Marcio Aquino - Academia.edu (original) (raw)

Papers by Marcio Aquino

In addition to the common practice of eliminating the (first order) ionospheric effect, for insta... more In addition to the common practice of eliminating the (first order) ionospheric effect, for instance, by the ionosphere-free observable, this work shows a method of accounting for the remaining (higher order) ionosperic effects, which lead to residual range errors (RREs) in GNSS positioning. An investigation on the higher (second and third) order ionospheric effects (Ion2 and Ion3) in the European region during the high and low periods of the solar cycle is presented in this work. Days are selected for analysis in terms of the planetary K index (measure of disturbances in the geomagnetic field), Kp, which provides a reasonable threshold to include and exclude the effect of geomagnetic storms on the state of the ionosphere. The stations analyzed in this work are selected from the International GNSS Service (IGS) network in Europe, with a geographical distribution in terms of latitude (mid and high latitudes, including the auroral region) and longitude. This work investigates RREs due to Ion2 and Ion3 by using the program Rinex_HO (Marques et al. 2007) which estimates these errors and the total electron content (TEC) along line of sight for each receiver/satellite link. It also creates new GPS observation files that are corrected for these higher order ionospheric effects. Thereby it is possible to assess the effect of correcting the GPS observations for the higher order ionospheric terms in the station coordinates estimation. In this paper the precise point positioning (PPP) approach was used for analysis.

Under perturbed conditions caused by intense solar wind magnetosphere coupling, the ionosphere ma... more Under perturbed conditions caused by intense solar wind magnetosphere coupling, the ionosphere may become highly turbulent and irregularities, typically enhancements or depletions of the electron density embedded in the ambient ionosphere, can form. Such irregularities cause diffraction effects, mainly due to the random fluctuations of the refractive index of the ionosphere, on the satellites signals passing through them and consequent perturbations may cause GNSS navigation errors and outages, abruptly corrupting its performance. Due to the morphology of the geomagnetic field, whose lines are almost vertical at high latitude, polar areas are characterized by the presence of significant ionospheric irregularities having scale sizes ranging from hundreds of kilometers down to a few centimeters and with highly dynamic structures. The understanding of the effect of such phenomena is important, not only in preparation for the next solar cycle (24), whose maximum is expected in 2012, but also for a deeper comprehension of the dynamics of the high-latitude ionosphere. We analyze the fluctuations in the carrier frequency of the radio waves received on the ground, commonly referred to as ionospheric amplitude and phase scintillations, to investigate the physical processes causing them. The phase scintillations on GNSS signals are likely caused by ionospheric irregularities of scale size of hundreds of meters to few kilometers. The amplitude scintillations on GNSS signals are caused by ionospheric irregularities of scale size smaller than the Fresnel radius, which is of the order of hundreds of meters for GNSS signals, typically embedded into the patches. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Institute of Engineering Surveying and Space Geodesy (IESSG) of the University of Nottingham manage the same kind of GISTM (GPS Ionospheric Scintillation and TEC Monitor) receivers over the European high and mid latitude regions and over Antarctica. The GISTM receivers consist of NovAtel OEM4 dual-frequency receivers with special firmware specifically able to compute in near real time the amplitude and the phase scintillation from the GPS L1 frequency signals, and the ionospheric TEC (Total Electron Content) from the GPS L1 and L2 carrier phase signals. From this ground-based network, we are able to capture the dynamics of ionospheric plasma in a wide latitudinal range, from auroral to cusp/cap regions, considering the contribution of both hemispheres, in a bi-polar framework. The data collection started in 2001 and is still in progress. The results, obtained by statistically analyzing a large data sample over a wide period, show the effect of ionospheric disturbances on the GNSS signals, evidencing the different contributions of the auroral and the cusp/cap ionosphere and highlighting possible scintillation scenarios over polar regions.

Nowadays, Global Navigation Satellite Systems (GNSS), especially the Global Positioning System (G... more Nowadays, Global Navigation Satellite Systems (GNSS), especially the Global Positioning System (GPS), represent one of the most used techniques for geodetic positioning. The functional models related with the GNSS observables are better understood than the stochastic models, considering that the development of the latter is more complex. Usually, the stochastic models are used in a simplified form, as the standard models, which assume that all the GNSS observables are statistically independent and have the same variance. However, the stochastic models may be investigated in more detail, considering for example, the effects of ionospheric scintillation. The high latitudes regions experiment strong influence of the ionospheric effects, in particular ionospheric scintillation. Considering the availability of specially designed GNSS receivers that provide ionospheric scintillation parameters, these effects can be mitigated through improved stochastic models. This paper presents the methodology and results from GPS relative and point positioning considering ionospheric scintillation in the stochastic modeling. Two programs have been developed to obtain the results from relative and point positioning: "GPSeq" (currently under development at the FCT/UNESP Sao Paulo State University - Brazil) and "pp_sc" (developed in a collaborative project between FCT/UNESP and Nottingham University - UK). The point positioning approach can be realized considering an epoch by epoch solution and the relative positioning using a Kalman Filter and the LAMBDA method to solve the Double Differences ambiguities. Both programs have the option to estimate the ionospheric residuals as one stochastic process using the white noise or random walk correlation models. In both cases it is also possible to use the L1/L2 ion-free linear combination. The stochastic modeling considering ionospheric scintillation has been implemented based in the models of Conker et al. (2003), following the approach described in Aquino et al. (2008). Data from a network of GPS Ionospheric Scintillation and TEC Monitor (GISTM) receivers set up in Northern Europe was used in the experiments as can be seen in De Franceschi et al. (2006) and Romano et al. (2008). The point positioning results have shown improvements of the order of 5 to 20 percent when considering the proposed stochastic modeling. In relative positioning, improvements of the order of 20 percent have been achieved. These and further results will be discussed in this paper.

Annales Geophysicae, 2011

After removal of the Selective Availability in 2000, the ionosphere became the dominant error sou... more After removal of the Selective Availability in 2000, the ionosphere became the dominant error source for Global Navigation Satellite Systems (GNSS), especially for the high-accuracy (cm-mm) demanding applications like the Precise Point Positioning (PPP) and Real Time Kinematic (RTK) positioning. The common practice of eliminating the ionospheric error, e.g. by the ionosphere free (IF) observable, which is a linear combination of observables on two frequencies such as GPS L1 and L2, accounts for about 99 % of the total ionospheric effect, known as the first order ionospheric effect (Ion1). The remaining 1 % residual range errors (RREs) in the IF observable are due to the higher - second and third, order ionospheric effects, Ion2 and Ion3, respectively. Both terms are related with the electron content along the signal path; moreover Ion2 term is associated with the influence of the geomagnetic field on the ionospheric refractive index and Ion3 with the ray bending effect of the ionosphere, which can cause significant deviation in the ray trajectory (due to strong electron density gradients in the ionosphere) such that the error contribution of Ion3 can exceed that of Ion2 (Kim and Tinin, 2007). The higher order error terms do not cancel out in the (first order) ionospherically corrected observable and as such, when not accounted for, they can degrade the accuracy of GNSS positioning, depending on the level of the solar activity and geomagnetic and ionospheric conditions (Hoque and Jakowski, 2007). Simulation results from early 1990s show that Ion2 and Ion3 would contribute to the ionospheric error budget by less than 1 % of the Ion1 term at GPS frequencies (Datta-Barua et al., 2008). Although the IF observable may provide sufficient accuracy for most GNSS applications, Ion2 and Ion3 need to be considered for higher accuracy demanding applications especially at times of higher solar activity. This paper investigates the higher order ionospheric effects (Ion2 and Ion3, however excluding the ray bending effects associated with Ion3) in the European region in the GNSS positioning considering the precise point positioning (PPP) method. For this purpose observations from four European stations were considered. These observations were taken in four time intervals corresponding to various geophysical conditions: the active and quiet periods of the solar cycle, 2001 and 2006, respectively, excluding the effects of disturbances in the geomagnetic field (i.e. geomagnetic storms), as well as the years of 2001 and 2003, this time including the impact of geomagnetic disturbances. The program RINEX_HO (Marques et al., 2011) was used to calculate the magnitudes of Ion2 and Ion3 on the range measurements as well as the total electron content (TEC) observed on each receiver-satellite link. The program also corrects the GPS observation files for Ion2 and Ion3; thereafter it is possible to perform PPP with both the original and corrected GPS observation files to analyze the impact of the higher order ionospheric error terms excluding the ray bending effect which may become significant especially at low elevation angles (Ioannides and Strangeways, 2002) on the estimated station coordinates.

Ionospheric effects are one of the main barriers to achieve high accuracy GNSS positioning and na... more Ionospheric effects are one of the main barriers to achieve high accuracy GNSS positioning and navigation, including precise point positioning (PPP), relative carrier phase based positioning and Differential GNSS (DGNSS), as well as Satellite Based Augmentation System (SBAS). Measurements made on two different frequencies allow the correction of the first order ionospheric effects by means of the widely used ionospheric-free linear combination. However, through this process, the second and third order ionospheric effects, which may cause errors of the order of centimeters in the GNSS measurements, still remain unmodeled. Furthermore, effects such as ionospheric scintillations, caused by time-varying electron density irregularities in the ionosphere that occur more often at equatorial and high latitudes, in particular during solar maxima, also degrade the quality of positioning and navigation. Several approaches have been proposed to mitigate ionospheric effects on GNSS, which in general involve improvements to the functional and/or stochastic models of the Least Squares Adjustment used to estimate position. In this contribution we present techniques recently developed that combine both functional and stochastic models, in order to reduce such effects and their propagation on positioning quality. We present results of the application of these developments on GNSS PPP and relative positioning.

Electronics in Marine International Symposium, 2007

The effect of the ionosphere on the signals of global navigation satellite systems (GNSS), such a... more The effect of the ionosphere on the signals of global navigation satellite systems (GNSS), such as the global positionig system (GPS) and the proposed European Galileo, is dependent on the ionospheric electron density, given by its total electron content (TEC). Ionospheric time-varying density irregularities may cause scintillations, which are fluctuations in phase and amplitude of the signals. Scintillations occur more often at equatorial and high latitudes. They can degrade navigation and positioning accuracy and may cause loss of signal tracking, disrupting safety-critical applications, such as marine navigation and civil aviation. This paper addresses the results of initial research carried out on two fronts that are relevant to GNSS users if they are to counter ionospheric scintillations, i.e. forecasting and mitigating their effects. On the forecasting front, the dynamics of scintillation occurrence were analysed during the severe ionospheric storm that took place on the evening of 30 October 2003, using data from a network of GPS ionospheric scintillation and TEC monitor (GISTM) receivers set up in Northern Europe. Previous results [I] indicated that GPS scintillations in that region can originate from ionospheric plasma structures from the American sector. In this paper we describe experiments that enabled confirmation of those findings. On the mitigation front we used the variance of the output error of the GPS receiver DLL (delay locked loop) to modify the least squares stochastic model applied by an ordinary receiver to compute position. This error was modelled, as a function of the S4 amplitude scintillation index measured by the GISTM receivers. An improvement of up to 21% in relative positioning accuracy was achieved with this technique.

2014 IEEE/ION Position, Location and Navigation Symposium - PLANS 2014, 2014

ABSTRACT Global Navigation Satellite Systems (GNSS) signals traversing small scale irregularities... more ABSTRACT Global Navigation Satellite Systems (GNSS) signals traversing small scale irregularities present in the ionosphere may experience fast and unpredictable fluctuations of their amplitude and phase. This phenomenon can seriously affect the performance of a GNSS receiver, decreasing the position accuracy and, in the worst scenario, also inducing a total loss of lock on the satellite signals. This paper proposes an adaptive Kalman Filter (KF) based Phase Locked Loop (PLL) to cope with high dynamics and strong fading induced by ionospheric scintillation events. The KF based PLL self-tunes the covariance matrix according to the detected scintillation level. Furthermore, the paper shows that radio frequency interference can affect the reliable computation of scintillation parameters. In order to mitigate the effect of the interference signal and filter it out, a wavelet based interference mitigation algorithm has been also implemented. The latter is able to retrieve genuine scintillation indices that otherwise would be corrupted by radio frequency interference.

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014

Drifting ionospheric electron density irregularities may lead to the scintillation of transionosp... more Drifting ionospheric electron density irregularities may lead to the scintillation of transionospheric radio waves, as in the case of signals broadcast from artificial satellites. Scintillations can not only degrade signal quality but also cause receiver loss of lock on GNSS satellites, therefore posing a major threat to GNSS based applications demanding high levels of accuracy, availability and integrity, including EGNOS-based applications notably in low latitude areas. The problem is particularly acute in Latin America and will be further amplified with the next solar maximum, predicted for 2013. The CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project, led by Septentrio NV and co-funded by the European GNSS Supervisory Authority (GSA) through the European 7th Framework Program, will tackle this problem. The aim of the CIGALA project is to develop ionospheric scintillation mitigation countermeasures to be implemented in Septentrio's professional multi-frequency multi-constellation GNSS receivers and tested in Latin America. The project will leverage research and development activities coordinated between European and Brazilian experts and will involve a wide scale ionospheric measurement and test campaigns that will be conducted in Brazil with the support of several local academic and industrial partners. Details on the objectives, current status, and workflow of the project will be presented and discussed.

Annales Geophysicae, 2009

Ionospheric scintillation may present significant effects on GPS, mainly in equatorial and aurora... more Ionospheric scintillation may present significant effects on GPS, mainly in equatorial and auroral regions, and during times of high solar flux. In the auroral regions scintillation occurrence mostly relates to geomagnetic activity and can affect GNSS users even at sub-auroral (and potentially mid-latitude) regions, with impact ranging from degradation of accuracy to loss of signal tracking. Recent work at Nottingham investigated the impact of ionospheric scintillation and Total Electron Content (TEC) gradients on GNSS users, through a network of four GPS Ionospheric Scintillation Monitors set up in the UK and Norway. Statistical analyses of the scintillation and TEC data, aiming to characterise ionospheric scintillation over Northern Europe, were also carried out. Critically to GNSS users these studies covered, in particular, aspects of availability and integrity, through the assessment of occurrence of loss of lock on GPS satellites due to high scintillation levels. However, accuracy aspects have also been investigated, through the analysis of standalone GPS, DGPS, EGNOS aided DGPS and carrier phase errors, which have been correlated with observed scintillation levels and geomagnetic indices. Horizontal errors in GPS C/A code point-positioning were seen to correlate to enhancement in the background TEC observed during times of occurrence of high scintillation. DGPS positioning accuracy was seen to be affected by TEC gradients occurring at auroral and sub-auroral latitudes, especially under enhanced geomagnetic activity. Missing corrections in the EGNOS ionospheric grid during periods of occurrence of high phase scintillation suggested an inability of the EGNOS reference stations to track one or both of the GPS signals of some satellites. In this paper the main focus is on carrier phase positioning experiments, which revealed an increase in the measurement noise and positioning accuracy degradation significantly correlated with the occurrence of high phase scintillation.

Journal of Geodesy, 2009

Ionospheric scintillations are caused by time- varying electron density irregularities in the ion... more Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and \({\sigma_{\varphi}}\) represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.

Position Location and Navigation IEEE Symposium, 2006

International Association of Geodesy Symposia, 2011

ABSTRACT The Global Positioning System (GPS) transmits signals in two frequencies. It allows the ... more ABSTRACT The Global Positioning System (GPS) transmits signals in two frequencies. It allows the correction of the first order ionospheric effect by using the ionosphere free combination. However, the second and third order ionospheric effects, which combined may cause errors of the order of centimeters in the GPS measurements, still remain. In this paper the second and third order ionospheric effects, which were taken into account in the GPS data processing in the Brazilian region, were investigated. The corrected and not corrected GPS data from these effects were processed in the relative and precise point positioning (PPP) approaches, respectively, using Bernese V5.0 software and the PPP software (GPSPPP) from NRCAN (Natural Resources Canada). The second and third order corrections were applied in the GPS data using an in-house software that is capable of reading a RINEX file and applying the corrections to the GPS observables, creating a corrected RINEX file. For the relative processing case, a Brazilian network with long baselines was processed in a daily solution considering a period of approximately one year. For the PPP case, the processing was accomplished using data collected by the IGS FORT station considering the period from 2001 to 2006 and a seasonal analysis was carried out, showing a semi-annual and an annual variation in the vertical component. In addition, a geographical variation analysis in the PPP for the Brazilian region has confirmed that the equatorial regions are more affected by the second and third order ionospheric effects than other regions.

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014

In addition to the common practice of eliminating the (first order) ionospheric effect, for insta... more In addition to the common practice of eliminating the (first order) ionospheric effect, for instance, by the ionosphere-free observable, this work shows a method of accounting for the remaining (higher order) ionosperic effects, which lead to residual range errors (RREs) in GNSS positioning. An investigation on the higher (second and third) order ionospheric effects (Ion2 and Ion3) in the European region during the high and low periods of the solar cycle is presented in this work. Days are selected for analysis in terms of the planetary K index (measure of disturbances in the geomagnetic field), Kp, which provides a reasonable threshold to include and exclude the effect of geomagnetic storms on the state of the ionosphere. The stations analyzed in this work are selected from the International GNSS Service (IGS) network in Europe, with a geographical distribution in terms of latitude (mid and high latitudes, including the auroral region) and longitude. This work investigates RREs due to Ion2 and Ion3 by using the program Rinex_HO (Marques et al. 2007) which estimates these errors and the total electron content (TEC) along line of sight for each receiver/satellite link. It also creates new GPS observation files that are corrected for these higher order ionospheric effects. Thereby it is possible to assess the effect of correcting the GPS observations for the higher order ionospheric terms in the station coordinates estimation. In this paper the precise point positioning (PPP) approach was used for analysis.

Under perturbed conditions caused by intense solar wind magnetosphere coupling, the ionosphere ma... more Under perturbed conditions caused by intense solar wind magnetosphere coupling, the ionosphere may become highly turbulent and irregularities, typically enhancements or depletions of the electron density embedded in the ambient ionosphere, can form. Such irregularities cause diffraction effects, mainly due to the random fluctuations of the refractive index of the ionosphere, on the satellites signals passing through them and consequent perturbations may cause GNSS navigation errors and outages, abruptly corrupting its performance. Due to the morphology of the geomagnetic field, whose lines are almost vertical at high latitude, polar areas are characterized by the presence of significant ionospheric irregularities having scale sizes ranging from hundreds of kilometers down to a few centimeters and with highly dynamic structures. The understanding of the effect of such phenomena is important, not only in preparation for the next solar cycle (24), whose maximum is expected in 2012, but also for a deeper comprehension of the dynamics of the high-latitude ionosphere. We analyze the fluctuations in the carrier frequency of the radio waves received on the ground, commonly referred to as ionospheric amplitude and phase scintillations, to investigate the physical processes causing them. The phase scintillations on GNSS signals are likely caused by ionospheric irregularities of scale size of hundreds of meters to few kilometers. The amplitude scintillations on GNSS signals are caused by ionospheric irregularities of scale size smaller than the Fresnel radius, which is of the order of hundreds of meters for GNSS signals, typically embedded into the patches. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Institute of Engineering Surveying and Space Geodesy (IESSG) of the University of Nottingham manage the same kind of GISTM (GPS Ionospheric Scintillation and TEC Monitor) receivers over the European high and mid latitude regions and over Antarctica. The GISTM receivers consist of NovAtel OEM4 dual-frequency receivers with special firmware specifically able to compute in near real time the amplitude and the phase scintillation from the GPS L1 frequency signals, and the ionospheric TEC (Total Electron Content) from the GPS L1 and L2 carrier phase signals. From this ground-based network, we are able to capture the dynamics of ionospheric plasma in a wide latitudinal range, from auroral to cusp/cap regions, considering the contribution of both hemispheres, in a bi-polar framework. The data collection started in 2001 and is still in progress. The results, obtained by statistically analyzing a large data sample over a wide period, show the effect of ionospheric disturbances on the GNSS signals, evidencing the different contributions of the auroral and the cusp/cap ionosphere and highlighting possible scintillation scenarios over polar regions.

Nowadays, Global Navigation Satellite Systems (GNSS), especially the Global Positioning System (G... more Nowadays, Global Navigation Satellite Systems (GNSS), especially the Global Positioning System (GPS), represent one of the most used techniques for geodetic positioning. The functional models related with the GNSS observables are better understood than the stochastic models, considering that the development of the latter is more complex. Usually, the stochastic models are used in a simplified form, as the standard models, which assume that all the GNSS observables are statistically independent and have the same variance. However, the stochastic models may be investigated in more detail, considering for example, the effects of ionospheric scintillation. The high latitudes regions experiment strong influence of the ionospheric effects, in particular ionospheric scintillation. Considering the availability of specially designed GNSS receivers that provide ionospheric scintillation parameters, these effects can be mitigated through improved stochastic models. This paper presents the methodology and results from GPS relative and point positioning considering ionospheric scintillation in the stochastic modeling. Two programs have been developed to obtain the results from relative and point positioning: "GPSeq" (currently under development at the FCT/UNESP Sao Paulo State University - Brazil) and "pp_sc" (developed in a collaborative project between FCT/UNESP and Nottingham University - UK). The point positioning approach can be realized considering an epoch by epoch solution and the relative positioning using a Kalman Filter and the LAMBDA method to solve the Double Differences ambiguities. Both programs have the option to estimate the ionospheric residuals as one stochastic process using the white noise or random walk correlation models. In both cases it is also possible to use the L1/L2 ion-free linear combination. The stochastic modeling considering ionospheric scintillation has been implemented based in the models of Conker et al. (2003), following the approach described in Aquino et al. (2008). Data from a network of GPS Ionospheric Scintillation and TEC Monitor (GISTM) receivers set up in Northern Europe was used in the experiments as can be seen in De Franceschi et al. (2006) and Romano et al. (2008). The point positioning results have shown improvements of the order of 5 to 20 percent when considering the proposed stochastic modeling. In relative positioning, improvements of the order of 20 percent have been achieved. These and further results will be discussed in this paper.

Annales Geophysicae, 2011

After removal of the Selective Availability in 2000, the ionosphere became the dominant error sou... more After removal of the Selective Availability in 2000, the ionosphere became the dominant error source for Global Navigation Satellite Systems (GNSS), especially for the high-accuracy (cm-mm) demanding applications like the Precise Point Positioning (PPP) and Real Time Kinematic (RTK) positioning. The common practice of eliminating the ionospheric error, e.g. by the ionosphere free (IF) observable, which is a linear combination of observables on two frequencies such as GPS L1 and L2, accounts for about 99 % of the total ionospheric effect, known as the first order ionospheric effect (Ion1). The remaining 1 % residual range errors (RREs) in the IF observable are due to the higher - second and third, order ionospheric effects, Ion2 and Ion3, respectively. Both terms are related with the electron content along the signal path; moreover Ion2 term is associated with the influence of the geomagnetic field on the ionospheric refractive index and Ion3 with the ray bending effect of the ionosphere, which can cause significant deviation in the ray trajectory (due to strong electron density gradients in the ionosphere) such that the error contribution of Ion3 can exceed that of Ion2 (Kim and Tinin, 2007). The higher order error terms do not cancel out in the (first order) ionospherically corrected observable and as such, when not accounted for, they can degrade the accuracy of GNSS positioning, depending on the level of the solar activity and geomagnetic and ionospheric conditions (Hoque and Jakowski, 2007). Simulation results from early 1990s show that Ion2 and Ion3 would contribute to the ionospheric error budget by less than 1 % of the Ion1 term at GPS frequencies (Datta-Barua et al., 2008). Although the IF observable may provide sufficient accuracy for most GNSS applications, Ion2 and Ion3 need to be considered for higher accuracy demanding applications especially at times of higher solar activity. This paper investigates the higher order ionospheric effects (Ion2 and Ion3, however excluding the ray bending effects associated with Ion3) in the European region in the GNSS positioning considering the precise point positioning (PPP) method. For this purpose observations from four European stations were considered. These observations were taken in four time intervals corresponding to various geophysical conditions: the active and quiet periods of the solar cycle, 2001 and 2006, respectively, excluding the effects of disturbances in the geomagnetic field (i.e. geomagnetic storms), as well as the years of 2001 and 2003, this time including the impact of geomagnetic disturbances. The program RINEX_HO (Marques et al., 2011) was used to calculate the magnitudes of Ion2 and Ion3 on the range measurements as well as the total electron content (TEC) observed on each receiver-satellite link. The program also corrects the GPS observation files for Ion2 and Ion3; thereafter it is possible to perform PPP with both the original and corrected GPS observation files to analyze the impact of the higher order ionospheric error terms excluding the ray bending effect which may become significant especially at low elevation angles (Ioannides and Strangeways, 2002) on the estimated station coordinates.

Ionospheric effects are one of the main barriers to achieve high accuracy GNSS positioning and na... more Ionospheric effects are one of the main barriers to achieve high accuracy GNSS positioning and navigation, including precise point positioning (PPP), relative carrier phase based positioning and Differential GNSS (DGNSS), as well as Satellite Based Augmentation System (SBAS). Measurements made on two different frequencies allow the correction of the first order ionospheric effects by means of the widely used ionospheric-free linear combination. However, through this process, the second and third order ionospheric effects, which may cause errors of the order of centimeters in the GNSS measurements, still remain unmodeled. Furthermore, effects such as ionospheric scintillations, caused by time-varying electron density irregularities in the ionosphere that occur more often at equatorial and high latitudes, in particular during solar maxima, also degrade the quality of positioning and navigation. Several approaches have been proposed to mitigate ionospheric effects on GNSS, which in general involve improvements to the functional and/or stochastic models of the Least Squares Adjustment used to estimate position. In this contribution we present techniques recently developed that combine both functional and stochastic models, in order to reduce such effects and their propagation on positioning quality. We present results of the application of these developments on GNSS PPP and relative positioning.

Electronics in Marine International Symposium, 2007

The effect of the ionosphere on the signals of global navigation satellite systems (GNSS), such a... more The effect of the ionosphere on the signals of global navigation satellite systems (GNSS), such as the global positionig system (GPS) and the proposed European Galileo, is dependent on the ionospheric electron density, given by its total electron content (TEC). Ionospheric time-varying density irregularities may cause scintillations, which are fluctuations in phase and amplitude of the signals. Scintillations occur more often at equatorial and high latitudes. They can degrade navigation and positioning accuracy and may cause loss of signal tracking, disrupting safety-critical applications, such as marine navigation and civil aviation. This paper addresses the results of initial research carried out on two fronts that are relevant to GNSS users if they are to counter ionospheric scintillations, i.e. forecasting and mitigating their effects. On the forecasting front, the dynamics of scintillation occurrence were analysed during the severe ionospheric storm that took place on the evening of 30 October 2003, using data from a network of GPS ionospheric scintillation and TEC monitor (GISTM) receivers set up in Northern Europe. Previous results [I] indicated that GPS scintillations in that region can originate from ionospheric plasma structures from the American sector. In this paper we describe experiments that enabled confirmation of those findings. On the mitigation front we used the variance of the output error of the GPS receiver DLL (delay locked loop) to modify the least squares stochastic model applied by an ordinary receiver to compute position. This error was modelled, as a function of the S4 amplitude scintillation index measured by the GISTM receivers. An improvement of up to 21% in relative positioning accuracy was achieved with this technique.

2014 IEEE/ION Position, Location and Navigation Symposium - PLANS 2014, 2014

ABSTRACT Global Navigation Satellite Systems (GNSS) signals traversing small scale irregularities... more ABSTRACT Global Navigation Satellite Systems (GNSS) signals traversing small scale irregularities present in the ionosphere may experience fast and unpredictable fluctuations of their amplitude and phase. This phenomenon can seriously affect the performance of a GNSS receiver, decreasing the position accuracy and, in the worst scenario, also inducing a total loss of lock on the satellite signals. This paper proposes an adaptive Kalman Filter (KF) based Phase Locked Loop (PLL) to cope with high dynamics and strong fading induced by ionospheric scintillation events. The KF based PLL self-tunes the covariance matrix according to the detected scintillation level. Furthermore, the paper shows that radio frequency interference can affect the reliable computation of scintillation parameters. In order to mitigate the effect of the interference signal and filter it out, a wavelet based interference mitigation algorithm has been also implemented. The latter is able to retrieve genuine scintillation indices that otherwise would be corrupted by radio frequency interference.

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014

Drifting ionospheric electron density irregularities may lead to the scintillation of transionosp... more Drifting ionospheric electron density irregularities may lead to the scintillation of transionospheric radio waves, as in the case of signals broadcast from artificial satellites. Scintillations can not only degrade signal quality but also cause receiver loss of lock on GNSS satellites, therefore posing a major threat to GNSS based applications demanding high levels of accuracy, availability and integrity, including EGNOS-based applications notably in low latitude areas. The problem is particularly acute in Latin America and will be further amplified with the next solar maximum, predicted for 2013. The CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project, led by Septentrio NV and co-funded by the European GNSS Supervisory Authority (GSA) through the European 7th Framework Program, will tackle this problem. The aim of the CIGALA project is to develop ionospheric scintillation mitigation countermeasures to be implemented in Septentrio's professional multi-frequency multi-constellation GNSS receivers and tested in Latin America. The project will leverage research and development activities coordinated between European and Brazilian experts and will involve a wide scale ionospheric measurement and test campaigns that will be conducted in Brazil with the support of several local academic and industrial partners. Details on the objectives, current status, and workflow of the project will be presented and discussed.

Annales Geophysicae, 2009

Ionospheric scintillation may present significant effects on GPS, mainly in equatorial and aurora... more Ionospheric scintillation may present significant effects on GPS, mainly in equatorial and auroral regions, and during times of high solar flux. In the auroral regions scintillation occurrence mostly relates to geomagnetic activity and can affect GNSS users even at sub-auroral (and potentially mid-latitude) regions, with impact ranging from degradation of accuracy to loss of signal tracking. Recent work at Nottingham investigated the impact of ionospheric scintillation and Total Electron Content (TEC) gradients on GNSS users, through a network of four GPS Ionospheric Scintillation Monitors set up in the UK and Norway. Statistical analyses of the scintillation and TEC data, aiming to characterise ionospheric scintillation over Northern Europe, were also carried out. Critically to GNSS users these studies covered, in particular, aspects of availability and integrity, through the assessment of occurrence of loss of lock on GPS satellites due to high scintillation levels. However, accuracy aspects have also been investigated, through the analysis of standalone GPS, DGPS, EGNOS aided DGPS and carrier phase errors, which have been correlated with observed scintillation levels and geomagnetic indices. Horizontal errors in GPS C/A code point-positioning were seen to correlate to enhancement in the background TEC observed during times of occurrence of high scintillation. DGPS positioning accuracy was seen to be affected by TEC gradients occurring at auroral and sub-auroral latitudes, especially under enhanced geomagnetic activity. Missing corrections in the EGNOS ionospheric grid during periods of occurrence of high phase scintillation suggested an inability of the EGNOS reference stations to track one or both of the GPS signals of some satellites. In this paper the main focus is on carrier phase positioning experiments, which revealed an increase in the measurement noise and positioning accuracy degradation significantly correlated with the occurrence of high phase scintillation.

Journal of Geodesy, 2009

Ionospheric scintillations are caused by time- varying electron density irregularities in the ion... more Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and \({\sigma_{\varphi}}\) represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.

Position Location and Navigation IEEE Symposium, 2006

International Association of Geodesy Symposia, 2011

ABSTRACT The Global Positioning System (GPS) transmits signals in two frequencies. It allows the ... more ABSTRACT The Global Positioning System (GPS) transmits signals in two frequencies. It allows the correction of the first order ionospheric effect by using the ionosphere free combination. However, the second and third order ionospheric effects, which combined may cause errors of the order of centimeters in the GPS measurements, still remain. In this paper the second and third order ionospheric effects, which were taken into account in the GPS data processing in the Brazilian region, were investigated. The corrected and not corrected GPS data from these effects were processed in the relative and precise point positioning (PPP) approaches, respectively, using Bernese V5.0 software and the PPP software (GPSPPP) from NRCAN (Natural Resources Canada). The second and third order corrections were applied in the GPS data using an in-house software that is capable of reading a RINEX file and applying the corrections to the GPS observables, creating a corrected RINEX file. For the relative processing case, a Brazilian network with long baselines was processed in a daily solution considering a period of approximately one year. For the PPP case, the processing was accomplished using data collected by the IGS FORT station considering the period from 2001 to 2006 and a seasonal analysis was carried out, showing a semi-annual and an annual variation in the vertical component. In addition, a geographical variation analysis in the PPP for the Brazilian region has confirmed that the equatorial regions are more affected by the second and third order ionospheric effects than other regions.

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014

Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, 2014