- Design Principles
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.