Short-term prediction of rain attenuation level and volatility in Earth-to-Satellite links at EHF band

This paper shows how nonlinear models originally developed in the finance field can be used to predict rain attenuation level and volatility in Earth-to-Satellite links operating at the Extremely High Frequencies band (EHF, 20–50 GHz). A common approach to solving this problem is to consider that the prediction error corresponds only to scintillations, whose variance is assumed to be constant. Nevertheless, this assumption does not seem to be realistic because of the heteroscedasticity of error time series: the variance of the prediction error is found to be time-varying and has to be modeled. Since rain attenuation time series behave similarly to certain stocks or foreign exchange rates, a switching ARIMA/GARCH model was implemented. The originality of this model is that not only the attenuation level, but also the error conditional distribution are predicted. It allows an accurate upper-bound of the future attenuation to be estimated in real time that minimizes the cost of Fade Mitigation Techniques (FMT) and therefore enables the communication system to reach a high percentage of availability. The performance of the switching ARIMA/GARCH model was estimated using a measurement database of the Olympus satellite 20/30 GHz beacons and this model is shown to outperform significantly other existing models. The model also includes frequency scaling from the downlink frequency to the uplink frequency. The attenuation effects (gases, clouds and rain) are first separated with a neural network and then scaled using specific scaling factors. As to the resulting uplink prediction error, the error contribution of the frequency scaling step is shown to be larger than that of the downlink prediction, indicating that further study should focus on improving the accuracy of the scaling factor

A Quorum-Sensing Factor in Vegetative Dictyostelium Discoideum Cells Revealed by Quantitative Migration Analysis

Background: Many cells communicate through the production of diffusible signaling molecules that accumulate and once a critical concentration has been reached, can activate or repress a number of target genes in a process termed quorum sensing (QS). In the social amoeba Dictyostelium discoideum, QS plays an important role during development. However little is known about its effect on cell migration especially in the growth phase. Methods and Findings: To investigate the role of cell density on cell migration in the growth phase, we use multisite timelapse microscopy and automated cell tracking. This analysis reveals a high heterogeneity within a given cell population, and the necessity to use large data sets to draw reliable conclusions on cell motion. In average, motion is persistent for short periods of time (tƒ5min), but normal diffusive behavior is recovered over longer time periods. The persistence times are positively correlated with the migrated distances. Interestingly, the migrated distance decreases as well with cell density. The adaptation of cell migration to cell density highlights the role of a secreted quorum sensing factor (QSF) on cell migration. Using a simple model describing the balance between the rate of QSF generation and the rate of QSF dilution, we were able to gather all experimental results into a single master curve, showing a sharp cell transition between high and low motile behaviors with increasing QSF. Conclusion: This study unambiguously demonstrates the central role played by QSF on amoeboid motion in the growt

Migration d'amibe de dictyostelium : rôle de l'adhésion et de la détection de Quorum

This thesis focuses on the analysis of the role of adhesion between substrate and cell and factors of Quorum sensing on the migration of Dictyostelium amoeba. Tools to automate the recordings of videomicroscopy and image analysis have been developed to work with very large samples of cells and toquantify cell migration. A microfluidic device for cell detachment in hydrodynamic flow combined witha motorized stage has allowed a statistical study of adhesion but also the dynamics of detachment. The analysis of the migration of Dictyostelium in non nutritive medium highlights the role of density on celldifferentiation and migration capacity. We observe the presence of a maximum speed of migration after6 hours of starvation. We show that the adhesion to glass is twice as low in deprivation buffer as inthe nutrient medium. The experiences of migration in growth medium revealed the presence of a factorof detection of density secreted by the cells and regulating their random migration. The diffusion coefficient, the persistence of the movement and morphology of cells vary depending on the concentrationof this factor. This factor does not affect cell adhesion but only the dynamics of detachment. Finally, the testing protocol developed allowed us to make a comparative study of migration by varying otherparameters such as surface or the chemical composition of experimental medium. This work concludesby outlining the possible role of adhesion to the migration of Dictyostelium in nutrient medium.Cette thèse est centrée sur l’analyse du rôle de l’adhésion cellule-substrat et des facteurs de détectionde Quorum sur la migration amibienne de Dictyostelium . Des outils pour automatiser les enregistrementsde vidéoomicroscopie et l’analyse d’image ont été développés afin de travailler avec de trèsgrands échantillons de cellules et de quantifier la migration cellulaire. Un dispositif microfluidique dedétachement cellulaire sous flux hydrodynamique combiné une platine motorisée a permis une étude statistique de l’adhésion mais aussi de la dynamique de détachement. L’analyse de la migration de Dictyostelium en milieu non nutritif met en évidence le rôle de la densité sur la différentiation des celluleset leur capacité de migration. Nous observons la présence d’une vitesse maximale de migrationaprès 6h de carence. Nous montrons que l’adhésion sur verre est deux fois plus faible en milieu carenc´equ’en milieu nutritif. Les exp´eriences de migration en milieu nutritif ont révélé la présence d’un facteur de détection de densité sécrété par les cellules et régulant leur migration aléatoire. Le coefficient de diffusion, la persistance du mouvement et la morphologie des cellules varient en fonction de la concentrationde ce facteur. Ce facteur ne modifie pas l’adhésion cellulaire mais uniquement la dynamique de d´etachement. Enfin, le protocole d’analyse développé nous a permis de faire une étude comparative de la migration en faisant varier d’autres paramètres tel que la surface ou la composition chimique du milieu expérimental. Ce travail se conclue en exposant le possible rôle de l’adhésion sur la migrationchez Dictyostelium en milieu nutritif

Neural network model for atmospheric attenuation retrieval between 20 and 50 GHz by means of dual-frequency microwave radiometers

International audienceThe propagation of signals through the atmosphere plays a major role in the quality of communications between ground terminals and satellites. Its characteristics have to be known accurately for appropriate communications equipment to be selected. In the band of frequencies used by operators in the future generation of satellites (beyond 20 GHz), the quality of transmission is especially affected by the attenuation of received signals because of rain, and by other less significant but much more frequent effects due to atmospheric gases, and nonprecipitating water. These phenomena have a direct impact on the availability ratio of a link between a ground terminal and a satellite. Our main goal in this study is to measure the atmospheric attenuation, using dual-frequency ground-based radiometers measuring the sky radiation at different pointing directions, so as to perform a statistical study. A new algorithm, based on a neural approach, is thus developed for estimating atmospheric attenuation, in various meteorological conditions, for several elevation angles and for frequencies between 20 and 50 GHz, from dual-frequency radiometric measurements. A validation of the obtained algorithm is performed on Olympus experimental data for the 20 and 30 GHz channels. At the end of this paper some applications are then presented to underline the usefulness of this new algorithm. The applicability of the algorithm to satellite beacon calibration in Ka or Q band with accuracy of 0.1 dB is shown. Preliminary joint statistics between attenuation at various pointing directions obtained at 40 GHz show what improvement can be expected from satellite diversity in the case of satellite constellations

The Effect of Rain-No Rain Intermittency on the Estimation of the Universal Multifractals Model Parameters

International audienceThe multifractal properties of rain are investigated within the framework of universal multifractals. The database used in this study includes measurements performed over several months in different locations by means of a disdrometer, the dual-beam spectropluviometer (DBS). An assessment of the effect of the rain-no rain intermittency shows that the analysis of rain-rate time series may lead to a spurious break in the scaling and to erroneous parameters. The estimation of rain multifractal parameters is, therefore, performed on an event-by-event basis, and they are found to be significantly different from those proposed in scientific literature. In particular, the parameter H, which has often been estimated to be 0, is more likely to be 0.53, thus meaning that rain is a fractionally integrated flux (FIF). Finally, a new model is proposed that simulates high-resolution rain-rate time series based on these new parameters and on a simple threshold

Assessment of rain fade mitigation techniques in the EHF band on a Syracuse 3 20/44-GHz low elevation link

International audienceAn Earth-to-satellite propagation experiment in the EHF band has been carried out within the framework of the Syracuse 3 program, which is a new generation French military SATCOM system. The originality of this experiment resides in the link's frequencies (20 GHz downlink and 44 GHz uplink) and its low elevation angle (17°). The first part of the article presents a statistical analysis of attenuation data providing the long-term statistics, frequency scaling ratios and fade durations. These results are compared to standard ITU models. The second part of the article is dedicated to the short-term forecasting of rain fade, useful for the implementation of Fade Mitigation Techniques (FMT). Firstly, the downlink attenuation is predicted based on a non-linear ARIMA-GARCH model. The prediction result is then separated into several physical components (gases, clouds and rain) that are scaled to the uplink frequency using specific frequency scaling factors. The performance of the model is assessed based on Syracuse 3 20/44-GHz data collected during a period of 1 year

Short-term prediction of atmospheric attenuation in Q band from Ka-band measurements on earth-to-satellite links

International audienceFade mitigation techniques on Earth-satellite links require the estimation of channel propagation conditions. This is generally done by using downlink measurements at a lower frequency to derive the channel uplink propagation conditions at higher frequencies. Moreover, when atmospheric conditions are changing, the ground stations have to change their own configuration (increase or decrease transmit power and/or data rates for example). These changes involve estimating the uplink propagation conditions in advance. The proposed algorithm allows the margin to be estimated a few seconds in advance from the downlink so that a given percentage of availability of the uplink is reached. For that purpose, we have used a recently developed prediction method based on an ARMA/GARCH model (de Montera et al., 2008) and scaling frequency models. From the downlink attenuation, first the attenuation is first predicted with a prediction horizon ranging between 2 and 30 seconds. The contributions of gases, clouds and rain are then estimated using a neural network (Barthes et al., 2006) and scaled using specific scaling factors. Finally, the attenuation and its variance for the uplink are estimated and the margin is derived. This algorithm allows an accurate upper-bound of the future attenuation to be estimated in real time, which minimizes the cost of fade mitigation techniques and therefore enables the communication system to reach an appropriate percentage of availability. After a presentation of the method, we describe results obtained from the Syracuse 3 EHF measurements (20/44 GHz). We show that the method gives good results for a prediction horizon in the range 2-30 seconds. We conclude the paper by showing that for a prediction horizon of 10 seconds and for attenuations greater than 5 dB the error due to frequency scaling is larger than the prediction error, so that future improvements should focus on scaling frequency models

Statistical analysis of the Syracuse 3 satellite EHF propagation experiment

International audienceAn Earth-to-satellite propagation experiment operating in the EHF band was set up as part of the Syracuse 3 program. The Syracuse 3 satellites are the new generation of French military SATCOM systems. This experiment is a good opportunity to carry out attenuation measurements and increase knowledge about tropospheric propagation effects on EHF links at a low elevation angle (17deg). At these frequencies (20 & 44 GHz), the attenuation due to the troposphere can be very strong. This paper presents yearly statistics of attenuation level, frequency scaling ratio and fade durations. A comparison with ITU models is given