Χρήση του Φάσματος Συχνοτήτων 70-500GHz στις δορυφορικές επικοινωνίες

Στα πλαίσια αυτής της διπλωματικής εργασίας έγινε μία προσπάθεια να επικεντρώσουμε το ενδιαφέρον μας στις συχνότητες που αφορούν τις δορυφορικές επικοινωνίες. Στο πρώτο κεφάλαιο γίνεται μία ιστορική αναδρομή στις δορυφορικές επικοινωνίες και σε θέματα που αφορούν τη δομή τους, την αρχιτεκτονική τους, τις τροχιές, τις υπηρεσίες και τις εφαρμογές τους. Στο δεύτερο κεφάλαιο γίνεται μία παρουσίαση των δορυφορικών επικοινωνιών με τη χρήση συχνοτήτων σε TeraHertz τιμές. Παράλληλα, αναφερόμαστε στα κίνητρα για τη χρήση των TeraHertz συχνοτήτων καθώς και στην ατμοσφαιρική απορρόφηση σε αυτές τις συχνότητες. Στο τρίτο κεφάλαιο αναφερόμαστε σε μήκη κύματος της τάξης των mm και στο μοντέλο καναλιού σε μήκη κύματος mm. Ταυτόχρονα, παρουσιάζονται δύο εφαρμογές γι’ αυτά τα μήκη κύματος (mmMAGIC Project και mmWave Backhaul και Fronthaul). Τέλος, στο τέταρτο κεφάλαιο εκθέτουμε μία σειρά από συμπεράσματα που αφορούν τη χρήση υπερυψηλών συχνοτήτων στις δορυφορικές επικοινωνίες.In the context of this dissertation, an attempt was made to focus our attention on the frequencies related to satellite communications. The first chapter provides a historical overview of satellite communications and issues related to their structure, architecture, trajectories, services and applications. The second chapter presents a presentation of satellite communications using frequencies in TeraHertz values. At the same time, we refer to the incentives for the use of TeraHertz frequencies as well as to the atmospheric absorption at these frequencies. In the third chapter we refer to wavelengths of the order of mm and to the channel model in wavelengths of mm. At the same time, two applications are presented for these wavelengths (mmMAGIC Project and mmWave Backhaul and Fronthaul). Finally, in the fourth chapter we present a series of conclusions regarding the use of high frequencies in satellite communications

Validation of OMI erythemal doses with multi-sensor ground-based measurements in Thessaloniki, Greece

The aim of this study is to validate the Ozone Monitoring Instrument (OMI) erythemal dose rates using ground-based measurements in Thessaloniki, Greece. In the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki, a Yankee Environmental System UVB-1 radiometer measures the erythemal dose rates every minute, and a Norsk Institutt for Luftforskning (NILU) multi-filter radiometer provides multi-filter based irradiances that were used to derive erythemal dose rates for the period 2005–2014. Both these datasets were independently validated against collocated UV irradiance spectra from a Brewer MkIII spectrophotometer. Cloud detection was performed based on measurements of the global horizontal radiation from a Kipp & Zonen pyranometer and from NILU measurements in the visible range. The satellite versus ground observation validation was performed taking into account the effect of temporal averaging, limitations related to OMI quality control criteria, cloud conditions, the solar zenith angle and atmospheric aerosol loading. Aerosol optical depth was also retrieved using a collocated CIMEL sunphotometer in order to assess its impact on the comparisons. The effect of total ozone columns satellite versus ground-based differences on the erythemal dose comparisons was also investigated. Since most of the public awareness alerts are based on UV Index (UVI) classifications, an analysis and assessment of OMI capability for retrieving UVIs was also performed. An overestimation of the OMI erythemal product by 3–6% and 4–8% with respect to ground measurements is observed when examining overpass and noontime estimates respectively. The comparisons revealed a relatively small solar zenith angle dependence, with the OMI data showing a slight dependence on aerosol load, especially at high aerosol optical depth values. A mean underestimation of 2% in OMI total ozone columns under cloud-free conditions was found to lead to an overestimation in OMI erythemal doses of 1–5%.While OMI overestimated the erythemal dose rates over the range of cloudiness conditions examined, its UVIs were found to be reliable for the purpose of characterising the ambient UV radiation impact

Towards continuously programmable networks

While programmability has been a feature of network devices for a long time, the past decade has seen significant enhancement of programming capability for network functions and nodes, spearheaded by the ongoing trend towards softwarization and cloudification. In his context, new design principles and technology enablers are introduced (Section 7.2) which reside at: (i) service/application provisioning level, (ii) network and resource management level, as well as (iii) network deployment and connectivity level

V2X Communication over Cellular Networks: Capabilities and Challenges

Vehicular communications is expected to be one of the key applications for cellular networks during the following decades. Key international organizations have already described in detail a number of related use cases, along with their requirements. This article provides a comprehensive analysis of these use cases and a harmonized view of the requirements for the latest and most advanced autonomous driving applications. It also investigates the extent of support that 4G and 5G networks can offer to these use cases in terms of delay and spectrum needs. The paper identifies open issues and discusses trends and potential solutions

3D laser scanning and digital restoration of an archaeological find

The current paper demonstrates the digital recreation and 3D printing of a missing fragment of an ancient ceramic pottery following digitization using a three dimensional laser scanning. The resulting pointcloud of the laser scans was treated with a series of advanced software for the creation of surfaces and ultimately for a digital model. An analytical methodology is presented revealing the step by step approach, which is an innovative way of recreating a missing fragment. Such approach aims to demonstrate the level of contribution that the ever evolving computer based technologies and 3D printing could bring to cultural heritage. The reverse engineering method presented for the reconstruction of a ceramic pottery, which is a part of the larger field of digital archaeology, is believed to benefit a variety of interested parties including 3D CAD users and designers, archaeologists and museum curators

3D laser scanning and digital restoration of an archaeological find

The current paper demonstrates the digital recreation and 3D printing of a missing fragment of an ancient ceramic pottery following digitization using a three dimensional laser scanning. The resulting pointcloud of the laser scans was treated with a series of advanced software for the creation of surfaces and ultimately for a digital model. An analytical methodology is presented revealing the step by step approach, which is an innovative way of recreating a missing fragment. Such approach aims to demonstrate the level of contribution that the ever evolving computer based technologies and 3D printing could bring to cultural heritage. The reverse engineering method presented for the reconstruction of a ceramic pottery, which is a part of the larger field of digital archaeology, is believed to benefit a variety of interested parties including 3D CAD users and designers, archaeologists and museum curators

NILU-UV multi-filter radiometer total ozone columns: comparison with satellite observations over Thessaloniki, Greece

This study aims to construct and validate a neural network (NN) model for the production of high frequency (~1 min) ground-based estimates of total ozone column (TOC) at a mid-latitude UV and ozone monitoring station in the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki (LAP/AUTh) for the years 2005–2014. In the first stage of model development, ~30,000 records of coincident solar UV spectral irradiance measurements from a Norsk Institutt for Luftforskning (NILU)-UV multi-filter radiometer and TOC measurements from a co-located Brewer spectroradiometer are used to train a NN to learn the nonlinear functional relation between the irradiances and TOC. The model is then subjected to sensitivity analysis and validation. Close agreement is obtained (R2 = 0.94, RMSE = 8.21 DU and bias = −0.15 DU relative to the Brewer) for the training data in the correlation of NN estimates on Brewer derived TOC with 95% of the coincident data differing by less than 13 DU. In the second stage of development, a long time series (≥1 million records) of high frequency (~1 min) NILU-UV ground-based measurements are presented as inputs to the NN model to generate high frequency TOC estimates. The advantage of the NN model is that it is not site dependent and is applicable to any NILU input data lying within the range of the training data. GOME/ERS-2, SCIAMACHY/Envisat, OMI/Aura and GOME2/MetOp-A TOC records are then used to perform a precise cross-validation analysis and comparison with the NILU TOC estimates over Thessaloniki. All 4 satellite TOC dataset are retrieved using the GOME Direct Fitting algorithm, version 3 (GODFIT_v3), for reasons of consistency. The NILU TOC estimates within ±30 min of the overpass times agree well with the satellite TOC retrievals with coefficient of determination in the range 0.88 ≤ R2 ≤ 0.90 for all sky conditions and 0.95 ≤ R2 ≤ 0.96 for clear sky conditions. The mean fractional differences are found to be −0.67% ± 2.15%, −1.44% ± 2.25%, −2.09% ± 2.06% and −0.85% ± 2.19% for GOME, SCIAMACHY, OMI and GOME2 respectively for the clear sky cases. The near constant standard deviation (~±2.2%) across the array of sensors testifies directly to the stability of both the GODFIT_v3 algorithm and the NN model for providing coherent and robust TOC records. Furthermore, the high Pearson product moment correlation coefficients (0.94 ≤ R ≤ 0.98) testify to the strength of the linear relationship between the satellite algorithm retrievals of TOC and ground-based estimates, while biases of less than 5 DU suggest that systematic errors are low. This novel methodology contributes to the ongoing assessment of the quality and consistency of ground and space-based measurements of total ozone columns