Scintillation on global navigation satellite signals and its mitigation

PhD ThesisThe scintillation effects on the Global Positioning system (GPS) or other GNSS (global navigation satellite system) receivers have been investigated by many researchers and several mitigation strategies have been proposed in this regard but the problem is not yet fully solved. This thesis covers the investigation of scintillation effects on GPS receivers and developing a mitigation approach which can play an important role in mitigating the effects of scintillation on these and other GNSS receivers. Firstly, a new GPS signal acquisition method known as the repetitive block acquisition (RBA) is presented which can be used to speed up the GPS signal acquisition in case fast acquisition is required. This acquisition method is implemented using coarse-acquisition (C/A) codes and tested by collecting real GPS data. The RBA method can also be used for other codes as well. It is rather difficult to show that how scintillation affects the acquisition process in a GPS receiver because mostly it results in tracking loop loss of lock due to cycle slip. However, during strong amplitude scintillation which is usually most important at low or near-equatorial latitudes, deep power fades resulting from amplitude scintillation result in the selection of long data records which leads to slow acquisition due to long acquisition times. It is shown in this thesis that, by using the RBA method, the acquisition time can be reduced to a fairly low level by reducing the number of computations involved in acquisition compared to other well-known methods such as the parallel FFT-based method and zero padding method (ZP). Secondly, the scintillation effects on the GPS tracking loop have also been investigated in this thesis and, based on this investigation, a new improved analogous phase scintillation index, σw φa, has been designed to more accurately represent the phase scintillation intensity at European high latitudes. This is then also validated using the real GPS data from Trondheim, Norway (63.41o N, 10.4o E). The σw φa uses dual frequency (L1 & L2) based vi time and spatial variations of total electron contents (TEC) at 1 Hz for estimating the phase scintillation values. For deriving the σw φa, the low frequency TEC fluctuations due to Doppler shift of the satellite/receiver motion and also due to the slowly varying background ionosphere need to be removed in order to consider only the high frequency TEC fluctuations which are responsible for scintillation due to the fast moving electron density irregularities which is done by using the wavelet transform. The σw φa is really an improved version of σφa where, rather than using time-invariant digital high pass filters (HPF), which according to several researchers are in-appropriate for filtering the non-stationary raw GPS signals affected by the ionospheric scintillation, a wavelet-based filtering technique is used. Although, the wavelet transform has been used previously in detrending raw amplitude and phase observations at 50 Hz for estimating the scintillation indices (amplitude and phase), due to the high sample data rate it may not be desirable to use this transform due to its very high computational cost. Since, σw φa uses TEC data at 1 Hz so this problem has been overcome. The performance of the new improved index (σw φa) is investigated and is also compared with the previously proposed σφa and σφ indices using one whole year of data from a GPS receiver at Trondheim, Norway (63.41o N, 10.4o E). The raw TEC observations and the σw φa index are then used in estimating the tracking phase jitter using two different methods. The phase jitter helps in defining the tracking thresholds for the tracking loops in a receiver which is useful in updating the tracking loop parameters during scintillation conditions as required in robust GPS/GNSS receiver designs because the phase jitter decides how wide the tracking (and thus the noise) bandwidth should be allowed in the tracking loop for the tracking to remain efficient. It is shown that if the phase jitter is estimated using the new proposed methods, generally a better estimate can be obtained compared to the previously proposed phase jitter estimation methods which employs σφa and σφ indices. These new phase jitter estimation methods can further be used in GPS/GNSS receivers for updating the tracking loop parameters during scintillation conditions and hence can serve as a good alternative for mitigating the effects of scintillation on GPS/GNSS receivers.Higher Education Commission (HEC) of Pakistan and the Sukkur Institute of Business Administration, Pakistan

Disentangling ionospheric refraction and diffraction effects in GNSS raw phase through fast iterative filtering technique

We contribute to the debate on the identification of phase scintillation induced by the ionosphere on the global navigation satellite system (GNSS) by introducing a phase detrending method able to provide realistic values of the phase scintillation index at high latitude. It is based on the fast iterative filtering signal decomposition technique, which is a recently developed fast implementation of the well-established adaptive local iterative filtering algorithm. FIF has been conceived to decompose nonstationary signals efficiently and provide a discrete set of oscillating functions, each of them having its frequency. It overcomes most of the problems that arise when using traditional time–frequency analysis techniques and relies on a consolidated mathematical basis since its a priori convergence and stability have been proved. By relying on the capability of FIF to efficiently identify the frequencies embedded in the GNSS raw phase, we define a method based on the FIF-derived spectral features to identify the proper cutoff frequency for phase detrending. To test such a method, we analyze the data acquired from GPS and Galileo signals over Antarctica during the September 2017 storm by the ionospheric scintillation monitor receiver (ISMR) located in Concordia Station (75.10° S, 123.33° E). Different cases of diffraction and refraction effects are provided, showing the capability of the method in deriving a more accurate determination of the σϕ index. We found values of cutoff frequency in the range of 0.73–0.83 Hz, providing further evidence of the inadequacy of the choice of 0.1 Hz, which is often used when dealing with ionospheric scintillation monitoring at high latitudes

Disentangling ionospheric refraction and diffraction effects in GNSS raw phase through fast iterative filtering technique

We contribute to the debate on the identification of phase scintillation induced by the ionosphere on the global navigation satellite system (GNSS) by introducing a phase detrending method able to provide realistic values of the phase scintillation index at high latitude. It is based on the fast iterative filtering signal decomposition technique, which is a recently developed fast implementation of the well-established adaptive local iterative filtering algorithm. FIF has been conceived to decompose nonstationary signals efficiently and provide a discrete set of oscillating functions, each of them having its frequency. It overcomes most of the problems that arise when using traditional time–frequency analysis techniques and relies on a consolidated mathematical basis since its a priori convergence and stability have been proved. By relying on the capability of FIF to efficiently identify the frequencies embedded in the GNSS raw phase, we define a method based on the FIF-derived spectral features to identify the proper cutoff frequency for phase detrending. To test such a method, we analyze the data acquired from GPS and Galileo signals over Antarctica during the September 2017 storm by the ionospheric scintillation monitor receiver (ISMR) located in Concordia Station (75.10° S, 123.33° E). Different cases of diffraction and refraction effects are provided, showing the capability of the method in deriving a more accurate determination of the σϕ index. We found values of cutoff frequency in the range of 0.73–0.83 Hz, providing further evidence of the inadequacy of the choice of 0.1 Hz, which is often used when dealing with ionospheric scintillation monitoring at high latitudes

Survey on Signal Processing for GNSS under Ionospheric Scintillation: Detection, Monitoring, and Mitigation

Ionospheric scintillation is the physical phenomena affecting radio waves coming from the space through the ionosphere. Such disturbance is caused by ionospheric electron density irregularities and is a major threat in Global Navigation Satellite Systems (GNSS). From a signal processing perspective, scintillation is one of the most challenging propagation scenarios, particularly affecting high-precision GNSS receivers and safety critical applications where accuracy, availability, continuity and integrity are mandatory. Under scintillation, GNSS signals are affected by amplitude and phase variations, which mainly compromise the synchronization stage of the receiver. To counteract these effects, one must resort to advanced signal processing techniques such as adaptive/robust methods, machine learning or parameter estimation. This contribution reviews the signal processing landscape in GNSS receivers, with emphasis on different detection, monitoring and mitigation problems. New results using real data are provided to support the discussion. To conclude, future perspectives of interest to the GNSS community are discussed

Estimation Techniques and Mitigation Tools for Ionospheric effects on GNSS Receivers

Navigation is defined as the science of getting a craft or person from one place to another. The development of radio in the past century brought fort new navigation aids that enabled users, or rather their receivers, to compute their position with the help of signals from one or more radio-navigation system . The U.S. Global Positioning System (GPS) was envisioned as a satellite system for three-dimensional position and velocity determination fulfilling the following key attributes: global coverage, continuous/all weather operation, ability to serve high-dynamic platforms, and high accuracy. It represents the fruition of several technologies, which matured and came together in the second half of the 20th century. In particular, stable space-born platforms, ultra-stable atomic frequency standards, spread spectrum signaling, and microelectronics are the key developments in the realization and success of GPS. While GPS was under development, the Soviet Union undertook to develop a similar system called GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS). Both GLONASS and GPS were designed primarily for the military, but have transitioned in the past decades towards providing civilian and Safety-of-Life services as well. Other Global Navigation Satellite Systems (GNSS) are now being developed and deployed by governments, international consortia, and commercial interests. Among these are the European system Galileo and the Chinese system Beidou. Other regional systems are the Japanese Quasi-Zenith Satellite System and the Indian Gagan. GNSS have become a crucial component in countless modern systems, e.g. in telecommunication, navigation, remote sensing, precise agriculture, aviation and timing. One of the main threats to the reliable and safe operation of GNSS are the variable propagation conditions encountered by GNSS signals as they pass through the upper atmosphere of the Earth. In particular, irregular concentration of electrons in the ionosphere induce fast fluctuations in the amplitude and phase of GNSS signals called scintillations. The latter can greatly degrade the performance of GNSS receivers, with consequent economical impacts on service providers and users of high performance applications. New GNSS navigation signals and codes are expected to help mitigate such effects, although to what degree is still unknown. Furthermore, these new technologies will only come on line incrementally over the next decade as new GNSS satellites become operational. In the meantime, GPS users who need high performance navigation solution, e.g., offshore drilling companies, might be forced to postpone operations for which precision position knowledge is required until the ionospheric disturbances are over. For this reason continuous monitoring of scintillations has become a priority in order to try to predict its occurrence. Indeed, it is a growing scientific and industrial activity. However, Radio Frequency (RF) Interference from other telecommunication systems might threaten the monitoring of scintillation activity. Currently, the majority of the GNSS based application are highly exposed to unintentional or intentional interference issues. The extremely weak power of the GNSS signals, which is actually completely buried in the noise floor at the user receiver antenna level, puts interference among the external error contributions that most degrade GNSS performance. It is then of interest to study the effects these external systems may have on the estimation of ionosphere activity with GNSS. In this dissertation, we investigate the effect of propagation issues in GNSS, focusing on scintillations, interference and the joint effect of the two phenomena

Ionospheric scintillation monitoring and modelling

This paper presents a review of the ionospheric scintillation monitoring and modelling by the European groups involved in COST 296. Several of these groups have organized scintillation measurement campaigns at low and high latitudes. Some characteristic results obtained from the measured data are presented. The paper also addresses the modeling activities: four models, based on phase screen techniques, with different options and application domains are detailed. Finally some new trends for research topics are given. This includes the wavelet analysis, the high latitudes analysis, the construction of scintillation maps and the mitigation techniques

Analysis and Detection of Outliers in GNSS Measurements by Means of Machine Learning Algorithms

L'abstract è presente nell'allegato / the abstract is in the attachmen

Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS

Among the numerous potential sources of GNSS signal degradation, multipath takes on a prominent position. Unlike other errors like ionospheric or tropospheric path delays which can be modeled or significantly reduced by differential techniques, multipath influences cannot be mitigated by such approaches. Although a lot of multipath mitigation techniques have been proposed and developed in the past among them many receiver internal approaches using special signal processing algorithms multipath (especially multipath with small geometric path delays) still remains a major error source. This is why multipath has been a major design driver for the definition of the Galileo signal structure carried out in the past years and the subsequent signal optimization activities. This thesis tries to provide a broad and comprehensive insight into various aspects of multipath propagation, mitigation and monitoring (without claiming to be exhaustive). It contains an overview of the most important aspects of multipath propagation, including the discussion of different types of multipath signals (e.g. specular vs. diffuse multipath, satellite vs. receiver multipath or hardware-induced multipath), typical characteristics such as periodic signal variations whose frequency depends on the satellite-antenna-reflector geometry and the impact on the signal tracking process within a GNSS receiver. A large part of this thesis is dedicated to aspects of multipath mitigation, first providing a summary of the most common multipath mitigation techniques with a special focus on receiver-internal approaches such as the narrow correlation technique, double-delta correlator implementations, the Early-Late Slope (ELS) technique or Early/Early tracking implementations. However, other mitigation approaches such as using arrays of closely spaced antennas or multipath-limiting antennas are discussed as well. Some of these techniques are used for subsequent multipath performance analyses considering signals of the (modernized) GPS and Galileo. These analyses base on a new methodology to estimate typical and meaningful multipath errors making use of multipath error envelopes that are scaled in a suitable way to account for different multipath environments. It will be shown that typical (mean) multipath errors can be derived from these scaled envelopes by computation of the envelopes running average and that these mean multipath errors are of the same order as multipath errors obtained from complex statistical channel models. Another part of this thesis covers various aspects of multipath detection and monitoring. First, current techniques for multipath detection and monitoring are described and discussed with respect to their benefits and drawbacks or their real-time capability. Among the considered approaches are techniques like code minus carrier monitoring, SNR monitoring, the use of differenced observations or spectral and wavelet analysis. Following this introductory overview, a completely new approach for real-time multipath monitoring by processing multi-correlator observations will be introduced. Previously being used primarily for the detection of Evil Waveforms (signal failures that originate from a malfunction of the satellites signal generation and transmission hardware), the same basic observations (linear combinations of correlator outputs) can be used for the development of a multi-correlator-based real-time multipath monitoring system. The objective is to provide the user with instant information whether or not a signal is affected by multipath. The proposed monitoring scheme has been implemented in the form of a Matlab-based software called RTMM (Real-Time Multipath Monitor) which has been used to verify the monitoring approach and to determine its sensitivity.Die Qualität eines Satellitensignals wird durch eine Vielzahl potenzieller Fehlerquellen negativ beeinflusst. Neben atmosphärischen Einflüssen tragen Mehrwegeeinflüsse einen wesentlichen Anteil zum Gesamtfehlerbudget der Satellitennavigation bei. Während eine ganze Reihe von Fehlereinflüssen durch geeignete Modellierung oder differenzielle Verfahren deutlich reduziert werden können, ist dies durch die räumliche Dekorrelation der Mehrwegeeffekte nicht möglich. Obwohl in der Vergangenheit eine Vielzahl von Verfahren zur Mehrwegereduzierung vorgeschlagen und entwickelt wurden, stellen Mehrwegesignale noch immer eine wesentliche, stets zu berücksichtigende Fehlerquelle dar. Aus diesem Grund spielten die zu erwartenden Mehrwegefehler auch eine sehr wichtige Rolle im Zuge der Definition sowie der Optimierung der Galileo-Signalstruktur und können somit als wesentliches Design-Kriterium angesehen werden. Die vorliegende Arbeit gibt einen umfassenden Einblick in verschiedene Aspekte der Mehrwegeausbreitung, -reduzierung sowie der Detektion und der Überwachung auftretender Mehrwegeeffekte. Die Arbeit beschreibt zunächst die wichtigsten Aspekte der Mehrwegeausbreitung, wobei beispielsweise unterschiedliche Arten von Reflexionen oder unterschiedliche Entstehungsarten ebenso diskutiert werden wie typische Auswirkungen von Mehrwegesignalen wie die Entstehung periodischer Signalvariationen. Solche Signalvariationen sind in starkem Maße abhängig von der durch die Satellitenposition, dem Antennenstandpunkt und der Lage des Reflexionspunktes definierten Geometrie. Die Frequenz dieser Signalvariationen wird für unterschiedliche geometrische Verhältnisse berechnet. Zudem werden der Einfluss bzw. die Auswirkungen einer Mehrwegeausbreitung auf den Signalverarbeitungsprozess in einem GNSS Empfänger aufgezeigt. Einen weiteren Schwerpunkt dieser Arbeit bilden die derzeit gebräuchlichen Methoden zur Reduzierung von Mehrwegeeinflüssen. Dabei werden zunächst die wichtigsten empfängerinternen Ansätze vorgestellt. Aber auch Methoden wie die Verwendung von Antennenarrays oder spezieller Antennen bleiben nicht unberücksichtigt. Einige dieser Methoden bilden im Folgenden die Grundlage für die Bestimmung von typischen Mehrwegefehlern. Dazu wird eine neuartige Methodik vorgestellt, um aus Hüllkurven des Mehrwegefehlers aussagekräftige mittlere Mehrwegefehler zu bestimmen. Hierzu werden die Hüllkurven mit Hilfe einiger aus statistischen Kanalmodellen abgeleiteter Parameter in geeigneter Weise skaliert, um unterschiedlichen Mehrwegeumgebungen Rechnung zu tragen. Es wird gezeigt, dass die mit Hilfe dieser relativ einfachen und effizienten Methode ermittelten Mehrwegefehler in derselben Größenordnung liegen wie die aus komplexen statistischen Kanalmodellen ermittelten Fehler. Einen weiteren Themenkomplex stellen Methoden zur Detektion und zum Monitoring von Mehrwegeeinflüssen dar. Dabei werden zunächst derzeit verwendete Ansätze vorgestellt und hinsichtlich ihrer Vor- und Nachteile sowie hinsichtlich ihrer Echtzeitfähigkeit diskutiert. In Anschluss daran wird ein neuartiger Ansatz zur Detektion und zum Monitoring von Mehrwegesignalen in Echtzeit vorgestellt, der auf der Auswertung von Multikorrelatorbeobachtungen basiert. Ziel dieser Entwicklung ist es, einen potenziellen Nutzer sofort darüber informieren zu können, wenn ein Signal mit Mehrwegefehlern behaftet ist. Der vorgeschlagene Ansatz wurde in Form einer Matlab-basierten implementiert, welche im Folgenden zur Verifizierung und zur Bestimmung der Empfindlichkeit des Verfahrens verwendet wird

Investigation of scintillation effects in European Galileo Signals

Ionospheric scintillations are known to be rather challenging in Global Navigation Satellite Systems (GNSS) receivers. The scintillation effects include rapid variations in signal phase and amplitude, which may hinder the receiver to acquire and track the signal and may cause a loss of lock at GNSS receiver. This thesis focuses on the scintillation effects on the European GNSS namely, Galileo. Abrupt phase variations during transmission cause deep power fades called canonical fading, half cycle slips and frequency unlock. Phase locked loop designs that are currently available helps in reducing the scintillation effects to some extent, though this is complicated when scintillation is severe. This thesis focuses on investigating some of the scintillation effects on Galileo signal during acquisition. The considered performance criteria are the detection performance and the root mean square error at the receiver. For implementing this task, this thesis uses two toolboxes, namely Cornell Scintillation toolbox for generating synthetic scintillation time histories and TUT MBOC tracking model for simulating and studying the scintillation effects at the receiver. Cornell Scintillation toolbox generates synthetic amplitude and phase time histories based on two input parameters namely, scintillation intensity and decorrelation time that show how rapidly the signal amplitude and phase change. TUT MBOC acquisition and tracking simulator generates Galileo E1 signal that undergoes MBOC modulation and it is transmitted through multipath Nakagami-m fading channels. The thesis work focused on merging the scintillation time histories generated by Cornell scintillation toolbox with the TUT MBOC acquisition tracking algorithm, by adding the scintillation to fading channel. By calculating the Line of Sight (LOS) phase delay, the acquisition of the received signal is performed with and without scintillations. The obtained results with and without scintillations are compared and studied in order to evaluate the impact of scintillations on the European GNSS

Robust GNSS Point Positioning in the Presence of Cycle Slips and Observation Gaps

Among the various factors limiting accurate positioning with a Global Navigation Satellite System (GNSS) is the inherent code error level on a code observation, cycle slip occurrence on a phase observation, inadequate accuracy in the broadcast ionospheric model for single-frequency receivers; and the occurrence of observation gaps, which are short duration satellite outages (temporal loss of an observed satellite). The existing Cycle Slip Detection and Correction (CSDC) techniques are usually multi-satellite based; quite computationally intensive; and are often marred by the inherent code errors from the included code observations. Also, existing code-carrier smoothing techniques employed to mitigate code errors are limited by cycle slip occurrences on phase observations. In this research, algorithms are proposed in order to facilitate simple, efficient and real-time cycle slip detection, determination and correction, on a standalone single- or dual-frequency receiver; to enable cycle-slip-resilient code errors mitigation; and to improve the broadcast ionospheric model for single-frequency receivers. The proposed single-satellite and phase-only-derived CSDC algorithms are based on adaptive time differencing of short time series phase observables. To further provide robustness to the impact of an observation gap occurrence for an observed satellite, post-gap ionospheric delay is predicted assuming a linearly varying ionospheric delay over a short interval, which consequently enables the dual-frequency post-gap cycle slip determination and code error mitigation. The proposed CSDC algorithms showed good performance, with or without simulated cycle slips on actual data obtained with static and kinematic GNSS receivers. Over different simulated cycle slip conditions, a minimum of 97.3% correct detection and 79.8% correctly fixed cycle slips were achieved with single-frequency data; while a minimum of 99.9% correct detection and 95.1% correctly fixed cycle slips were achieved with dual-frequency data. The point positioning results obtained with the proposed methods that integrates the new code error mitigation and cycle slip detection and correction algorithms, showed significant improvement over the conventional code-carrier smoothing technique (i.e. a standalone Hatch filter, without inclusion of any cycle slip fixing method). Under different simulated cycle slip scenarios, the new methods achieved 25-42% single-frequency positioning accuracy improvement over the standalone Hatch filter, and achieved 18-55% dual-frequency positioning accuracy improvement over the standalone Hatch filter