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The customary procedure to compute the rate of total electron content index (ROTI) presents some limitations when using the geometry-free (GF) combination of Global Positioning System (GPS) L1 and L2 carriers tracked by geodetic receivers. First, the effect of the tracking strategy implemented by each receiver manufacturer to obtain the L2 carrier from code-less observations. Second, the impact of frequent cycle slips on the L2 carrier. These limitations hinder the monitoring and characterization of ionospheric scintillation. To overcome them, the present study proposes the calculation of ROTI from the individual (uncombined) L1 carrier-phase, ROTIL1, using the Geodetic Detrending (GD) post-processing methodology, in contrast to the conventional GF combination, ROTIGF. The analysis of the entire year 2020 shows that those two aforementioned limitations produce inconsistent ROTIGF values measured by pairs of close receivers from different manufacturers. In contrast, the distribution of ROTIL1 values shows a full consistency between different receivers, being significantly less affected by cycle slips and allowing a valid and well-grounded identification of scintillation. The study concludes that ROTIL1, calculated using a 60 s window from geodetic receivers operating at 1 Hz, provides a robust tool to monitor and characterize ionospheric scintillation world-wide and regardless of the type of receiver. In particular, a ROTIL1 threshold of 1.8 TECU/min is established as the minimum level of detectable scintillation in 2020, a year of low solar activity. The most intense scintillation periods in high-latitude regions are statistically characterized by the newly proposed ROTIL1., This work was supported in part by MCIN/AEI/10.13039/501100011033/Fondo Europeo de Desarrollo Regional (FEDER), Unión Europea (UE), under Project CNS2022-135383 and Project PID2022-138485OB-I00; and in part by European Space Agency (ESA) through the Open Space Innovation Platform program under Contract 4000137762/22/NL/GLC/ov, Peer Reviewed, Postprint (published version)

The effects of the Sun on the Earth’s upper atmosphere, known as the ionosphere, and their implications for satellite-ground communication and ground infrastructures are of significant importance not only for the aerospace industry but also for the communications field. In this thesis, a study on Solar Flares (SF) has been conducted, which are sudden emissions of electromagnetic radiation from the Sun lasting from minutes to hours. To investigate this phenomenon, a technique based on Global Navigation Satellite System (GNSS) has been employed, detecting one of the Solar Flares effects (SFe), which is specifically the enhancement of Total Electron Content (TEC) in the ionosphere, resulting in delays in GNSS signals. By understanding the typical behavior of the ionosphere, sudden variations in TEC can be discriminated, allowing for the establishment of thresholds to determine when a Solar Flare has occurred. The GNSS Solar Flare detector (GNSS-SF) had been previously tested over an 11 year period. However, in the year 2023, coinciding with a solar maximum, the objective of this thesis is to demonstrate that increasing the sampling rate from 30 seconds to 5 seconds improves the performance of the GNSS-SF. This was achieved through a statistical analysis spanning almost 330 days between the two implementations, comparing GNSS-SF detections with those provided by the Geostationary Operational Environmental Satellites (GOES). It has been concluded that the implementation at a 5-second rate yields superior results and detects Solar Flares that the 30 second implementation fails to detect.

Tesi en modalitat de compendi de publicacions, (English) This document presents the collection of four manuscripts published during my Doctoral academic formation, which main goal has been the real-time implementation of tools to monitor the ionosphere using Global Navigation Satellite System (GNSS) signals. Despite the fact that there is a vast literature on ionospheric modelling, the state-of-the-art becomes narrow when referring to real-time developments, especially fulfilling precise requirements on accuracy, performance, coverage, and confidence in the generated products. The main contribution of this work to the scientific community is the deployment of ionospheric-related products to monitor in real-time the state of the ionosphere. The first and second publications targeted the implementation of a novel strategy based on a definition of a GNSS Solar Flare (SF) monitor to automatically confirm Solar Flare Effects (Sfe) in geomagnetism. In the first scientific article, it is inspected the methodology used to fine-tune (adapt) a SF monitor, working with an eleven years period of data to statistically correlate detected SF using GNSS signals with respect to SFe. The results demonstrated that the proposed GNSS Solar Flare monitor can confirm Sfe events when traditional Sfe detectors are not able to respond categorically. The second publication details the methodological approach for defining the proposed GNSS Solar Flare monitor, focusing on the theoretical formulation of the Slant Total Electron Content (STEC) obtained. The third contribution used GNSS signals to detect the presence of Medium Scale Travelling Ionospheric Disturbance (MSTID) within a network of permanent GNSS stations that provide the high-accuracy positioning service known as Network-Real-Time Kinematics (NRTK). The effects of a MSTID are characterized in terms of fluctuations in the electron density in the iono- sphere, experienced differently by each one of the GNSS stations used as reference receivers within the NRTK, and resulting in a d, (Español) Este documento se centra en la implementación de herramientas para monitorizar la ionosfera terrestre por medio del uso de señales GNSS. Si bien se puede encontrar una literatura bastante amplia sobre el modelado de la ionosfera, son reducidos los trabajos relacionados con aplicaciones en tiempo real, particularmente si se busca cumplir con requerimientos específicos vinculados con la precisión, el rendimiento, el cubrimiento y la certidumbre de los productos generados. La principal contribución de esta tesis doctoral es la generación en tiempo real de productos para monitorizar el estado de la ionosfera. Las primeras dos publicaciones se centran en el desarrollo de una nueva metodología fundamentada en un detector de Fulguraciones Solares (SF) basado en mediciones GNSS para la confirmación de los efectos de un SF (SFe) detectados por sensores geomagnéticos. En el primer artículo, se expone el procedimiento para el diseño y ajuste del detector GNSS de SF (GNSS-SF), empleando para ello un periodo de once años de datos con los cuales correlacionar estadísticamente SF detectados por medio del detector GNSS y los eventos SFe detectados en magnetismo. Los resultados demuestran que el detector GNSS-SF es capaz de confirmar eventos SFe cuando estos últimos no son categóricamente validados por los instrumentos magnéticos. La segunda publicación detalla el enfoque metodológico desarrollado para definir el detector GNSS-SF que se propone en el artículo. La tercera publicación emplea señales GNSS para detectar la presencia de perturbaciones de escala media que se desplazan en la ionosfera (MSTID) en una red de estaciones GNSS fijas que brindan servicios de posicionamiento muy preciso (NRTK). Los efectos de una MSTID pueden ser caracterizados por medio de la fluctuación en el contenido electrónico de la ionosfera, experimentado de manera diferente por cada una de las estaciones GNSS usadas como referencia dentro del servicio NRTK, ocasionando una degradación en el pos, Postprint (published version)

Tesi en modalitat de compendi de publicacions, (English) This document presents the collection of four manuscripts published during my Doctoral academic formation, which main goal has been the real-time implementation of tools to monitor the ionosphere using Global Navigation Satellite System (GNSS) signals. Despite the fact that there is a vast literature on ionospheric modelling, the state-of-the-art becomes narrow when referring to real-time developments, especially fulfilling precise requirements on accuracy, performance, coverage, and confidence in the generated products. The main contribution of this work to the scientific community is the deployment of ionospheric-related products to monitor in real-time the state of the ionosphere. The first and second publications targeted the implementation of a novel strategy based on a definition of a GNSS Solar Flare (SF) monitor to automatically confirm Solar Flare Effects (Sfe) in geomagnetism. In the first scientific article, it is inspected the methodology used to fine-tune (adapt) a SF monitor, working with an eleven years period of data to statistically correlate detected SF using GNSS signals with respect to SFe. The results demonstrated that the proposed GNSS Solar Flare monitor can confirm Sfe events when traditional Sfe detectors are not able to respond categorically. The second publication details the methodological approach for defining the proposed GNSS Solar Flare monitor, focusing on the theoretical formulation of the Slant Total Electron Content (STEC) obtained. The third contribution used GNSS signals to detect the presence of Medium Scale Travelling Ionospheric Disturbance (MSTID) within a network of permanent GNSS stations that provide the high-accuracy positioning service known as Network-Real-Time Kinematics (NRTK). The effects of a MSTID are characterized in terms of fluctuations in the electron density in the iono- sphere, experienced differently by each one of the GNSS stations used as reference receivers within the NRTK, and resulting in a d, (Español) Este documento se centra en la implementación de herramientas para monitorizar la ionosfera terrestre por medio del uso de señales GNSS. Si bien se puede encontrar una literatura bastante amplia sobre el modelado de la ionosfera, son reducidos los trabajos relacionados con aplicaciones en tiempo real, particularmente si se busca cumplir con requerimientos específicos vinculados con la precisión, el rendimiento, el cubrimiento y la certidumbre de los productos generados. La principal contribución de esta tesis doctoral es la generación en tiempo real de productos para monitorizar el estado de la ionosfera. Las primeras dos publicaciones se centran en el desarrollo de una nueva metodología fundamentada en un detector de Fulguraciones Solares (SF) basado en mediciones GNSS para la confirmación de los efectos de un SF (SFe) detectados por sensores geomagnéticos. En el primer artículo, se expone el procedimiento para el diseño y ajuste del detector GNSS de SF (GNSS-SF), empleando para ello un periodo de once años de datos con los cuales correlacionar estadísticamente SF detectados por medio del detector GNSS y los eventos SFe detectados en magnetismo. Los resultados demuestran que el detector GNSS-SF es capaz de confirmar eventos SFe cuando estos últimos no son categóricamente validados por los instrumentos magnéticos. La segunda publicación detalla el enfoque metodológico desarrollado para definir el detector GNSS-SF que se propone en el artículo. La tercera publicación emplea señales GNSS para detectar la presencia de perturbaciones de escala media que se desplazan en la ionosfera (MSTID) en una red de estaciones GNSS fijas que brindan servicios de posicionamiento muy preciso (NRTK). Los efectos de una MSTID pueden ser caracterizados por medio de la fluctuación en el contenido electrónico de la ionosfera, experimentado de manera diferente por cada una de las estaciones GNSS usadas como referencia dentro del servicio NRTK, ocasionando una degradación en el pos, Postprint (published version)

This technical note provides a perspective of the results obtained in the first real-time campaign of the IONO4HAS model conducted between 49 days of the year 2022. The goal is to quantify the performance in terms of operation and positioning., Preprint

Global Navigation Satellite System signals have been used for years to study high-frequency fluctuations (f>0.1Hz)intheionosphere. The customary procedure uses the geometry-free (GF) combination ofL1andL2carriers, for which it is necessaryto acquire theL2GPS signal. Initially,L2had to be acquired from a codeless signal, L2P(Y), using several techniques, someof them requiring the aid ofL1. New GPS satellites transmit the newC2civil code, which can be used to acquire directlyL2,i.e. L2C. Several publications have reported differences in the GF combination when it is computed from L2P(Y) or L2C.Using two ionospheric scintillation monitoring receivers (ISMRs), these differences were shown to be related to how theyacquireL2, i.e. if the receiver acquiresL2with theL1aid. However, ISMRs are scarce, so the extension of such a study is notstraightforward. The present work uses the geodetic detrending technique to identify whether a conventional geodetic-gradereceiver acquiresL2with the aid ofL1. The study employs six different receiver types with measurements stored in RINEXformats version 2 and 3. In both formats, we are able to identify ifL2signal is acquired withL1aid. In this way, we showthat some receiver types heavily underestimate high-frequency ionospheric fluctuations when using the GF combination.Our results show that the ionosphere-free combination of these carrier phases is not free from high-frequency ionosphericfluctuations, but in some receivers, almost 90% of the high-frequency effects inL1remain in such combination., Postprint (published version)

The first phase of the RT-WMIS activity comprises two Work Packages (WPs). WP 10 is aimed at performing a revision of the state-of-the-art RT products and requirements for the RT-WMIS activity. On the other hand, WP 20 is devoted to carry out the software design for the subsequent implementation of the RT-WMIS tool. The present technical note (TN-1) describes the activities developed in the framework of WP 10., Preprint

The Tonga volcano eruption at 04:15 UT on 2022-01-15 released enormous amounts of energy into the atmosphere, causing very significant geophysical variations not only in the immediate proximity of the epicenter but also globally across the whole atmosphere. The released energy generates perturbations on the electronic density present on the atmosphere, known as Traveling Ionospheric Disturbances (TIDs), i.e., wave-like propagating irregularities that alter the electron density environment and play an important role spreading radio signals propagating through the ionosphere. Geomagnetic and ionospheric phenomena affect communications, GNSS, and other systems on which our technological society depends. The present final master’s thesis aims to analyse some of the most relevant TIDs characteristics (such as propagation speed, time, etc.) as a function of the distance from the eruption in Global Navigation Satellite System (GNSS) signals. First, evaluate Receiver Independent Exchange Format (RINEX) files, i.e., a data interchange format for raw satellite navigation system data, and then, compute Prefit Residuals, i.e., the input data for the navigation equations. With this data, TIDs will be analysed with a series of indices that sample in post-process the disturbance of the ionosphere, developed by Research group of Astronomy and GEomatics of the Universitat Politecnica de Catalunya (gAGE)., La erupción del volcán Tonga a las 04:15 UT del 15-01-2022 liberó enormes cantidades de energía a la atmósfera, desencadenando variaciones geofísicas muy significativas no solo en la proximidad inmediata del epicentro sino también a nivel mundial en toda la atmósfera. Estas variaciones se conocen como perturbaciones ionosféricas viajeras (TID, por sus si-glas en inglés), es decir, irregularidades de propagación en forma de onda que alteran el entorno de densidad de electrones y desempeñan un papel importante en la propagación de señales de radio que se propagan a través de la ionosfera. Los fenómenos geomagnéticos e ionosféricos afectan a las comunicaciones, GNSS y otros sistemas de los que depende nuestra sociedad tecnológica. La presente tesis final de máster tiene como objetivo analizar algunas de las características más relevantes de los TIDs (como la velocidad de propagación, el tiempo, etc.) en función de la distancia desde la erupción en las señales del Sistema Global de Navegación por Satélite (GNSS). Primero, evaluando los archivos de Receiver Independent Exchange Format (RINEX), es decir, un formato de intercambio de datos para los datos brutos del sistema de navegación por satélite, y luego, calculando los residuos de preajuste, es decir, los datos de entrada para las ecuaciones de navegación. Con estos datos, se analizarán las TIDs con una serie de índices que muestrean en postproceso la perturbación de la ionosfera, desarrollados por el grupo de investigación de Astronomía y GEomática de la Universitat Politecnica de Cata-lunya (gAGE)., Incoming

The Galileo High Accuracy Service (HAS) is a new capability of the European Global Navigation Satellite System that is currently under development. The Galileo HAS will start providing satellite orbit and clock corrections (i.e. non-dispersive effects) and soon it will also correct dispersive effects such as inter-frequency biases and, in its full capability, ionospheric delay. We analyse here an ionospheric correction system based on the fast precise point positioning (Fast-PPP) and its potential application to the Galileo HAS. The aim of this contribution is to present some recent upgrades to the Fast-PPP model, with the emphasis on the model geometry and the data used. The results show the benefits of integer ambiguity resolution to obtain unambiguous carrier phase measurements as input to compute the Fast-PPP model. Seven permanent stations are used to assess the errors of the Fast-PPP ionospheric corrections, with baseline distances ranging from 100 to 1000 km from the reference receivers used to compute the Fast-PPP corrections. The 99% of the GPS and Galileo errors in well-sounded areas and in mid-latitude stations are below one total electron content unit. In addition, large errors are bounded by the error prediction of the Fast-PPP model, in the form of the variance of the estimation of the ionospheric corrections. Therefore, we conclude that Fast-PPP is able to provide ionospheric corrections with the required ionospheric accuracy, and realistic confidence bounds, for the Galileo HAS., Open Access funding provided thanks to the CRUECSIC agreement with Springer Nature. The present work was supported in part by the European Space Agency contract IONO4HAS 4000128823/19/NL/AS, by the project RTI2018-094295-B-I00 funded by the MCIN/AEI 10.13039/501100011033 which is co-founded by the FEDER programme and by the Horizon 2020 Marie Skłodowska-Curie Individual Global Fellowship 797461 NAVSCIN., Peer Reviewed, Postprint (published version)

The main objective of the TechTIDE project (warning and mitigation technologies for travelling ionospheric disturbances effects) is the development of an identification and tracking system for travelling ionospheric disturbances (TIDs) which will issue warnings of electron density perturbations over large world regions. The TechTIDE project has put in operation a real-time warning system that provides the results of complementary TID detection methodologies and many potential drivers to help users assess the risks and develop mitigation techniques tailored to their applications. The TechTIDE methodologies are able to detect in real time activity caused by both large-scale and medium-scale TIDs and characterize background conditions and external drivers, as an additional information required by the users to assess the criticality of the ongoing disturbances in real time. TechTIDE methodologies are based on the exploitation of data collected in real time from Digisondes, Global Navigation Satellite System (GNSS) receivers and Continuous Doppler Sounding System (CDSS) networks. The results are obtained and provided to users in real time. The paper presents the achievements of the project and discusses the challenges faced in the development of the final TechTIDE warning system., Postprint (published version)

Medium-scale traveling ionospheric disturbances (MSTIDs) are fluctuations in the plasma density that propagate through the upper layer of the atmosphere at velocities of approximately 100 m/s and periods reaching some tens of minutes. Due to their wavelengths, MSTIDs can degrade the performance of differential positioning techniques, such as real-time kinematics (RTK) or network-RTK (NRTK). This paper defines a novel methodology as a tool for relating the errors in NRTK positioning based on an MSTIDs indicator using the second difference in time of the slant total electron content (STEC). The pro-posed methodology performs integer ambiguity resolution (IAR) on the undifferenced measurements instead of using double-differenced carrier-phase measurements, as it is usual in RTK and NRTK. Statistical tests are applied to evaluate the degradation in the position errors caused by the impacts of MSTIDs on RTK and NRTK positioning over a data set spanning one year gathered from the CATNET network; a dual-frequency network of fixed permanent GNSS receivers located at the mid-latitudes of northeastern Spain. With the development of the proposed methodology for measuring the position degradation, another results of the present research are the establishment of thresholds for the proposed MSTIDs index, which can be used to monitor the positioning solution and to warn users when the measurements are affected by MSTIDs events, relating the position error to MSTIDs that affect not only the user receivers but also of the reference receivers within the network., Peer Reviewed, Postprint (published version)

Hernández-Pajares and García-Rigo have written a document criticizing our paper “Confirming geomagnetic Sfe by means of a solar flare detector based on GNSS. J Space Weather Space Clim 9: A42. https://doi.org/10.1051/swsc/2019040” (Curto et al., 2019). The main goal of our paper was to define a methodology based on GNSS measurements that is able to detect solar flares (SF) in an automatic way. This methodology was used to confirm Sfe (SF effects) detected by geomagnetism in an unsupervised manner. In their document, Hernández-Pajares and García-Rigo posed two objections related to the correctness and the novelty of the methodology used in our paper. This document is a reply to these objections and concludes that they are not relevant, Peer Reviewed, Postprint (published version)

This document (D2.3) presents the final products of the TID identification codes in TechTIDE released after adjustments driven by WP5 (Assessment of the TID impact on aerospace and ground systems) results, to efficient support specific systems operations (such EGNOS, N-RTK, HF communication and geolocation) and the mitigation of the TID effects., The document presents a report on the final products in TechTIDE. The TID identification codes in TechTIDE have been adjusted since its first release driven by the assessment of the TID impact on aerospace and ground systems (WP5) to efficient support specific systems operations (such EGNOS, N-RTK, HF communication and geolocation) and the mitigation of the TID effects. The new products and improvements resulted of users’ recommendations and WP5 results. The final TechTIDE products released after this adjustment, are described in this report., Preprint

The aim of this work package (WP) 5 is to specify the impact of TID in EGNOS, N-RTK and HF systems through statistical correlation of the performance data recorded from system operators and of TID detection results. Within the HF-TID-method in TechTIDE WP2 we use Digisonde-to-Digisonde measurements to determine characteristics of TIDs appearing in the region of the ionospheric reflection area between a transmitting and a receiving Digisonde. One of the TID characteristics is the Angle of Arrival (AoA) of the received radio signal, which can be divided into azimuth and elevation. In this analysis, we look for a correlation between TIDs and the variation of the azimuth of the received radio signal. In N-RTK, statistical tests have been applied over the entire data tests showing NRTK solutions are close to the solutions implementing a dual frequency solution methodology. Additionally, it was possible to correlate presence of MSTID with the degradation in the NRTK solutions., The aim of this WP5 is to specify the impact of TID in EGNOS, N-RTK and HF systems through statistical correlation of the performance data recorded from system operators and of TID detection results.Within the HF-TID-method in TechTIDE WP2 we use Digisonde-to-Digisonde measurements to determine characteristics of TIDs appearing in the region of the ionospheric reflection area between a transmitting and a receiving Digisonde. One of the TID characteristics is the Angle of Arrival (AoA) of the received radio signal, which can be divided into azimuth and elevation. In this analysis, we look for a correlation between TIDs and the variation of the azimuth of the received radio signal.In N-RTK, statistical tests have been applied over the entire data tests showing NRTK solutions are close to the solutions implementing a dual frequency solution methodology. Additionally, it was possible to correlate presence of MSTID with the degradation in the NRTK solutions, Preprint

Solar Flares (SF) refer to sudden increases of electromagnetic radiation from the Sun lasting from minutes to hours. Irradiance in the Extremely Ultra-Violet (EUV) or X band is enhanced and it can produce a sudden over-ionization in the ionosphere, which can be tracked by several techniques. On the one hand, this over-ionization increases the ionospheric delays of GNSS signals in such a way as can be monitored using measurements collected by dual-frequency GNSS receivers. On the other hand, this over-ionization of the ionosphere is the origin of electrical currents which, in turn, induce magnetic fields which can be monitored with ground magnetometers. In this work we propose the use of a GNSS Solar Flare Monitor (GNSS-SF) for its utility to confirm the presence of ionospheric ionization which is able to produce Solar Flare Effects (Sfe) in geomagnetism. A period of 11 years (2008–2018) has been analyzed and contingency tables are shown. Although most of the GNSS-SF detections coincide with SF and most of the Sfe have a detected origin in the ionosphere, there are some paradoxes: sometimes small flares produce disturbances which are clearly detected by both methods while other disturbances, originated by powerful flares, go by virtually unnoticed. We analyzed some of these cases and proposed some explanations. We found that suddenness in the variation is a key factor for detection. Threshold values of the velocity of change to remove the background noise and the use of the acceleration of change instead of the velocity of change as the key performance detector are other topics we deal with in this paper. We conclude that the GNSS-SF detector could provide warnings of ionization disturbances from SF covering the time when the Sfe detectors are “blind”, and can help to confirm Sfe events when Sfe detectors are not able to give a categorical answer., Peer Reviewed, Postprint (published version)

EU H2020 project TechTIDE deriverable, reporting on the design of the adjustments and upgrades required in order to develop the TID identification algorithms. The added value products that will result from the detection methods will be specified and designed. Each detection method developer will design a validation methodology that will lead to the definition of the confidence metrics., TechTIDE project, funded by the European Commission Horizon 2020 research and innovation program [AD-1], will establish a pre-operational system to demonstrate reliability of a set of TID (Travelling Ionospheric Disturbances) detection methodologies to issue warnings of the occurrence of TIDs over the region extended from Europe to South Africa. TechTIDE warning system will estimate the parameters that specify the TID characteristics and the inferred perturbation, with all additional geophysical information to the users to help them assess the risks and to develop mitigation techniques, tailored to their application. This document is TechTIDE D2.1 “Report on the design and specifications of the TID algorithms and products” and it is an output of TechTIDE Task 2.1 (Specifications for the TID algorithms and the resulting products) of the WP2 (TID identification methodologies) which has the final goal to release the basic algorithms for the TID identification and the value-added products for implementation in the TechTIDE warning system. The document presents the design of adjusted and upgraded TID detection codes, the design of the value-added products, and the validation plan. The design of the adjustments and the upgrades of the different methods are based on the initial requirements gathered among potential users affected by TIDs [RD-1]. Some requirements were brought in from ESA Space Situational Awareness Space Weather (SSA SWE) [RD-2] users' requirements. This way, TID algorithms and product outputs will try to adapt to assess ESA SSA SWE Service Network prerequisites., Preprint

The Ground based Augmentation System (GBAS) is being developed to enable precision approach and landing operations using the Global Positioning System (GPS). Each GBAS installation provides services through a GBAS Ground Facility (GGF) which is located at the airport it serves. By monitoring the GPS signals, measurements, and navigation messages, the GGF is able to exclude unhealthy satellites and broadcast real-time range correction messages for healthy satellites to users via a VHF data link. Airborne users apply these corrections to remove errors that are common between the GGF and the aircraft. The GGF is also responsible for warning the aircraft of any potential integrity threats. One source of potential errors is the satellite broadcast ephemeris message, which users decode and use to compute GPS satellite positions. In GBAS, potential GPS ephemeris faults are categorized into two types, A and B, based upon whether or not the fault is associated with a satellite manoeuvre. Both type A and type B events can cause differential ranging errors. However, the ramifications of these failure classes differ in both likelihood of occurrence and means of detection. The likelihood of type B failures is relatively higher than type A because orbit ephemeris uploads and broadcast ephemeris changeovers are frequent (nominally once per day and once every 2 h, respectively, for each satellite), whereas spacecraft manoeuvres are rare (no more than once or twice per year). This work focuses on aviation navigation threats caused by Type B faults for Category I (Cat-I) aircraft Precision Approach (PA). To detect and mitigate these threats, we investigate GGF monitors based on comparing projected satellite position estimates based on Zero-Order and First-Order Hold (ZOH, FOH) estimators, using prior days of data. Objectives: − Background literature review to know the state of the Art. − To develop algorithms to linearly compute projected satellite estimated based on a pr