Satellite propagation
- an overview of the phenomena that affect satellite propagation and the way
the radio signals propagate to and from the satellite.
Satellites are used for a wide variety of applications from
satellite TV broadcasting, and navigation in the case of GPS to photography,
weather monitoring and many more applications. For the transmission and
reception of signals, satellites normally use frequencies above 500 MHz, and
often much higher. At these frequencies the transmissions to and from the
satellites might be thought to be immune to the effects of the atmosphere.
However this is not the case and effects are introduced primarily by two areas
of the atmosphere. One is the troposphere and the other is the ionosphere.
Accordingly satellite propagation or the radio propagation characteristics of
the path between the ground and the satellite of great importance.
The atmosphere
The atmosphere can be divided into several areas. It is found
that the temperature varies according to the height. Initially the temperature
falls until altitudes of around 10 km are reached. At this point the temperature
is around -50 or -60 Celsius. It is around this point that the temperature
starts to rise again. The region below this inflexion point is known as the
troposphere.
The second area that affects radio signals is known as the
ionosphere. This is a region of the atmosphere that starts at altitudes of
around 50 km, and extends to more than 400 km. In this region radiation from the
Sun mainly in the form of ultraviolet light strikes the gas molecules and atoms
causing them to ionise producing a positive ion and a negative electron. These
electrons then affect radio signals, effectively reflecting those in the short
wave bands and often returning them to Earth. However this region also affects
the signals passing to and from satellites.
Beyond the ionosphere the signals can be considered to be in
free space, and the region between the upper reaches of the troposphere and the
ionosphere is often temred "inner free space." This region too has little
effect.
There are a number of different of radio signal propagation
effects that are introduced into satellite radio signals by the troposphere and
ionosphere. Transmission in free space has unity refractive index and is
loss-less (apart from the spreading effect that reduces the signal power over a
fixed area with distance away from the source, but no power is actually lost).
The troposphere and ionosphere have refractive indices that
differ from unity. The troposphere is greater than unity and the ionosphere is
less than unity and as a result refraction and absorption occur. The inner free
space region also has little effect.
Faraday rotation
Faraday rotation is an effect that affects satellite
propagation. Faraday rotation results from the fact that the ionosphere is a
magneto-ionic region. The Faraday rotation of a signal causes different elements
of a signal to travel in different ways, particularly rotating the plane of
polarisation. This can create some problems with reception. A linearly polarised
signal can be considered as two contra-rotating circularly polarised signals.
The phase velocities of these two signals vary in a magnetic medium such as the
ionosphere and as a result the polarisation of the signal changes. The degree of
change is dependent upon the state of the ionosphere and it follows the same
pattern as that experienced for HF ionospheric communications changing over the
course of the day, with the seasons and over the sunspot cycle.
Ionospheric scintillations
Another satellite propagation effect introduced by the
ionosphere is termed "ionospheric scintillations." These scintillations manifest
themselves as a variety of variations of amplitude, phase, and polarisation
angle. They can also change the angle of arrival of the signals. These
variations change over a period of between one to fifteen seconds, and they can
affect signals well into the microwave region.
The variations are caused primarily by the variations in
electron density arising in the E region, often as a result of sporadic E but
also in the F layer where a spreading effect is the cause. The level of
scintillation is dependent upon a number of factors including the location of
the earth station and the state of the ionosphere, as a result of the location,
the sunspot cycle, the level of geomagnetic activity, latitude, and local time
of day.
The scintillations are more intense in equatorial regions,
falling with increasing latitude away from the equator but then rising at high
latitudes, i.e. in the auroral zone or the region where auroras take place. The
effects are also found to decrease with increasing frequency, and generally not
noticeable above frequencies of 1 - 2 GHz. As such they are not applicable to
many direct broadcast television signals, although they may affect GPS, and some
communications satellites.
Tropospheric satellite propagation effects
There are a number of satellite propagation effects that the
troposphere introduces including signal bending as a result of refraction,
scintillation, and attenuation.
The signal refraction in the troposphere is in the opposite
sense to that in the ionosphere. This is because the refractive index in the
troposphere is greater than unity, and it is also frequency independent. The
signal refraction gives them a greater range than would be expected as a result
of the direct geometric line of sight. Tropospheric ducting and extended range
effects that are experienced by terrestrial VHF and UHF communications may also
be experienced when low angles of elevation are used.
Scintillations induced by the troposphere are often greater
than those seen as a result of the ionosphere. They occur as a result of the
turbulence in the atmosphere where areas of differing refractive index move
around as a result of the wind or convection currents. The degree to which the
scintillations occur is dependent upon the angle of inclination, and above
angles of around 15 degrees the effect can normally be ignored. At angles
between 5 and 10 degrees the changes can often be around 6 dB at frequencies of
around 5 GHz.
Doppler shift
Frequency changes as a result of the Doppler shift principle
may be in evidence with signals from some satellites. This form of satellite
propagation effect is important because it plays a major part in the systems
design of a satellite system. The shifts resulting from the Doppler phenomenon
need to be taken into account as part of the overall design.
Satellites in Low Earth Orbits move very quickly and as a
result a Doppler frequency shift is apparent in many cases. With the satellite
moving towards the earth station the frequency appears higher than nominal, and
then as it moves away the apparent frequency falls. The degree of shift is
dependent upon a number of factors including the speed of the satellite (more
correctly its speed relative to the earth station) and the frequencies in use.
Shifts of the order of 10 kHz may be experienced. As most satellites operate in
a cross mode configuration, the Doppler shift is not just applicable to the band
on which the signal is received, but to the cumulative effect of the uplink and
downlink transmissions. In many instances the effects will subtract because of
the way the satellite mixing process is configured.
Summary
Although satellites generally operate at frequencies that may
be thought to be immune from tropospheric and ionospheric disturbance, these
regions still have a significant effect and this needs to be taken into account
when designing satellite systems. As a result satellite propagation, i.e. the
radio signal propagation to and from the satellites of great importance.
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