The requirements and the effects of impairments

For excellent quality in the home reception of TV signals from a satellite, a signal to noise ratio (SNR) of 45-50 dB will be required. An SNR of 40 dB will provide an acceptable picture. With the technological advances made since the World Broadcasting-Satellite Administrative Radio Conference (1977), systems noise temperatures of less than 500 K are in prospect. This will mean antenna diameters smaller than the 0.9 m specified in 1977. Since signals will arrive from two widely separated directions, a viewer wishing to receive programs from both must have two antennas or one capable of being slewed to the desired direction. One-way attenuation of the direct broadcast signal will derive from three sources: oxygen and water vapor in the atmosphere, absorption in cloud and fog, and rain and snow.