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  • Communications Effects of Space Weather
  • Ground station interference about All Band for Space Weather

Ground station interference about All Band for Space Weather

  • A satellite ground station is responsible for receiving signals from a satellite and possibly transmitting commands and/or communication material to a satellite. For the reception of satellite signals the ground station employs an antenna to "capture" the satellite signal. All antennas have a beamwidth, an angular range over which they can detect a signal, which is determined by the size or aperture of the antenna and the frequency of operation. The larger the antenna, the smaller will be its beamwidth, and the higher the frequency of operation, the smaller the beamwidth. Any received signal has to compete with background noise. A parameter known as the signal to noise ratio (SNR) determines whether the antenna receives a useable signal. For normal communications, the power of the signal of interest must be at least 10 dB (a factor of ten) above whatever background noise is present, to be useable. Noise in a system comes from two sources; internal and external. Every receiver generates some internal noise. This may be minimised by careful design, but cannot be entirely eliminated, and eventually sets a limit on all communication. External noise enters via the receiving antenna, and comes from any sources (other than the desired satellite) that may coincidentally lie within the beamwidth of the antenna. The sun, with a temperature that may vary from 6000 to 2 million degrees, is a strong source of radio noise, moving across the sky daily and possibly entering the beam of the receive antenna. For geosats this tends to happen around the equinoxes (March and September), when the declination (celestial latitude) of the sun equals the apparent declination of the geosat. When this occurs, the satellite signal must compete against the solar noise signal. Even at times of low solar activity, this signal is typically about 20 decibels (a factor of 100 in power) above the typical C-band (4 GHz) satellite TV transponder. All satellite communications are subject to "sun-outages" described above. Systems that have small beamwidths and high SNR's will be most resistant to a sun-outage. Systems with large beamwidths and low signal to noise ratios will be more affected. The sun's radio noise also increases with frequency, so K band systems will often be at greater risk than C band systems.
  • Even satellite communication systems which have proven robust on occasions of quiet solar conditions, may be affected during an energetic solar event where the radio noise output at all frequencies can rise by several orders of magnitude.

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