1,721,114 research outputs found
Tropospheric Attenuation: A Challenge for Future Earth-Space Communications Systems
No full textIn approximately 60 years, Earth-space communications have evolved from early broadcasting systems to the modern complex interactive ones able to deliver worldwide broadband coverage. This was made possible by the gradual transition of carrier frequencies from the S band (2-4 GHz) to the Ka band (26.5-40 GHz), which is still partially taking place. Unfortunately, the increase in the carrier frequency is also accompanied by the growing impact of the troposphere, which induces several negative effects, attenuation being the most detrimental one. If the troposphere already poses clear limitation to modern Earth-space communications, even more it will to future systems, which are foreseen to operate at even higher carrier frequencies. This contribution discusses such challenges, specifically targeting next generation Earth-space communication systems operating at V band (40-75 GHz), W band (75-110 GHz) and, possibly, even up to the sub-THz band
Modeling the space-time evolution of rain fields: Electromagnetic wave propagation applications
No full textSpace-time (ST) multiexponential cell (MultiEXCELL), a ST correlated rainfall model oriented to the analysis of radio-propagation impairments (1 km × 1 km spatial resolution and 1-min temporal resolution), is presented. The model is developed on the basis of a comprehensive rain field database collected by the Spino d'Adda, Italy, weather radar. Rain cells are modeled using an exponentially shaped profile, whose main parameters evolve over time according to simple yet effective analytical expressions. The spatial correlation of the rain rate across the field is preserved by reproducing the natural aggregative process of single rain cells into larger clusters, while the temporal evolution of the rain field is achieved by combining the structural change of the rain cells and their displacement across the field, at the same time taking the realistic evolution of the fractional rainy area into account. When tested against local data, the time series generated by ST MultiEXCELL show to correctly reproduce the spatial and temporal correlation of the rain rate. These encouraging results suggest the use of ST MultiEXCELL as a comprehensive tool for the design and performance assessment of Earth-space and terrestrial millimeter-wave systems implementing advanced mitigation techniques relying on the uneven distribution of the rain rate in space and time
Modelling 3D Dynamic Cloud Fields to Investigate the Performance of FSO SatCom Systems
No full textWith increasing data rate requirements on SatCom systems there is a push to use higher frequency carriers. Although free space optical communications would meet these demands, they are severely limited by the weather conditions affecting the ground stations. Therefore, it is important to model these phenomena to predict the possibility of link outages so that an appropriate fade mitigation technique can be applied. This contribution aims to extend a model in the literature (Stochastic Model of Clouds) to include the time variability of the cloud field. The temporal evolution is studied using satellite observations and is introduced into the model by inputting ERA5 data chronologically, which has been linearly interpolated to a temporal resolution of 1 minute. The new model is then used to study the effect of site diversity for the mitigation of cloud attenuation on a GEO-Earth optical link operating at 1.55 mu mathrm{m}
A site diversity model for non-geosynchronous systems: Development and preliminary results
No full textAn analytical site diversity model for non-geosynchronous (GSO) systems is presented. The model allows predicting the joint rain attenuation statistics along Earth-space links at different elevation angles, from which the site diversity gain can be also calculated. The main advantage offered by the proposed prediction model is that it takes into account the variability of rain attenuation correlation function as a function of the elevation angle (besides the obvious one on the site separation distance), which, in turn, allows its application also to non-GSO system. The proposed model, fully described by analytical expressions, offers satisfactory accuracy when applied to predict the joint rain attenuation CCDFs along non-GSO links with ground station in a tropical site
Investigating spatial behaviour: An application of space analysis to criminal investigations
No full textGeographical profiling (GP) is an investigative methodology which uses the crime-related scenes of crimininal series to determine the area in which the offender might live or conducts relevant activities (e.g.,work and exercise).The idea underlyng GP is that the offender tends to use familiar places and his own spatial representations to carry out criminal activities. This methodology is often applied on serial crimes such as rapes, homicides and fire setters , but also on single crimes involving more than a place and it is based on the connection between the geographic information , the knowledge of the victim and crime frame. The analyst uses tools and digital representations visualised as maps in which it is attempt to indicate where it is more probable that the offender lives and can be used by investigators to reduce the searching areas, select investigative strategies such as assign priority on a list of suspects , implement a search based on addresses , patrolling an so on. The aim is to reconstruct the cognitive map of the offender , to study the distance variable in its explorative strategies and mobility. In this study , we will present simulation data on real cases to asses whether the output map could be a useful tool in crime investigations. Results are discussed in the light of cognitive mapping and crime pattern approaches
Vertical product differentiation and adverse selection: an experimental note
No full textIn three identical laboratory markets, sellers possess products whose quality is both exogenously and endogenously determined. Buyers can observe products' quality only in the last session of each experiment. It is also assumed an uneven distribution of income among buyers. We study whether a separating equilibrium arises in such a context, as in traditional models of vertical product differentiation, thus reducing adverse selection outcomes
Prediction Model for the Effective Operation of Earth-space Links Using Up-Link Power Control
No full textA new model for the application of Up-Link Power Control in satellite communication systems is presented. The model consists in the combination of an ARIMA model to forecast the rain rate (5 minutes in the future) and of the Enhanced Synthetic Storm Technique (E-SST) to generate time series of the rain attenuation at two frequencies, from which the frequency scaling ratio can be calculated for the real time application of Up-Link Power Control. The performance of the model, tested against the Alphasat Ka- and Q-band propagation data collected in Milan, indicate that the proposed frequency scaling approach is accurate enough to allow the real-time effective application of ULPC in EHF satellite communication systems
Enhancement of the Synthetic Storm Technique for the Prediction of Rain Attenuation Time Series at EHF
Full text linkA physically based rain attenuation prediction model for Earth-space links, namely the enhanced synthetic storm technique (E-SST), is presented. Different from the original SST, the E-SST receives as input detailed information on the rain height and on the storm velocity, and it discriminates between stratiform and convective rain events having a different impact on the link. The rain attenuation prediction accuracy of E-SST, both as applied directly and as embedded into a more accurate frequency scaling technique, is evaluated against a full year of propagation data collected by the NASA equipment installed at Politecnico di Milano in the frame of the Alphasat Aldo Paraboni propagation experiment. To this aim, a novel methodology to isolate the contribution of rain attenuation from the received beacon power is devised and presented. Results indicate that E-SST represents an accurate and reliable tool for the prediction of rain attenuation at EHF, both on a statistical basis (direct application) and on an event basis (frequency scaling)
Radiometric Estimation of Tropospheric Attenuation: A Mixed Physically Based/Machine Learning Approach
Full text linkA mixed physically based/machine learning (ML) approach to measure tropospheric attenuation A in all-weather conditions by means of microwave radiometers (MWRs) is proposed. The key idea is to combine the advantages originating from the accurate radiometric A retrievals, provided by the well-established Cosmic background (CB) approach in clear-sky conditions, with the benefits coming from ML techniques. The latter aim at estimating A in rainy situations through a simplified approach able to overcome the issues posed by more complex techniques such as the standard solution of the radiative transfer equation or the Sun tracking (ST) microwave technique. To this aim, an artificial neural network (ANN) is devised to turn the antenna noise temperatures measured by a four-channel MWR (from Ka- to W-band) into tropospheric attenuation at the frequencies of the radiometric channels, namely 23.8, 31.4, 72.5, and 82.5 GHz. The network is properly trained and tested by taking advantage of the concurrent CB and ST measurements collected by the RpG radiometer deployed at Politecnico di Milano, Milan, Italy, under the ESA-funded WRAD project. The proposed approach to retrieve the tropospheric attenuation is intended to overcome the limits associated both with the ST technique (only measurements during the day, link elevation strictly bound to the Sun ecliptic) and to the CB one (unreliable measurements in rainy conditions)
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