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Introduction to satellite communication:
Satellite is a physical object that orbits or revolves around some celestial body. A balance between the inertia of the revolving satellite and gravitational pull keeps the satellite in orbit.
The communication payload plays an important part in providing a radio relay for links between Earth-stations. The payload basically consist of two distinct parts such as the repeaters and the antennas. In this section we will highlight the main characteristics of payload and there function. The brief overview of repeater architecture and its function is also emphasised. Satellites mainly consist of types of transponders [receivers and transmitters]. The satellite is only useful if the antenna (on-board) are efficient enough and there is a proper link between between the ground-station and satellites. In the end we will try to broaden our views and knowledge about different types of antenna beams used in satellites.
Important characteristics and functions of the communication payload:
Function of a Payload:
As narrow beam-width antennas are used by both, the satellite and the earth-stations, the signals with correct polarization should be accepted by the payload. Whereas signals from dis-similar polarization should be rejected.
The space may also contain some environmental noise, hence the payload should have capabilities to filter out error signals.
The non-linearities of the amplifier should be reduced by limiting the amplification of error signals.
The payload should be capable of translating the uplink carrier into appropriate downlink carrier.
If the payload consist of many transponders, it should be capable of distinguishing with the earth-station with correct polarization.
The payload should be extremely sensitive to the weak uplink signals (usually in milliwatts).
It should withstand high range of power translation and modifications.
It should effectively use frequency reuse concept and should avoid interference with adjacent channels.
B. Important characteristic parameters of a satellite payload:
* The efficiency of the satellite in delivering the channel functions after decades.
* The amphibious nature of the amplifier.
* The selectivity and figure of merit of the transponders amplifier.
* The power flux density achieved in the sighted area.
* The achieved power in the coverage area.
* The antenna polarization at the transmitter and the receiver.
Using satellite as an microwave relay link:
Uplink signal Downlink signal
Earth 's Horizon
An earth station transmits information to the satellite. The satellite contains a receiver which picks up the transmitted signal, amplifies it and translates it to another frequency.
The translated new frequency (down link signal) is then retransmitted to the receiving station on the earth.
The original information being send from the base station to the satellite is called the uplink signal (original signal), and the retransmitted signals from the satellite to the base station is called the downlink signal(new translated signal).
A typical uplink frequency is 6 GHz and a common downlink frequency is 4 GHz.
Satellite Frequency allocation:
225 - 390 MHz
350 - 530 MHz
1530 - 2700 MHz
2500 - 2700 MHz
3400 - 6425 MHz
7250 - 8400 MHz
10.95 - 14.5 GHz
17.70 - 21.20 GHz
27.50 - 31.00 GHz
36.00 - 46.00 GHz
46.00 - 56.00 GHz
56.00 - 100 GHz
The relationship between the uplink and downlink radio frequency characteristics:
[C/No]á¹¯Ë‰á´µ = [C/No]á¹·Ë‰á´µ + [C/No]á¸Ë‰á´µ
The figure of merit corresponds to the uplink carrier to noise ratio whereas Isotropic radiated power (EIRP) corresponds to the downlink carrier to noise ratio.
G/T C = A [G/T]Ë‰á´µ + B [EIRP]Ë‰á´µ
Min Max EIRP
Why satellite is also called an Transponder:
Receiving Antenna Transmitting Antenna
Power amplifier Mixer
Transponder is a combination of transmitter and receiver. The basic function of a transponder is amplification and frequency translation. A typical communication satellite has 12, 24 or more transponder.
The transponder cannot send and receive on the same frequency because the transmitter's strong signal would damage the sensitive receiver and block out the very weak uplink signal. By using widely spaced transmit and receive frequencies, no interference is encountered.
Types of transponders used:
Single conversion transponder:
The term "single conversion" refers to the fact that only a single frequency translation process from the received signal to the transmitted signal takes place within the satellite.
BPFReceiving Antenna Mixer Transmitting Antenna
The received signal is extremely weak, hence the amplification is provided with the help of low noise amplifier [LNA].
Then the frequency is translated with the help of the mixer and the local oscillator. The L.O's frequency can be adjusted with respect to the incoming signal. The mixer output is amplified again and applied to the band pass filter [BPF].
The purpose of the BPF is to channelize the output and to remove all but the desired downlink signal.
Finally the downlink signal is amplified by a high pass amplifier [HPA], usually a Travelling Wave Tube [TWT].
The output is filtered at the channel frequencies to eliminate harmonics and inter-modulation products that might have been produced due to the non-linearities of TWT. Finally the resulting output is fed to the downlink antenna.
Double conversion transponder:
Receiving Antenna Mixer Mixer Transmitting Antenna
Here the first mixer translates the incoming signal into an IF. The IF out of the first mixer is fed to the IF amplifier, where gain can be magnified.
The output of the IF amplifier is fed to other mixer which translates the signal to the output frequency and the signal passes through the number of stages and is applied to the downlink antenna.
The main advantage of the double-conversion transponder is that greater flexibility in filtering and amplification can be achieved. Amplification and selectivity at the lower IF level is far easier to obtain.
Receiving antenna Mixer Baseband Signal Transmitting Antenna
The uplink signal is amplified and frequency translated and applied to the de-modulator. The output of the demodulator is the baseband signal.
The baseband signal can be any basic signal such as voice, telephone conversation, TV signal or digital data.
The baseband signal is fed to a modulator along with the carrier at the downlink frequency. The modulation technique used is FM. The output of the modulator is a new carrier containing the same information. The signal is than amplified, filtered and transmitted over the downlink.
The transmitter and the receiver section can be clearly optimized, without worrying about the interaction that normally occurs in more conventional configuration.
The amplification in a regenerative repeater is also easier to obtain at the very low baseband frequencies, the circuits are simpler and less expensive.
The S/N ratio of the transponder improves by 2-3 dB.
In the multi-transponder satellites it is easier to switch baseband signals than to switch higher frequency signals.
Thus we can say that transponders basically consist of an amplifier and antennas in a proper orientation such that the antenna always faces the earth-station and the communication is not harness due to any losses. Non-linearities may also be included due to the filters and antennas used on the payload.
Non-Linearities due to the amplifier:
The primary functions of the repeaters is amplification of the signal power level and to increase the coverage area. The repeaters operate over a range of 500 MHz - 2 GHz which are divided into sub-channels. The bandwidth of these channels is several tens of MHz.
Sâ‚€ = aSá¼° + bSi³ + cSiâµ
Power transfer characteristics in single carrier operation:
If an un-modulated(original) carrier is applied as the input of a device, it produce one as the angular frequency while others are made as the multiple of the angular frequency termed as the harmonics. These harmonics can be eliminated by the filtering.
Normalizes characteristics input and output back-off:
The amplifier is operated near the saturation level (i.e. between the saturation level and cut-off region) so as to obtain saturation characteristics respectively.
The characteristic thus relates the magnitude Y = Poâ±/(Poâ±)sat and the magnitude X = Pá¼°/(Pá¼°)sat.
AM/AM conversion coefficient:
When there is a smooth increase in input power the gain remains constant, but when it reaches near the saturation region, there is a drop in the gain and these is when the conversion coefficient decreases as seen from the above graph.
It is defined as the ratio of the (Pout / Pin). It remains nearly constant in the linear part of the graph, where it can be termed as Gss. Whereas the gain decreases at saturation and is called as Gsat.
Point of compression to 1dB:
It is the linear portion when the gain falls by 1dB at saturation.
Power transfer characteristics in multicarrier operation:
When there are more than one carrier transmitted from the vicinity there are chances that there might be resultant inter-modulation products are generated. Only the odd inter-modulation products decreases with the increase in order. The most error-full are the third order products.
Characterisation of non-linearities using two un-modulated carriers of equal amplitude:
The maximum capacity (power) of the device can be effectively divided between multiple carriers, also the inter-modulation product is shared between the multiple carriers and the power difference between single carrier technique and multiple power technique is 4 to 5 dB.
The capture effect in third order inter-modulation:
Considering an multicarrier operation in which there is a difference in carrier power between the two carriers by âˆ†(Pá¼°), hence at the output the carrier is increased by âˆ†(Po). This phenomena is called capture effect.
The uplink and the downlink channels occupy 500 MHz of bandwidth. There are [12 just an example] separate transmit and receive channels each 36 MHz wide. There are 4 MHz guard band between channels that are to minimise adjacent channel interference.
500 MHz [Receive C channels]
5925 MHz 6425 MHz
36 MHz 4 MHz 40 MHz
3720 3760 3800 3840 3880 3920 3960 4000 4040 4080 4120 4160
3700 MHz 4200 MHz
500 MHz [Transmit C channels]
Improving bandwidth efficiency by increasing channel capacity:
By using transmitting and receiving antennas that are vertically or horizontally polarised or that uses left-or-right-hand polarization, two completely separate of transponders operating on the same frequency can be used simultaneously. One set of  transponders will have a vertically polarised or left-hand circular polarized antennas. The other set will use horizontal or right-hand circular polarization. By careful positioning and orientation of the antenna, one set of signals will not interfere with the other.
In this technique, very narrow beam width antennas are used to focus the downlink signals to specific areas of the earth. Such antennas are referred to as spot beam antennas. By using such antennas on the spacecraft, the signals can be confined to a particular area.
In this way, different earth stations can use the same frequencies. They do not interfere with one another because of the highly directional antennas. In this way the total bandwidth or information-carrying capacity of the satellite can be doubled.
SunThe Payload subsystems:
Attitude Control Subsystem
Telemetry tracking & control Subsystem
Charger & Batteries
Regulators: Protection & Combining
Dc to all subsystem Dc and Ac to special subsystem
Telemetry Antennas Communication Subsytem
on-board sensor control signals to all subsystems
The heart of a payload is the communication subsystems. This is a set of transponders that receives the uplink signals and retransmits them to earth. A transponder is a repeater that implements a wideband communication channels which can carry many simultaneous communication transmissions.
The transmitter is well supported by additional house-keeping subsystems such as Power subsystem, the telemetry tracking and command subsystems, the antennas and propulsion and attitude stabilization subsystems.
The solar panel supplies the electrical power for the spacecraft. They drive regulators that distribute Dc power to all subsystems. They also charge the batteries that operate the satellite during Eclipse period. Both Dc/Ac inverters and DC/Dc converters are used to supply special voltages to some subsystems.
It consist of multiple transponders. These receives the uplink signals, amplifies them, translate them in frequency and amplify them again for transmission as downlink signals. The transponder share a common antenna subsystem for both reception and transmission.
Telemetry, Tracking and Command Subsystem:
It will try to monitor on-board conditions, such as temperature and battery voltage and transmits this data back to the ground station for analysis. The ground station may then issue orders to the satellite by transmitting a signal to the command subsystem, which then is used to control many spacecraft functions, such as firing the Jet thrusters etc.
The Jet thrusters and the Apogee Kick Motor [AKM] are part of the propulsion subsystems. They are controlled by commands from the ground.
Attitude Control Subsystems:
The attitude-control subsystem provides stabilization in orbit and senses changes in orientation. It fires the jet thrusters to perform attitude adjustment and station keeping manoeuvres that keeps the satellite in its assigned orbital positions.
Details of Attitude control:
Not only the satellite should be placed in orbit, but also the antennas on it should correctly point the earth station for proper communication. It is also necessary in some satellite to keep the solar panel pointed towards the sun so that the maximum power is produced at all times. There are 2 types.
Three axis stabilization.
Outer surface of satellite
Covered with solar cells
Jet thrusters fired to start
Isolation or adjust it
Spin direction of satellite
Axis of satellite
Once the satellite is in the proper orbit ,a jet thruster is fired to begin spinning the satellite. A typical spin-stabilized rotates at approximately 100 rotations/min.
Once the satellite is spinning, it remains very stable. The spinning causes a gyroscopic or flywheel effect that keeps the satellite pointed in one direction.
A typical position may be with the axis of the satellite pointed towards the earth. An antenna mounted at the axis will remain fixed in that position.
The antenna mounted on the cylinder is independent because if the antenna is also connected to spin cabinet, the antenna will also start to rotate and change the position with respect to earth. Therefore de-spinning is done for proper orientation of the satellite antenna towards earth. The gyroscopic effect holds the satellite in position.
Three axis stabilization method:
Vertical axis from the earth
In the three axis stabilized system, three heavy flywheel or reaction wheels, one for each axis are spun by motors to provide a gyroscopic effect to stabilized the satellite.
Any pitch, roll or yaw is corrected by firing jet thrusters in proper direction and by controlling flywheel motor speed.
Three axis stabilization system is far more accurate in attitude control and positioning than a spin-stabilized satellite. In those application in which pinpoint accuracy of an antenna pointing must be maintained, a three axis stabilization system is used.
Providing Electrical power to the payload:
There are two ways in which the payload can produce electrical power.
Satellite solar arrays in Spin stabilized satellite and Body-stabilized satellite:
Solar cell array Rotation
Deployable solar panel
Central housing or body
Spin stabilized satellite
Today huge solar panels are capable of generating many kilowatts power. A key requirement is that the solar panels should always be pointed towards the sun.
In cylindrical shaped satellite, the solar cells surround the entire unit and therefore some portion of them is always exposed to sunlight.
In body stabilized or three axis satellite, individual solar panels are manipulated with various controls to ensure that they are correctly oriented with respect to sun.
The Dc power is typically used to charge nickel-cadmium batteries that acts as a buffer.
At times when the satellite goes into an eclipse or when the solar panels are not properly positioned, the batteries takes over temporarily and keeps the satellite operating.
The batteries aren't large enough to power the satellite for a long period of time, they are simply used as backup system for eclipses, initial satellite orientation and stabilization or emergency conditions.
The Dc voltages are passed through on voltage regulator circuits before being used to power individual electronic circuits. Most electronic equipment, works best with fixed, stable voltages, therefore regulators are incorporated in most satellite system.
The voltage higher than produced by the solar panel can be generated by the use of Dc/Dc converters for the operation of TWT's.
Details of Telemetry, Tracking and Command subsystem:
All satellite have a Telemetry, Tracking and Command (TTC) subsystem that allows a ground station to monitor and control conditions in the satellite.
Sample & Hold
Analog MuxTelemetry unit:
Shift RegisterSignal conditioning Amplifiers
Analog sensor From digital mux
Digital transducers To shift regeister
The telemetry system is used to report the status of the on-board subsystems to the ground station.
The telemetry system typically consist of various electronic sensors for measuring temperature, radiation level, power supply voltages and other key operating characteristics.
Both analog and digital sensors may be used. The sensors are selected by a multiplexer and are converted to a digital signal, which then modulates an internal transmitter.
Thus the transmitter sends the telemetry information back to the earth station where it is recorded and monitored.
With this information, the ground station is then able to determine the operational status of the satellite at all times.
Command and Control Unit:
It permits the ground station to control the satellite.
Control unit, CPU memory and I/O
Controls signals to all subsystem
The satellite contains a command receiver which receives control signals from an earth station transmitter. The control signals are typically made up of various digital codes that tell the satellite what to do.
Various commands may initiate a telemetry sequence, activate thrusters for attitude correction, re-orient an antenna or perform other operations as required by the special equipment specific to the mission. Usually the control signals are processed by the on-board computers.
The computer contains a built-in ROM with a master control program. The master control program operates the computer and causes all other subsystems to be operated as required.
The computer may also used to make necessary computations and decisions. Information collected from the telemetry system may be first processed by the computer before it is sent to the ground station. The memory of the computer may also be used to store data temporarily before it is processed or transmitted back to earth.
The computer may also serve as an event timer or clock. The computer is a versatile control element that can typically be re-programmed via the command and control system to carry out any additional function that may be required.
Details of Propulsion subsystem:
A propulsion subsystem is usually the Apogee Kick Motor [AKM] used to put the satellite into the final orbit, or it may be one or more liquid or solid propellant rocket that could be used to change the orbit of a satellite or remove the satellite from the orbit.
Details of thermal control subsystem:
Black thermal blanket (behind the
All the heat absorbed by the solar panel is not converted into electrical energy. This is accumulated as heat.
The side of the satellite which is facing the sun is normally around 150â°C whereas the backside is -150â° C, This leads loading and buckling sound. Hence the black thermal blanket which has high emissivity surface is normally used to absorb or radiate heat into the space.
The thermal control subsystem maintains the subsystem and payload temperatures within the allowable temperature ranges. Excessive heat generated by the satellite is dissipated to cold space by means of radiation.
The active thermal control is based on heaters regulated by software via thermistor.
Passive thermal control composes of heat pipes, multilayer insulation blankets, radiators paints and surface finishes maintaining temperature level of the overall carrier component within an acceptable value.
Details of Antennas used on satellites:
The main functions are as follows:
To collect only those radio wave signals which has the correct polarization with that of the satellite antenna and also should match the band of the Transponders channel.
Should avoid picking undesired signals from the region outside the frequency band.
To transmit only those frequencies which are included in the given band and with the correct polarization.
The beam of the antenna should be confined in such a way that frequency reuse should be possible.
The side lobes of the antenna should be as narrower as possible.
There should be high isolation between the orthogonal polarization used.
Types of Antennas used in Satellite communication:
Rectangular horn and circular horn are most common.
The horn antennas are one of the basic types of directional antenna. If we want a very narrow beam of antenna, then the aperture should be large enough.
Horn suffers poor side lobe characteristics, which can be improved with the corrugation in the internal side of the horn.
Spot beam or the shaped beams can be produced with the help of reflecting antenna.
The source is normally at the centre or the focal point, the signals hits the reflecting surface and radiates into space.
The depth of the paraboloid is normally specified by the F/D ratio:
In the reflecting antennas there are chances that the beams can be blocked at the apertures, but in the lens antennas the radiating apertures are in front of the source arrays and thus eliminates the blocking of beam.
The propagation delay is minimum at the periphery whereas maximum along the axis. The plane wave is generated from the spherical wave. One of the disadvantage of lens antennas is large mass and occupies lot of space.
The radiating aperture consist of a many radiating element over the given area. As there are large number of elements, the final radiation is a combination of amplitude and phase of waves. As there are many elements the beam of the arrays should be as narrower as possible to avoid interference between the responses. The radiating elements can be micro-strips, dipoles, and printed elements etc.
The power dividers and phase splitters helps in changing the phase and amplitude of the radiating elements.
Total radiation pattern is given by:
Total Pattern = [Pattern factor] * [Array factor]
Variable power divider
To transmitter or receiver
Redundancies included to increase reliability:
Attenuators isolators Two position switches
The satellite launching is an very expensive research area. So it is very important to make the system as reliable as possible. There are many mechanical parts which may ware out due to aging, high heat produce in the cabinet or other environmental effects.
Hence it is very important to always have redundancies of the components which can be immediately switched if the previous stage have become faulty. This can reduce the chances of the satellite to become useless if anything goes wrong.
Components which are generally used for designing the payload:
Integrated circuits (planar)
Important considerations when designing the payload:
Space craft should be designed to withstand the space environment over the whole lifetime of the satellite.
Spacecraft should be tested in an environment created just as space to withstand shocks and atmospheric conditions.
Satellite should be designed to withstand:
Severe acoustic and vibration environment.
Peaks - Immediately when rocket fires
When launch should abandoned.
Mechanical shocks due to sudden movements.
Thermal environment changes.
Atmospheric pressure decreases during launch.
Electromagnetic interference occurs.
Applications of Satellites:
Global Positioning System [GPS]
Thus we can say that Satellites plays an very important role and care should be taken that the boon to the men doesn't turn to be the curse. There has been a boom in this field due to which it is extremely difficult to find the space where it can be launched. There are launchers all over the world whose sincere efforts help us in getting the knowledge of the celestial bodies which lies in space.
Redundancies are also applied to avoid satellite being lost in space. After research there are some components which are preferred over others. There is a large scope of development in this field. Companies like NASA, INTELSAT are paying great interest in developing the satellites and making it more efficient.