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On the Ionospheric Influence on GNSS Radio Occultation Signals: Modelling and Assessment
Blekinge Institute of Technology, Faculty of Engineering, Department of Mathematics and Natural Sciences.ORCID iD: 0000-0002-7769-8641
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 [en]
Remote Sensing, Radio Occultation, Ionosphere, Scintillation, Wave Optics Propagator, Spectral Analysis, Plasma Bubble
National Category
Remote Sensing
Research subject
Systems Engineering
Identifiers
URN: urn:nbn:se:bth-22836ISBN: 978-91-7295-439-7 (print)OAI: oai:DiVA.org:bth-22836DiVA, id: diva2:1651829
Public defence
2022-05-25, 413A + Zoom, Campus Gräsvik, Karlskrona, 08:15 (English)
Opponent
Supervisors
Projects
NRFP
Funder
Swedish National Space BoardAvailable from: 2022-04-13 Created: 2022-04-13 Last updated: 2022-05-02Bibliographically approved
List of papers
1. A Simulation Study of the Effect of Ionospheric Vertical Gradients on the Neutral Bending Angle Error for GNSS Radio Occultation
Open this publication in new window or tab >>A Simulation Study of the Effect of Ionospheric Vertical Gradients on the Neutral Bending Angle Error for GNSS Radio Occultation
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2017 (English)In: Progress in Electromagnetics Research Symposium, IEEE , 2017, p. 1540-1545Conference paper, Published paper (Refereed)
Abstract [en]

Radio Occultation based on Global Navigation Satellite System signals (GNSS RO) is an increasingly important remote sensing technique. Its measurements are used to derive parameter of the Earth's atmosphere, e.g., pressure, temperature and humidity, with good accuracy. The systematic residual error present on the data processing is related to ionospheric conditions, such as the distribution of electrons and the resultant vertical gradient. This study investigates the relationship between these parameters and the residual ionospheric error (RIE) on the retrieved bending angle in the stratosphere. Chapman function combined to sinusoidal perturbations are used to model electron density profiles and compared to RO retrievals of the ionosphere to perform the investigation. The results confirmed that the major ionospheric influence on the retrieval error is related to the F-layer electron density peak, whereas small-scale vertical structures play a minor role.

Place, publisher, year, edition, pages
IEEE, 2017
Series
Progress in Electromagnetics Research Symposium, ISSN 1559-9450
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:bth-16124 (URN)000428518301099 ()978-1-5386-1211-8 (ISBN)
Conference
Progress in Electromagnetics Research Symposium - Fall (PIERS - FALL), Singapore
Available from: 2018-04-26 Created: 2018-04-26 Last updated: 2023-04-17Bibliographically approved
2. GNSS Radio Occultation Simulation Using Multiple Phase Screen Orbit Sampling
Open this publication in new window or tab >>GNSS Radio Occultation Simulation Using Multiple Phase Screen Orbit Sampling
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2020 (English)In: IEEE Geoscience and Remote Sensing Letters, ISSN 1545-598X, E-ISSN 1558-0571, Vol. 17, no 8, p. 1323-1327, article id 8869926Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2020
Keywords
Orbits; Atmospheric modeling; Ionosphere; Refractive index; Global navigation satellite system; Low earth orbit satellites; Diffraction; Ionosphere; multiple phase screen (MPS); radio occultation (RO); remote sensing; wave optics propagator (WOP)
National Category
Remote Sensing
Identifiers
urn:nbn:se:bth-18897 (URN)10.1109/LGRS.2019.2944537 (DOI)000552271800007 ()
Projects
National Space Engineering Program (NRFP-3), grant 241/15, Swedish National Space Agency (Rymdstyrelsen)
Funder
Swedish National Space Board, 241/15
Available from: 2019-11-11 Created: 2019-11-11 Last updated: 2022-04-13Bibliographically approved
3. Evaluation of Ionospheric Scintillation in GNSS Radio Occultation Measurements and Simulations
Open this publication in new window or tab >>Evaluation of Ionospheric Scintillation in GNSS Radio Occultation Measurements and Simulations
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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
Keywords
Remote Sensing, Radio Occultation, Ionosphere
National Category
Remote Sensing
Identifiers
urn:nbn:se:bth-20332 (URN)10.1029/2019RS006996 (DOI)000567926300001 ()
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
4. Supervised Detection of Ionospheric Scintillation in Low-Latitude Radio Occultation Measurements
Open this publication in new window or tab >>Supervised Detection of Ionospheric Scintillation in Low-Latitude Radio Occultation Measurements
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2021 (English)In: Remote Sensing, E-ISSN 2072-4292, Vol. 13, no 9, article id 1690Article in journal (Refereed) Published
Abstract [en]

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. 

Place, publisher, year, edition, pages
MDPI, 2021
Keywords
Remote Sensing, Radio Occultation, Ionosphere, Scintillation, Machine Learning
National Category
Signal Processing Meteorology and Atmospheric Sciences
Research subject
Systems Engineering
Identifiers
urn:nbn:se:bth-21351 (URN)10.3390/rs13091690 (DOI)000650753100001 ()2-s2.0-85105493546 (Scopus ID)
Funder
Swedish National Space Board, NRPF-4
Note

open access

Available from: 2021-04-26 Created: 2021-04-26 Last updated: 2023-08-28Bibliographically approved
5. Detection and localization of F-layer ionospheric irregularities with the back-propagation method along the radio occultation ray path
Open this publication in new window or tab >>Detection and localization of F-layer ionospheric irregularities with the back-propagation method along the radio occultation ray path
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2023 (English)In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 16, no 7, p. 1849-1864Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Copernicus Publications, 2023
National Category
Meteorology and Atmospheric Sciences
Research subject
Systems Engineering
Identifiers
urn:nbn:se:bth-22798 (URN)10.5194/amt-2022-57 (DOI)000962705900001 ()2-s2.0-85152796342 (Scopus ID)
Projects
Swedish National Space Board, NRFP-4
Funder
Swedish National Space Board
Available from: 2022-03-28 Created: 2022-03-28 Last updated: 2023-05-01Bibliographically approved

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Ludwig Barbosa, Vinícius

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Citation style
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