Michael Hatzopoulos

Monitoring and Forecasting the Ionosphere Over Europe: The DIAS Project

Knowledge of the state of the upper atmosphere, and in particular its ionospheric part, is very important in several applications affected by space weather, especially the communications and navigation systems that rely on radio transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data collection endeavour called the European Digital Upper Atmosphere Server (DIAS) was initiated in 2004 by a consortium formed around several European ionospheric stations that transmit in real-time ionospheric parameters automatically scaled. The DIAS project is a collaborative venture of eight institutions funded by the European Commission eContent Programme. The project seeks to improve access to digital information collected by public European institutes and to expand its use. The main objective of the DIAS project is to develop a pan-European digital data collection describing the state of the upper atmosphere, based on real-time information and historical data collections provided by most of the operating ionospheric stations in Europe. Various groups of users require data specifying upper atmospheric conditions over Europe for nowcasting and forecasting purposes. The DIAS system is designed to distribute such information. The successful operation of DIAS is based on the effective use of observational data in operational applications through the development of new added-value ionospheric products and services that best fit the needs of the market. DIAS is a unique European system, and its continuous operation will efficiently support radio propagation services with the most reliable information. DIAS began providing services to users in August 2006. The Need for Accurate Ionospheric Products Radio frequency communications and satellite positioning and navigation systems are applications most affected by ionospheric disturbances. Such disturbances can cause drastic and large-scale changes in the usable ranges of high frequency (HF) or below HF bands affecting standard ground-to-ground and submarine communication systems. The characteristics of an ionospheric propagation channel, whether it is HF or transionospheric frequencies, are highly variable on timescales ranging from a few seconds to the 11-year solar cycle. Even during its quietest periods, the Sun produces electromagnetic radiation and solar wind, both of which can affect a variety of geomagnetic and ionospheric phenomena, which in turn affect radio waves propagating through the ionosphere. Hence day-to-day and hour-to-hour changes in propagation channel characteristics can occur.

Parallel optimization of large join queries with set operators and aggregates in a parallel environment supporting pipeline

Proposes a parallel optimizer for queries containing a large number of joins, as well as set operators and aggregate functions. The platform for the execution is a shared-disk multiprocessor machine supporting bushy parallelism and pipeline processing. Our model partitions the query into almost independent subtrees that can be optimized simultaneously, and it applies an enhanced variation of the iterative improvement technique on those subtrees which contain a large number of joins; this technique is parallelized, too. In order to estimate the cost of the states constructed during the optimization of join subtrees, cost formulae are developed that estimate the cost of relational algebra operators when executed across coalescing pipes.

Agents for Querying Distributed Statistical Databases Over the Internet

Distributed database techniques and the Internet provide producers of statistics with a means to publish their data and metadata widely and make them available to a variety of users. Data matching to a user query and data access as well as data harmonization are some of the problems that should be solved. Intelligence is required in various stages of query answering and data matching. Moreover, the breadth and distributed nature of the Internet urge for a distributed approach. Agents seem to be the means by which both intelligence and distributed processing can be achieved. This paper presents a distributed approach for answering queries on statistical data that exist over the Internet using a multi-agent framework.

Database maintenance efficiency using differential files

Abstract Severance and Lohman[1] proposed that all database amendments (i.e. additions, deletions and updates) are localized into a relatively small storage area, called a differential file. When the differential file grows sufficiently large, a reorganization incorporates all changes into a new generation of the database. This paper proposes a modification of the differential file approach in that the differential file only stores record additions. It is also concerned with the selection of the optimum reorganization points for both the original and the modified differential file approaches.

The file allocation problem under dynamic usage

Abstract The file allocation problem considers a file and a fully connected network having n nodes. The problem assumes that the overall file usage over a unit time period is known and it asks for the optimal set of network sites at which to locate copies of the file. This paper considers the same problem but it assumes that the behavior of the user access patterns changes over v planning periods in a manner, known in advance. A model is presented which shows that there are (2n − 1)v possible file allocations. To assist the searching of this large solution space four theorems are presented which are subsequently utilized to analyze the problem and to solve an example case.

Interactive Multimedia Documents: A Modeling, Authoring and Rendering Approach

In this paper we present our research and development experience in the context of Interactive Multimedia Documents (IMDs). We define a rich model for such documents covering the issues of interaction and spatiotemporal compositions as means of representing the functionality of an IMD. We exploit the event concept to represent interaction, while complex interactions are covered by a rich algebraic and spatiotemporal event composition scheme. Based on the model we implemented an authoring methodology. Thereafter we present a generic framework for rendering (presenting) IMDs putting emphasis to the handling of interaction and to the temporal synchronization of media objects. The rendering system is implemented as a client-server architecture using Java and accompanying technologies. The implementation is suitable for WWW enabled interactive multimedia documents.

The Determination of the Optimum Database Maintenance Points

The initial database search cost deteriorates due to record additions, deletions and updates performed during the system operation. Previous studies proposed strategies for selecting the optimum maintenance points by making various assumptions about the rate of database deterioration, the database planning period, the maintenance interval, etc. However, the studies did not express the optimum maintenance points as a function of the database physical structure. The paper assumes (a) that the rate of additions to the database equals the rate of deletions for each database record type, and (b) that maintenance is performed for the entire database at fixed time intervals and it shows how the maintenance points may be determined for various primitive physical database structures. The results are subsequently ultilized to select the optimum maintenance points for arbitrary database structures.

Using a medical digital library for education purposes

Athens Medical School is one of the largest medical research institutions in Greece, where educational activities are combined with every-day practice in the University Hospital. The laboratories operating in the University Hospital produce a large amount of research material, mainly consisting of medical images and videos in digital format. In this paper, we present the Digital Library System built to support the collections of Athens Medical School laboratories. The system facilitates access through the Web to medical research material and on-line presentations/lessons. Laboratory collections have different requirement regarding digital material structure and metadata modelling. Thus, while designing the Digital Library architecture, the organisation and administration of dynamically created collections was explored We introduced the term dynamic collection management to denote the support of automated collection definition and management within an integrated Digital Library environment. Laboratory staff is responsible for selecting the material to be added in the library, while they also participate in metadata creation. Furthermore, they use this material to create presentation or on-line tutorials, also stored in the Digital Library. To facilitate the aforementioned functionality the Digital Library system also supports a complex workflow environment.

i-Mu.S.E.-Interactive Multimedia Scenario Editor

Presents a rich model for interactive multimedia presentations and an implemented authoring system based on the model. The model covers the issues of interaction in terms of external and internal events and also the spatiotemporal composition of media objects in the context of the presentation. The authoring is carried out in three consecutive phases related to: (i) selection and transformations of the media objects that are to participate, (ii) definitions of the events (atomic and complex) that the presentations are to consume, and (iii) specification of the scenario of the presentation in terms of scenario tuples. Furthermore, in order for the presentation specifications to be rendered, appropriate algorithms have to be developed. We present the transformation of the declarative specifications into procedures. The authoring system was fully implemented using the authoring tool Asymetrix/Toolbook 4.0 under Windows 95.

Allocation of copies of s distinct files in an information network

Abstract The problem of deciding on the optimal allocation of copies of a file in a computer network has been solved under various sets of assumptions. This study considers s distinct files (which may well be s physical partitions of a database) and assumes that the selected allocation must not violate any disk space constraints and must not result in overloaded and/or underloaded nodes in the network. The problem solved asks therefore for the optimal allocation of the files by prohibiting the allocation of more than k files ( k ≤ s ) in a node. Two models are developed which solve the problem for the cases where 1. (a) a single copy, and 2. (b) multiple copies of each file may appear in the optimal allocation.