Session - Modelling the Earth's ionosphere and solutions to counter ionospheric threats to GNSS applications
Marcio Aquino, Cathryn Mitchell, Giorgiana De Franceschi
The Earth's ionosphere is a complex system that is driven by many different factors such as solar radiation, electric and
magnetic fields, and neutral atmosphere dynamics. New models and data to realise the state of the Earth's ionosphere are
of interest to radio system users whose signals are affected by ionospheric propagation, in particular navigation and
communications systems operating below 3 GHz. Global Navigation Satellites Systems (GNSS), which have become essential
in support of a growing number of activities now embedded in modern society, are especially vulnerable to signal propagation
through the ionosphere. In this context this session addresses models and solutions to mitigate ionospheric threats to GNSS
and related applications. The TRANSMIT project, an FP7 funded Marie Curie Initial Training Network,
has focused on the understanding and development of new models that can be tested for their usefulness in
addressing radio system specification with a view to support the development of concepts and operational tools
that could contribute to a service to assist European users in countering GNSS vulnerability to ionospheric phenomena.
Results of this project are expected to be showcased in this session, as well as any other initiatives in this area.
The session welcomes work in the areas of ionospheric tomography and imaging, radio occultation, scintillation and
interference resilient receiver tracking models and implementations, real time positioning algorithms (e.g. for
Precise Point Positioning) to mitigate ionospheric threats affecting legacy and new GNSS signals, scintillation and
TEC prediction models and operational tools, as well as any other related topics. The main aim of the session is to
stimulate discussion and encourage new collaborative work.
Invited Talks and First Class Posters
Monday November 17, 16:00 - 18:00
| 1 |
Oral - invited |
4:00 pm |
The Added Value of New GNSS for
Ionosphere Monitoring |
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Warnant, R1; Wautelet, G1; Lonchay, M1 |
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1University
of Liege |
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Local variability in the
ionosphere Total Electron Content due to different types of ionospheric
“disturbances” can strongly degrade the performances of GNSS-based high
accuracy positioning. Real time GNSS applications are particularly vulnerable
to these phenomena. In this context,
the University of Liege developed the so-called GPSTECMON software: based on
GPS dual frequency measurements performed at a single station, GPSTECMON
monitors the Total Electron Content and detects in real time the occurrence
of ionospheric disturbances which can pose a threat to GNSS high accuracy
applications. Then, the reconstructed information on the ionospheric activity
is exploited to offer different services to GNSS users through an operational
web site (http://www.gnss-ulg.be). At European mid-latitudes, the most
frequent disturbances are Medium-Scale Travelling Ionospheric
Disturbances. At the present time,
ionosphere monitoring using GPS dual frequency L1/L2 measurements has
different weaknesses: - Limited TEC accuracy: it ranges from 2 to 5 TECU;
this is mainly due to the fact that the “classical” dual frequency TEC
reconstruction techniques need code measurements to solve phase ambiguities;
multipath, noise and hardware biases affecting code measurements limit the
accuracy of the reconstructed TEC. In practice, an uncertainty of 5 TECU on
TEC corresponds to an uncertainty of about 80 cm on the computation of the
ionospheric error on the L1 carrier. Therefore, increased TEC accuracy is
highly desirable. - Limited TEC spatial resolution: it depends on the number
of satellites in view; in Liege (Belgium), the number of GPS satellites in
view ranges from 8 to 15 with an average of 11. A proper modeling of local
variability in TEC due to TID’s or to geomagnetic storms requires an
increased spatial resolution. - Observational bias in the detection of
irregularities: GPS based detection of moving structures like TIDs is limited
due to satellite orbital movement. Indeed, most of GNSS satellites and, in
particular, GPS satellites are placed on Medium Earth Orbits (MEO) at an
altitude around 20 000 km. This means that they have a velocity with respect
to the ionosphere. TIDs are moving structures and therefore have also a
velocity with respect to the ionosphere. In practice TID detection will be
affected by the relative velocity between the TID and the satellite. For a
given TID, the fact that the satellite has a velocity which is parallel,
anti-parallel (worst case) or perpendicular to the TID velocity has an
influence. For example, if the TID has a velocity which is anti-parallel and
of the same order of magnitude than the satellite velocity, there is a high
probability that the TID will not be detected. In other words, the study of
ionospheric disturbances using GPS satellites has an “observational bias”
which makes their modelling more difficult.
The availability of new GNSS emitting new and more accurate signals
and also the combination of the different GNSS will allow the development of
improved data processing strategies for ionosphere monitoring and, as a
consequence, improved services for GNSS users. The paper analyses the added
value of new GNSS for ionosphere monitoring. In particular, the paper
discusses the use of satellites with geostationary or inclined geosynchronous
orbits to remove the orbital bias in the detection of ionospheric
irregularities. |
| 2 |
Oral - invited |
4:20 pm |
TRANSMIT from an Industrial
Partner’s Perspective |
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de Jong, K1; Visser, H1; Memarzadeh, Y1 |
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1Fugro
Intersite B.V. |
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Fugro is a world leading
provider of precise offshore Global Navigation Satellite System (GNSS)
positioning services. Particularly in areas along the geomagnetic equator and
in arctic regions, where many of Fugro’s activities take place, these
services are frequently affected by effects due to space weather, such as
ionospheric scintillations. When asked
to participate in TRANSMIT as an industrial partner in 2008, Fugro therefore
immediately realized the benefits such a project could have for its
operations. In this presentation we
will focus on the benefits gained over the years. These include: - A better
understanding of scintillations from different points of view and for different
applications, thanks to contacts with highly motivated researchers and other
partners, at workshops, but also at the personal level. - Involvement in state of the art research,
based on our own data. - Access to data from scintillation monitors, which
not only proved useful to detect scintillations, but also other effects, such
as high phase noise and satellite clock anomalies. - New modules of our data
analysis software, developed by TRANSMIT fellows, which are now routinely
used in our daily network monitoring. - Increased awareness of the need of a
data archiving system for our network data, as it provides a lot of valuable
ionospheric information. The overall
conclusion is that TRANSMIT indeed has proved to be a very beneficial
experience for Fugro. We hope we will be able to continue and extend our
cooperation with project partners in the future. |
| 3 |
Oral - invited |
4:40 pm |
TRANSMIT Prototype: Tools for
Mitigation of Ionospheric Threats to GNSS |
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Sato, H1; Hlubek, N1 |
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1DLR |
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The TRANSMIT prototype is a
web-based demonstrator of the research results from the EC FP7 funded project
Training Research and Applications Network to Support the Mitigation of
Ionospheric Threats (TRANSMIT), a Marie Curie Initial Training Network (ITN).
The applications implemented in the prototype aim to provide awareness of
current ionospheric threats and improve solutions for the mitigation of
ionospheric impacts for users of Global Navigation Satellite Systems (GNSS)
and related services and applications.
The prototype demonstrator consists of three processors formed by six
applications (called TRANSMIT processors). The processors have been developed
by the TRANSMIT research fellows exploiting the varied expertise in their
hosting institutions. In order to maximize the prototype research products,
TRANSMIT processors are designed to be able to exchange their outcomes and
use them as inputs to other related applications via the prototype network.
The research topics covered by the processors are: -Scintillation index S4 and TEC modeling
-Accuracy in precise point positioning
-Tracking architecture for robust receivers -TEC map tools
-Ionospheric model comparison -Asymmetry in ionosphere The data flow in the prototype system is
based on a cross-institutional network approach. The main concept of the
prototype network is to clearly divide the functions of the partner
institutions. The user portal and demonstrator is provided by the Space
Weather Application Center – Ionosphere (SWACI) hosted by the German
Aerospace Center. The data archive is hosted by the Italian National
Institute of Geophysics and Volcanology (INGV). The processors are hosted by the respective
institutions, where the applications were developed, distributed over Europe.
The TRANSMIT prototype service is accessible by sending queries and receiving
the processed results via internet.
In this paper we will present the overview of the TRANSMT prototype
and demonstrate selected outcome examples. |
| 4 |
Oral |
5:00 pm |
An Industry based Perspective on
the Effects of Solar Radio Bursts on GNSS Receivers |
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Vadakke Veettil, S1; Aquino, M1; de Jong , K2; Visser, H2 |
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1University
of Nottingham; 2Fugro Intersite B.V. |
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Intense solar radio bursts
occurring in the L-Band can impact the tracking performance of Global
Navigation Satellite Systems (GNSS) receivers located in the sunlit
hemisphere of the Earth. Significant decrease in the GNSS signal
carrier-to-noise ratio (C/N0) is observed, which can lead to the complete
loss of lock on the satellite signals. Previous experimental evidence has
revealed that high-precision GPS positioning on Earth’s entire sunlit side
was partially disrupted during solar radio bursts. Hence, solar radio bursts
are a potential threat to safety-critical systems based on GNSS. Consequently
monitoring these events is important for suitable warnings to be issued in
support to related services and applications. Despite such relevant
experimental evidence, not enough emphasis or research effort has been given
to this phenomenon, which is characterized by low probability of occurrence,
but also by high impact when it occurs. This paper presents the results from
investigations on the impact of the solar radio burst of 24 September 2011 on
GNSS receivers from an industrial point of view. It exploits data from the
GNSS based service provider Fugro, collected by reference station receivers.
The impact of this radio burst on the availability of Fugro’s real-time
precise point positioning (PPP) services and on the quality of the L-band
data link used to broadcast this service are presented and discussed. |
| 5 |
Oral |
5:15 pm |
Space Weather
Effects over EGNOS Performance in the North of Europe |
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Pintor, P1; Roldan, R2; Gomez, J1; De la Casa, C1; Fidalgo, R M1 |
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1ESSP;
2ESSP SAS |
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Solar cycle #24 is generating a
period of high solar activity. This increased activity affects the
geomagnetic behavior of the ionosphere. The continuous observation of EGNOS
performances since the beginning of this solar cycle and particularly the
focus on EGNOS performance degradation has shown a dependency due to the
behavior of the ionosphere at high latitudes.
The dependence of EGNOS performance with the variations observed in
the ionospheric behavior is known, and has been especially relevant since the
beginning of the solar activity increase linked to the current solar cycle.
This kind of events affects not only EGNOS but also other SBAS systems under
geomagnetic storm conditions. This is considered as an intrinsic limitation
in single frequency SBAS systems. In the past, some areas of Europe were more
sensitive to the variations in the behavior of the ionosphere. The current
EGNOS release (2.3.2), deployed in October 2013, increased the robustness of
EGNOS against this kind of events, correcting some issues which were observed
during autumn of 2012. A new release ESR2.4.1 will be deployed during 2015,
including additional improvements related to ionosphere monitoring. However,
even if these new releases provide a higher stability to ionospheric
disturbances, some degradation can still be expected during periods with very
high geomagnetic activity. Several indicators can be used to measure the
status of the ionosphere such as the K and A indexes or the SSN. The
dependence of EGNOS performance with the indicators is known and has been
especially relevant since the increase of the solar activity in 2011. Other
indicators, such as the disturbance storm time (Dst), have been investigated
as an alternative mean to detect ionospheric events. However, the use of
these indicators presents also some limitations such as temporal resolution,
geographical applicability or different level of correlations wrt EGNOS
performance. These limitations show
that the information provided by these indexes needs to be used carefully in
order to understand the impact of the ionospheric behavior in the performance
of SBAS. The effect of the ionospheric
disturbances can be easily observed through the observation of the Horizontal
Protecion Level (HPL). The HPL is a bound of the horizontal error and is
calculated with the EGNOS information broadcast according to MOPS 229D. This
parameter is an integrity monitor to decide if a specific operation (for
example, an LPV) can be initiated, and so, it can be used to measure, during
a given period, the performance of the system, in terms of availability, for
a particular operation. EGNOS
performance appeared to be degraded with values of HPL well above the 40m
operational limit in the North of Europe during several periods along the
solar cycle #24. EGNOS integrity was always guaranteed every single epoch for
the whole service area but its availability was reduced. From SBAS standards,
it is known SBAS systems model its ionospheric corrections and integrity
information from TEC estimated by its algorithms. TEC disturbances are known
to be also correlated to geomagnetic storms and local time and these storms,
at the same time can be traced to exchanges between the Earth’s magnetosphere
and the interplanetary conditions originated by the Sun. In order to analyze
the geomagnetic conditions affecting EGNOS, the method of analysis focuses on
the negative variations of interplanetary magnetic field z component (Bz) and
solar wind speed sudden increases and other factors. This paper provides a summary of the
different analyses performed by ESSP relative to the existing correlation
between the EGNOS performance measured over the north region of the EGNOS
service area, some ionospheric indicators and some solar events. |
| 1 |
First class poster |
5:30 PM |
Characterization of High
Latitude GPS Sensed Ionospheric Irregularities: Case Studies |
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Ghoddousi-Fard, R1; Prikryl, P1; Oksavik, K2; van der Meeren, C2; Lahaye, F1; Danskin, D1 |
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1Natural
Resources Canada; 2Birkeland Centre for Space Science, Department of Physics and
Technology, University of Bergen / Arctic Geophysics, The University Centre
in Svalbard |
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Rate of change of 1Hz GPS phase
differences at two frequencies are being monitored in near-real-time and used
as proxy ionospheric disturbance indices at the Canadian Geodetic Survey
(CGS) of Natural Resources Canada from about 165 globally distributed stations
including Real-Time International GNSS Service stations as well as other high
rate stations operated by CGS. It has been shown that such indices correlate
well with the phase scintillation index during weak to moderate phase
scintillation over high latitudes.
GPS phase rate variations over Canada and adjacent regions have been
analyzed during 2013 and early 2014. With more than 40 stations being
monitored over the region a significant coverage of ionospheric pierce points
during each 24 hours in geomagnetic latitude and local hour is achieved. Maps
of occurrence of phase scintillation as a function of local hour and
geomagnetic latitude are studied toward characterization of occurrence of
irregularities. A number of
scintillation events over polar, auroral and sub-auroral latitudes correlated
with coronal mass ejections or high-speed solar wind streams have been
identified and their analysis is complemented with observations from
dedicated scintillation receivers from Canadian High Arctic Ionospheric
Network (CHAIN) and a new multi-constellation receiver network in Svalbard. |
| 2 |
First class poster |
5:35 PM |
GNSS Observational Bias in the
Frame of Ionospheric Studies |
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Wautelet, G1; Warnant, R1 |
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1University
of Liège |
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Ionospheric disturbances such as
Traveling Ionospheric Disturbances (TIDs) affect high-accuracy positioning
with Global Navigation Satellite Systems (GNSS). Besides, GNSS satellites are
also considered as “satellites of opportunity” for ionospheric research.
Indeed, each constellation being made up of about 30 satellites, receivers
are able to track several satellites simultaneously, and therefore to perform
multiple ionospheric observations at a given epoch. In the frame of
ionospheric studies with GNSS, it is common use to assimilate the ionosphere
to a simple geometric shape. Generally, ionospheric plasma is assumed to be
confined into a spherical shell, infinitesimally thin, containing all free
electrons. The intersection of this layer and a given satellite-to-receiver
link is called the Ionospheric Pierce Point (IPP). Using moving satellites to
monitor the Total Electron Content (TEC) and its variations in time and space
induces some observational biases. Indeed, there is a relative movement between
the moving observer (the satellite), the moving structure (the ionospheric
disturbance) and the receiver station which is also moving due to the Earth
rotation. As a consequence, one-way measurements are affected by the relative
movement between the IPPs and the ionospheric disturbance. More precisely,
only the projection of the velocity vector of the IPPs v on the direction of
propagation of the disturbance k matters. In other words, one can expect no
relative effect if both directions are perpendicular, i.e. if v · k = 0. The
consequence resulting from this relative motion, which is elevation-dependent
(IPP velocity may change with a ratio of 1:7 according to satellite
elevation), is that apparent TID wavelength and period are distorted with
respect to their true value. Besides this purely geometrical effect,
ionospheric measurements with GNSS also suffer from the so-called aliasing
effect, related to the observation sampling rate. Indeed, a signal (here a
TID) is resolved properly if the time step between measurements is not kept
shorter than its timescale (i.e. its period). As a result, long sampling
rates will make the observed signal as if it has a longer period than it
actually has. In addition, the amplitude of a TID is also distorted if the
latter is not properly resolved by GNSS observations, which leads generally
to under-estimated values. In this context, the aim of this paper is to
identify geometrical conditions leading to distortions in the observation of
TIDs with GNSS, which are the most common source of ionospheric disturbances
over mid-latitudes. Based on simulation of several GNSS constellations (GPS,
GLONASS, Galileo, Beidou) but also of measurements related to geostationary
satellites, it is proposed to study the reconstruction of a simulated TID
observed from a single GNSS station on Earth. The TID is modeled by a simple
planar wave in the TEC, with a given wavelength, period and direction of
propagation. Analysis of geometric and aliasing effect will be separately
assessed by considering several values of these propagation parameters,
considering both small-scale (wavelengths smaller than 50km) and large-scale
TIDs (wavelengths larger than 1000km). The different orbit configurations of
GNSS constellations (Medium-Earth Orbit for GPS, GLONASS, Galileo and Beidou,
Inclined Geosynchronous Orbit for Beidou or Geostationary Orbit for EGNOS,
WAAS and Beidou) offer a variety of different geometrical conditions,
resulting in different observational biases. Because of their low IPP
velocity, inclined geosynchronous and geostationary orbits are expected to
better reconstruct TIDs than medium-earth ones used by GPS, GLONASS or
Galileo systems. |
| 3 |
First class poster |
5:40 PM |
Mitigation of the Impact of
Scintillations on GNSS Positioning in the Scope of the TRANSMIT Project
Prototype |
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Kieft, P |
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University of Nottingham |
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TRANSMIT (Training Research and
Applications Network to Support the Mitigation of Ionospheric Threats) is an
EU funded Marie Curie Actions project. The final outcome of the TRANSMIT
project is a prototype of a web based service, whose functionality comes from
an underlying distributed system, comprising a database at INGV (Italian
National Institute for Geophysics and Vulcanology) , processors (at the
participating universities) and a web front-end at DLR (German Aerospace
Centre). This ‘prototype’ service aims to provide the users of Global
Navigation Satellite Systems (GNSS) and related applications with information
on ionospheric threats, and a demonstration of the capabilities of improved
ionospheric perturbations mitigation solutions. This paper concentrates on the research
behind one of the processors, as well as its functionality and implementation
in the TRANSMIT prototype, which aims to demonstrate a tool to mitigate the
impact of ionospheric scintillation on the GNSS positioning computation. Small scale (in the order of a few hundred
meters), time varying irregularities in electron density in the ionosphere
can cause scattering and diffraction of the radio signals passing through
them. The result for the observer is a signal which is rapidly changing in
phase (phase scintillation) and intensity (amplitude scintillation). The
impact for a GNSS receiver can range from a complete loss of lock (signal no
longer available), cycle slips on the phase measurements or a degraded
accuracy of the phase and code ranges that can lead to poor positioning
accuracy. The scintillation
mitigation method during the positioning computation stage is limited to the
repair of cycle slips and finding the ‘best’ stochastic model which will
achieve a balance between the decreased accuracy of the affected ranges and
the degradation of the geometry, which in itself can lead to degradation in
positioning accuracy. The reduction of the amount of ‘losses of lock’ is
improved through a novel PLL design (demonstrated within another TRANSMIT
prototype processor). This processor
of the TRANSMIT prototype service will perform a position calculation for
different, currently used stochastic models (e.g.: elevation based,
signal-to-noise-ratio based), and will provide a comparison of their
performance during scintillation events with the TRANSMIT algorithms. The
TRANSMIT algorithms comprise a stand-alone solution and an augmented solution
using scintillation information from a network of scintillation monitoring
receivers. The user of this processor of the prototype service will be able
to choose from a set of selected events, start the calculation process
(Single Point Positioning or Precise Point Positioning) and will be presented
with the (comparison) results. |
| 4 |
First class poster |
5:45 PM |
Designing a GNSS
Carrier Tracking Architecture Robust to Ionospheric Scintillation |
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Susi, M1; Andreotti, M2; Aquino, M1 |
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1University
of Nottingham; 2Novatel |
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Electron density irregularities
in the ionosphere are responsible for random amplitude and phase fluctuations
of trans-ionospheric signals, a phenomenon known as scintillation. Amplitude
scintillation may produce deep signal fading and push the signal intensity
below the receiver tracking threshold. Furthermore, phase scintillation
increases the carrier Doppler shift and can even make it larger than the
phase locked loop (PLL) bandwidth. As
a consequence large phase errors, cycle slips or even losses of signal lock
can occur. Moreover, even if this phenomenon usually affects only a portion
of the sky, it can still decrease the final position solution accuracy and
also impair GNSS operation when the number of healthy satellite links is not
enough. A possible approach to
mitigate scintillation effects on GNSS is to increase the robustness of GNSS
receivers and in particular of their PLL, that is the receiver part most
sensitive to scintillation effects. A conventional PLL requires a short
integration time or a wide carrier loop bandwidth to follow the fast dynamics
due to phase scintillation. On the
other hand a long integration time or a narrow carrier loop bandwidth should
be preferred to reduce the noise produced by amplitude scintillation. To
solve this trade off, the use of adaptive tracking schemes, such as Kalman
Filter (KF) based PLLs, represents a suitable strategy. The use of a KF
allows optimizing the loop filter by minimizing the phase error. However to
ensure the optimality of the KF, the correct dynamic model should be defined.
This is not a trivial task in the presence of variable working conditions,
such as under the occurrence of scintillation. In fact, the assumed dynamic
model would no longer be suitable when the noise and dynamic levels change
significantly due to scintillation,.
In order to overcome this issue, this paper proposes a self-tuning KF
based PLL. The algorithm monitors the scintillation level by computing on the
fly some key parameters as p, T, namely the slope and strength of the phase
scintillation spectrum, and the carrier to noise ratio. Then the above parameters
are used to tune the covariance matrix and the measurement noise of the KF on
the fly. The proposed algorithm,
implemented into a software receiver, is assessed using both high latitude
and equatorial scenarios. Specifically, high frequency signal perturbations
were extracted from real data collected at high latitudes (in Bronnoysund,
Norway, ~65oN geographic latitude) through a NovAtel professional
scintillation monitoring receiver (the GSV4004B). Finally, these
perturbations were superimposed onto GPS signals using a Spirent GS8000
hardware simulator. Moreover, Intermediate Frequency (IF) GPS and Galileo
real equatorial data collected in Vietnam during a period of high solar
activity are used to assess the
algorithm in equatorial scenarios. The above data were collected using a USRP
N210 front end driven by a rubidium clock.
The tracking performance of the proposed algorithm is compared to that
of the Septentrio PolaRxS professional scintillation monitoring receiver, in
terms of the resulting phase error, cycle slips and loss of lock occurrence.
Finally, the effectiveness of the scintillation parameters computation is
also tested by using the Septentrio PolaRxS receiver as benchmark. |
| 5 |
First class poster |
5:50 PM |
Effect of Interference in the
Estimation of Ionospheric
Scintillation Indices with GNSS. |
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Romero, R1; Dovis, F1 |
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1Politecnico
di torino |
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Irregular electron
concentrations in the ionosphere are known to cause rapid fluctuations in the
amplitude and phase of satellite signals, a phenomenon known as
scintillation. The occurrence of scintillation depends on solar and
geomagnetic activity, season, local time, geographic location and frequency
of the satellite signal. For Global
Navigation Satellite System (GNSS) receivers, ionospheric scintillations can
considerably degrade the performance by inducing cycle slips, losses of lock
of the signals and reduced accuracy of the PVT solution. Ionospheric Scintillation Monitoring
Receivers (ISMR) are specialized receivers able to track and monitor
scintillations in order to collect data that can be used to analyze the
phenomenon. This is normally done by computing the S4 and Phi60, namely the
amplitude and phase scintillation indices, in a minute by minute basis.
Within this work we deal with a specific environment of an ISMR where the
monitoring of scintillation activity is threatened by the presence of Radio
Frequency Interference (RFI) in the operation area. RFI is, among the
different error sources that corrupt satellite navigation waveforms, a
particularly harmful error since in some cases it cannot be mitigated by a
simple correlation process. This is indeed a problem that may affect the
detection of ionospheric scintillation when monitored by GNSS signals, and
has been quantified in several scenarios based on different characteristics
of the satellite signal, the interference and the receiver configuration
. The work proposed here is being
developed within the framework of TRANSMIT (Training Research and
Applications Network to Support the Mitigation of Ionospheric Threats), a
Marie Curie Initial Training Network aiming to develop new techniques to
detect and monitor Ionospheric threats with the introduction of new
prediction and forecasting models, mitigation tools and improved system
design. In particular, the work
package (WP) of the TRANSMIT
project titled “Scientific and
Industrial Application Reliant on GNSS”,
is dedicated to the assessment of ionospheric effects on
GNSS and related applications and is steered by industrial
requirements. |
| 6 |
First class poster |
5:55 PM |
Mitigating the Effects of
Spherical Symmetry Hypothesis from
Ionospheric Radio Occultation (RO) Data Inversion with the help of
Model Data |
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Shaikh, M1; Nava, B2; Notarpietro, R1 |
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1Politecnico
di Torino; 2The
Abdus Salam International Center for Theoretical Physics |
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Spherical symmetry is a common
assumption applied to the Radio Occultation (RO) observations in the
ionosphere. The corresponding techniques are very effective for RO data
inversion especially in the case of small horizontal electron density
gradients present in the ionosphere. Nevertheless, during geomagnetic
disturbed periods or in specific geographic regions, like the equatorial
anomaly region, large electron density gradients may be experienced which
could lead to the failure of spherical symmetry hypothesis producing
erroneous electron density (Ne(h)) profiles as output. We had performed a
thorough study on the qualitative and qualitative aspects of Ionospheric
asymmetry. As a result, we developed an asymmetry index [1] that, with the
help of a suitable background ionosphere, is able to quantify the level of
asymmetry present in the ionosphere with respect to time, location and
prevailing solar and geomagnetic activity. This work is an attempt to
mitigate the effects of spherical symmetry hypothesis in RO data inversion
algorithms. We have implemented a simple technique based on the NeQuick2
model adaptation to RO-derived TEC. It relies on the minimization of a cost
function involving experimental and model-derived TEC data to determine the
NeQuick2 input parameters at the wanted locations and time. These parameters
are then used to evaluate the electron density profile Ne(h) along the ray
perigee positions associated to the relevant RO event. We performed our
analysis on one month’s experimental data (approximately 29,000 RO events in
September 2006) downloaded from the COSMIC mission database
(http://tacc.cwb.gov.tw/cdaac/index.html). In particular the differences
between peak densities derived from the profiles obtained with model-assisted
technique and the corresponding experimental peak densities obtained from
collocated ionosondes have been considered. The results indicate that, even
if the technique was able to avoid the presence of negative electron density
values in the reconstructed profiles, it could not improve in all cases the
results of spherical symmetry hypothesis. In this paper, we have presented a
description of the adopted approach together with our initial results towards
developing the model assisted RO data inversion. In the scenario of TRANSMIT project
prototype implementation, we are going to evaluate global asymmetry maps (not
shown to the users) with their associated inversion error (DVTEC and DNmF2)
plots considering background ionosphere computed using NeQuick2, IRI and
MIDAS. By querying these global maps of asymmetry, and for a given geometry
of RO event, we will try to predict the expected level of asymmetry present
in the ionosphere and its potential impact on RO inverted products using
estimates of delta_NmF2 and delta_VTEC. Also, initial results obtained in
attempt to develop the model assisted RO data inversion (currently being
developed) will also be shown. |
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