Satellite Multipath Propagation Channel Model for Links in the 1-10 GHz Range Zoran Blažević 1 , Igor Zanchi 2 , Ivan Marinović 3 1, 2, 3 Dept. of Electronics, University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, Ruđera Boškovića bb, 21000 Split, Croatia E-mail: 1 zblaz@fesb.hr, 2 izanchi@unist.hr, 3 imarin@fesb.hr Abstract: The paper presents a multipath propagation model for satellite channels, which includes in calculation the influence of the large-scale tropospheric refraction. This influence, as well as the influence of scattered fields for a specific scenario of multipath environment, is assessed via diagrams of the excess power, phase and Doppler power spectra calculated by the model for a L band frequency of 1501,5 MHz. 1. INTRODUCTION Mainly due to the possibility of achievement of a global coverage and their availability, satellite radio systems provide some essential advantages over their ground counterparts in a wide variety of services. Thus services like positioning, TV, forecast or maritime radionavigation and even Internet can no longer be imagined without implementations of the satellite technology. As the propagation modelling of a satellite channel is very often the first step in the design of a satellite system, existence of efficient procedures for predicting the receiving power of the signal transmitted by the satellite could be very helpful in fulfilling the task. However, the satellite radio propagation is coupled with the propagation through the atmosphere where the phenomena such as refraction, attenuation or depolarization could take place. Basically, two atmosphere layers can have a significant impact on the propagation: the troposphere and the ionosphere layer. The allocated frequencies for satellite systems are from the microwave portion of the spectrum from 1 to 40 GHz for which the ionosphere refraction is insignificant [7], so that only the phenomena that occur in the troposphere should be considered. Since the tropospheric propagation effects such as the attenuation by rain and clouds, scintillation and depolarization are not significant for the frequencies below 10 GHz, for the considered frequency range of 1-10 GHz only the large-scale tropospheric refraction effect remains. The famous model for the troposphere refraction that could serve a need for the propagation modelling is the effective Earth radius model [6,7], which is already well implemented in design of various ground radio systems. This model has also been used for the modelling of the satellite propagation channel, e.g. in [2] where the constant k-factor for the standard atmosphere of 4/3 has been applied. In [1], this model is updated by the implementation of the effective Earth radius that is corrected to the satellite elevation angle. However in the both applications the troposphere has been supposed to be spread beyond the satellite orbit, which is an assumption very far from the truth. Therefore, a more accurate procedure for predicting the large-scale refraction effect explained in the first part of Section 2 is implemented in the paper. In addition to the troposphere refraction, a significant impact of the environment that surrounds a receiver on the ground is usually noted because of the effects such like blockage, shadowing and multipath from the object settled on the ground or nearby. As those phenomena for the satellite signal take place only during its voyage via the last fraction of the radio-path, in the following text they are referred to as the "last-mile" propagation phenomena. Accordingly, the corresponding model is referred to as the "last-mile" propagation model. Those "last-mile" propagation effects have been analysed in a great number of the research papers. In order to simplify the analysis that follows let us keep our attention on the models based on the radar equation exclusively, such as in [3,4]. Thus, the "last-mile" model from [3] is upgraded for a specific scenario of the multipath propagation in the second part of Section 2. In Section 3, a comparison of the results obtained by the model is given. 2. SATELLITE PROPAGATION CHANNEL MODEL A global scenario of a satellite-ground link is presented in Fig. 1. The z'-axis is defined by the point of the satellite zenith with reference to the receiver and the centre of Earth, and x'-axis is perpendicular to the orbital plane. The satellite- to-ground geometry in the plane determined by the vectors R S and R E that define current positions of the satellite transmitter and the receiver from Fig. 1 respectively is shown in Fig. 2. Values of the Earth radius R E and of the relative troposphere height h T depend on the portion of the planet surface considered. In the analysis that follows, they are assumed to be 6366 km and 16 km respectively. The satellite travels via its orbit that may be inclined with reference to the equatorial plane and emits the signal toward a receiver settled somewhere on the Earth surface. During its propagation toward the receiver, it is assumed that the wave transmitted by the satellite passes uninterrupted prior reaches the troposphere margin. During its propagation through the troposphere the signal is subjected to refraction. The large-scale refraction occurs due to the changes in the mean value of the vertical refraction index of the troposphere,