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Depolarization measurements performed in circular polarization at centimeter wavelength have frequently shown depolarization values lower than those theoretically expected by application of the Oguchi theory, which assumes equioriented ellipsoidal raindrops. This discrepancy can be explained by considering that in reality the drop-axes are oriented at a given instant according to a certain statistical distribution the spreading of which causes a decrease in the anisotropy (differential propagation constants along the principal planes), which is the prime source of rain depolarization (both in circular and in linear polarization). The reduction of anisotropy with respect to the equioriented drops model is experimentally assessed and the spreading necessary in a particular model distribution for the same reduction is determined in this communication. The study is based upon simultaneous measurements of attenuation and depolarization in circular polarization gathered in 1974-1975 during the Magnola experiment (Italy) on a low-elevation terrestrial path ( 9\deg ) at 11.6 GHz. The results confirmed that the equiorientation of raindrops is too severe an assumption, being the true anisotropy 20-30 percent (on average) less than the expected value in case of equiorientation. This figure corresponds to an average spreading of about 20-30\deg which is in agreement with the results given by other authors.

Publisher Summary This chapter provides an overview of the features and the objectives of the Italian Sirio satellite program and the characteristics of its systems. The principal objective is to investigate the applicability of frequencies above 10 GHz to satellite communication systems through an SHF propagation and communication experiment. Sirio is a spin-stabilized geostationary satellite with a mechanically despun antenna for the propagation and communication experiment. The nominal spin rate is 90 rpm. The Sirio program is developed in the frame of the Italian space program by the Consiglio Nazionale delle Ricerche (CNR). Project management is at the Servizio Attivita Spaziali (SAS) of CNR, while the experimental activity is under the scientific direction of the Centro Telecomunicazioni Spaziali of CNR at the Politecnico di Milano. Prime contractorship for the Sirio satellite has been entrusted to Compagnia Industriale Aerospaziale while the contractorship for the Sirio Operation Control Center and for the Sirio SHF ground stations has been entrusted to Telespazio. Telespazio is also giving technical and administrative assistance to CNR for the development of the program. NASA will take care of the satellite launch.

The Italian space program includes the implementation of a preoperational communication satellite, named Italsat. The paper explains the motivations of the Italsat program and gives a concise description of the related satellite and ground equipment. The preoperational system will allow to implement communication services with Italian coverage at 20 30 GHz (using advanced techniques such as SS-TDMA and onboard regeneration) and to perform propagation experiments at 40 50 GHz with European coverage.

Publisher Summary This chapter provides an overview of the features and the objectives of the Italian Sirio satellite program and the characteristics of its systems. The principal objective is to investigate the applicability of frequencies above 10 GHz to satellite communication systems through an SHF propagation and communication experiment. Sirio is a spin-stabilized geostationary satellite with a mechanically despun antenna for the propagation and communication experiment. The nominal spin rate is 90 rpm. The Sirio program is developed in the frame of the Italian space program by the Consiglio Nazionale delle Ricerche (CNR). Project management is at the Servizio Attivita Spaziali (SAS) of CNR, while the experimental activity is under the scientific direction of the Centro Telecomunicazioni Spaziali of CNR at the Politecnico di Milano. Prime contractorship for the Sirio satellite has been entrusted to Compagnia Industriale Aerospaziale while the contractorship for the Sirio Operation Control Center and for the Sirio SHF ground stations has been entrusted to Telespazio. Telespazio is also giving technical and administrative assistance to CNR for the development of the program. NASA will take care of the satellite launch.

In connection with the present development trends of the technology, and with the subsequent service rendering capability, the role of communication satellites in future is discussed. Three basic types of systems are considered: - for Mobile communications - for Point-to-Multipoint Fixed Communications - for Point-to-Point Fixed Communications The emphasis is put on point-to-point systems, which marked the beginning of satellite communications; for these systems it is easier to predict future developments. The use of frequency bands below 15 GHz will propably cover all requirements coming from Intelsat and from emerging countries. In the case of regional or domestic systems for highly developed countries, such as the U.S.A. and Europe, the local constraints will probably impose the use of the 2030 GHz bands, which would allow installing the Earth terminals close to traffic centers, with dramatic reduction in channel costs. Key features of these systems should be: - use of well stabilized platform with multibeam antennas - TDMA with switching onboard - Voice Signals Processing - Space Diversity, where needed