| 1. |
- Anderson, Dan, 1943, et al.
(author)
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Galerkin approach to approximate solutions of some nonlinear oscillator equations
- 2010
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In: American Journal of Physics. - : American Association of Physics Teachers (AAPT). - 0002-9505 .- 1943-2909. ; 78:9, s. 920-924
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Journal article (peer-reviewed)abstract
- An analysis based on the Galerkin method is given of some nonlinear oscillator equations that have been analyzed by several other methods, including harmonic balance and direct variational methods. The present analysis is shown to provide simple yet accurate approximate solutions of these nonlinear equations and illustrates the usefulness and the power of the Galerkin method. (C) 2010 American Association of Physics Teachers. [DOI: 10.1119/1.3429974]
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| 2. |
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| 3. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(author)
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Detection and localization of F-layer ionospheric irregularities with the back-propagation method along the radio occultation ray path
- 2023
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In: Atmospheric Measurement Techniques. - : Copernicus Publications. - 1867-1381 .- 1867-8548. ; 16:7, s. 1849-1864
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Journal article (peer-reviewed)abstract
- The back propagation (BP) method consists of diffractive integrals computed over a trajectory path, projecting a signal to different planes. It unwinds the diffraction and multipath, resulting in minimum disturbance on the BP amplitude when the auxiliary plane coincides with the region causing the diffraction. The method has been previously applied in GNSS Radio Occultation (RO) measurements showing promising results in the location estimate of ionospheric irregularities but without complementary data to validate the estimation. In this study, we investigate with wave optics propagator (WOP) simulations of an equatorial C/NOFS occultation with scintillation signatures caused by an equatorial plasma bubble (EPB), which was parametrized with aid of collocated data. In addition, a few more test cases were designed to assess the BP method regarding size, intensity and placement of single and multiple irregularity regions. The results show a location estimate accuracy of 10 km (single bubble, reference case), where in multiple bubble scenarios only the strongest disturbance would be resolved properly. The minimum detectable disturbance level and the estimation accuracy depend on the receiver noise level, and in the case of several bubbles on the distance between them. The remarks of the evaluation supported the interpretation of results for two COSMIC occultations.
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| 4. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(author)
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Evaluation of Ionospheric Scintillation in GNSS Radio Occultation Measurements and Simulations
- 2020
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In: Radio Science. - : Wiley-Blackwell Publishing Inc.. - 0048-6604 .- 1944-799X. ; 55:8
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Journal article (peer-reviewed)abstract
- Like any other system relying on trans-ionospheric propagation, GNSS Radio Occultation (GNSS-RO) is affected by ionospheric conditions during measurements. Regions of plasma irregularities in F-region create abrupt gradients in the distribution of ionized particles. Radio signals propagated through such regions suffer from constructive and destructive contributions in phase and amplitude, known as scintillations. Different approaches have been proposed in order to model and reproduce the wave propagation through ionospheric irregularities. We present simulations considering an one-component inverse power-law model of irregularities integrated with Multiple Phase Screen (MPS) propagation. In this work, the capability of the scintillation model to reproduce features in the signal amplitude of low latitude MetOp measurements in the early hours of DOY 76, 2015 (St. Patrick’s Day geomagnetic storm) is evaluated. Power spectral density (PSD) analysis, scintillation index, decorrelation time and standard deviation of neutral bending angle are considered in the comparison between the simulations and RO measurements. The results validate the capability of the simulator to replicate an equivalent total integrated phase variance in cases of moderate to strong scintillation.
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| 5. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(author)
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GNSS Radio Occultation Simulation Using Multiple Phase Screen Orbit Sampling
- 2020
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In: IEEE Geoscience and Remote Sensing Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1545-598X .- 1558-0571. ; 17:8, s. 1323-1327
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Journal article (peer-reviewed)abstract
- Wave optics propagators (WOPs) are commonlyused to describe the propagation of radio signals through earth’satmosphere. In radio occultation (RO) context, multiple phasescreen (MPS) method has been used to model the effects of theatmosphere in Global Navigation Satellite System (GNSS) signalsduring an occultation event. WOP implementation includes,in addition to MPS, a diffraction integral as the final step tocalculate the radio signal measured in the low-earth orbit (LEO)satellite. This approach considers vacuum as the propagationmedium at high altitudes, which is not always the case when theionosphere is taken into account in simulations. An alternativeapproach is using MPS all the way to LEO in order to samplethe GNSS signal in orbit. This approach, named MPS orbitsampling (MPS-OS), is evaluated in this letter. Different scenariosof setting occultation assuming a short segment of the LEO orbithave been simulated using MPS and MPS-OS. Results have beencompared to Abel transform references. Furthermore, a longsegment scenario has been evaluated as well. A comparison ofbending angle (BA) and residual ionospheric error (RIE) showsthe equivalence between MPS and MPS-OS results. The mainapplication of MPS-OS should be in occultation events with longsegments of orbit and including ionosphere, in which a standardWOP may not be appropriate.
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| 6. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(author)
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Location of Ionospheric Irregularities in Extended GNSS-RO Measurements Using Back Propagation Method
- 2023
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In: 2023 35th General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2023. - : Institute of Electrical and Electronics Engineers (IEEE). - 9789463968096
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Conference paper (peer-reviewed)abstract
- Besides providing electron density profiles (EDP), GNSS Radio Occultation (GNSS-RO) measurements allow monitoring the frequency and the areas where ionospheric scintillations occur. In this work, RO measurements composing an experimental data set are processed with the back propagation (BP) method to estimate the location of sporadic E-clouds and equatorial plasma bubbles (EPB). The data set includes non-conventional measurements tracked up to 600 km (generally around 80 km), covering F-region heights, shortly before MetOp-A was decommissioned. Results indicate the combination of extended occultations and the BP method is promising for monitoring the occurrence and characterizing ionospheric irregularities in the F-region and the E-region. © 2023 International Union of Radio Science.
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| 7. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(author)
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Supervised Detection of Ionospheric Scintillation in Low-Latitude Radio Occultation Measurements
- 2021
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In: Remote Sensing. - : MDPI. - 2072-4292. ; 13:9
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Journal article (peer-reviewed)abstract
- Global Navigation Satellite System (GNSS) Radio Occultation (RO) has provided high- quality atmospheric data assimilated in Numerical Weather Prediction (NWP) models and climatol- ogy studies for more than 20 years. In the satellite–satellite GNSS-RO geometry, the measurements are susceptible to ionospheric scintillation depending on the solar and geomagnetic activity, seasons, geographical location and local time. This study investigates the application of the Support Vector Machine (SVM) algorithm in developing an automatic detection model of F-layer scintillation in GNSS-RO measurements using power spectral density (PSD). The model is intended for future analyses on the influence of space weather and solar activity on RO data products over long time periods. A novel data set of occultations is used to train the SVM algorithm. The data set is composed of events at low latitudes on 15–20 March 2015 (St. Patrick’s Day geomagnetic storm, high solar flux) and 14–19 May 2018 (quiet period, low solar flux). A few conditional criteria were first applied to a total of 5340 occultations to define a set of 858 scintillation candidates. Models were trained with scintillation indices and PSDs as training features and were either linear or Gaussian kernel. The investigations also show that besides the intensity PSD, the (excess) phase PSD has a positive contribution in increasing the detection of true positives.
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| 8. |
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| 9. |
- Rasch, Joel, 1982
(author)
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An approximate general model for multipactor in curved geometries
- 2014
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In: 41st EPS Conference on Plasma Physics. ; , s. P1.128-
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Conference paper (peer-reviewed)abstract
- In order to formulate practical guidelines to avoid multipactor in a quadri-filar helix antennawe have developed a simplified statistical treatment of the electron dynamics between twoopposing conducting surfaces of arbitrary curvature. The model allows us to judge the effectof the curvature and the inhomogeneous electric field on the electron density between theconductors. It is found that the parameter regions in these types of systems where multipactorwill be possible are quite restricted. In particular it was found that multipactor will not bepossible in any realistic helix antenna configuration.
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| 10. |
- Rasch, Joel, 1982, et al.
(author)
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Gas breakdown in inhomogenous microwave electric fields
- 2009
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In: Journal of Physics D: Applied Physics. - 1361-6463 .- 0022-3727. ; 42:20, s. 205203-
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Journal article (peer-reviewed)abstract
- The main physical properties of gas breakdown in inhomogeneous high frequency microwave electric fields are investigated using both analytical and numerical analysis. In particular, the interplay between diffusion and attachment in redistributing electrons from high field regions to low field regions and the concomitant effect on the breakdown threshold is studied using three different examples of ionization profiles. The results give a clear physical picture of the dependence of breakdown electric field on pressure showing a high pressure branch determined by conditions in the high field region, a low pressure branch determined by the properties of the low field region only, and a more or less smooth transition region for intermediate pressures. Simple estimates for the extension of the transition region are given in good agreement with the numerical simulations, and a comparison of the theoretical breakdown threshold with previous experimental results also shows good agreement.
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