 horizontal polarisation doubles the usage of the 500 MHz bandwidth) [9]. Path Loss in Space As discussed above, most satellite communications systems are designed to operate with a line-of-sight between the transmitter and receiver. Natural structures generally will not block this line-of-sight, in contrast to terrestrial communications where natural and man-made objects may block a link. Satellite Communications DLA = Driver/limited/amp Wideband receiver (500 MHz bandwidth) Figure 11.6 Transponder functions However, there exists another limitation which is faced by all telecommunications systems. This is signal weakening as a signal traverses a carrier medium. The distance of a satellite from the Earth can be from hundreds to tens of thousands of kilometres, and the path loss over this distance can be very large. Therefore the signal arriving at a receiver is very weak compared with locally added noise. Signal amplification through strong antennas on the ground and on- board the satellite is necessary. Such antennas must face a point in space where the satellite is - meaning that the satellite needs to be stationary with respect to an observer on the Earth. Although a line-of-sight exists between a transmitter on the Earth and a satellite in space, the signals transmitted to and from the satellite do travel a very long distance. Only a small portion of the signal energy transmitted through an antenna arrives at a receiver, as the trans- mitted energy is dispersed in space. The ratio of received signal power compared with trans- mitted signal power is a function of the distance between the two and the frequency of operation, as well as characteristics of the medium. For line-of-sight communications through space the path loss (L) in decibels is calculated from the following equation: =32.4+201og(d)+20log(f) where d is the distance in kilometres and f is the carrier frequency in MHz. Example 11.1 What is the path loss for a signal in the C band (4.0 GHz) from a satellite positioned 36 000 km from the receiver ant