915 ANNALS OF GEOPHYSICS, SUPPLEMENT TO VOL. 47, N. 2/3, 2004 COST 271 Action – Effects of the upper atmosphere on terrestrial and Earth-space communications: introduction BRUNO ZOLESI ( 1 ) and LJILJANA R. CANDER ( 2 ) ( 1 ) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy ( 2 ) Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K. The COST 271 Action («Effects of the Upper Atmosphere on Terrestrial and Earth-space Com- munications») within the European ionospheric community has the objectives, embodied in the Memorandum of Understanding (MoU): to study the influence of upper atmospheric conditions on terrestrial and Earth-space communications, to develop methods and techniques to improve ionos- pheric models over Europe for telecommunication and navigation applications and to transfer the re- sults to the appropriate Radiocommunication Study Groups of the International Telecommunication Union (ITU-R) and other national and international organizations dealing with the modern commu- nication systems. This introductory paper summarises briefly the background and historical context of COST 271 and outlines the main objectives, working methods and structure. It also lists the par- ticipating countries and institutions, the Management Committee (MC) Meetings, Workshops and Short-term Scientific Missions. In addition, the paper discusses the dissemination of the results and the collaboration among the participating institutions and researchers, before outlining the content of the Final Report. 1. BACKGROUND AND HISTORICAL CONTEXT Temporal and spatial changes in the upper atmosphere can act to limit and degrade the perform- ance of terrestrial and Earth-space radio systems in many different ways. Command, control and com- munication systems involving transionospheric propagation may be disrupted; global positioning net- works compromised and surveillance (optical and radar) systems affected. In addition, conditions in near-Earth space have other adverse effects on modern technology, ranging from the tracking and the lifetimes of satellites to the induction of damaging currents in power grids and transcontinental pipelines. The vulnerabilities of systems to changes in the upper atmosphere incorporate many differ- ent aspects. These include: space-weather effects on communication, navigation and surveillance sys- tems; solar cycle phenomena and their impact on operational systems; modelling, forecasting, and pre- diction services; ionosonde sounder methods and measurements; applications of ionospheric tomog- raphy; transionospheric effects including scintillation; ionospheric propagation for high frequency communication systems; longwave propagation systems and effects; Global Positioning System (GPS) and ionospheric Total Electron Content (TEC) studies. The European ionospheric community has long been aware that co-operative research on an international basis is essential to deal with such complex issues. In particular, international co-operation is required for the collection of data, in both real-time and retrospective modes, the development and verification of new methods to improve the perform- ance of both operational and future terrestrial and Earth-space communications systems and the ex- change of expertise on space plasma effects on Global Navigation Satellite Systems (GNSS).