Satellite Communication Systems
– Design Principles
Frequency and Propagation Considerations – Introduction
In chapter one we learnt that a satellite communication network utilizes radio links for inter-connections. The design of radio links is one of the most important consideration in implementing a satellite communication system. A large number of parameters must be considered and optimized. An important consideration in a link design is the selection of the operational frequency.
In this chapter we will develop a basic understanding of some of the basic criteria in the selection of frequency. The choice is mainly governed by propagation considerations, the need for various services to coexist without causing undue interference to one another, radio regulations and the prevalent state of technology. Additional factors governing a radio link design will be discussed in chapter 4.
An important requirement in selecting the operational frequency is that radio signals suffer minimum degradation while propagating through the intervening medium. Degradations include attenuation, scintillation, noise contamination, etc. Other important considerations include, the need to comply by the radio regulation to ensure co-existence of various systems and maximize the utilization of the limited frequency resource; economic considerations which is related to the state of technology; and the availability of the desired bandwidth in the preferred frequency band. An additional constraint, when expanding the bandwidth capacity of an operational system, is the need to ensure that the frequencies be chosen close to the operational frequency to minimize the need to upgrade existing equipment.
In general, an attempt is made to minimize the transmitted power because the transmitter cost is significant and directly related to its power requirements, especially on a spacecraft; and the potential of interference to other radio systems is reduced with low transmitted power.
In the following sections we will discuss the issues related to harmonizing the use of radio spectrum, followed by a detailed discussion of propagation effects at frequencies used in satellite communications