Satellite Communications and Space Weather

Introduction

Satellite communication is normally thought of as a robust means of communication, not sensitive to environmental impacts. This perception is not totally accurate. Satellite communication can be and is affected by the environment in which it operates.

Space environmental effects on satellite communication can be separated into (1) effects on the space element (ie the satellite), (2) effects on the ground element (ie the Earth station), and (3) effects on the signals propagating through the Earth's lower and upper atmosphere.

Effects on the satellite depend on the orbit of the satellite. Geosynchronous satellites in the highest orbits are susceptible to bursts of high energy particles that are infrequently emitted from the sun. These particles may cause (1) memory upsets, (2) dielectric charging and (3) radiation damage to components. This can result in operational glitches, degradation of service or in extreme cases, loss of the satellite.

Satellites in low Earth orbits are less susceptible to particle damage, except over the polar regions, but may suffer increased orbital decay (and consequent reduced lifetime) when increased solar activity occurs.

Ground station downlink sensitivity is affected by noise sources in the beam of the receiving antenna. This can include sky noise and solar noise. The effect is dependent on frequency of operation.

The propagating signal may be affected by its passage through the ionosphere (upper atmosphere) or the troposphere (lower atmosphere). These effects depend significantly on frequency, but include signal absorption, scintillation, Faraday rotation and bandwidth decoherence. Geographic location and signal propagation path can determine the extent to which the signal is affected.

The following sections discuss each of the above effects in more detail.

Satellite effects

Space is not the benign environment that was once thought. It is traversed by small pieces of matter (meteoroids) and also by a large and variable radiation flux. The radiation field in Earth orbit comes from three sources, galactic cosmic radiation, trapped radiation belts (the Van Allen belts) and solar radiation. The first two sources are particulate radiation, mostly protons and electrons. Solar radiation is both electromagnetic and particulate (atomic and sub-atomic).

Galactic cosmic radiation consists mostly of very high energy protons that form a constant low level background radiation source. These particles are not particularly significant for communications satellites, but present a possible hazard for long duration manned spaceflights to other planets. Occasionally they may be responsible for memory upsets in communication satellites.

http://www.ips.gov.au/Educational/1/3/2

Solar Radio Frequency Interference and Scintillation

The Sun is a strong, highly variable, broad-band radio source. At times, the Sun is within a side-lobe or even the main beam of a ground antenna looking at a satellite, usually pointed within about 1 degree of the Sun. If the Sun happens to produce a large radio burst during that time, the signal from the spacecraft can be overwhelmed. Large solar radio bursts occur most frequently during solar maximum years. An operator should be aware of when the Sun is in close proximity to the satellite being tracked. The SWPC, through reports from U. S. Air Force radio observatories, catalogs solar radio burst occurrences. (See Activity Summaries and Solar Event Lists )

At times, the ionosphere becomes highly irregular causing satellite signals to band inhomogeneously when they transit this disturbed medium, and scintillate at the receiver. Strong geomagnetic storms can cause scintillation in the auroral zones. Additionally, scintillation is problematic for signals traversing the equatorial ionosphere. In this area, large rising turbulent plumes form in the afternoon and evening ionosphere, resulting in rapidly varying, significant signal loss. Not only does this affect telemetry up/downlink but, GPS users can lose tracking of enough spacecraft so as to make location finding difficult.


http://www.swpc.noaa.gov/info/Satellites.html