On the Mitigation of Ionospheric Scintillation in Advanced GNSS Receivers

Ionospheric scintillation is one of the major threats and most challenging propagation scenarios affecting Global Navigation Satellite Systems (GNSS) and related applications. The fact that this phenomenon causes severe degradations only in equatorial and high latitude regions has led to very few contributions dealing with the fundamental scintillation mitigation problem, being of paramount importance in safety critical applications and advanced integrity receivers. The goal of this paper is twofold, first to bring together the most relevant contributions on GNSS receiver design under scintillation conditions, and then, to propose a new GNSS carrier tracking framework and scintillation mitigation methodology. Scintillation complex gain components are modeled as AR processes and embedded into the state-space formulation, providing the filter the capability to distinguish between dynamics and phase scintillation contributions. In addition, the actual need of robust solutions is solved by using an adaptive filtering approach and directly operating with the baseband received signal. Simulation results, using both synthetic and real scintillation data, are provided to support the theoretical discussion and to show the performance improvements of such new approach

Modified Costas Loop for Carrier Phase Tracking in GPS Receivers

The carrier phase received at the receivers of the Global Positioning System (GPS) links is used to detect navigation data and to precisely determine the position, speed and time corresponding to the user's equipment. Therefore, subsystems for carrier phase tracking are crucial parts in all GPS receivers. When the propagation conditions are favorable, the method frequently used for phase tracking is based on Digital Phase-Locked Loop (DPLL)) and implemented through the discrete Costas loop operating under the modulated L1 carrier, in the case of a GPS receiver. This technique is quite simple, well known and very suitable for implementation in low-cost receivers. In this article, we revisit the traditional Costas loop design and point out some issues that affect the phase tracking performance of this loop. In order to overcome these problems, we propose some modifications to the traditional Costas loop. The resulting architecture presents better performance and complexity equivalent to the original loop. Another contribution of this work is the mathematical analysis to evaluate the performance of the new architecture when operating on an Additive White Gaussian Noise (AWGN) channel. Various results from computational simulations carried out with the two architectures, in different operating scenarios, including AWGN, dynamic stress and ionospheric scintillation are presented and discussed. We conclude that the new architecture outperforms the traditional Costas loop in terms of the variance of the estimated phase error, root mean squared error of the estimated phase and robustness to cycle-slip and loss of lock

Analysis of scintillation measurement errors in GNSS signals

Scintillation is a disturbance induced by the ionosphere, the farther layer of the atmosphere in which free ions (electrons) are present. These electrons vary in quantity according to many factors related to Solar activity and affect instantaneously and simultaneously both in amplitude and in phase the radio navigation signal. Therefore, the impact of scintillation is typically assessed by means of amplitude and a phase scintillation index. However, multipath reflections induce a signal amplitude variation very similar to what scintillations do, leading to potentially wrong scintillation indices estimations. In the thesis, a technique for clearing out the amplitude scintillation index from the multipath effect is presented and analyzed. The results obtained led by the technique, demonstrate the effect of Multipath in the S4 index and the presence of scintillation masked by it.L'objectiu d'aquest projecte és demostrar la presència d'errors en les mesures d'índexs de scintilaltion en el senyal GPS L1 C / A a causa de la presència de reflexió multipath al receptor i proposar un mètode per reduir aquests errors. La scintilaltion és una perturbació induïda per la ionosfera, la capa més llarga de l'atmosfera en la qual estan presents els ions lliures (electrons). Aquests electrons varien en quantitat segons molts factors relacionats amb l'activitat solar i afecten instantàniament i simultàniament tant en amplitud com en fase de senyal de radiocomunicació. Per tant, l'impacte del scintilaltion sol avaluar-se mitjançant amplitud i un índex de scintilaltion de fase. No obstant això, els reflectors de múltiples vies indueixen una variació d'amplitud de senyal molt similar a la que fan els scintilaltions, donant lloc a estimacions d'índexs de scintilaltion potencialment incorrectes. En la tesi es presenta i analitza una tècnica per eliminar l'índex de scintilaltion d'amplitud de l'efecte multipath. Per a la demostració, es van utilitzar les dades recollides durant els primers deu dies del mes de setembre de 2017, a latituds polars durant una tempesta solar. Les dades es van recollir a un interval d'Hertz (1 Hz) per a períodes de cinquanta (50) minuts; i inclou l'índex de scintilaltion d'amplitud i l'índex de scintilaltion de fase del senyal GPS L1 C / A. Amb aquestes dades, el comportament del senyal s'ha analitzat durant les vint-i-quatre hores del dia i, juntament amb un altre detector de scintilaltion, s'han comparat les dades durant els dies i les hores més concloents de la setmana. Els resultats obtinguts han estat dividits per block de temps i blocs de comportament del senyal, diferenciats en tres blocs diferents segons la quantitat de scintillation present. El processament de dades va confirmar que en els casos seleccionats multipath indueix un creixement considerable del valor de l'índex d'escintilació d'amplitud i que hi ha una correlació en aquesta tendència entre dies consecutius. S'ha demostrat que al alinear correctament les sèries temporals de mesuraments, és possible eliminar la contribució de la ruta múltiple i, per tant, subratllar l'única contribució del scintilaltion real. A més, s'ha enfrontat la necessitat d'un gran enfocament de programació de dades per analitzar totes les dades adquirides.Outgoin