Satellite design and construction
- the elements that need to be considered for satellite design and
construction, and satellite systems design and other factors relating to the
design of a satellite.
Satellite design and construction is a particularly
specialised business. The requirements for satellites are very stringent and
satellites must be capable of operating in extreme conditions whilst still
maintaining the highest standards of reliability because they cannot be
retrieved for maintenance or repair. Apart from the general factors relating to
satellite design, the circuitry required for their operation such as the
transmitters and receivers, satellites also contain a number of systems used for
what is called station keeping. All of these functions, whether for performing
the primary role of the satellite, or for ensuring that it reliably maintains
its position and function are all important and must be included in the design
of the satellite.
Satellite position maintenance
Satellites need to be kept in the correct position. Although
they may be placed in exactly the correct orbit after they are launched, the
variations in the Earth's gravitational field and other factors may cause them
to drift out of their correct position. As a result it is necessary to
reposition them periodically.
Small thrusters are used to perform this operation. Often
they consist of canisters of a gas which when released with a catalyst gives a
form of rocket propulsion to move the satellite back on station. Often the
service life of a satellite is determined by the amount of fuel for
repositioning the satellite rather than the reliability of the electronics.
The other problem with a satellite is that its attitude will
change. This is of great importance because directive antennas or cameras are
often used, and the satellite needs to be orientated in the correct direction
for them. The basic method of gaining the correct orientation is to use the
thrusters. However the attitude will change comparatively quickly. The most
common method to overcome this is to use the gyroscopic effect. Sometimes a
large flywheel may be made to spin inside the satellite. This can be inefficient
in its use of the weight of the satellite. To overcome this other cylindrical
satellites actually rotate a portion of the body, often an inner cylindrical
section so that the antennas mounted on the outer section do not revolve.
Satellite power
Electrical power is also required by the satellite for its
electronic circuitry and other electrical systems. Although the power
requirements for some satellites may be relatively modest, this is certainly not
the case for satellites such as direct broadcast (DBS) or satellite television
broadcasting satellites. Although they do not transmit the same levels of power
that are used for terrestrial broadcasting, they still consume considerable
amounts of power.
The power is supplied by the large arrays of photo or solar
cells. Some cylindrical satellites have them positioned around the outer area on
the cylinder so that some part of the body is always exposed to sunlight. Others
have large extending panels that are orientated to collect the maximum amount of
light. Today these panels are capable of producing the many kilowatts of power
required for the high power output stages used in many transponders.
Batteries are also needed for the periods when the satellite
is in darkness. These need to charged by the solar cells so that when the
satellite passes out of the sunlight it can still remain operational. This
naturally places an additional burden on the solar cells because they need to be
able to power not only the satellite itself, but also charge the batteries. This
may double the power they have to supply during periods of sunlight.
Satellite antennas
The antennas used on satellites are particularly important.
They are the only means through which communication can take place with the
ground. For geostationary satellites directional antennas are generally used.
These are used because power consumption on the satellite has to be minimised
wherever possible. Directional antennas provide gain and enable the best use to
be made of the available transmitted power. Additionally they enable the signals
from the earth to be received with the best signal to noise ratio. In view of
the long path lengths required for geostationary satellites, there is a
considerable path loss and the antenna gain is used to improve the received
signal strength. It also helps reduce the reception of solar and cosmic noise
that would further degrade the received signal. In a geostationary orbit the
earth subtends only 18 degrees of arc. Any power not falling into this area is
wasted.
As a result, parabolic reflector or "dish" antennas are
widely used. Horn antennas are also popular and in some cases phased arrays may
be employed, especially where coverage of a specific area of the globe is
required. However the use of directional antennas does mean that the orientation
of the antenna is crucial, and any perturbation of the alignment of the
satellite can have a major effect on its operation, both in reception and
transmission.
The situation is different for low earth orbit or LEO
satellites. These satellites are not in geostationary orbit and they move across
the sky. Additionally they may need to be received by several users at any time
and this means that they cannot use directive antennas. Additionally the earth
subtends around half the celestial sphere and as a result users may be separated
by angles ranging from zero to almost 180 degrees. Fortunately the satellites
are much closer to the earth and path losses are very significantly less,
reducing the need to high gain antennas.
Environmental
The environment in which satellites operate is particularly
harsh. Combined with the need for exceedingly high levels of reliability
resulting from the near impossible task of repair, this means that every detail
of the design and operation under these conditions must be carefully considered.
In the first instance the temperatures range over very wide
extremes. The surfaces exposed to the sun are heated by solar radiation and will
rise to very high temperatures, whereas the other side that is not heated will
be exceedingly cold. Only conduction will give any heating effect under these
circumstances. The temperature of the whole of the satellite will also fall when
it is in darkness.
There are a number of other effects that must be considered.
Solar radiation itself has an effect on some materials, causing them to degrade.
Notice must also be taken of meteorites. Very small ones cause the surfaces to
be eroded slightly, but larger ones may penetrate the body of the satellite
causing significant damage. To overcome this satellites are protected by
specially designed outer layers. These consist of sheets of metal which are
slightly separated giving a cushioning effect when any meteorites impact on the
satellite. Cosmic particles also degrade the performance of satellites.
Particularly during solar flares the increase in solar particle flow can degrade
solar cells, reducing their efficiency.
Ground stations
Ground stations also need an effective antenna system. For
communication with satellites in geostationary orbit the antenna remains fixed,
except if there is a need to change to another satellite. Accordingly parabolic
reflectors are often used. This can be seen from the number of satellite TV
antennas that are in use. These are a form of parabolic reflector. This type of
antenna is widely sued for example with direct broadcast satellite TV. The
antennas seen on the sides of houses are almost exclusively parabolic
reflectors. However it is possible to use other types such as arrays of Yagi
antennas. Here they are stacked (one on top of the other) and bayed (side bay
side) to give additional gain.
For some low earth orbit satellites the ground station
antenna systems are designed to be able to track the satellite in azimuth and
elevation. This is typically achieved by automatically tracking the satellite as
it moves across the sky. This is normally achieved by taking a signal level
output from the receiver. By ensuring that it is maintained at its peak level
the satellite will be tracked. Many low earth orbit satellites are required for
systems such as positioning or even telephone style communications. Here
directional antennas are not used as the user will not want to re-orientate the
antenna all the time. Instead almost non-directional antennas are used and the
transmitter powers and receiver sensitivities designed to give a sufficient
level of signal to noise ratio. This is the case for GPS where several
satellites need to be received at the same time. Accordingly receivers are
designed to accommodate the very low signal levels.
Satellite design summary
Satellites are in everyday use around the globe. Not only are
functions such as GPS widely used, but so are many other forms of satellite
including communications satellites, weather satellites, geophysical satellites
and many forms of satellites. Without them, our lives would be very different,
and we have come to rely on them. Accordingly their design must be made to be
very reliable as they are not easy to repair, even if it is viable - which is
normally not the case, and they operate under extremely harsh conditions.
Accordingly the design of a satellite is critical. The satellite design
specifications need to be totally correct, and the basic satellite design needs
to take all aspects into consideration, making any satellite design project
particularly exacting. However if correct, the satellite will normally be able
to give many years of service and pay for the large investment in the satellite
design and construction.
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