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Evaluation of Ionospheric Scintillation in GNSS Radio Occultation Measurements and Simulations
Blekinge Institute of Technology, Faculty of Engineering, Department of Mathematics and Natural Sciences.ORCID iD: 0000-0002-7769-8641
Blekinge Institute of Technology, Faculty of Engineering, Department of Mathematics and Natural Sciences.ORCID iD: 0000-0002-2856-6140
Molflow, SWE.
RUAG Space AB, SWE.
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2020 (English)In: Radio Science, ISSN 0048-6604, E-ISSN 1944-799X, Vol. 55, no 8, article id e2019RS006996Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Wiley-Blackwell Publishing Inc., 2020. Vol. 55, no 8, article id e2019RS006996
Keywords [en]
Remote Sensing, Radio Occultation, Ionosphere
National Category
Remote Sensing
Identifiers
URN: urn:nbn:se:bth-20332DOI: 10.1029/2019RS006996ISI: 000567926300001OAI: oai:DiVA.org:bth-20332DiVA, id: diva2:1463298
Funder
Swedish National Space Board, NRFP‐3 dnr: 241/15
Note

Open access

Available from: 2020-09-01 Created: 2020-09-01 Last updated: 2022-04-13Bibliographically approved
In thesis
1. Effects of Small-Scale Ionospheric Irregularities on GNSS Radio Occultation Signals: Evaluations Using Multiple Phase Screen Simulator
Open this publication in new window or tab >>Effects of Small-Scale Ionospheric Irregularities on GNSS Radio Occultation Signals: Evaluations Using Multiple Phase Screen Simulator
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Radio Occultation (RO) is a remote sensing technique which uses Global Navigation Satellite System (GNSS) signals tracked by a Low-Earth Orbit (LEO) satellite to sound the earth's atmosphere both in low (troposphere, stratosphere) and high (ionosphere) altitudes. GNSS-RO provides global coverage and SI traceable measurements of atmospheric data with high-vertical resolution. Refractivity, dry temperature, pressure and water vapour profiles retrieved from RO measurements have a relevant contribution in Numerical Weather Prediction (NWP) systems and in climate-monitoring.

Due to the partial propagation through the ionosphere, a systematic bias is added to the lower atmospheric data product. Most of this contribution is removed by a linear combination of data for two frequencies. In climatology studies, one can apply a second-order correction - so called κ-correction - which relies on a priori information on the conditions in the ionosphere. However, both approaches do not remove high-order terms in the error due to horizontal gradient and earth's geomagnetic fields. The remaining residual ionospheric error (RIE) and its systematic bias in RO atmospheric data is a well-known issue and its mitigation is an open research topic.

In this licentiate dissertation, the residual ionospheric error after the standard correction is evaluated with computational simulations using a wave optics propagator (WOP). Multiple Phase Screen (MPS) method is used to simulate occultation events in different ionospheric scenarios, e.g. quiet and disturbed conditions. Electron density profiles (EDP) assumed in simulations are either defined by analytical equations or measurements. The disturbed cases are modelled as small-scale irregularities within F-region in two different ways: as sinusoidal fluctuations; and by using a more complex approach, where the irregularities follow a single-slope power-law that yields moderate to strong scintillation in the signal amplitude. Possible errors in MPS simulations assuming long segment of orbit and ionosphere are also evaluated.

The results obtained with the sinusoidal disturbances show minor influence in the RIE after the standard correction, with the major part of the error due to the F-region peak. The implementation of the single-slope power-law is validated and the fluctuations obtained in simulation show good agreement to the ones observed in RO measurements. Finally, an alternative to overcome limitations in MPS simulations considering occultations with long segment of orbit and ionosphere is introduced and validated.

The small-scale irregularities modelled in F-region with the power-law can be added in simulations of a large dataset subjected to κ-correction, in order to evaluate the RIE bending angle and the consequences in atmospheric parameters, e.g. temperature.

Place, publisher, year, edition, pages
Karlskrona: Blekinge Tekniska Högskola, 2019
Series
Blekinge Institute of Technology Licentiate Dissertation Series, ISSN 1650-2140 ; 16
Keywords
GNSS Radio Occultation (GNSS-RO); Ionosphere; Scintillation
National Category
Remote Sensing
Identifiers
urn:nbn:se:bth-18907 (URN)978-91-7295-392-5 (ISBN)
Presentation
2019-12-12, 08:30 (English)
Opponent
Supervisors
Projects
NRPF-3, Rymdstyrelsen, 241/15
Funder
Swedish National Space Board
Available from: 2019-11-15 Created: 2019-11-14 Last updated: 2020-11-16Bibliographically approved
2. On the Ionospheric Influence on GNSS Radio Occultation Signals: Modelling and Assessment
Open this publication in new window or tab >>On the Ionospheric Influence on GNSS Radio Occultation Signals: Modelling and Assessment
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Radio Occultation (RO) is a well-established remote sensing technique that uses Global Navigation Satellite System (GNSS) signals to sound the Earth’s atmosphere. GNSS-RO measurements provide high-resolution, vertical profiles of physical parameters from the lower atmosphere (troposphere and stratosphere), e.g., refractivity, dry temperature, pressure, and water vapour, with primary application in weather forecasting and climatology models. The upper atmosphere (ionosphere) is also sounded during measurements, in which information about total electron content, electron density profiles, and scintillation indices compose the RO ionospheric data product.

The ionosphere is a dispersive medium composed of ionized particles. It is extensively conditioned by Solar activity and shows seasonal, geographical, and day- and night-time variation. Despite the benefit of the upper atmospheric data, the ionosphere influences the retrievals in the lower atmosphere by (i) adding an inherent systematic bias in bending angles, i.e., residual ionospheric error (RIE), and (ii) disturbing the signal amplitude and phase, i.e., scintillation, in the presence of irregularities regions on the electron density along the ray path, e.g., equatorial plasma bubbles. In this dissertation, both aspects are investigated by modelling the equatorial ionosphere, and its small-scale irregularities in simulations of occultation events to (i) reproduce the effects observed in measurements and (ii) assess methods that can extract information about the ionosphere and support its monitoring and modelling.

The multiple phase screen method was applied to model the GNSS signal propagation through quiet and disturbed ionospheric conditions. The small-scale irregularities in the F-region were modelled by a single slope power law to yield moderate to strong scintillation in the signals. Results were assessed according to the amplitude and phase scintillation indices, RIE, the standard deviation of the retrieved bending angles, and power spectral density (PSD). A subset of these parameters was taken as features to train a classifier based on the support vector machine algorithm. The purpose of this model was to detect RO measurements affected by ionospheric scintillation. Specifically, those in which PSD could provide further information about the irregularities according to the scintillation theory. Additionally, the back propagation (BP) method and its capability to estimate the mean distance between the receiver and irregularities were evaluated.

Applying spectral analysis techniques to RO measurements may contribute to the characterization of small-scale irregularities in equatorial plasma bubbles. The results from simulations applying the single-slope power law to model the irregularities showed a good agreement with the selected cases. The automatic detection of occultations affected by ionospheric irregularities has achieved similar performance to models trained with ground-based measurements. Furthermore, the BP method can add the estimation of the mean location to the spectral analysis information. Such tools can enlarge the amount of ionospheric data retrieved -- especially for occultations with extended vertical range and when combined with other sounding techniques.

Place, publisher, year, edition, pages
Karlskrona: Blekinge Tekniska Högskola, 2022
Series
Blekinge Institute of Technology Doctoral Dissertation Series, ISSN 1653-2090 ; 2022:03
Keywords
Remote Sensing, Radio Occultation, Ionosphere, Scintillation, Wave Optics Propagator, Spectral Analysis, Plasma Bubble
National Category
Remote Sensing
Research subject
Systems Engineering
Identifiers
urn:nbn:se:bth-22836 (URN)978-91-7295-439-7 (ISBN)
Public defence
2022-05-25, 413A + Zoom, Campus Gräsvik, Karlskrona, 08:15 (English)
Opponent
Supervisors
Projects
NRFP
Funder
Swedish National Space Board
Available from: 2022-04-13 Created: 2022-04-13 Last updated: 2022-05-02Bibliographically approved

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Ludwig Barbosa, ViníciusSievert, ThomasPettersson, MatsVu, Viet Thuy

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