Category: satellite Communication

Satellite Communication – Subsystems In satellite communication system, various operations take place. Among which, the main operations are orbit controlling, altitude of satellite, monitoring and controlling of other subsystems. A satellite communication consists of mainly two segments. Those are space segment and earth segment. So, accordingly there will be two types of subsystems namely, space segment subsystems and earth segment subsystems. The following figure illustrates this concept. As shown in the figure, the communication takes place between space segment subsystems and earth segment subsystems through communication links. Space Segment Subsystems The subsystems present in space segment are called as space segment subsystems. Following are the space segment subsystems. AOC Subsystem TTCM Subsystem Power and Antenna Subsystems Transponders Earth Segment Subsystems The subsystems present in the ground segment have the ability to access the satellite repeater in order to provide the communication between the users. Earth segment is also called as ground segment. Earth segment performs mainly two functions. Those are transmission of a signal to the satellite and reception of signal from the satellite. Earth stations are the major subsystems that are present in earth segment. We will discuss about all these subsystems of space segment and earth segment in following chapters. Learning working make money

Earth Orbit Satellites Satellite should be properly placed in the corresponding orbit after leaving it in the space. It revolves in a particular way and serves its purpose for scientific, military or commercial. The orbits, which are assigned to satellites with respect to earth are called as Earth Orbits. The satellites present in those orbits are called as Earth Orbit Satellites. We should choose an orbit properly for a satellite based on the requirement. For example, if the satellite is placed in lower orbit, then it takes less time to travel around the earth and there will be better resolution in an onboard camera. Similarly, if the satellite is placed in higher orbit, then it takes more time to travel around the earth and it covers more earth’s surface at one time. Following are the three important types of Earth Orbit satellites − Geosynchronous Earth Orbit Satellites Medium Earth Orbit Satellites Low Earth Orbit Satellites Now, let us discuss about each type of earth orbit satellites one by one. Geosynchronous Earth OrbitSatellites A Geo-synchronous Earth Orbit (GEO) Satellite is one, which is placed at an altitude of 22,300 miles above the Earth. This orbit is synchronized with a side real day (i.e., 23 hours 56 minutes). This orbit can have inclination and eccentricity. It may not be circular. This orbit can be tilted at the poles of the earth. But, it appears stationary when observed from the Earth. These satellites are used for satellite Television. The same geo-synchronous orbit, if it is circular and in the plane of equator, then it is called as Geostationary orbit. These Satellites are placed at 35,900kms (same as Geosynchronous) above the Earth’s Equator and they keep on rotating with respect to earth’s direction (west to east). The satellites present in these orbits have the angular velocity same as that of earth. Hence, these satellites are considered as stationary with respect to earth since, these are in synchronous with the Earth’s rotation. The advantage of Geostationary orbit is that no need to track the antennas in order to find the position of satellites. Geostationary Earth Orbit Satellites are used for weather forecasting, satellite TV, satellite radio and other types of global communications. The following figure shows the difference between Geo-synchronous and Geo-stationary orbits. The axis of rotation indicates the movement of Earth. Note − Every Geostationary orbit is a Geo-synchronous orbit. But, the converse need not be true. Medium Earth Orbit Satellites Medium Earth Orbit (MEO) satellites will orbit at distances of about 8000 miles from earth”s surface. Signals transmitted from a MEO satellite travel a shorter distance. Due to this, the signal strength at the receiving end gets improved. This shows that smaller and light weight receiving terminals can be used at the receiving end. Transmission delay can be defined as the time it takes for a signal to travel up to a satellite and back down to a receiving station. In this case, there is less transmission delay. Because, the signal travels for a shorter distance to and from the MEO satellite. For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satellite requires 0.25 seconds for a round trip, then MEO satellite requires less than 0.1 seconds to complete the same trip. MEOs operate in the frequency range of 2 GHz and above. These satellites are used for High speed telephone signals. Ten or more MEO satellites are required in order to cover entire earth. Low Earth Orbit Satellites Low Earth Orbit LEO) satellites are mainly classified into three categories. Those are little LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth”s surface. These satellites are used for satellite phones and GPS. This relatively short distance reduces transmission delay to only 0.05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Twenty or more LEO satellites are required to cover entire earth. Little LEOs will operate in the 800 MHz (0.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and Mega-LEOs operates in the 20-30 GHz range. The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and yields to the capability of real-time, low delay video transmission scheme. The following figure depicts the paths of LEO, MEO and GEO Orbital Slots Here, a question may arise that with more than 200 satellites that are in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space? To answer this problem (question), international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit, where the communications satellites can be located. These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites due to the huge demand for orbital slots. Learning working make money

Satellite Communication Tutorial Job Search If communication takes place between any two earth stations through a satellite, then it is called as satellite communication. In this communication, electromagnetic waves are used as carrier signals. These signals carry the information such as voice, audio, video or any other data between ground and space and vice-versa. This tutorial is meant to provide the readers an overview of Satellite Communication and how it works. Audience This tutorial will be suitable for all those readers who wish to learn the fundamental concepts of satellite communication. After completing this tutorial, you will be able to learn the significance of satellite communication and its role in present scenario. Prerequisites The readers should have prior knowledge on the basic concepts of analog and digital communication systems, in order draw benefit from this tutorial. Learning working make money

Discuss Satellite Communication If communication takes place between any two earth stations through a satellite, then it is called as satellite communication. In this communication, electromagnetic waves are used as carrier signals. These signals carry the information such as voice, audio, video or any other data between ground and space and vice-versa. This tutorial is meant to provide the readers an overview of Satellite Communication and how it works. Learning working make money

Satellite Communication – Transponders 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. Hence, the word ‘Transponder’ is obtained by the combining few letters of two words, Transmitter (Trans) and Responder (ponder). Block diagram of Transponder 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. The block diagram of transponder is shown in below figure. 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. Low Noise Amplifier (LNA) amplifies the weak received signal. 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. Types of Transponders Basically, there are two types of transponders. Those are Bent pipe transponders and Regenerative transponders. Bent Pipe 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 Transponders Regenerative transponder performs the functions of Bent pipe transponder. i.e., frequency translation and amplification. 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. Learning working make money

Satellite Communication – TTCM Subsystem Telemetry, Tracking, Commanding and Monitoring (TTCM) subsystem is present in both satellite and earth station. 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. We can make this TTCM subsystem into the following three parts. Telemetry and Monitoring Subsystem Tracking Subsystem Commanding Subsystem Telemetry and Monitoring Subsystem The word ‘Telemetry’ means measurement at a distance. Mainly, the following operations take place in ‘Telemetry’. Generation of an electrical signal, which is proportional to the quantity to be measured. Encoding the electrical signal. Transmitting this code to a far distance. Telemetry subsystem present in the satellite performs mainly two functions − receiving data from sensors, and transmitting that data to an earth station. Satellites have quite a few sensors to monitor different parameters such as pressure, temperature, status and etc., of various subsystems. In general, the telemetry data is transmitted as FSK or PSK. 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 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 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. This commanding subsystem is responsible for turning ON / OFF of other subsystems present in the satellite based on the data getting from telemetry and tracking subsystems. 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. Since, the first one transmits the satellite’s information to earth station and second one receives command signals from earth station. Learning working make money

Satellite Communication – Services The services of satellite communication can be classified into the following two categories. One-way satellite communication link service Two-way satellite communication link service Now, let us discuss about each service one by one One-way Satellite Communication Link Service In one-way satellite communication link service, the information can be transferred from one earth station to one or more earth stations through a satellite. That means, it provides both point to point connectivity and point to multi point connectivity. Below figure shows an example of one-way satellite communication link service. Here, the communication takes place between first earth station (transmitter) and second earth station (receiver) on earth’s surface through a satellite in one direction. Following are some of the one-way satellite communication link services. Broadcasting satellite services like Radio, TV and Internet services. Space operations services like Telemetry, Tracking and Commanding services. Radio determination satellite service like Position location service. Two-way Satellite Communication Link Service In two-way satellite communication link, the information can be exchanged between any two earth stations through a satellite. That means, it provides only point to point connectivity. The following figure shows an example of two-way satellite communication link service. Here, the communication takes place between first earth station (transmitter) and second earth station (receiver) on earth’s surface through a satellite in two (both) directions. Following are some of the two-way satellite communication link services. Fixed satellite services like Telephone, Fax and Data of high bit rate services. Mobile satellite services like Land mobile, Maritime and Aero mobile communication services. Learning working make money

Earth Segment Subsystems 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. Later, digital modulation techniques, namely Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) are used for transmitting those signals. Because, both voice and video signals are used to represent in digital by converting them from analog. Block Diagram of Earth Station 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. The block diagram of digital earth station is shown in below figure. 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. Transmitter 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. In satellite communication, the Intermediate Frequency (IF) can be chosen as 70 MHz by using a transponder having bandwidth of 36 MHz. Similarly, the IF can also be chosen as 140 MHz by using a transponder having bandwidth of either 54 MHz or 72 MHz. 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. Receiver 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. So, in order to amplify this signal, Low Noise Amplifier (LNA) is used. Due to this, there is an improvement in Signal to Noise Ratio (SNR) value. RF signal can be down converted to the Intermediate Frequency (IF) value, which is either 70 or 140 MHz. 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. Earth Station Antenna 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. Tracking Subsystem 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. Those are automatic tracking, manual tracking & program tracking. Learning working make money

Look Angles & Orbital Perturbations Earth station will receive the maximum signal level, if it is located directly under the satellite. Otherwise, it won’t receive maximum signal level and that signal level decreases as the difference between the latitude and longitude of earth station increases. So, based on the requirement we can place the satellite in a particular orbit. Now, let us discuss about the look angles. Look Angles The following two angles of earth station antenna combined together are called as look angles. Azimuth Angle Elevation Angle Generally, the values of these angles change for non-geostationary orbits. Whereas, the values of these angles don’t change for geostationary orbits. Because, the satellites present in geostationary orbits appear stationary with respect to earth. These two angles are helpful in order to point at the satellite directly from the earth station antenna. So, the maximum gain of the earth station antenna can be directed at satellite. We can calculate the look angles of geostationary orbit by using longitude & latitude of earth station and position of satellite orbit. Azimuth Angle The angle between local horizontal plane and the plane passing through earth station, satellite and center of earth is called as azimuth angle. The formula for Azimuth angle ($alpha$) is $$alpha: = 180^0 + Tan^{-1}left(frac{Tan G}{TanL}right)$$ Where, L is Latitude of earth station antenna. G is the difference between position of satellite orbit and earth station antenna. The following figure illustrates the azimuth angle. Measure the horizontal angle at earth station antenna to north pole as shown in figure. It represents azimuth angle. It is used to track the satellite horizontally. Elevation Angle The angle between vertical plane and line pointing to satellite is known as Elevation angle. Vertical plane is nothing but the plane, which is perpendicular to horizontal plane. The formula for Elevation angle ($beta$) is $$beta = Tan^{-1}left(frac{cosG.cosL-0.15}{sqrt{1-cos^2G.cos^2L}}right)$$ We can calculate the elevation angle by using above formula. The following figure illustrates the elevation angle. Measure the vertical angle at earth station antenna from ground to satellite as shown in the figure. It represents elevation angle. Orbital Perturbations Following are the orbital perturbations due to gravitational and non-gravitational forces or parameters. Irregular gravitational force around the Earth due to non-uniform mass distribution. Earth’s magnetic field too causes orbital perturbations. Main external perturbations come from Sun and Moon. When a satellite is near to these external bodies, it receives a stronger gravitational pull. Low-orbit satellites get affected due to friction caused by collision with atoms and ions. Solar radiation pressure affects large GEO satellites, which use large solar arrays. Self-generated torques and pressures caused by RF radiation from the antenna. Most satellites use a propulsion subsystem in order to maintain a proper spin axis direction and control the altitude of the satellite against perturbation forces. Learning working make money

Power & Antenna Subsystems In this chapter, let us discuss about Power systems from which various subsystems of satellite gets power and Antenna subsystems one by one. Power Systems 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. Solar Cells 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. Solar Arrays 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. Rechargeable Batteries 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. Antenna Subsystems 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. Satellite Antennas The antennas, which are used in satellite are known as satellite antennas. There are mainly four types of Antennas. They are: Wire Antennas Horn Antennas Array Antennas Reflector Antennas Now, let us discuss about these antennas one by one. Wire Antennas 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.e., l = λ/2), such an antenna is called as half-wave dipole antenna. 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. Horn 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. Array Antennas 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 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. Learning working make money