SATELLITE
COMMUNICATION
SYSTEMS
INTRODUCTION
Essentially, satellite radio communications systems are an extension of the relay systems
that have been developed for terrestrial communications, with the difference that the
receiver/transmitter, known in this case as the transponder, is now located in space. In
spite of the high costs involved, the flexibility and advantages gained allow satellites
to make a valuable contribution to world-wide communications. To some extent, the high
costs are due to the lack of service access to the satellite. This involves the use of
on-board equipment redundancy and the provision of back-up satellite, to ensure continuity
of service. The high level of reliability of current technology is such that the
operational lifetime of a satellite is now in the order of 15 years.
A single satellite can provide communications coverage for almost one third of the earth's
surface using much less radio frequency power than would be required for an equivalent
terrestrial system. All the propagation parameters are well defined, well understood and
capable of being accurately modeled mathematically. Even though the signal attenuation due
to the long path lengths involved is high, it is fairly constant. A signal fade margin
allowance of only about 3 to 5 dBi needs to be made to account for the variability due to
local atmosphere and weather conditions. For the equivalent earth-bound system, an
allowance of more than 30 dBi may be necessary.
In this chapter, we shall try to provide the reader with some basic
knowledge about the components of a satellite communication system such as: earth station,
antenna, orbits, inter-satellite link and spacecraft. A discussion of link analysis and
reliability of such systems is also included.
13.1. EARTH STATION
A satellite communication depends mainly on two components: the satellite itself, located
usually at 36,000KM from see surface where there is no gravity, cooperating with an earth
station connected in the frequency band 4 - 6 GHZ. An earth station is any transmitting or
receiving system that sends signals to, or receives signals from, a satellite. The earth
station may be located on a ship at sea or on an aircraft. These earth stations cost
millions of dollars and are used to link major cities in domestic and international
satellite communication systems.
The earth station is connected directly with local exchanges and with the international
exchange by microwave or optical fiber links. It is important that earth stations do not
suffer interference from other communication systems operating in the same frequency band
and that transmitting stations do not cause interference to other systems. Earth station
multiplexes many voice or data signals together and provide links to many countries
simultaneously and that requires base band and IF equipment and frequency coordination.
The main components of an earth station are:
1. Antennas (Satellite antenna, Microwave antenna)
2. Control room
3. Terrestrial link
4. Ground Communication Equipment and RF section
5. Wave guide section
Antenna axis
Local Horizon
Power
supply
Monitoring
Duplexer Tracking
and
Control
RF
IF Baseband signals
High power
Modulator
(from users)
Amplifier
RF
IF Baseband signals
Front end
Demodulator
(to users)
(low noise)
Figure 13.1 - The architecture of an earth station
The antenna is the most visible part of an earth station. It can be as large as 30m or as
small as 0.7m. Figure 13.1 shows a typical architecture of an earth station.
13.1.1. Earth Station Design
An earth station is characterized by a figure of merit given by the gain to noise
temperature ratio (G/T). The main equation is:
(C/N) = Pt ( Gt / kB ) [l / 4 p R] ( Gr / Ts )
where
C/N : the carrier to noise ratio in the receiving channel which sets the
communications capacity and performance link (5®25dB)
Pt : transmitted power
Gt : transmitting antenna gain
Gr : receiving antenna gain
Ts : the system noise temperature at the frequency of transmission and
it is desirable to reduce it to zero to obtain a high gain to noise temperature ratio.
The noise sources that affect Ts are the atmosphere, the receiving system wave guide and
the low frequency amplifiers.
The place of an earth station must be chosen such as to satisfy the following criteria:
1. Hard land for the heavy equipment
2. Far away from volcanoes
3. Far away from earthquakes
4. Far away from the city (15KM)
5. Proximity the power supplies
13.1.2. Standards Defined by International Organizations
The international organizations for satellite communications have defined various
standards for earth stations operating in connection with the satellites they operate.
These standards specify numerous parameters particularly the figure of merit and services
rendered.
INTELSAT
International Telecommunication Satellite (INTELSAT) operates 16 spacecrafts The network
is controlled from the spacecraft technical control center in Washington DC. INTELSAT has
109 members and provides service to over 600 earth stations in more than 149 countries,
territories and dependencies. Technical details of INTELSAT spacecraft and member country
earth station operation are adopted by a board of governors which follows general rules
established by meeting of signatories.
INTELSAT manages 36000 voice circuits and 2 TV channels and classifies earth stations as
follows:
· Standard A : first generation, more than 250 stations of this type are operated in the
world.
· Standard B : makes routes of the order of tens of channels profitable which is not the
case with standard A stations.
· Standard C : it is an expensive type of station, principally suited to heavy traffic
routes.
· Standard D : for telephone and low density links.
· Standard E and F : for international business service.
· Standard Z and G : for transponder reservation services for national and international
use respectively.
INMARSAT
International Marine Satellite (INMARSAT) helps in case of mobile communication (ships,
airplanes). A small mobile station fixed on the moving body is able to communicate with
INMARSAT then to an earth station. If the mobile is not visible to the satellite it uses
an intermediate earth station. Services provided by INMARSAT cover:
· On land : journalists - traveling - executive - agencies - government official - trains
- companies - construction workers
· At sea : fishing boats - yachts - cargo ships - drilling rings - petrol prospect
Different standards have been defined for stations on ships:
· Standard A : permits transmission of several data and telephone channels in SCPC/FM
mode.
· Standard B : permits transmission of low rate data and telephony with data rate
compression.
· Standard C : permits transmission of very low rate messages. It also provides the
possibility of exchanging messages between a boat and a terrestrial subscriber. In
addition it allows transmission of distress signals within a worldwide distress and
maritime security system.
Comparison between INMARSAT and INTELSAT
INMARSAT INTELSAT
· uses access control system (ACS) to control and prove the call time duration
· uses time division multiplexing (TDM) and digital speech
interpolation DSI
· uses L and C bands in its communication (L with mobile, C with earth station)
· uses C band only
· uses 1.6and 1.5 GHZ in L band
and 6.4 GHZ in C band · uses 6.4 GHZ for up and down link
EUTELSAT
European Satellite Communication (EUTELSAT) operates earth stations of
several types:
· Stations for transmission of high capacity telephone trunks and television.
· Stations used within the Satellite Multi Service (SMS) system and that conform to
EUTELSAT standards 1 and 2.
· Fixed stations with diameters between 8 and 12m and transportables of the order of 4m
used for various applications such as the establishment of an emergency network in
disaster areas with transportable stations.
13.1.3. MAADI Earth Stations
An example of satellite earth stations can be found in Egypt in the Maadi earth stations
center located in Maadi about 15KM from Cairo. This center contains 9 stations as follows:
Station name Built in Type
AOR 1976 analogue
IOR 1984 analogue
INMARSAT 1987 analogue
IBS 1989 digital
ARABSAT 1990 digital
GUPCO 1990 digital
ESC 1993 digital
GPS 1995 digital
· AOR, IOR are used by INTELSAT for communication between the world earth stations.
· INMARSAT is used for ships and airplanes to communicate with each other and to
communicate with earth stations.
· IBS (International Business service).
· GPS (Global Position Service) use private satellites for there communications
13.2. ANTENNAS
Microwaves transmitted inside waveguides do not produce any fields
outside the waveguide. Hence, they do not produce any power. However a waveguide that is
open at one end will generate external fields. Antennas radiate spherical waves but the
intensity is usually not the same in all directions. In general, the impedance of the
waveguide will not match the source so a horn or feed is designed to match the source
impedence and will radiate the maximum power.
13.2.1. Types of Antennas
Satellites use horns for global beams. The antenna most commonly used
for satellite communications is a parabolic antenna. It consists of a feed (similar to a
horn) and a parabolic reflector. The simplest antenna system is a dish paraboloid with a
waveguide feed at its focus as shown in figure 13.2. The main advantage of parabolic
reflectors is that they work over a wide range of frequencies.
Signal Feed antenna
input
Aperture plane
Figure 13.2 - Center-fed paraboloid
Large earth station antennas usually have the feed located in the reflected beam. The
reflectors are large and the feeds are small. Therefore, the feed intercepts only a small
part of the reflected beam. For small antennas and satellite parabolic antennas the offset
design of the feed is commonly used to increase the efficiency (h) and reduce the
scattered radiation to the sides (called the side lobes). In this design, the feed is
placed outside the reflected beam since the reflector is smaller and the feed is larger.
13.2.2. Antenna Gain
A transmitter focuses the transmitted power toward the distant
receiver. If this power is not focused, it will radiate equally in all directions
(isotropic radiation). The antenna gain is the ratio of transmitter power required for an
isotropic radiator to the actual transmitted power. Theoretically, a point source and a
parabolic reflector radiate all the power in exactly the same direction. If this principle
could be realized, the antenna gain would be infinite (ideal antennas). In practice there
is an antenna gain limitation based on the wavelength. The antenna gain (G), which is the
peak or maximum gain in the best direction, for a parabolic reflector is:
G = ( 4 p A h ) / l2
where A: Antenna cross-sectional area in meter squared.
l: The wavelength in meter.
h: The antenna efficiency. (For more details see section 13.2.4)
13.2.3. Off-axis Antenna Gain
A high gain antenna has a sharp peak in the main direction and a rapid decrease in gain at
other angles (Fig 13.3). The main lobe has the highest gain (44.5 dBi) on-axis, the two
sides look symmetrical. Each local maximum is called a side lobe, the two nearin side
lobes, around (-0.5 & +0.5 degrees), are at 42 dBi about 1 degree off-axis, the gain
is about 26 dBi. At 8 degrees the gain is 0 dBi that is equivalent to an isotropic
radiation.
Antenna gain (dBi)
47
40
33
Near-in sidelobe Near-in sidelobe
26
19
12
5
-2
-10 -8 -6 -4 -2 0 2 4 6 8 10
Off - axis angle (deg)
Figure 13.3 - Typical earth station antenna pattern.
13.2.3.1. Antenna beam width and side lobe interference
The beam width must be narrow and low side lobes are required. Usually the power in the
side lobe is insignificant compared to the total power in the main lobe. As earth station
antennas become smaller to reduce the cost, the chances of the side lobe interference
increase.
There can also be interference with terrestrial services using the same frequencies since
the satellite communications share the 4 and 6 GHZ bands with microwave relay stations
(Fig 13.4a).
Satellite
interference
Figure 13.4 a - Terrestrial interference.
Many earth stations are placed in isolated locations to reduce the effect of this
interference as shown in figure 13.4b. In a few cases signal cancellation is used, that is
a small signal with an opposite phase is used to cancel the interference at a selected
location (Fig 13.4c).
Satellite
Figure 13.4 b - Site shielding.
Satellite
+
-
Figure 13.4 c - Interference cancellation.
13.2.4. Antenna Efficiency
For conventionally illuminated parabolic reflectors, such as those used in large earth
stations, typical efficiencies are 0.65 to 0.75. The efficiency may be increased by using
super conductive surfaces. The global efficiency of the antenna is the product of several
factors which take of illumination loss, spillover loss, surface tolerance and resistive
mismatch losses
h = h i* h s * h f *.....
where:
hi : Illumination efficiency, which specifies how the power from the feed is spread over
the reflector surface. It is unity for uniform illumination which leads to high level of
secondary lobes. Illumination can be attenuated at the reflector boundaries,.
hs : The spill over efficiency is defined as the ratio of the energy radiated by the
primary source that is intercepted by the reflector to the total energy radiated by the
primary source,
hf : Surface tolerance efficiency. A good parabolic reflector should have as smooth
surface as possible, but for larger wave lengths the surface tolerance can be relaxed. All
these factors cause gain reduction.
13. 3. LINK ANALYSIS
This short note deals with the transmission of radio waves between two earth stations, one
transmitting and the other receiving, via a satellite. The link consists of two sections,
the up link from the earth station to the satellite and the down link from the satellite
to the receiving earth station.
13. 3.1. The Characteristic Parameters of an Antenna
13. 3.1.1. Gain
The gain of an antenna is the ratio of the power radiated (or received) per unit solid
angle by the antenna in a given direction to the power radiated (or received) per unit
solid angle by an isotropic antenna fed with the same power. The gain is maximum in the
direction of maximum radiation.
13. 3.1.2. The radiation pattern
The radiation pattern represents the variations of gain with direction.
For an antenna with a circular reflector, this pattern has rotational symmetry and is
completely represented within a plane in Polar coordinate form or Cartesian coordinate
form.
13. 3.1.3. Polarization
The wave radiated by an antenna consists of an electric field component
and a magnetic field component that are orthogonal and perpendicular to the direction of
propagation. The polarization of the wave is defined by the direction of the electric
field that is not fixed and has a variable amplitude. Polarization can be represented by
the following parameters:
1) Direction of rotation with respect to the direction of propagation: Right-hand
(clockwise) or Left-hand (counter clockwise)
2) Axial rotation: AR= Emax / Emin that is the ratio of the major and minor axes of the
ellipse in case of elliptical polarization. This ratio equals 1 when the ellipse is a
circle (circular polarization) and equals infinity when the ellipse reduces to one axis
(linear polarization).
3) Inclination of the ellipse: angle between the ellipse plane and the equator.
Two waves are in orthogonal polarization if either electric fields describe identical
ellipses in opposite directions. An antenna designed to transmit or receive a wave of
given polarization can neither transmit nor receive in the orthogonal polarization. This
property enables two simultaneous links to be established at the same frequency between
the same two locations. This is described as frequency re-use by orthogonal polarization.
To achieve this, either two polarized antennas must be provided or preferably one antenna
which operates with the two specified polarization may be used. However, we must take into
consideration the imperfection of the antennas and the possible depolarization of the
waves by the transmission medium. These effects lead to mutual interference of the two
links.
13. 3.2. Signal to Noise Ratio at the Receiver Input
The signal to noise ratio enables the relative magnitude of the
received signal to be specified with respect to the noise present at the receiver input.
For specifying this relative magnitude, several ratios can be considered:
· The ratio of signal power to noise power (C/N)
· The ratio of the signal power to the spectral density of the noise (C/No) in Hz. It has
the advantage with respect to the ratio C/N of not in any way presupposing the band width
used.
· The ratio of the signal power to the noise temperature. This ratio is derived from the
ratio C/No by multiplication by Boltzmann's constant k (C/T) in w/k.
The ratio C/No corresponds to the most widespread practice.
13. 3.2.1. The antenna noise temperature
There are two cases to be considered: that of a satellite antenna (the
up link) and that of an earth station antenna (the down link).
1) The satellite antenna (the up link)
The noise captured by the antenna is noise from the earth and from outer space. The beam
width of a satellite antenna is equal to or less than the angle of view of the earth from
the satellite that is 17.5° for a geostationary satellite. Under these conditions, the
major contribution is that from the earth.
2) The earth station antenna (the down link)
The noise captured by the antenna consists of noise from the sky and noise due to
radiation from the earth. At frequencies greater than 2 GHz, the greatest contribution is
that of the non-ionized region of the atmosphere which, being an absorbent medium, is a
noise source.
13. 3. 3. Influence of the Propagation Medium
From the point of view of wave propagation at the frequency range from
1 to 30 GHz, only two regions of the atmosphere have an influence: the troposphere and the
ionosphere. The troposphere is from the ground up to a 15 Km altitude. The ionosphere is
situated between around 70 and 1000 Km. The regions where their influence is maximum are
in the vicinity of the ground for the troposphere and at an altitude of the order of 400
Km for the ionosphere. The predominant effects are those caused by absorption and
depolarization due to tropospheric precipitation (rain and snow). These are particularly
significant for frequencies greater than 10 GHz. There are other effects beside the
influence of atmospheric regions.
1) Attenuation by atmospheric gases:
Attenuation due to the gases in the atmosphere depends on the frequency, the elevation
angle, the altitude of the station and the water vapor concentration. It is negligible at
frequencies less than 10 GHz.
2) Attenuation by sandstorms:
The specific attenuation is inversely proportional to the visibility and depends strongly
on the humidity of the particles.
3) Refraction:
The troposphere and the ionosphere have different refractive indices. The refractive index
of the troposphere decreases with altitude, is a function of meteorological conditions and
independent of frequency while that of the ionosphere depends on frequency and the
electronic content of this ionosphere. The effect of refraction is to cause curvature of
the trajectory of the wave, variation of wave velocity and hence propagation time.
4) The Faraday effect:
The ionosphere causes a rotation of the plane of polarization of a linearly polarized
wave. The angle of rotation is inversely proportional to the square of the frequency. It
is a function of the electronic content of the ionosphere and consequently varies with
time, the season and the solar cycle. However, since cyclic variations are predictable, we
can compensate this effect by a consequent rotation of the antenna polarization.
Nevertheless, some perturbations (geomagnetic storms …) are sudden and unpredictable.
5) Cross polarization due to ice crystals:
Ice clouds, where high altitude ice crystals are in a region close to the 0°C isotherm,
are also the cause of cross-polarization. However, in contrast to rain and other
hydrometeors, this effect is not accompanied by attenuation.
6) Influence of the ground - multipath effects:
When the earth station antenna is small and hence has a beam with a large angular width,
the received signal can be the result of a wave received directly and a wave of equivalent
amplitude received after reflection on the ground or environmental obstacles. This effect
does not exist when the earth station is equipped with an antenna which is sufficiently
directional to eliminate the reflected wave.
13.3.4.Propagation Time
The propagation time of signals from one terminal to another is the sum
of the propagation times on the space link (station-to-station) tSS and the propagation
times on the network at departure and arrival tN .The propagation time on the space link
is given by the relation :
tSS = ( RU + RD ) / C
where RU and RD are the distances from the earth station to the satellite on the up and
down links and C is the velocity of light.
The propagation time on the network can be calculated using an expression given by the
CCITT as:
tN = 12 + 0.004 d (msec)
where d is the distance in kilometers.
13. 4.TYPES OF ORBITS
Orbits are divided into three main categories depending on the plane of inclination with
respect to the equator (Fig 13.5):
· Geostationary orbits.
· Polar orbits.
· Inclined orbits.
Polar Orbit
Geostationary Orbit
EARTH
Figure 13.5 - Geostationary and Polar orbits
13. 4.1. Geostationary Orbits
Satellites in geostationary orbits appear to be stationary over the
earth because the time taken from the satellite to move around the earth is equal to the
time taken by the earth to rotate once on its axis. This time is known as the sidereal
day, and is equal 23 hour, 56 minutes and four seconds (three minutes and 56 seconds
shorter than a solar day).
13. 4.1.1.Geostationary orbits calculations
The geostationary orbit lies directly above the equator and must orbit the earth at a
constant speed. To satisfy this condition, Kepler's second law requires the orbit to be
circular. We will assume a satellite with mass (m), the centripetal force applied on it is
given by the equation:
F = ( m v2 ) / (a + h)
Where v is the velocity of the satellite,
a is the earth radius (6378.165 Km), and
h is the satellite height relative to the equator.
On the other hand the gravitational force is calculated from the equation F = m g ' ,
where g ' is the gravitational acceleration at a height h and is equal to [g (a / (a +
h)]. Using both equations we get the following relation for the velocity:
v = a ? [g /(a + h)]
Let the satellite period be t , where t = 2 p / w and v = w (a + h) .Thus the relation
between the period and the radius is given by:
t = [2 p ( a + h)] / [a ? g]
This equation shows that as the orbit altitude increases, the required satellite velocity
decreases while the orbit period increases. In case of geostationary orbits, t must equal
exactly one sidereal day, this occurs at an altitude given by h = 35,784 Km
13. 4.1.2.Number of satellites needed
The number and type of satellites to be used in a satellite relay
network depend on the network coverage desired and the line-of-sight range of the
satellite (this range depends on its height). In case of geostationary satellites,
although three satellites can provide global coverage, the desire to satisfy an increased
communication demand and other reasons make four or more satellites with closer spacing an
obvious consideration for a global system.
13. 4.1.3. Propagation delay of geostationary satellite
A disadvantage of a geostationary satellite link is the long
propagation delay caused by great distances between the earth stations and the satellite.
For a one way channel this delay is about 270 ms. Thus in a two-way telephone
conversation, an interval of more than half a second occurs between speaking and receiving
an answer. In practice, the delay is not objectionable to telephone users connected by a
single satellite link. Connections made between two links in tandem, however would be
unsatisfactory. Consequently, the CCITT "Committee Consultative Internationale de
Telegraph et Telephone" recommends that very long intercontinental connections be
made over a tandem connection of a satellite link one way and a submarine cable the other
way.
13. 4.1.4. Doppler shift in geostationary orbit
In this special case the Doppler shift is negligible, this is obviously
seen from the equation:
f ' = f ( 1 + v / c )
where f ' is the received frequency,
f is the transmitted frequency,
v is the relative speed between satellite and earth, and
c is the speed of light.
For geostationary satellites v is equal to zero which leads to: f ' = f .
13. 4.2. Polar Orbits
In this case the angle between the equatorial plane and the satellite
orbital plane is 90 degrees. Geostationary satellites provide distorted images of the
polar regions with poor spatial resolution because they orbit in the equatorial plane.
Unlike them, polar-orbiting satellites provide a more global view of the earth. Whereas
there is only one geostationary orbit (and limited number of satellites) an infinite
number of polar orbits is possible. They may have different altitude starting by 800 to
900 Km
13. 4.2.1. Usage of polar orbiting satellites
Polar satellites are used to report on weather generating factors such
as total ozone, clear radiance, incoming and radiated heat, cloud cover and heights. For
weather forecasting it is particularly important to gather such information for the polar
regions of the earth.
In addition to providing environmental data services, they are used to help locate ships
and aircraft in distress. This service is known as Sarsat, for search and rescue
satellite. In this operation the satellite receives a signal from an emergency transmitter
carried by the plane or ship. This transmission is in the VHF/UHF range at a precisely
controlled frequency. The satellite moves at some velocity and results in a Doppler shift
in the received frequency at the satellite. This effect enables locating the plane or ship
in distress.
13. 5. INTER SATELLITES LINK
An inter satellite link (ISL) is a particular beam of a multibeam satellite. This beam is
directed not towards the earth but towards another satellite. Three classes of inter
satellite links can be distinguished
· (GEO - LEO) links between geostationary earth orbit and low earth orbit satellites.
· (GEO - GEO) links between geostationary satellites.
· (LEO - LEO) links between low orbit satellites.
13. 5.1. (GEO - LEO) Links
This type serves to establish a permanent relay via a geostationary
satellite between one or more earth stations and a group of satellites proceeding in a low
orbit (at altitudes of the order of 500 to 1000 Km).
13. 5.2. (GEO - GEO) Links
An intersatellite link permits earth stations of two networks to be
interconnected and hence the geographical coverage of the two satellites to be combined
(Fig 13.6).
Satellite 1 ISL
Satellite 2
Figure 13-6. GEO - GEO link
The alternative solutions are:
· To install an interconnecting earth station equipped with two antennas in the common
region of the two coverage if it exists (Fig 13.7).
Satellite 1 Satellite 2
Figure 13.7 - Common coverage area.
· To make the connection by means of the terrestrial network (Fig 13.8).
Satellite 1 Satellite 2
s
Figure 13.8 - Connection by means of terrestrial network
13.5.2.1. The capacity of the system
The link increases the capacity of the system. Consider a multibeam
satellite as shown in figure 13.9, it will be assumed that the traffic demand increases
and exceeds the capacity of the satellite.
Satellite 1
Satellite 1 ISL
Satellite 2
1
3 1
3
2
2
Figure 13.9 - Multibeam satellite Figure 13.10 -
Capacity increase with ISL
It is therefore necessary to launch a satellite of greater capacity and this implies
risks, development, costs and the availability of a suit launcher. Alternatively a second
satellite identical to the first could be launched and provided with intersatellite
transponders. We can equip the station of region three (assumed to be generating the
excess traffic), with a second antenna and retain the same configuration for regions one
and two as shown in figure 13.10.
13. 5.3. (LEO - LEO) links
The disadvantage of an orbiting satellite (limited duration of
communication time and relatively small coverage) can be reduced in a network containing a
large number of satellites which are interconnected by intersatellite links and equipped
with a means of switching between beams.
13.6. MULTIPLE ACCESS
Multiple access is about the techniques that facilitate information interchange between
several stations on the same network through the satellite. In the case of single beam
satellite, the carriers transmitted by all the network stations are received by the
satellite and the stations can receive the carriers transmitted by the satellite antenna.
There are three possible techniques to achieve multiple access:
1. Frequency Division Multiple Access (FDMA)
2. Time Division multiple Access (TDMA)
3. Code Division Multiple Access (CDMA)
To avoid the interference between the carriers it is necessary for earth station receivers
to be able to discriminate between the received carriers.
13.6.1. Frequency Division Multiple Access
FDMA is characterized by continuous access to the satellite in a given frequency band. The
bandwidth of a repeater channel is divided into sub-bands. Each sub-band is assigned to
the carriers transmitted by an earth station. In this case, the discrimination of earth
station is a function of the location of the carrier energies in the frequency domain. The
main advantage of FDMA is the simplicity of realization but its disadvantages are:
1. lack of flexibility in case of reconfiguration
2. loss of capacity when the number of accesses increase
3. the need to control the transmitting power of earth stations.
13.6.2. Time Division Multiple Access
TDMA is characterized by access to the channel during a time slot. The earth stations
transmit discontinuously during a time TB. This transmission is inserted within longer
time structure of duration TF (frame period) which is the same time structure within which
all stations transmit. The main advantages of TDMA technique are:
1. at each instant the channel amplifies a signal
2. there is no need to control the transmitting power of the stations
3. all stations transmit and receive on the same frequency.
The disadvantages of this technique are:
1. the need for synchronization.
2. the need to dimension the station for transmission at high throughput.
In this case, the discrimination of earth station is a function of temporal location of
the carrier energies.
13.6.3. Code Division Multiple Access
CDMA operates on the principle of spread spectrum transmission, with code division
multiple access network stations transmitting coded information continuously and together
on the same frequency band. The interference between the transmission of different
stations is resolved by decoding the received signals to recover the useful information of
the selected transmitter. The advantages of CDMA are:
1. it is simple because it does not need any transmission synchronization between stations
there need only synchronization to receive the sequence of the received carrier,
2. its protection against the interference from other systems: interference is due to
multiple paths, so, the CDMA is better to use in networks of small stations for satellite
communication with mobiles.
The main disadvantage of CDMA technique is the need of a large bandwidth for a the total
network. To discriminate the received carriers in earth station we must add signature to
the discriminator.
13.6.4.Applicability of Multiple Access Techniques
There is a large variety of solutions to the problem of multiple access
to a repeater by a group of network stations. The choice of access type depends above all
on economic considerations. However, general guide lines can be given according to the
type of traffic:
· For traffic characterized by long messages, implying continuos or quasi-continuos
transmission of a carrier, FDMA, TDMA and CDMA access techniques are the most appropriate.
This includes, for example, telephone traffic, television transmission and video
conferencing.
· If the same volume of traffic per carrier is large and the number of accesses is small
(trunking), FDMA has the advantage of operational simplicity.
· When the traffic per carrier is small and the number of accesses is large, FDMA loses
much in efficiency of usage of the space segment and TDMA and CDMA are the best
candidates. However, TDMA requires relatively costly earth station equipment
· For small stations exposed to inter-system interference, CDMA may be preferred despite
its low throughput.
Figure 13.11 shows a comparison of the throughput for different multiple access
techniques. In this figure, a 100% throughput corresponds to the capacity considering one
access only (one carrier within a single transponder).
throughput
%
100
TDMA
50
FDMA
CDMA
0
1 20
40 60
number of accesses
Figure 13.11 - Comparison of throughput for different multiple access techniques.
13.7. SPACECRAFT
The spacecraft must provide a stable platform in a geostationary orbit
for a period of 5 to 10 years. The main function of the spacecraft is performed by the
communication subsystem. This subsystem is usually composed of one or more antennas which
receive and transmit over wide bandwidths at a microwave frequency, and a set of receivers
and transmitters (transponder) that amplify and transmit the incoming signals.
In addition to the communication subsystem, the spacecraft carries
other controlling and monitoring subsystems. All communication satellite systems derive
their electric power from solar cells covering large areas of the spacecraft.
13.7.1. Telemetry, Tracking and Command Systems
Telemetry, tracking and command systems (TT&C) provide the means of monitoring and
controlling the satellite operation (Fig 13.12).
Satellite
TT&C antenna
Receiving Transmitting
antenna antenna
Telemetry
Telecommand
receiver
transmitter
Tracking
system
Data
processor
Computer
for attitude Controller
and orbital control
Ephemeris data
Figure 13.12 - Typical tracking, telemetry and command system.
These three functions are usually integrated into a single subsystem and kept separate
from the main communication, although they operate in the same frequency band (6 and 4
GHz). Telemetry means measurement made at a distance and transmitted to an observer.
Tracking is observing and collecting data to plot the moving path of an object. Command
means controlling is established and maintained.
13.7.1.1 Telemetry system
This system collects data from many sensors and sends it to a
controlling earth station. The sensors monitor the pressure in fuel tanks, voltage and
current in power conditioning unit, current drawn by each subsystem, and critical voltages
and currents in communication electronics, besides many temperature sensors. The status of
each subsystem and positions of switches in the communication system are also reported.
Devices used to maintain spacecraft attitude are also monitored.
Telemetry data are usually digitized and transmitted as frequency or
phase shift keying (FSK or PSK) of a low power telemetry carrier using time division
modulation (TDM) techniques. A low rate data is issued so that the receiver at the earth
station has a narrow bandwidth to maintain a high carrier to noise ratio. The TDM contain
thousands of bits of data. At the controlling earth station a computer monitors, stores
and decodes the telemetry data so that the status of any system or sensor on the
spacecraft can be determined.
13.7.1.2 Tracking
Velocity and acceleration sensors on the spacecraft are used to
establish the change in orbit from last known position by integration of the data. The
earth station observes the Doppler shift of the telemetry carrier to determine the rate at
which the range is changing. With accurate angular measurements from the earth station
antenna, range is used to determine the orbital elements. The determination of the range
is achieved by transmitting a sequence of pulses to the satellite and observing the time
delay before the pulses are received.
13.7.1.3 Command
Command is used to make changes in attitude ,corrections to the orbit and to control the
communication system. During launch, it controls the firing of boost motors and to spin a
spinner. Also, during the launch phase and injection into the geostationary orbit, the
main TT&C system is not used because the spacecraft does not have the correct
attitude. A backup system is used instead.
13.7.2. Attitude and Orbital Control System
Antennas on the spacecraft must be kept pointing towards the earth,
frequently within 0.1° or 0.01°. The major disturbances on geostationary satellites are
torques due to solar radiation pressure and misalignment of thrusters. Many sensors are
used to detect pointing errors that are corrected by changing the speed or axis direction
of a rotating wheel. A control system takes the information from the sensors and provides
commands to the torque generator(Fig 13.13). Wheels change the satellite orientation and
thrusters generate the desired external torque (to save fuel magnetic coils are used).
Sensors
Control
Torque generators
Earth sensor
Reaction wheels
or
Sun sensor
Momentum wheels
Control
Thrusters :
Star sensor
System
hydrazine or
bipropellant
RF sensor
Magnetic
torquing
coils
Gyroscopes
TT&C interface
Figure 13.13 - Attitude control system
13.8. RELIABILITY OF SATELLITE COMMUNICATIONS SYSTEMS
The reliability of a system is defined by the probability of correct operation of the
system during a given lifetime. The reliability of a complete satellite communication
system depends on the reliability of its principle constituent. For complex equipment such
as satellites two types of breakdown must be considered : coincidental breakdown and
breakdowns resulting from usage.
The instantaneous failure rate l(t) is defined as the limit (as the time interval tends to
zero) of the ratio of the number of pieces of equipment that fail in the time interval
concerned to the number of pieces of equipment in a correct operating state at the start.
For most electronic components, the probability of failure is higher at the beginning of
life (burn in period), and, as the component ages, failure becomes more likely leading to
a bathtub shape for the failure rate with time as shown in figure 13.14.
Failure rate
burn in
aging
Time
Figure 13.14 - Variation of failure rate with time.
13.8.1 The Probability of Survival or Reliability
The reliability, also called probability of survival, is given by :
t
R ( t ) = Exp [ - ? l ( u ) du ]
0
where R ( t ) is the probability of survival or
reliability at time t , and
l ( t ) is the failure rate of the piece of equipment at time t.
If during the useful time of the equipment l ( t ) is constant, then:
R(t) = Exp [ - l t ]
The mean life time or Mean Time To Failure (MTTF) is the mean time T of the occurrence of
the first failure after entering the service and is given by:
¥ ¥
T = ? t f ( t ) dt = ? R ( t ) dt
0 0
If the failure rate l is constant then T = 1 / l .
For a satellite the maximum mission life at the end of which the service is no longer
provided can be defined as the time after which the probability of survival is practically
zero and can be directly related to MTTF.
CONCLUSION
Over the last two decades, satellite systems have revolutionized the
pattern of a border-less world communication. They became now part of our environment.
Everyday we receive and transmit information via satellite, often without knowing it. The
availability of the service is high, typically 99.5%.
Satellite telecommunication will have to face the increasing
competition of fiber optic ground networks in the next 10 to 20 years. Installation of
these networks has started and, in time, the most industrialized countries will be
entirely cabled. Such networks offer both wide bandwidth and high capacity; features that
have so far been characteristics of satellites.
However, the competition will force the operators of satellite systems to offer
specialized services which will use the characteristics of satellite communication more
specifically. Examples are broadcasting and data collection, access to mobile vehicles,
radiolocation and so on. Whatever, one can be assured that satellites will continue to
occupy an important place as means of communications.
REFRENCES:
1. Gary D. Gordan and Walter L. Morgan,"Principles of Communications
Satellites", Wiley, 1993.
2. Maral and M.Bousquet, "Satellite Communication systems", Wiley, 2nd edition,
1993.
3. J.Dunlop and D.G. Smith, "Telecommunications Engineering"
4. Dennis Roddy, "Satellite Communications"
5. Robert M. Gagliardi, "Satellite Communications"
6. B.P.Lathi, "Modern Digital and Analog Communication Systems", Holt, Rinehart
and Winston, 2nd edition, 1989.
7. Timothy Pratt and Charles W. Bostian, "Stellite Communications", Wiley, 1986.