A review is given of the current state of the art in the prediction of rain attenuation on slant paths. Different types of research effort are identified and discussed. A critical assessment of data analysis methods and theoretical models is made to identify areas where more effort is needed. Various types of data available are discussed (direct attenuation measurements, radiometric data, rain rate, and radar reflectivity) as well as the models and methods available to date to relate these to attenuation on a satellite path. Finally, prediction methods for worldwide use are reviewed. While great progress is being made in the physical modeling of the atmosphere, statistical prediction methods are still very general and coarse. In the future, the emphasis should be on the development of detailed models to predict the variability of attenuation statistics within a coverage area. In this connection there is a need for coordination in the comparison of experimental data. In Europe, projects of international cooperation for this purpose are being carried out. 2. PRIME ATTENUATION DATA Measurements of attenuation using satellite beacons have been carried out during the last 5 years Paper number 1 SO 149. 0048-6604 / 81 / 0910-0149501.00 745 746 G. BRUSSAARD on a fairly large scale in North America, Japan, and Europe. For the United States and Canada the first bpportunity to measure satellite-borne beacon signals at frequencies above 10 GHz was provided f•by ATS 5 at 15.3 GHz. However, problems with the satellite reduced the usefulness of the results. Since then, the ATS 6 satellite, followed by the CTS and COMSTAR satellites, has provided ample opportunity for the collection of primary attenuation data in the range 10-30 GHz [Arnold et al., 1979; Ippolito, 1975]. For Europe the opportunity arose when ATS 6 was moved to a position over Africa from July 1976 to July 1977. In a well-coordinated effort several European research centers participated in the experiments, the results of which have been brought together in a publication by the European Space Agency, which had overall responsibility for the coordination [Brussaard and Battrick, 1977]. Since then, the Italian SIRIO satellite and the European OTS satellite, both still in orbit, have been providing long-term data in the 10-to 20-GHz range. A first review of SIRIO measurements has been published [Alta Frequenza, 1979]. OTS measurements are coordinated by the organization Interim Eutelsat in which European telecommunica-tions administrations cooperate. Eutelsat has recently initiated an attempt to bring together statistical results of attenuation and cross-polarization measurements.
