The geosynchronous orbit is now used by many satellites for routine
telecommunication and broadcast purposes. Such satellites appear to be
stationary as viewed from the ground, and are able to provide coverage over
large areas. Although such communication is not subject to the vagaries
of the ionosphere, it is subject to interference from the Sun. Around the
time of equinoxes each year, the Sun tends to pass behind the line of sight
of geostationary satellites at some time during the day. Since the sun emits
strong radio signals across a wide range of frequencies, it is often
difficult to pick out the desired satellite signal from the radio noise of
the sun.
The level of interference that will be experienced depends upon
the frequency of operation, the antenna beamwidth,
the receiver bandwidth, the acceptable signal to noise ratio, and the level
of solar activity at the time.
The Sun's output at VHF is relatively low and few communications systems
are affected unless they are very wideband or there happens to be a large solar radio
flare at the time the Sun is in the antenna beam. The Sun's radiation output
is greater at higher frequencies however. At 4 GHz the solar
radio emission exceeds the signal typically received from a television
broadcast satellite by about 20 dB (a factor of 100).
The graph below indicates the days on which the maximum solar interference
will occur for a given latitude. A significant but lesser interference
will occur for about a week on either side of this date. The duration of
the interference reaches a maximum of about 30 minutes on the day of maximum effect,
decreasing to either side. The time of day at which the interference
occurs depends upon the relative position of the satellite. A satellite
in the western sky will be subject to interference in the afternoon and in
the eastern sky in the morning.
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