ntennas have 146 elements. figure of merit (G/T) of the antenna and receiver combinations of the various earth stations involved. Wide deviation FM is widely used for analogue television signals and frequency division multiplex (FDM) multi-channel telephone baseband aggregates. When used for single speech channels, a valuable saving in power can be obtained by suppressing the carrier when the speaker is silent. The concentration of spectral energy in the neighbourhood of the carrier frequency of an FM signal may necessitate the application of a carrier energy dispersal waveform to the baseband of a television emission or a high capacity FDM telephone emission, to meet the PFD constraint referred to earlier, if the down-link frequency allocation is shared with terrestrial radio services. For digital signals, phase shift keying (PSK) is most commonly used, 2-phase or 4-phase. For 2-phase PSK a clear sky C/N ratio of 8.4dB, plus a small rain and implementation margin, is sufficient 14/11 GHz spot beam and for 4-phase PSK the ratio should be 3dB larger. However, some 6/4 GHz zone beam form of forward error correction (FEC) is often used, especially 6/4 GHz hemi beam where the G/T of the earth station receivers is low and this permits satisfactory operation with a significantly lower C/N ratio. Such emissions carry all kinds of digital signals, ranging from single speech channels, through time division multiplex (TDM) multi- Figure 51.10 Approximate aerial beam coverage of channel telephone aggregates and data systems of a wide range of INTELSAT V spacecraft for the Atlantic Ocean region information rates to digital television signals, although the last mentioned are usually subjected to some form of bit rate reduction video signal processing. 51.4.5 Modulation techniques Wide band digital emissions may exhibit strong spectral lines under idle circuit conditions, and it may be necessary to add to the Up to the present time the choice of modulation technique for use modulating signal at the transmitting earth station a