Ground station interference

A satellite ground station is responsible for receiving signals from a satellite and possibly transmittingcommands and/or communication material to a satellite. For the reception of satellite signals the groundstation employs an antenna to "capture" the satellite signal. All antennas have a beamwidth, an angular rangeover which they can detect a signal, which is determined by the size or aperture of the antenna and thefrequency of operation. The larger the antenna, the smaller will be its beamwidth, and the higher the frequencyof 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 ofthe signal of interest must be at least 10 dB (a factor of ten) above whatever background noise is present, tobe useable.

Noise in a system comes from two sources; internal and external. Every receiver generates some internalnoise. This may be minimised by careful design, but cannot be entirely eliminated, and eventually sets a limiton all communication. External noise enters via the receiving antenna, and comes from any sources (otherthan the desired satellite) that may coincidentally lie within the beamwidth of the antenna. The sun, with atemperature that may vary from 6000 to 2 million degrees, is a strong source of radio noise, moving acrossthe sky daily and possibly entering the beam of the receive antenna. For geosats this tends to happen aroundthe equinoxes (March and September), when the declination (celestial latitude) of the sun equals the apparentdeclination 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) abovethe typical C-band (4 GHz) satellite TV transponder.

All satellite communications are subject to "sun-outages" described above. Systems that have smallbeamwidths and high SNR's will be most resistant to a sun-outage. Systems with large beamwidths and lowsignal to noise ratios will be more affected. The sun's radio noise also increases with frequency, so K bandsystems 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 beaffected during an energetic solar event where the radio noise output at all frequencies can rise by severalorders of magnitude.

Effects on the propagating signal

Ionospheric Effects

The ionosphere is a region of the upper atmosphere that extends from about 70 to 500 km in altitude. It is aregion where some of the atoms have had their outer electrons removed by extreme ultra-violet (EUV) and X-ray radiation coming from the sun. The atmosphere is thus said to be partially ionised - hence the name,ionosphere. An ionised gas is also referred to as a plasma. A plasma is conductive, and because of this it willinteract with electromagnetic signals (eg radio waves) that pass through it. In fact, below a certain frequency(the plasma frequency, which is proportional to the square root of the plasma electron density), a plasma willreflect a radio signal incident upon it. It is this property that allows the propagation of high frequency (HF) orshortwave signals around the globe. Above the plasma frequency, a signal will not be reflected, but will still berefracted or bent as is traverses the plasma. The higher the frequency of the radio signal, the less the bending.

The ionosphere is not a uniform layer of plasma. Its density changes with time of day, altitude, latitude, seasonand solar activity. It may contain irregularities such as patches, clumps, and troughs of ionisation. It is also adispersive medium, that is, one through which signals of different frequencies travel at slightly differentvelocities. At the lowest altitudes it also tends to absorb radio waves rather than reflect them. All thesecharacteristics cause a variety of effects on propagating electromagnetic signals.

 

Direct absorption at low altitudes and reflection at higher altitudes only occurs for signals below about 30 MHz,so these are not usually a problem for satellite communications. However, refraction (bending) and dispersionare important issues for satellite links. In a uniform ionosphere, refraction is an important consideration forradars tracking space objects as it causes them to see the object in a position displaced from the true one. Thisis the same as looking at an object immersed in water which appears displaced from where it really is, due tothe refraction (bending) of light. Dispersion causes signal delay and differential delay in widebandcommunication systems that can be a problem. Another phenomenon, the Faraday effect, occurs when a signalpropagates through a plasma in the presence of a magnetic field. The result is a rotation of the plane ofpolarisation of a plane polarised signal.

When the ionosphere contains irregularities, we are faced with the phenomenon of scintillation. This effect isconsidered the following section.

Scintillation

Ionospheric scintillation is a rapid fluctuation in the signal strength of a trans-ionospheric signal (eg fromsatellite to ground station). Effectively scintillation introduces an additional low frequency noise component onthe signal.

Scintillation, which is similar to the visible twinkling of stars in the night sky, is caused by small-scaleirregularities in the ionosphere. That is, instead of a uniform layer of ionisation, certain regions of theionosphere are subject to patches of lower or higher density ionisation.

These irregularities preferentially form in two different regions over the Earth - the polar, or more correctly theauroral regions (both north and south), and the equatorial regions. In the polar regions, ionosphericirregularities are caused by particles precipitating down into the ionosphere from the magnetosphere (the sameparticles that produce the visible aurora). Flows of these particles cause bubbles and troughs which are notstable and at whose edges scintillations are the strongest. Auroral scintillations may occur at any time of day,but tend to be stronger at night, and when geomagnetic activity is high (ie during geomagnetic storms). In theequatorial regions, after sunset, bubbles of ionisation form at the bottom of the ionosphere and rise upwardduring the night forming vertical plumes (which can also be moving horizontally). Signals that propagate nearthe edges of these plumes are subject to the most intense scintillations. Equatorial scintillations are thusbasically a night-time phenomenon, with most of the plumes disappearing by midnight, although some dopersist into the early morning hours. Equatorial scintillations increase in strength as the sun's extreme ultra-violet (EUV) and X-ray output increases (which produces a thicker and more strongly ionised ionosphere). Thustheir intensity follows the approximately 11 year solar cycle. They also display a 27 day periodicity due to thesolar rotation, since the EUV producing plage is distributed unevenly across the solar surface.

Both the phase and intensity of a trans-ionospheric signal are affected by scintillations. Intensity fluctuations,which may occasionally be large enough to cause deep signal fades, are not caused by signal absorption withinthe ionospheric irregularities, but rather by a phase change of various parts of the signal wavefront.Constructive and destructive interference of various signal paths as observed on the ground, produce theobserved changes in signal strength.

It is equatorial scintillations that are of most significance for geosynchronous satellites. These are found to be

greatest within 20 degrees north and south of the geomagnetic equator, which is close to the geographicequator. A signal that transits the ionosphere at a geomagnetic latitude of around 15 to 20 degrees will bemost affected. The geomagnetic equator changes as a function of longitude. Over parts of the world from themiddle east to Australia, the geomagnetic equator is well north of the geographic equator.

The shaded areas show the approximate regions that are generally affected by medium to strong scintillations.The most intensely affected regions lie toward the north and south edges of the shaded areas.

To determine whether a particular signal path will be affected by ionospheric scintillation, it is necessary todetermine the geomagnetic latitude of the ionospheric penetration point (IPP) of the trans-ionospheric signal.The table below shows the IPP for signals received at Darwin (Australia) from a range of geosynchronoussatellites. Note that many of these are close to 20 degrees south (geomagnetic), and will thus be subject toionospheric scintillation, which will increase to the east and also toward solar maximum as overall electrondensity increases. Scintillation effects, when they occur, will become apparent just after local sunset, and willdiminish after local midnight. By 3 am local time, all signal scintillations should have disappeared.

Ionospheric scintillations are worse at lower frequencies than high. VHF frequencies, such as the 240 MHzfrequencies used in military communications, suffer the most, while L- band is moderately affected, and onlythe strongest scintillations affect C-band and above.

Faraday Rotation

 

When a plane polarised radio signal travels through a plasma, such as the ionosphere, in which a magneticfield (such as the Earth's magnetic field) is also present, the plane of polarisation of the signal is rotated. Theamount of rotation is proportional to the magnitude of the magnetic field and the total ionisation through whichthe signal passes. It is also inversely proportional to the square of the frequency of the signal.

Low frequencies thus suffer significantly greater rotation than high frequencies. As the ionosphere becomesmore ionised, due to increasing solar EUV and X-ray flux, signal rotation also increases. Signals traversing theionosphere at low elevation angles will be affected more than signals propagating near the zenith, because thetotal distance traversed through the ionosphere will be greater.

At VHF frequencies and high solar activity levels, the plane of polarisation may be rotated through many times360 degrees. At C-band frequencies (4 GHz) the signal rotation will rarely exceed a few degrees.

The problem caused by a rotation of the plane of polarisation of a signal is two-fold. If a ground station isexpecting to receive a horizontally polarised wave, then a 90 degree rotation could result in a total loss ofsignal. However, much smaller rotations can cause problems in satellite communication systems that use bothvertically and horizontally polarised signals of the same frequency (ie, spectral re-use through orthogonalpolarisations). In this case a small rotation will barely affect the received signal strength of the desiredpolarisation, but it will allow some of the orthogonally polarised signal (the undesired channel) to cross-coupleinto the antenna. This results in interference to the desired signal. A rotation as small as 5 degrees may couplein the undesired channel to a level only 20 dB below the desired channel.

The graph below shows a typical variation of ionospheric total electron content throughout a day of highbackground solar activity, the resulting Faraday rotation that is produced in a C-band satellite TV downlink, andthe orthogonal power ratio coupling that results. The scale on the right indicates the ratio of the cross-couplinginterference to the main signal, in this case reaching around -20 dB (or a ratio of 1 to 100).

Tropospheric Absorption and other Effects

It is not only the upper atmosphere that can cause problems with satellite communication links. Troposphericweather conditions, "conventional" weather occuring in the lower 10 km of the atmosphere, can also causelosses in signals propagating between satellite and ground stations.

Water vapour is particularly damaging to signals above about 2 GHz, causing absorption of signals whichbecomes greater as the frequency increases. K-band signals (10-20 GHz) are particularly susceptible, andprecipitation in the vicinity of satellite ground stations can cause total loss of signal. Again, signals with lowelevations are more affected than those propagating near the zenith, because the wave has to follow a longerpath through the atmosphere.

At frequencies above 20 GHz we start to encounter resonant absorption at specific frequencies. Oxygen, inparticular, will absorb electromagnetic energy only at certain well-defined frequencies. These frequenciescorrespond exactly to the energies required to lift the Oxygen atoms into higher energy states. Satellitecommunication links are designed to avoid these well known frequency bands.