A new tracking approach for multipath mitigation based on antenna array

In Global Navigation Satellites Systems (GNSS), multipaths (MP) are still one of the major error sources. The additional signal replica due to reflection will introduce a bias in conventional Delay Lock Loops (DLL) which will finally cause a strong positioning error. Several techniques, based on Maximum Likelihood estimation (ML), have been developed for multipaths mitigation/estimation such as the Narrow correlator spacing [1] or the Multipath Estimating Delay-Lock-Loop (MEDLL) [2] algorithm. These techniques try to discriminate the MP from the Line Of Sight Signal (LOSS) on the time and frequency domains and thus, short delay multipaths (<0.1Chips) can not be completely mitigated. Antenna array perform a spatial sampling of the wave front what makes possible the discrimination of the sources on the space domain (azimuth and elevation). As the time-delay domain and space domain can be assumed independent, we can expect to mitigate/estimate very short delay MP by using an antenna array. However, we don't want to increase too much the size, the complexity and the cost of the receivers and thus, we focus our study on small arrays with a small number of antennas: typically a square 2x2 array. Consequently, conventional beamforming (space Fast Fourier Transform) is not directive enough to assure the mitigation of the multipaths, and then this first class of solutions was rejected. In order to improve the resolution, adaptive beamformers have also been tested. However, the LOSS and the MP signal are strongly correlated and thus, classical adaptive algorithms [3] are not able to discriminate the sources. These preliminary studies have shown that the mitigation/estimation of multipaths based on the space domain will exhibit limited performances in presence of close sources. Then, in order to propose robust algorithms, we decided to investigate a space-time-frequency estimation of the sources. Space Alternating Generalized Expectation maximisation (SAGE) algorithm [4], which is a low-complexity generalization of the Expectation Maximisation (EM) algorithm, has been considered. The basic concept of the SAGE algorithm is the hidden data space [4]. Instead of estimating the parameters of all impinging waves in parallel in one iteration step as done by the EM algorithm, the SAGE algorithm estimates the parameters of each signal sequentially. Moreover, SAGE algorithm breaks down the multi-dimensional optimization problem into several smaller problems. In [5], it can be seen that SAGE algorithm is efficient for any multipaths configurations (small relative delays, close DOAs) and space-time-frequency approach is clearly outperforming classical time-frequency approaches. Notwithstanding, SAGE algorithm is a post processing algorithm. Thus, it's necessary to memorise in the receiver the incoming signal in order to apply SAGE estimation. For example, if we want to process 10ms of signal with a 10MHz sampling rate, we need to store a matrix of m*105 with m the number of antennas. In such condition, we can understand than SAGE algorithm is hardly implemented in real time. The challenge is then to find a new type of algorithms that reach the efficiency of the SAGE algorithms, but with a reduced complexity in order to enable real time processing. Furthermore, the implementation should be compatible with conventional GNSS tracking loops (DLL and PLL). To cope with these two constraints, we propose to apply the SAGE algorithm on the post-correlated signal. Indeed, the correlation step can be seen as a compression step and thus, the size of the studied signal is strongly reduced. In such a way, SAGE algorithm is able to provide estimates of the relative delay and Doppler of the received signals with respect to the local replicas. Thus, a post correlation implementation of SAGE can be seen as a discriminator for both the DLL and the PLL

A new multipath mitigation method for GNSS receivers based on antenna array

the potential of small antenna array for multipath mitigation in GNSS systems is considered in this paper. To discriminate the different incoming signals (Line of sight and multipaths), a new implementation of the well known SAGE algorithm is proposed. This allows a significant complexity reduction and it is fully compatible with conventional GNSS receivers. Theoretical study thanks to the Cramer Rao Bound derivation and tracking simulation results (in static and dynamic scenarios) show that the proposed method is a very promising approach for the multipath mitigation problem in GNSS receivers

HAPEE, a statistical approach for ionospheric scintillation prediction in the polar region

International audienceThrough several studies, CNES, ONERA, NSC and NMA have sought to define a prediction model for ionospheric variations that can disturb e.g. GNSS-based systems. The studies have focused on the high-latitude region, with a particular focus on the auroral oval. A first model was proposed in 2014. The model was a simple empirical model driven by the Kp geomagnetic index, and where the main output was the instantaneous mean Rate-of-TEC Index (ROTI) value. The model was found to not be sufficiently reliable to be used as an operational prediction model. In 2019, an updated model is proposed, where the main inputs are now the solar wind parameters pressure (the solar wind pressure p) and B z (the z component of the solar wind magnetic field). Moreover, a distribution of predicted ROTI or σ φ index is provided instead of a mean value. Thus, the model allows estimating the percentage of time of occurrence for a level of ROTI (or σ φ) to be exceeded in the next 5 minutes or 1 hour, or the exceeded ROTI (or σ φ) for a corresponding percentage of time. This empirical approach is based on 10 years of GNSS/scintillation data collected by more than 15 GNSS stations in Norway

Comparison of SAGE and classical multi-antenna algorithms for multipath mitigation in real-world environment

The performance of the Space Alternating Generalized Expectation Maximisation (SAGE) algorithm for multipath mitigation is assessed in this paper. Numerical simulations have already proven the potential of SAGE in navigation context, but practical aspects of the implementation of such a technique in a GNSS receiver are the topic for further investigation. In this paper, we will present the first results of SAGE implementation in a real world environmen

Array processing for GNSS receiver in urban environement [i.e environment]

Dans les systèmes de positionnement par satellite (GNSS), les réflexions multiples, caractéristiques des canaux de propagation urbains, posent de gros problèmes dans la bonne estimation de la position. Bien que de nombreuses solutions aient été proposées pour lutter contre les multi-trajets (Narrow correlator, MEDLL), les multi-trajets à faibles retards relatifs (<0.1Chips) sont toujours problématique. Plus récemment, l'utilisation de réseaux d'antennes adaptatifs a été proposée pour lutter contre les multi-trajets. En effet, l'échantillonnage spatial du front d'onde réalisé à partir de plusieurs antennes permet de discriminer les sources dans le domaine spatial, et cela quels que soient les retards des multi-trajets. Cette thèse a donc pour objectif de définir des méthodes de réjection des multi-trajets qui tirent partie de la dimension des directions d'arrivées (DOA) apportée par un réseau d'antenne. Le cahier des charges impose que le réseau utilisé soit de petite taille (typiquement réseau carré 2×2), et demande un algorithme robuste aux défauts technologiques.Inspiré des méthodes de réjection d'interférence, les premières solutions proposées ont été les techniques d'antennes adaptatives afin de filtrer spatialement les multi-trajets Cependant, en raison de la petite taille du réseauainsi que de la corrélation intrinsèque entre les multi-trajets et le signal direct, ces méthodes ont rapidement été mises en défaut. En particulier, les multi-trajets spatialement proches du trajet direct restent toujours problématiques.Afin de tirer partie de la dimension spatiale apportée par un réseau d'antennes et sans être trop dépendant de l'espacement angulaire entre les sources, nous avons choisi de joindre à l'estimation des DOA, l'estimation des retards et fréquences Doppler de chaque trajet reçu. L'algorithme SAGE, issu de la théorie du maximum de vraisemblance, a été utilisé afin d'estimer de façon jointe les paramètres des sources. De plus, nous avons proposé une nouvelle implémentation de SAGE qui permet de réduire d'un facteur 500 la complexité de l'algorithme tout en conservant les mêmes performances d'estimation. Les simulations ont montré une nette amélioration de la réjection des multi-trajets par rapport aux méthodes mono-antenne et multi-antennes adaptatives, tout en conservant une complexité calculatoire raisonnable. Cette méthode a fait l’objet d’un brevet.Nous avons ensuite étudié l'influence des défauts technologiques (couplage, défaut de chaîne RF …), numériques (quantification) et des défauts intrinsèques à l'algorithme SAGE (estimation du nombre de multi-trajets) sur les performances d'estimation, et proposé différentes méthodes de compensation. Nous retiendrons que des filtres FIR ont été utilisés pour compenser les défauts large bande de la chaîne RF, et que le couplage peut être estimé à partir de la connaissance des DOA des signaux utiles. Les simulations ont montré qu'avec ces méthodes de compensation, l'algorithme SAGE affichait des performances très proches de celles dans le cas parfait (sans défaut). Un algorithme d'estimation du nombre de multi-trajets a aussi été proposé, et les résultats en simulation dynamique (en utilisant des modèles de canal existants) ont montré une bonne adaptation aux différentes situations.Pour finir, il est important de noter que les outils développés durant cette thèse peuvent être utilisés dans le cadre de l'étude du canal de propagation des signaux GNSS, en identifiant et estimant les multi-trajets susceptibles de perturber l'estimation de la position du récepteur.In Global Navigation Satellite Systems (GNSS) applications, multipath (MP) errors are still one of the major error sources in conventional receivers. The additional signal replicas due to reflections introduce a bias in Delay Lock Loops (DLL), which finally leads to a positioning error. Several techniques have been developed for multipath mitigation or estimation such as the Narrow Correlator Spacing or the Multipath Estimating Delay-Lock-Loop (MEDLL) algorithm.However, these techniques suffers from high sensitivity to noise, and can not mitigate short delay multipath (<0.1 chip).More recently, the use of antenna array algorithms has been proposed for multipath mitigation. Antenna arrays perform a spatial sampling that makes possible the discrimination of sources in the space domain (azimuth and elevation).However, in conventional receivers, little room remains for antenna integration, and only a small number of antenna elements can be integrated. This study will therefore focus on algorithms for a 2x2 square antenna array. Moreover, theproposed solutions have to be robust against technological defects.Two solutions are investigated to mitigate multipath with an antenna array. The first one tries to filter the multipaths in the space domain in order to "clean" the incoming signal of all the multipaths. However, the results obtained with this solution are quite mitigated. Indeed, the small size of the array implies a low space resolution, and the correlation between the LOS signal and the multipaths strongly degrade the performance of high resolution algorithms. Thus, close spaced multipath are still a problem.In the second approach, a set of parameters (amplitudes, times-delays, Doppler shifts, elevations and azimuths) of all the incoming sources are estimated. The main difference with the first approach is that, instead of filtering the sources on the space domain only, the different incoming paths are filtered on space, time and frequency domains. To estimate the parameters of all the sources, SAGE algorithm, which is a low-complexity generalization of maximum likelihood theory, has been considered. Moreover, a new implementation of the SAGE algorithm has been investigated in order to reduce the complexity by a factor 500, without loss of estimation performances. The simulations show a real improvement in the multipath mitigation compared to mono antenna algorithms and beamformerapproaches.The impact of technological defects (mutual coupling, RF channel mismatch …), numerical defect (quantization) and SAGE defect (estimation of the number of path) on the estimation performances were also investigated, and severalcompensation algorithms were proposed. The wide band effects of the RF filter were compensated by FIR equalizer, and mutual coupling can be estimated thanks to the knowledge of the satellites DOA. Simulations show that the estimationperformance of the SAGE algorithm after array calibration are very close than the performance in perfect system. Last, we proposed an algorithm to estimate the number of path, and dynamic simulations (by using channel model) show avery good adaptation of the algorithm.Last but not least, the tools developed in this PhD can be also useful in multipath modelling applications for GNSS

Modelling the radiowave propagation with a split-step wavelet method for radio occultation

International audienceThis paper introduces the split-step wavelet (SSW) propagation technique applied to radio occultation (RO). This technique accounts for both a complex atmosphere and the ground reflection. It is applied on a scenario between a GPS satellite and a low-elevation orbit satellite. The atmosphere is modelled using an ITU model. The Earth is considered as a sphere. We show that the atmosphere and ground effects are both visible at the receiving satellite. Therefore, SSW is a good candidate for RO propagation modelling

Caractérisation et modélisation de la scintillation ionosphérique sur les signaux GNSS en zone polaire et équatoriale

International audienceLa présente communication décrit un ensemble de travaux effectués à l'ONERA en partenariat avec l'université Paul Sabatier et le CNES sur la propagation électromagnétique à travers l'ionosphère turbulente. Le problème direct de la propagation à travers l'ionosphère turbulente est tout d'abord traité par le développement de modèles basés sur une description statistique du milieu ionosphérique inhomogène. Puis à partir du formalisme asymptotique développé, des mesures GNSS sont inversées pour obtenir des paramètres décrivant le milieu. Cette approche a été testée sur la base de données SAGAIE en zone équatoriale. Enfin un modèle dédié à la prédiction de la scintillation ionosphérique à hautes latitudes, régressé sur une base de données norvégienne acquises sur un cycle solaire, est proposé

Caractérisation et modélisation de la scintillation ionosphérique sur les signaux GNSS en zone polaire et équatoriale

International audienceLa présente communication décrit un ensemble de travaux effectués à l'ONERA en partenariat avec l'université Paul Sabatier et le CNES sur la propagation électromagnétique à travers l'ionosphère turbulente. Le problème direct de la propagation à travers l'ionosphère turbulente est tout d'abord traité par le développement de modèles basés sur une description statistique du milieu ionosphérique inhomogène. Puis à partir du formalisme asymptotique développé, des mesures GNSS sont inversées pour obtenir des paramètres décrivant le milieu. Cette approche a été testée sur la base de données SAGAIE en zone équatoriale. Enfin un modèle dédié à la prédiction de la scintillation ionosphérique à hautes latitudes, régressé sur une base de données norvégienne acquises sur un cycle solaire, est proposé

3D to 2D approximation effect on propagation modeling, impact on scintillation indices in polar region

International audienceIonospheric scintillations, in particular at equatorial and polar latitudes, are responsible of GNSS receiver loss of lock and accuracy decreasing. To improve the future GNSS systems, an understanding of the effect of the ionospheric irregularities is necessary, completed by an accurate propagation modeling across this layer. The inhomogeneous ionospheric layer responsible for the scintillation effects is classically represented by its spectral density function (or spectrum) in propagation modeling. The inhomogeneity spectrum is anisotropic due to Earth magnetic field influence and to induced ionospheric currents [6]. Propagation across this inhomogeneous layer can be modeled by asymptotic methods based on Rytov theory when the ionospheric turbulence is weak [8][9], and by numerical approach as Parabolic Wave Equation (PWE) resolution associated with Multiple Phase Screen (MPS) [5][3]. The PWE-MPS technique is valid in strong scattering regime and can consider a variability of ionospheric turbulence characteristics along the path. As PWE-MPS technique in 3D can be time and memory space consuming, some authors assume a dimensional reduction of the problem from 3D to 2D [1]. The latter is assumed to be valid in equatorial region, where the irregularities are highly elongated along the earth magnetic field, mainly perpendicularly to the Line Of Sight direction for earth satellite links [2][7]. Nevertheless, the validity of this approximation for other configurations, as for instance in polar region where the morphology of irregularities is different, is still open. This paper proposes to quantify the consequences of the dimensional reduction on the prediction of log-amplitude and phase variances from 2D numerical schemes

Ionospheric turbulent parameter inversion using scintillation log-amplitude spectra: performance and results

International audienceIonospheric scintillation is a threat for the availability of systems with high service disponibility such as GNSS services. Indeed, this phenomenon is characterized by an important fluctuation of the amplitude and phase of the signal in reception, that can lead to loss of lock and signal outages. Given the fact that scintillation is caused by small-scale irregularities of the ionospheric density (ranging from a few meters to tenth of kilometers), the phenomenon is usually localized, both temporally and spatially. Moreover, it has an important day-to-day variability. After decades of interest for that phenomenon, the forecasting of ionospheric scintillation is still in progress