Communication satellite manual

QUIZ: Q. What is passive satellite? These satellites simply reflect signal back to earth. What is active satellite? These electronically repeat the signal and send it back to earth. What is Non synchronous satellite? These satellites rotate around the earth in a low altitude elliptical or circular pattern. What is geosynchronous satellite? These satellite orbits in a circular pattern with an angular velocity equal to that of earth.

Explain ITU? International Telecommunication Union. Define Transponder. A microwave repeater, which receives, amplies, down converts and retransmits signals at a communication satellite.

Define Uplink. The earth station electronics and antenna which transmit information to a communication satellite. International Frequency Registration Board. What is CCIR? International Radio Consultative Committee. Ans: International Telegraph and Telephone consultative Committee. AIM:-To establish a direct communication link between Uplink transmitter and Downlink receiver using tone signal.

Antenna is system of elevated conductors which couples or matches the transmitter or receiver to free space. A transmitting antenna connecting to a transmitter by transmission line, forces electromagnetic waves in to free space which travel in space with velocity of light. Connect the satellite uplink transmitter to AC mains. Switch on the transmitter by mains switch and frequency display will come on. The transmitting frequency can be selected by up —down switch. The frequency can be changed from MHz.

The transmitter on —off switch will switch on —off the transmission. Place downlink receiver at a distance of m. Connect the downlink receiver to the AC mains and switch it on by mains switch. The downlink receiver frequency can be changed from MHz. The downlink receiver also has tuning potentiometer, which can be used to tune any frequency from MHz. Keep the tuning POT fully anticlockwise. The downlink receiver on —off switch will switch on —off the receiver.

Align both the transmitter and receiver antenna in line. Keep the uplink transmitter frequency to MHz. Keep the downlink receiver frequency to MHz. Now connect Tone out signal to tone input of the uplink transmitter by patch cord. Keep downlink receiver voice switch in the on position and you will be able to hear tone in the speaker of receiver.

This is a test link for direct communication between transmitter and receiver. Connect any other audio signal to the Audio2 of uplink transmitter and you will hear the music in the speaker of downlink receiver.

Connection should be tight. Switch off power supply after performing the experiment. What is X- band used for? Military purposes Q. What is C- band used for? Commercial purposes Q. Which frequency band is used for commercial comm.

Fewest propagation problems. Rain attenuation not sermons. What is advantage of satellite communication. Point —to — multipoint comm. What is EIRP? Effective Isotropic Radiated Power Q. What is figure of merit? What is complete link design? It is made up of uplink and downlink. On which link overall quality depends? Downlink design. What is analog base band signal? It is the voice signal in the form of voltage generated by a telephone set. The quality of signal is much improved with active satellite specially when distances between transmitter and receiver are considerable.

Place a satellite transponder between transmitter and receiver at a distance of m. Connect the satellite transponder to the AC mains and switch it on by mains switch. The receiver side of satellite Transponder has an on —off switch, which will switch off the receiver of the satellite. Similarly on —off switch on transmitter side will switch off transmitter of satellite.

Adjust transmitter uplink frequency to MHz and transponder receiver frequency also to MHz. Keep downlink frequency of Transponder to MHz. Keep the downlink receiver to MHz. Now connect a tone output to tone input of the uplink transmitter by patch cord. This is a test link for Active satellite communication.

Connect any other audio signal to the Audio 2 of uplink transmitter and you will hear the music in the speaker of downlink receiver. RESULT: -The above set up shows that a successful satellite communication link has been set up between transmitter and receiver. What is Multiplexing? Transmission of different signal in a form that these do not interfere with each other is termed multiplexing. What is Carson rule? What is SCPC? Single channel Per Carrier System. Where it is used for?

It is used for earth stations with relatively few channels. What are advantage of SCPC? It does not require expensive multiplexing and demultiplexing, hence cost reduced. What is bit? It is used for binary digit. What is baud? It is unit of signaling speed. What is digital base band signal? It is combination of logical ones and zero. What is orthogonal signal? If inner product Sr. What is ASK? Amplitude Shift Keying.

Set TV in AV mode. The TV monitor will display video and audio signal that you have connected to uplink transmitter input. Give another name of ASK. What is PSK? Phase Shift Keying. What are main problems with comm. Launching and putting the satellite in to geostationary orbit maintaining it.

What is apogee? The point where the satellite is farthest from the earth. What is perigee? The point where satellite is closest from the earth.

What are look angle? The look angles are the angles to which an earth station antenna must be pointed to communicate with the geosynchronous satellite. What is trace? The movement of the electron beam from left to right on a television screen. What is threshold? Power systems and rockets are present in this drum. Communication subsystem is placed on top of the drum. An electric motor drives this communication system. The direction of this motor will be opposite to the rotation of satellite body, so that the antennas point towards earth.

The satellites, which perform this kind of operation are called as de-spin. During launching phase, the satellite spins when the small radial gas jets are operated. After this, the de-spin system operates in order to make the TTCM subsystem antennas point towards earth station.

In this method, we can stabilize the satellite by using one or more momentum wheels. This method is called as three-axis method. These three axes define the altitude of satellite. Let X, Y and Z are another set of Cartesian axes. This set of three axis provides the information about orientation of the satellite with respect to reference axes.

If there is a change in altitude of the satellite, then the angles between the respective axes will be changed. In this method, each axis contains two gas jets. They will provide the rotation in both directions of the three axes. The second gas jet will be operated for same period of time, when the satellite reaches to the desired position.

So, the second gas jet will stop the motion of satellite in that axis direction. Orbit control subsystem is useful in order to bring the satellite into its correct orbit, whenever the satellite gets deviated from its orbit. The TTCM subsystem present at earth station monitors the position of satellite. If there is any change in satellite orbit, then it sends a signal regarding the correction to Orbit control subsystem.

Then, it will resolve that issue by bringing the satellite into the correct orbit. In this way, the AOC subsystem takes care of the satellite position in the right orbit and at right altitude during entire life span of the satellite in space. In general, satellite gets data through sensors. So, Telemetry subsystem present in the satellite sends this data to earth station s.

Therefore, TTCM subsystem is very much necessary for any communication satellite in order to operate it successfully. It is the responsibility of satellite operator in order to control the satellite in its life time, after placing it in the proper orbit. This can be done with the help of TTCM subsystem. Satellites have quite a few sensors to monitor different parameters such as pressure, temperature, status and etc. Telemetry subsystem is a remote controlled system.

It sends monitoring data from satellite to earth station. Generally, the telemetry signals carry the information related altitude, environment and satellite. Tracking subsystem is useful to know the position of the satellite and its current orbit.

Satellite Control Center SCC monitors the working and status of space segment subsystems with the help of telemetry downlink. And, it controls those subsystems using command uplink. We know that the tracking subsystem is also present in an earth station. It mainly focusses on range and look angles of satellite.

Number of techniques that are using in order to track the satellite. For example , change in the orbital position of satellite can be identified by using the data obtained from velocity and acceleration sensors that are present on satellite. The tracking subsystem that is present in an earth station keeps tracking of satellite, when it is released from last stage of Launch vehicle.

It performs the functions like, locating of satellite in initial orbit and transfer orbit. Commanding subsystem is necessary in order to launch the satellite in an orbit and its working in that orbit. This subsystem adjusts the altitude and orbit of satellite, whenever there is a deviation in those values.

It also controls the communication subsystem. In general, control codes are converted into command words. These command words are used to send in the form of TDM frames.

Initially, the validity of command words is checked in the satellite. After this, these command words can be sent back to earth station. Here, these command words are checked once again.

If the earth station also receives the same correct command word, then it sends an execute instruction to satellite. So, it executes that command. Functionality wise, the Telemetry subsystem and commanding subsystem are opposite to each other. In this chapter, let us discuss about Power systems from which various subsystems of satellite gets power and Antenna subsystems one by one. We know that the satellite present in an orbit should be operated continuously during its life span.

So, the satellite requires internal power in order to operate various electronic systems and communications payload that are present in it. Power system is a vital subsystem, which provides the power required for working of a satellite.

Mainly, the solar cells or panels and rechargeable batteries are used in these systems. Basically, the solar cells produce electrical power current from incident sunlight. Therefore, solar cells are used primarily in order to provide power to other subsystems of satellite.

We know that individual solar cells generate very less power. So, in order to generate more power, group of cells that are present in an array form can be used. There are two types of solar arrays that are used in satellites. Those are cylindrical solar arrays and rectangular solar arrays or solar sail. Cylindrical solar arrays are used in spinning satellites.

Only part of the cylindrical array will be covered under sunshine at any given time. Due to this, electric power gets generated from the partial solar array. This is the drawback of this type. The drawback of cylindrical solar arrays is overcome with Solar sail. This one produce more power because all solar cells of solar sail are exposed to sun light. During eclipses time, it is difficult to get the power from sun light. So, in that situation the other subsystems get the power from rechargeable batteries.

These batteries produce power to other subsystems during launching of satellite also. In general, these batteries charge due to excess current, which is generated by solar cells in the presence of sun light.

Antennas are present in both satellite and earth station. Now, let us discuss about the satellite antennas. Satellite antennas perform two types of functions. Those are receiving of signals, which are coming from earth station and transmitting signals to one or more earth stations based on the requirement.

In other words, the satellite antennas receive uplink signals and transmit downlink signals. We know that the length of satellite antennas is inversely proportional to the operating frequency.

The operating frequency has to be increased in order to reduce the length of satellite antennas. Therefore, satellite antennas operate in the order of GHz frequencies. The antennas, which are used in satellite are known as satellite antennas. There are mainly four types of Antennas. They are:. Wire antennas are the basic antennas. Mono pole and dipole antennas come under this category. These are used in very high frequencies in order to provide the communication for TTCM subsystem.

The length of the total wire, which is being used as a dipole, if equals half of the wave length i. Wire antennas are suitable for covering its range of access and to provide signal strength in all directions.

That means, wire antennas are Omni-directional antennas. An Antenna with an aperture at the end can be termed as an Aperture antenna. The edge of a transmission line when terminated with an opening, radiates energy. This opening which is an aperture, makes it as an aperture antenna. Horn antenna is an example of aperture antenna. It is used in satellites in order to cover more area on earth. Horn antennas are used in microwave frequency range.

The same feed horn can be used for both transmitting and receiving the signals. A device named duplexer, which separates these two signals. An antenna when individually can radiate an amount of energy, in a particular direction, resulting in better transmission, how it would be if few more elements are added it, to produce more efficient output.

It is exactly this idea, which lead to the invention of Array Antennas or Antenna arrays. Array antennas are used in satellites to form multiple beams from single aperture.

Reflector antennas are suitable for producing beams, which have more signal strength in one particular direction. That means, these are highly directional antennas. So, Parabolic reflectors increase the gain of antennas in satellite communication system. Hence, these are used in telecommunications and broadcasting.

If a Parabolic Reflector antenna is used for transmitting a signal, the signal from the feed, comes out of a dipole or a horn antenna, to focus the wave on to the parabola. It means that, the waves come out of the focal point and strikes the Paraboloidal reflector. This wave now gets reflected as collimated wave front.

If the same antenna is used as a receiver , the electromagnetic wave when hits the shape of the parabola, the wave gets reflected onto the feed point. The dipole or the horn antenna, which acts as the receiver antenna at its feed, receives this signal, to convert it into electric signal and forwards it to the receiver circuitry. The subsystem, which provides the connecting link between transmitting and receiving antennas of a satellite is known as Transponder. It is one of the most important subsystem of space segment subsystems.

Transponder performs the functions of both transmitter and receiver Responder in a satellite. Transponder performs mainly two functions.

Those are amplifying the received input signal and translates the frequency of it. In general, different frequency values are chosen for both uplink and down link in order to avoid the interference between the transmitted and received signals. We can easily understand the operation of Transponder from the block diagram itself. The function of each block is mentioned below. Duplexer is a two-way microwave gate. It receives uplink signal from the satellite antenna and transmits downlink signal to the satellite antenna.

Carrier Processor performs the frequency down conversion of received signal uplink. This block determines the type of transponder.

Power Amplifier amplifies the power of frequency down converted signal down link to the required level. Basically, there are two types of transponders. Those are Bent pipe transponders and Regenerative transponders. Bent pipe transponder receives microwave frequency signal. It converts the frequency of input signal to RF frequency and then amplifies it.

Bent pipe transponder is also called as repeater and conventional transponder. It is suitable for both analog and digital signals. Regenerative transponder performs the functions of Bent pipe transponder. In addition to these two functions, Regenerative transponder also performs the demodulation of RF carrier to baseband, regeneration of signals and modulation.

Regenerative transponder is also called as Processing transponder. It is suitable only for digital signals. The main advantages of Regenerative transponders are improvement in Signal to Noise Ratio SNR and have more flexibility in implementation. The earth segment of satellite communication system mainly consists of two earth stations.

Those are transmitting earth station and receiving earth station. The transmitting earth station transmits the information signals to satellite. Whereas, the receiving earth station receives the information signals from satellite. Sometimes, the same earth station can be used for both transmitting and receiving purposes.

In general, earth stations receive the baseband signals in one of the following forms. Voice signals and video signals either in analog form or digital form. Initially, the analog modulation technique, named FM modulation is used for transmitting both voice and video signals, which are in analog form. Because, both voice and video signals are used to represent in digital by converting them from analog. Designing of an Earth station depends not only on the location of earth station but also on some other factors.

The location of earth stations could be on land, on ships in sea and on aircraft. The depending factors are type of service providing, frequency bands utilization, transmitter, receiver and antenna characteristics.

We can easily understand the working of earth station from above figure. There are four major subsystems that are present in any earth station. Those are transmitter, receiver, antenna and tracking subsystem.

The binary digital information enters at base band equipment of earth station from terrestrial network. Encoder includes error correction bits in order to minimize the bit error rate.

Up converter performs the frequency conversion of modulated signal to higher frequency. This signal will be amplified by using High power amplifier. The earth station antenna transmits this signal. During reception , the earth station antenna receives downlink signal. This is a low-level modulated RF signal. In general, the received signal will be having less signal strength. Because, it is easy to demodulate at these intermediate frequencies.

The function of the decoder is just opposite to that of encoder. So, the decoder produces an error free binary information by removing error correction bits and correcting the bit positions if any. This binary information is given to base band equipment for further processing and then delivers to terrestrial network. The major parts of Earth station Antenna are feed system and Antenna reflector. These two parts combined together radiates or receives electromagnetic waves.

Since the feed system obeys reciprocity theorem, the earth station antennas are suitable for both transmitting and receiving electromagnetic waves. Parabolic reflectors are used as the main antenna in earth stations.

The gain of these reflectors is high. They have the ability of focusing a parallel beam into a point at the focus, where the feed system is located. The Tracking subsystem keeps track with the satellite and make sure that the beam comes towards it in order to establish the communication.

The Tracking system present in the earth station performs mainly two functions. Those are satellite acquisition and tracking of satellite. This tracking can be done in one of the following ways. A mesh type reflector can be used for focusing the signals into a dual feed-horn. It is having two separate outputs. From one output will get C-band signals and from other output will get Ku-band signals.

Television programming mostly originates as first generation signals. These signals are transmitted through satellite to network main end stations in C band. These signals are compressed and transmitted in digital form to cable and DBS providers.

C-band users can subscribe to pay TV channels. These subscription services are cheaper when compared to cable because of the availability of multiple-source programming.

The receiving antenna is directly fed into LNC. In general, the parabolic reflector is also used with the receiving horn antenna for more focusing of the beam. In general, the signal fed to the indoor unit is a wideband signal. The frequency of this signal lies between MHz and MHz. In indoor unit, this signal gets amplified by using an amplifier.

The amplified signal is applied to a tracking filter and down converter. IF amplifier amplifies the signal strength in order to demodulate it properly. These feeds are available separately for each sense of polarization. Due to this, all channels will be available at the indoor receiver, simultaneously. In this case, there is no need of separate receiver to each user. Because, all the carriers are demodulated in a common receiver-filter system.

After that, the channels are combined into a multiplexed signal. This signal is then transmitted through a cable to the subscribers users.

In satellite communication systems, there are two types of power calculations. Those are transmitting power and receiving power calculations. In general, these calculations are called as Link budget calculations. The unit of power is decibel. First, let us discuss the basic terminology used in Link Budget and then we will move onto explain Link Budget calculations. An isotropic radiator antenna radiates equally in all directions.

It is just a theoretical antenna. We can compare the performance of all real practical antennas with respect to this antenna.

Assume an isotropic radiator is situated at the center of the sphere having radius, r. We know that power flux density is the ratio of power flow and unit area. In general, the power flux density of a practical antenna varies with direction. The gain of practical antenna is defined as the ratio of maximum power flux density of practical antenna and power flux density of isotropic antenna. Equivalent isotropic radiated power EIRP is the main parameter that is used in measurement of link budget.

Mathematically , it can be written as. The difference between the power sent at one end and received at the receiving station is known as Transmission losses.

The losses can be categorized into 2 types. The losses which are constant such as feeder losses are known as constant losses.

No matter what precautions we might have taken, still these losses are bound to occur. Another type of loses are variable loss. The sky and weather condition is an example of this type of loss.

Means if the sky is not clear signal will not reach effectively to the satellite or vice versa. Therefore, our procedure includes the calculation of losses due to clear weather or clear sky condition as 1 st because these losses are constant.

They will not change with time. Then in 2 nd step, we can calculate the losses due to foul weather condition. There are two types of link budget calculations since there are two links namely, uplink and downlink. It is the process in which earth is transmitting the signal to the satellite and satellite is receiving it. Its mathematical equation can be written as. Here, Losses represent the satellite receiver feeder losses.

The losses which depend upon the frequency are all taken into the consideration. And this is possible when we get a clear sky condition. In this process, satellite sends the signal and the earth station receives it. In the above equation we have not included the signal bandwidth B.

However, if we include that the equation will be modified as follows. If we are taking ground satellite in to consideration, then the free space spreading loss FSP should also be taken into consideration. If antenna is not aligned properly then losses can occur. Similarly, when signal comes from the satellite towards earth it collides with earth surface and some of them get absorbed.

The designing of down link is more critical than the designing of uplink. Because of limitations in power required for transmitting and gain of the antenna.

That means, a satellite may have different service stations of its own located at different places on the earth. They send carrier signal for the satellite. In this situation, we do multiple access to enable satellite to take or give signals from different stations at time without any interference between them.

Following are the three types of multiple access techniques. In this type of multiple access, we assign each signal a different type of frequency band range. So, any two signals should not have same type of frequency range.

One perfect example of this type of access is our radio channels. We can see that each station has been given a different frequency band in order to operate. We want to access them through FDMA technique. So we assigned them different frequency bands. As shown in the figure, satellite station A has been kept under the frequency range of 0 to 20 HZ.

Similarly, stations B and C have been assigned the frequency range of Hz and Hz respectively. There is no interference between them. The main disadvantage of this type of system is that it is very burst. This type of multiple access is not recommended for the channels, which are of dynamic and uneven. Because, it will make their data as inflexible and inefficient.

As the name suggests, TDMA is a time based access. Here, we give certain time frame to each channel. Within that time frame, the channel can access the entire spectrum bandwidth. Suppose, we want to send five packets of data to a particular channel in TDMA technique.

So, we should assign them certain time slots or time frame within which it can access the entire bandwidth. In above figure, packets 1, 3 and 4 are active, which transmits data. Whereas, packets 2 and 5 are idle because of their non-participation. This format gets repeated every time we assign bandwidth to that particular channel.

Although, we have assigned certain time slots to a particular channel but it can also be changed depending upon the load bearing capacity. That means, if a channel is transmitting heavier loads, then it can be assigned a bigger time slot than the channel which is transmitting lighter loads. Another advantage of TDMA is that the power consumption will be very low.