelbach, Rüdiger Haas, Ansgar GreiweAbstract:The receiving properties of Radio Telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional Radio Telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the Radio Telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new Radio Telescopes, which are of a more compact and stiffer design and are able to move faster than conventional Radio Telescopes. These new Telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of Radio Telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin Telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about $$\pm \,1\,\hbox {mm}$$ ± 1 mm . To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about $$- \,0.5$$ - 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS Radio Telescopes. The uncertainty is almost $$200\,\upmu \hbox {m}$$ 200 μ m ( $$2\sigma $$ 2 σ ) and is derived by Monte Carlo simulations applied to the entire analysis process.
