Category: satellite Communication

Satellite Communication – Quick Guide Satellite Communication – Introduction In general terms, a satellite is a smaller object that revolves around a larger object in space. For example, moon is a natural satellite of earth. We know that Communication refers to the exchange (sharing) of information between two or more entities, through any medium or channel. In other words, it is nothing but sending, receiving and processing of information. If the 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. Soviet Union had launched the world”s first artificial satellite named, Sputnik 1 in 1957. Nearly after 18 years, India also launched the artificial satellite named, Aryabhata in 1975. Need of Satellite Communication The following two kinds of propagation are used earlier for communication up to some distance. Ground wave propagation − Ground wave propagation is suitable for frequencies up to 30MHz. This method of communication makes use of the troposphere conditions of the earth. Sky wave propagation − The suitable bandwidth for this type of communication is broadly between 30–40 MHz and it makes use of the ionosphere properties of the earth. The maximum hop or the station distance is limited to 1500KM only in both ground wave propagation and sky wave propagation. Satellite communication overcomes this limitation. In this method, satellites provide communication for long distances, which is well beyond the line of sight. Since the satellites locate at certain height above earth, the communication takes place between any two earth stations easily via satellite. So, it overcomes the limitation of communication between two earth stations due to earth’s curvature. How a Satellite Works A satellite is a body that moves around another body in a particular path. A communication satellite is nothing but a microwave repeater station in space. It is helpful in telecommunications, radio and television along with internet applications. A repeater is a circuit, which increases the strength of the received signal and then transmits it. But, this repeater works as a transponder. That means, it changes the frequency band of the transmitted signal from the received one. The frequency with which, the signal is sent into the space is called as Uplink frequency. Similarly, the frequency with which, the signal is sent by the transponder is called as Downlink frequency. The following figure illustrates this concept clearly. The transmission of signal from first earth station to satellite through a channel is called as uplink. Similarly, the transmission of signal from satellite to second earth station through a channel is called as downlink. Uplink frequency is the frequency at which, the first earth station is communicating with satellite. The satellite transponder converts this signal into another frequency and sends it down to the second earth station. This frequency is called as Downlink frequency. In similar way, second earth station can also communicate with the first one. The process of satellite communication begins at an earth station. Here, an installation is designed to transmit and receive signals from a satellite in an orbit around the earth. Earth stations send the information to satellites in the form of high powered, high frequency (GHz range) signals. The satellites receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite”s footprint is the area which receives a signal of useful strength from the satellite. Pros and Cons of Satellite Communication In this section, let us have a look at the advantages and disadvantages of satellite communication. Following are the advantages of using satellite communication: Area of coverage is more than that of terrestrial systems Each and every corner of the earth can be covered Transmission cost is independent of coverage area More bandwidth and broadcasting possibilites Following are the disadvantages of using satellite communication − Launching of satellites into orbits is a costly process. Propagation delay of satellite systems is more than that of conventional terrestrial systems. Difficult to provide repairing activities if any problem occurs in a satellite system. Free space loss is more There can be congestion of frequencies. Applications of Satellite Communication Satellite communication plays a vital role in our daily life. Following are the applications of satellite communication − Radio broadcasting and voice communications TV broadcasting such as Direct To Home (DTH) Internet applications such as providing Internet connection for data transfer, GPS applications, Internet surfing, etc. Military applications and navigations Remote sensing applications Weather condition monitoring & Forecasting Satellite Communication – Orbital Mechanics We know that the path of satellite revolving around the earth is known as orbit. This path can be represented with mathematical notations. Orbital mechanics is the study of the motion of the satellites that are present in orbits. So, we can easily understand the space operations with the knowledge of orbital motion. Orbital Elements Orbital elements are the parameters, which are helpful for describing the orbital motion of satellites. Following are the orbital elements. Semi major axis Eccentricity Mean anomaly Argument of perigee Inclination Right ascension of ascending node The above six orbital elements define the orbit of earth satellites. Therefore, it is easy to discriminate one satellite from other satellites based on the values of orbital elements. Semi major axis The length of Semi-major axis (a) defines the size of satellite’s orbit. It is half of the major axis. This runs from the center through a focus to the edge of the ellipse. So, it is the radius of an orbit at the orbit”s two most distant points. Both semi major axis and semi minor axis are represented in above figure. Length of semi major axis (a) not only determines the size of satellite’s orbit, but also the time period of revolution. If circular orbit is considered as a special case, then the length of semi-major axis

Satellite Communication – Useful Resources The following resources contain additional information on Satellite Communication. Please use them to get more in-depth knowledge on this. Useful Video Courses 69 Lectures 10.5 hours 14 Lectures 1 hours 63 Lectures 5 hours 25 Lectures 1 hours Featured 110 Lectures 1.5 hours 22 Lectures 49 mins Learning working make money

Multiple Access Techniques Sometimes a satellite’s service is present at a particular location on the earth station and sometimes it is not present. 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. FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) CDMA (Code Division Multiple Access) Now, let us discuss each technique one by one. FDMA 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. Hence, there won’t be any interference between them, even if we send those signals in one channel. 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. Let’s take three stations A, B and C. 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 30-60 Hz and 70-90 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. TDMA 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 Each station got a fixed length or slot. The slots, which are unused will remain in idle stage. 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. This is the biggest advantage of TDMA over FDMA. Another advantage of TDMA is that the power consumption will be very low. Note − In some applications, we use the combination of both TDMA and FDMA techniques. In this case, each channel will be operated in a particular frequency band for a particular time frame. In this case, the frequency selection is more robust and it has greater capacity over time compression. CDMA In CDMA technique, a unique code has been assigned to each channel to distinguish from each other. A perfect example of this type of multiple access is our cellular system. We can see that no two persons’ mobile number match with each other although they are same X or Y mobile service providing company’s customers using the same bandwidth. In CDMA process, we do the decoding of inner product of the encoded signal and chipping sequence. Therefore, mathematically it can be written as $$Encoded:signal = Orginal:data:: times:: chipping:sequence$$ The basic advantage of this type of multiple access is that it allows all users to coexist and use the entire bandwidth at the same time. Since each user has different code, there won’t be any interference. In this technique, a number of stations can have number of channels unlike FDMA and TDMA. The best part of this technique is that each station can use the entire spectrum at all time. Learning working make money

Examples of Earth Stations In this chapter, let us discuss about two examples of earth stations: Receive-only Home TV system and Community Antenna TV system. Receive Only Home TV System If broadcasting takes place directly to home TV receivers, then that type of service is called as Direct Broadcast Satellite (DBS) service. 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 block diagram of DBS TV receiver is shown in below figure. Outdoor Unit Outdoor unit mainly consists of receiving antenna and Low Noise Converter (LNC). Low Noise Converter (LNC) is nothing but the combination of Low Noise Amplifier (LNA) followed by a converter. 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. Indoor Unit In general, the signal fed to the indoor unit is a wideband signal. The frequency of this signal lies between 950 MHz and 1450 MHz. In indoor unit, this signal gets amplified by using an amplifier. The amplified signal is applied to a tracking filter and down converter. It selects the desired channel and converts its frequency to an Intermediate Frequency (IF) of 70 MHz. IF amplifier amplifies the signal strength in order to demodulate it properly. The baseband (demodulated) signal is used to generate a Vestigial Single Side Band (VSSB) signal. This signal is fed into one of VHF/UHF channels of a standard TV set. Frequency Modulation (FM) is used in DBS TV. Whereas, Amplitude Modulation (AM) in the form of VSSB is used in conventional TV. This is the major difference between DBS TV and conventional TV. Community Antenna TV System The Community Antenna TV (CATV) system uses a single outdoor unit and multiple feeds. These feeds are available separately for each sense of polarization. Due to this, all channels will be available at the indoor receiver, simultaneously. The block diagram of indoor unit of CATV system is shown in below figure. 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). Learning working make money

Satellite Communication – Link Budget 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. Basic Terminology An isotropic radiator (antenna) radiates equally in all directions. But, it doesn’t exist practically. It is just a theoretical antenna. We can compare the performance of all real (practical) antennas with respect to this antenna. Power flux density 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. Power flux density,$Psi_i$ of an isotropic radiator is $$Psi_i = frac{p_s}{4pi r^2}$$ Where, $P_s$ is the power flow. In general, the power flux density of a practical antenna varies with direction. But, it’s maximum value will be in one particular direction only. Antenna Gain 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. Therefore, the Gain of Antenna or Antenna gain, G is $$G = frac{Psi_m}{Psi_i}$$ Where, $Psi_m$ is the maximum power flux density of practical antenna. And, $Psi_i$ is the power flux density of isotropic radiator (antenna). Equivalent Isotropic Radiated Power Equivalent isotropic radiated power (EIRP) is the main parameter that is used in measurement of link budget. Mathematically, it can be written as $$EIRP = G::P_s$$ We can represent EIRP in decibels as $$left [ EIRP right ] = left [ G right ] + left [ P_s right ]dBW$$ Where, G is the Gain of Transmitting antenna and $P_s$ is the power of transmitter. Transmission Losses 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. Constant losses Variable losses 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 1st because these losses are constant. They will not change with time. Then in 2nd step, we can calculate the losses due to foul weather condition. Link budget calculations There are two types of link budget calculations since there are two links namely, uplink and downlink. Earth Station Uplink 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 $$left(frac{C}{N_0}right)_U = [EIRP]_U+left(frac{G}{T}right)_U – [LOSSES]_U -K$$ Where, $left [frac{C}{N_0}right ]$ is the carrier to noise density ratio $left [frac{G}{T}right ]$ is the satellite receiver G/T ratio and units are dB/K Here, Losses represent the satellite receiver feeder losses. The losses which depend upon the frequency are all taken into the consideration. The EIRP value should be as low as possible for effective UPLINK. And this is possible when we get a clear sky condition. Here we have used the (subscript) notation “U”, which represents the uplink phenomena. Satellite Downlink In this process, satellite sends the signal and the earth station receives it. The equation is same as the satellite uplink with a difference that we use the abbreviation “D” everywhere instead of “U” to denote the downlink phenomena. Its mathematical equation can be written as; $$left [frac{C}{N_0}right ]_D = left [ EIRP right ]_D + left [ frac{G}{T} right ]_D – left [ LOSSES right ]_D – K$$ Where, $left [frac{C}{N_0}right ]$ is the carrier to noise density ratio $left [frac{G}{T}right ]$ is the earth station receiver G/T ratio and units are dB/K Here, all the losses that are present around earth stations. In the above equation we have not included the signal bandwidth B. However, if we include that the equation will be modified as follows. $$left [frac{C}{N_0}right ]_D = left [ EIRP right ]_D + left [ frac{G}{T} right ]_D – left [ LOSSES right ]_D -K-B$$ Link Budget 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. so we take AML (Antenna misalignment losses) into account. Similarly, when signal comes from the satellite towards earth it collides with earth surface and some of them get absorbed. These are taken care by atmospheric absorption loss given by “AA” and measured in db. Now, we can write the loss equation for free sky as $$Losses = FSL + RFL+ AML+ AA + PL$$ Where, RFL stands for received feeder loss and units are db. PL stands for polarization mismatch loss. Now the decibel equation for received power can be written as $$P_R = EIRP + G_R + Losses$$ Where, $P_R$ stands for the received power, which is measured in dBW. $G_r$ is the receiver antenna gain. 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. Learning working make money

Global Positioning System Global Positioning System (GPS) is a navigation system based on satellite. It has created the revolution in navigation and position location. It is mainly used in positioning, navigation, monitoring and surveying applications. The major advantages of satellite navigation are real time positioning and timing synchronization. That’s why satellite navigation systems have become an integral part in most of the applications, where mobility is the key parameter. A complete operational GPS space segment contains twenty-four satellites in MEO. These satellites are made into six groups so that each group contains four satellites. The group of four satellites is called as one constellation. Any two adjacent constellations are separated by 60 degrees in longitude. The orbital period of each satellite is approximately equal to twelve hours. Hence, all satellites revolve around the earth two times on every day. At any time, the GPS receivers will get the signals from at least four satellites. GPS Codes and Services Each GPS satellite transmits two signals, L1 and L2 are of different frequencies. Trilateration is a simple method for finding the position (Latitude, Longitude, Elevation) of GPS receiver. By using this method, the position of an unknown point can be measured from three known points GPS Codes Following are the two types of GPS codes. Coarse Acquisition code or C/A code Precise code or P code The signal, L1 is modulated with 1.023 Mbps pseudo random bit sequence. This code is called as Coarse Acquisition code or C/A code and it is used by the public. The signal, L2 is modulated with 10.23 Mbps pseudo random bit sequence. This code is called as Precise code or P code and it is used in military positioning systems. Generally, this P code is transmitted in an encrypted format and it is called as Y code The P code gives better measurement accuracy when compared to C/A code, since the bit rate of P code is greater than the bit rate of C/A code. GPS Services Following are the two types of services provided by GPS. Precise Positioning Service (PPS) Standard Positioning Service (SPS) PPS receivers keep tracking of both C/A code and P code on two signals, L1 and L2. The Y code is decrypted at the receiver in order to obtain P code. SPS receivers keep tracking of only C/A code on signal, L1. GPS Receiver There exists only one-way transmission from satellite to users in GPS system. Hence, the individual user does not need the transmitter, but only a GPS receiver. It is mainly used to find the accurate location of an object. It performs this task by using the signals received from satellites. The block diagram of GPS receiver is shown in below figure. The function of each block present in GPS receiver is mentioned below. Receiving Antenna receives the satellite signals. It is mainly, a circularly polarized antenna. Low Noise Amplifier (LNA) amplifies the weak received signal Down converter converts the frequency of received signal to an Intermediate Frequency (IF) signal. IF Amplifier amplifies the Intermediate Frequency (IF) signal. ADC performs the conversion of analog signal, which is obtained from IF amplifier to digital. Assume, the sampling & quantization blocks are also present in ADC (Analog to Digital Converter). DSP (Digital Signal Processor) generates the C/A code. Microprocessor performs the calculation of position and provides the timing signals in order to control the operation of other digital blocks. It sends the useful information to Display unit in order to display it on the screen. Learning working make money

Satellite Communication – Orbital Mechanics We know that the path of satellite revolving around the earth is known as orbit. This path can be represented with mathematical notations. Orbital mechanics is the study of the motion of the satellites that are present in orbits. So, we can easily understand the space operations with the knowledge of orbital motion. Orbital Elements Orbital elements are the parameters, which are helpful for describing the orbital motion of satellites. Following are the orbital elements. Semi major axis Eccentricity Mean anomaly Argument of perigee Inclination Right ascension of ascending node The above six orbital elements define the orbit of earth satellites. Therefore, it is easy to discriminate one satellite from other satellites based on the values of orbital elements. Semi major axis The length of Semi-major axis (a) defines the size of satellite’s orbit. It is half of the major axis. This runs from the center through a focus to the edge of the ellipse. So, it is the radius of an orbit at the orbit”s two most distant points. Both semi major axis and semi minor axis are represented in above figure. Length of semi major axis (a) not only determines the size of satellite’s orbit, but also the time period of revolution. If circular orbit is considered as a special case, then the length of semi-major axis will be equal to radius of that circular orbit. Eccentricity The value of Eccentricity (e) fixes the shape of satellite’s orbit. This parameter indicates the deviation of the orbit’s shape from a perfect circle. If the lengths of semi major axis and semi minor axis of an elliptical orbit are a & b, then the mathematical expression for eccentricity (e) will be $$e = frac{sqrt{a^2 – b^2}}{a}$$ The value of eccentricity of a circular orbit is zero, since both a & b are equal. Whereas, the value of eccentricity of an elliptical orbit lies between zero and one. The following figure shows the various satellite orbits for different eccentricity (e) values In above figure, the satellite orbit corresponding to eccentricity (e) value of zero is a circular orbit. And, the remaining three satellite orbits are of elliptical corresponding to the eccentricity (e) values 0.5, 0.75 and 0.9. Mean Anomaly For a satellite, the point which is closest from the Earth is known as Perigee. Mean anomaly (M) gives the average value of the angular position of the satellite with reference to perigee. If the orbit is circular, then Mean anomaly gives the angular position of the satellite in the orbit. But, if the orbit is elliptical, then calculation of exact position is very difficult. At that time, Mean anomaly is used as an intermediate step. Argument of Perigee Satellite orbit cuts the equatorial plane at two points. First point is called as descending node, where the satellite passes from the northern hemisphere to the southern hemisphere. Second point is called as ascending node, where the satellite passes from the southern hemisphere to the northern hemisphere. Argument of perigee (ω) is the angle between ascending node and perigee. If both perigee and ascending node are existing at same point, then the argument of perigee will be zero degrees Argument of perigee is measured in the orbital plane at earth’s center in the direction of satellite motion. Inclination The angle between orbital plane and earth’s equatorial plane is known as inclination (i). It is measured at the ascending node with direction being east to north. So, inclination defines the orientation of the orbit by considering the equator of earth as reference. There are four types of orbits based on the angle of inclination. Equatorial orbit − Angle of inclination is either zero degrees or 180 degrees. Polar orbit − Angle of inclination is 90 degrees. Prograde orbit − Angle of inclination lies between zero and 90 degrees. Retrograde orbit − Angle of inclination lies between 90 and 180 degrees. Right Ascension of Ascending node We know that ascending node is the point, where the satellite crosses the equatorial plane while going from the southern hemisphere to the northern hemisphere. Right Ascension of ascending node (Ω) is the angle between line of Aries and ascending node towards east direction in equatorial plane. Aries is also called as vernal and equinox. Satellite’s ground track is the path on the surface of the Earth, which lies exactly below its orbit. The ground track of a satellite can take a number of different forms depending on the values of the orbital elements. Orbital Equations In this section, let us discuss about the equations which are related to orbital motion. Forces acting on Satellite A satellite, when it revolves around the earth, it undergoes a pulling force from the earth due to earth’s gravitational force. This force is known as Centripetal force (F1) because this force tends the satellite towards it. Mathematically, the Centripetal force (F1) acting on satellite due to earth can be written as $$F_{1} = frac{GMm}{R^2} $$ Where, G is universal gravitational constant and it is equal to 6.673 x 10-11 N∙m2/kg2. M is mass of the earth and it is equal to 5.98 x 1024 Kg. m is mass of the satellite. R is the distance from satellite to center of the Earth. A satellite, when it revolves around the earth, it undergoes a pulling force from the sun and the moon due to their gravitational forces. This force is known as Centrifugal force (F2) because this force tends the satellite away from earth. Mathematically, the Centrifugal force (F2) acting on satellite can be written as $$F_{2} = frac{mv^2}{R} $$ Where, v is the orbital velocity of satellite. Orbital Velocity Orbital velocity of satellite is the velocity at which, the satellite revolves around earth. Satellite doesn’t deviate from its orbit and moves with certain velocity in that orbit, when both Centripetal and Centrifugal forces are balance each other. So, equate Centripetal force (F1) and Centrifugal force (F2). $$frac{GMm}{R^2} = frac{mv^2}{R}$$ $$= > frac{GM}{R} = v^2$$ $$= > v

Satellite Communication – Kepler’s Laws We know that satellite revolves around the earth, which is similar to the earth revolves around the sun. So, the principles which are applied to earth and its movement around the sun are also applicable to satellite and its movement around the earth. Many scientists have given different types of theories from early times. But, only Johannes Kepler (1571-1630) was one of the most accepted scientist in describing the principle of a satellite that moves around the earth. Kepler formulated three laws that changed the whole satellite communication theory and observations. These are popularly known as Kepler’s laws. These are helpful to visualize the motion through space. Kepler’s First Law Kepler’s first law states that the path followed by a satellite around its primary (the earth) will be an ellipse. This ellipse has two focal points (foci) F1 and F2 as shown in the figure below. Center of mass of the earth will always present at one of the two foci of the ellipse. If the distance from the center of the object to a point on its elliptical path is considered, then the farthest point of an ellipse from the center is called as apogee and the shortest point of an ellipse from the center is called as perigee. Eccentricity “e” of this system can be written as − $$e = frac{sqrt{a^2 – b^2}}{a}$$ Where, a & b are the lengths of semi major axis and semi minor axis of the ellipse respectively. For an elliptical path, the value of eccentricity (e) is always lie in between 0 and 1, i.e. $0$ < $e$ < $1$, since a is greater than b. Suppose, if the value of eccentricity (e) is zero, then the path will be no more in elliptical shape, rather it will be converted into a circular shape. Kepler’s Second Law Kepler’s second law states that for equal intervals of time, the area covered by the satellite will be same with respect to center of mass of the earth. This can be understood by taking a look at the following figure. Assume, the satellite covers p1 and p2 distances in the same time interval. Then, the areas B1 and B2 covered by the satellite at those two instances are equal. Kepler’s Third Law Kepler’s third law states that, the square of the periodic time of an elliptical orbit is proportional to the cube of its semi major axis length. Mathematically, it can be written as follows − $$T^2:alpha:a^3$$ $$=> T^2=left(frac{4pi ^2}{mu }right) a^3$$ Where, $frac{4pi^2}{mu}$ is the proportionality constant. $mu$ is Kepler’s constant and its value is equal to 3.986005 x 1014m3 /sec2 $$1 = left(frac{2pi}{T}right)^2left(frac{a^2}{mu}right)$$ $$1 = n^2left(frac{a^3}{mu}right)$$ $$=> a^3 = frac{mu}{n^2}$$ Where, ‘n’ is the mean motion of the satellite in radians per second. Note − A satellite, when it revolves around the earth, undergoes a pulling force from the earth, which is gravitational force. Similarly, it experiences another pulling force from the sun and the moon. Therefore, a satellite has to balance these two forces to keep itself in its orbit. Learning working make money

Satellite Communication – AOC Subsystem We know that satellite may deviates from its orbit due to the gravitational forces from sun, moon and other planets. These forces change cyclically over a 24-hour period, since the satellite moves around the earth. Altitude and Orbit Control (AOC) subsystem consists of rocket motors, which are capable of placing the satellite into the right orbit, whenever it is deviated from the respective orbit. AOC subsystem is helpful in order to make the antennas, which are of narrow beam type points towards earth. We can make this AOC subsystem into the following two parts. Altitude Control Subsystem Orbit Control Subsystem Now, let us discuss about these two subsystems one by one. Altitude Control Subsystem Altitude control subsystem takes care of the orientation of satellite in its respective orbit. Following are the two methods to make the satellite that is present in an orbit as stable. Spinning the satellite Three axes method Spinning the satellite In this method, the body of the satellite rotates around its spin axis. In general, it can be rotated at 30 to 100 rpm in order to produce a force, which is of gyroscopic type. Due to this, the spin axis gets stabilized and the satellite will point in the same direction. Satellites are of this type are called as spinners. Spinner contains a drum, which is of cylindrical shape. This drum is covered with solar cells. 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. Three Axis Method In this method, we can stabilize the satellite by using one or more momentum wheels. This method is called as three-axis method. The advantage of this method is that the orientation of the satellite in three axes will be controlled and no need of rotating satellite’s main body. In this method, the following three axes are considered. Roll axis is considered in the direction in which the satellite moves in orbital plane. Yaw axis is considered in the direction towards earth. Pitch axis is considered in the direction, which is perpendicular to orbital plane. These three axes are shown in below figure. Let XR, YR and ZR are the roll axis, yaw axis and pitch axis respectively. These three axis are defined by considering the satellite’s position as reference. 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 first gas jet will be operated for some period of time, when there is a requirement of satellite’s motion in a particular axis direction. 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 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. Learning working make money

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