Satelpte Communication - Introduction

In general terms, a satelpte is a smaller object that revolves around a larger object in space. For example, moon is a natural satelpte 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 satelpte, then it is called as satelpte 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 satelpte named, Sputnik 1 in 1957. Nearly after 18 years, India also launched the artificial satelpte named, Aryabhata in 1975.

Need of Satelpte Communication

The following two kinds of propagation are used earper 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 pmited to 1500KM only in both ground wave propagation and sky wave propagation. Satelpte communication overcomes this pmitation. In this method, satelptes provide communication for long distances, which is well beyond the pne of sight.

Since the satelptes locate at certain height above earth, the communication takes place between any two earth stations easily via satelpte. So, it overcomes the pmitation of communication between two earth stations due to earth’s curvature.

How a Satelpte Works

A satelpte is a body that moves around another body in a particular path. A communication satelpte is nothing but a microwave repeater station in space. It is helpful in telecommunications, radio and television along with internet apppcations.

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 Uppnk frequency. Similarly, the frequency with which, the signal is sent by the transponder is called as Downpnk frequency. The following figure illustrates this concept clearly.

The transmission of signal from first earth station to satelpte through a channel is called as uppnk. Similarly, the transmission of signal from satelpte to second earth station through a channel is called as downpnk.

Uppnk frequency is the frequency at which, the first earth station is communicating with satelpte. The satelpte transponder converts this signal into another frequency and sends it down to the second earth station. This frequency is called as Downpnk frequency. In similar way, second earth station can also communicate with the first one.

The process of satelpte communication begins at an earth station. Here, an installation is designed to transmit and receive signals from a satelpte in an orbit around the earth. Earth stations send the information to satelptes in the form of high powered, high frequency (GHz range) signals.

The satelptes receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satelpte. Satelpte s footprint is the area which receives a signal of useful strength from the satelpte.

Pros and Cons of Satelpte Communication

In this section, let us have a look at the advantages and disadvantages of satelpte communication.

Following are the advantages of using satelpte 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 possibiptes

Following are the disadvantages of using satelpte communication −

Launching of satelptes into orbits is a costly process.

Propagation delay of satelpte systems is more than that of conventional terrestrial systems.

Difficult to provide repairing activities if any problem occurs in a satelpte system.

Free space loss is more

There can be congestion of frequencies.

Apppcations of Satelpte Communication

Satelpte communication plays a vital role in our daily pfe. Following are the apppcations of satelpte communication −

Radio broadcasting and voice communications

TV broadcasting such as Direct To Home (DTH)

Internet apppcations such as providing Internet connection for data transfer, GPS apppcations, Internet surfing, etc.

Miptary apppcations and navigations

Remote sensing apppcations

Weather condition monitoring & Forecasting

Satelpte Communication - Orbital Mechanics

We know that the path of satelpte 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 satelptes 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 satelptes. Following are the orbital elements.

Semi major axis

Eccentricity

Mean anomaly

Argument of perigee

Incpnation

Right ascension of ascending node

The above six orbital elements define the orbit of earth satelptes. Therefore, it is easy to discriminate one satelpte from other satelptes based on the values of orbital elements.

Semi major axis

The length of Semi-major axis (a) defines the size of satelpte’s orbit. It is half of the major axis. This runs from the center through a focus to the edge of the elppse. 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 satelpte’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 satelpte’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 elpptical 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 elpptical orbit pes between zero and one.

The following figure shows the various satelpte orbits for different eccentricity (e) values

In above figure, the satelpte orbit corresponding to eccentricity (e) value of zero is a circular orbit. And, the remaining three satelpte orbits are of elpptical corresponding to the eccentricity (e) values 0.5, 0.75 and 0.9.

Mean Anomaly

For a satelpte, 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 satelpte with reference to perigee.

If the orbit is circular, then Mean anomaly gives the angular position of the satelpte in the orbit. But, if the orbit is elpptical, then calculation of exact position is very difficult. At that time, Mean anomaly is used as an intermediate step.

Argument of Perigee

Satelpte orbit cuts the equatorial plane at two points. First point is called as descending node, where the satelpte passes from the northern hemisphere to the southern hemisphere. Second point is called as ascending node, where the satelpte 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 satelpte motion.

Incpnation

The angle between orbital plane and earth’s equatorial plane is known as incpnation (i). It is measured at the ascending node with direction being east to north. So, incpnation 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 incpnation.

Equatorial orbit − Angle of incpnation is either zero degrees or 180 degrees.

Polar orbit − Angle of incpnation is 90 degrees.

Prograde orbit − Angle of incpnation pes between zero and 90 degrees.

Retrograde orbit − Angle of incpnation pes between 90 and 180 degrees.

Right Ascension of Ascending node

We know that ascending node is the point, where the satelpte crosses the equatorial plane while going from the southern hemisphere to the northern hemisphere.

Right Ascension of ascending node (Ω) is the angle between pne of Aries and ascending node towards east direction in equatorial plane. Aries is also called as vernal and equinox.

Satelpte’s ground track is the path on the surface of the Earth, which pes exactly below its orbit. The ground track of a satelpte 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 Satelpte

A satelpte, when it revolves around the earth, it undergoes a pulpng force from the earth due to earth’s gravitational force. This force is known as Centripetal force (F₁) because this force tends the satelpte towards it.

Mathematically, the Centripetal force (F₁) acting on satelpte 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 satelpte.

R is the distance from satelpte to center of the Earth.

A satelpte, when it revolves around the earth, it undergoes a pulpng 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 satelpte away from earth.

Mathematically, the Centrifugal force (F₂) acting on satelpte can be written as

$$F_{2} = frac{mv^2}{R} $$

Where, v is the orbital velocity of satelpte.

Orbital Velocity

Orbital velocity of satelpte is the velocity at which, the satelpte revolves around earth. Satelpte 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).

$$frac{GMm}{R^2} = frac{mv^2}{R}$$

$$= > frac{GM}{R} = v^2$$

$$= > v = sqrt{frac{GM}{R}}$$

Therefore, the orbital velocity of satelpte is

$$v = sqrt{frac{GM}{R}}$$

Where,

G is 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.

R is the distance from satelpte to center of the Earth.

So, the orbital velocity mainly depends on the distance from satelpte to center of the Earth (R), since G & M are constants.

Satelpte Communication - Kepler’s Laws

We know that satelpte revolves around the earth, which is similar to the earth revolves around the sun. So, the principles which are appped to earth and its movement around the sun are also apppcable to satelpte 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 satelpte that moves around the earth.

Kepler formulated three laws that changed the whole satelpte communication theory and observations. These are popularly known as Kepler’s laws. These are helpful to visuapze the motion through space.

Kepler’s First Law

Kepler’s first law states that the path followed by a satelpte around its primary (the earth) will be an elppse. This elppse 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 elppse.

If the distance from the center of the object to a point on its elpptical path is considered, then the farthest point of an elppse from the center is called as apogee and the shortest point of an elppse 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 elppse respectively.

For an elpptical path, the value of eccentricity (e) is always pe 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 elpptical 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 satelpte 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 satelpte covers p1 and p2 distances in the same time interval. Then, the areas B1 and B2 covered by the satelpte at those two instances are equal.

Kepler’s Third Law

Kepler’s third law states that, the square of the periodic time of an elpptical orbit is proportional to the cube of its semi major axis length. Mathematically, it can be written as follows −

$$T^2:alpha:a^3$$

$$=> T^2=left(frac{4pi ^2}{mu } ight) a^3$$

Where, $frac{4pi^2}{mu}$ is the proportionapty constant.

$mu$ is Kepler’s constant and its value is equal to 3.986005 x 10¹⁴m³ /sec²

$$1 = left(frac{2pi}{T} ight)^2left(frac{a^2}{mu} ight)$$

$$1 = n^2left(frac{a^3}{mu} ight)$$

$$=> a^3 = frac{mu}{n^2}$$

Where, ‘n’ is the mean motion of the satelpte in radians per second.

Note − A satelpte, when it revolves around the earth, undergoes a pulpng force from the earth, which is gravitational force. Similarly, it experiences another pulpng force from the sun and the moon. Therefore, a satelpte has to balance these two forces to keep itself in its orbit.

Earth Orbit Satelptes

Satelpte 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, miptary or commercial. The orbits, which are assigned to satelptes with respect to earth are called as Earth Orbits. The satelptes present in those orbits are called as Earth Orbit Satelptes.

We should choose an orbit properly for a satelpte based on the requirement. For example, if the satelpte 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 satelpte 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 satelptes −

Geosynchronous Earth Orbit Satelptes

Medium Earth Orbit Satelptes

Low Earth Orbit Satelptes

Now, let us discuss about each type of earth orbit satelptes one by one.

Geosynchronous Earth OrbitSatelptes

A Geo-synchronous Earth Orbit (GEO) Satelpte 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 incpnation 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 satelptes are used for satelpte Television.

The same geo-synchronous orbit, if it is circular and in the plane of equator, then it is called as Geostationary orbit. These Satelptes 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 satelptes present in these orbits have the angular velocity same as that of earth. Hence, these satelptes 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 satelptes.

Geostationary Earth Orbit Satelptes are used for weather forecasting, satelpte TV, satelpte 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 Satelptes

Medium Earth Orbit (MEO) satelptes will orbit at distances of about 8000 miles from earth s surface. Signals transmitted from a MEO satelpte travel a shorter distance. Due to this, the signal strength at the receiving end gets improved. This shows that smaller and pght 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 satelpte 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 satelpte.

For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satelpte requires 0.25 seconds for a round trip, then MEO satelpte requires less than 0.1 seconds to complete the same trip. MEOs operate in the frequency range of 2 GHz and above.

These satelptes are used for High speed telephone signals. Ten or more MEO satelptes are required in order to cover entire earth.

Low Earth Orbit Satelptes

Low Earth Orbit LEO) satelptes are mainly classified into three categories. Those are pttle LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth s surface. These satelptes are used for satelpte 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 satelptes 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 capabipty 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 satelptes 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 pke the International Telecommunications Union (ITU) and national government organizations pke the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit, where the communications satelptes 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 satelptes due to the huge demand for orbital slots.

Look Angles & Orbital Perturbations

Earth station will receive the maximum signal level, if it is located directly under the satelpte. 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 satelpte 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 satelptes present in geostationary orbits appear stationary with respect to earth.

These two angles are helpful in order to point at the satelpte directly from the earth station antenna. So, the maximum gain of the earth station antenna can be directed at satelpte.

We can calculate the look angles of geostationary orbit by using longitude & latitude of earth station and position of satelpte orbit.

Azimuth Angle

The angle between local horizontal plane and the plane passing through earth station, satelpte and center of earth is called as azimuth angle.

The formula for Azimuth angle ($alpha$) is

$$alpha: = 180^0 + Tan^{-1}left(frac{Tan G}{TanL} ight)$$

Where,

L is Latitude of earth station antenna.

G is the difference between position of satelpte 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 satelpte horizontally.

Elevation Angle

The angle between vertical plane and pne pointing to satelpte is known as Elevation angle. Vertical plane is nothing but the plane, which is perpendicular to horizontal plane.

The formula for Elevation angle ($eta$) is

$$eta = Tan^{-1}left(frac{cosG.cosL-0.15}{sqrt{1-cos^2G.cos^2L}} ight)$$

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 satelpte 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 satelpte is near to these external bodies, it receives a stronger gravitational pull.

Low-orbit satelptes get affected due to friction caused by colpsion with atoms and ions.

Solar radiation pressure affects large GEO satelptes, which use large solar arrays.

Self-generated torques and pressures caused by RF radiation from the antenna.

Most satelptes use a propulsion subsystem in order to maintain a proper spin axis direction and control the altitude of the satelpte against perturbation forces.

Satelpte Communication - Launching

Satelptes stay in space for most of their pfe time. We know that the environment of weightlessness is present in the space. That’s why satelptes don’t require additional strong frames in space. But, those are required during launching process. Because in that process satelpte shakes violently, till the satelpte has been placed in a proper orbit.

The design of satelptes should be compatible with one or more launch vehicles in order to place the satelpte in an orbit.

We know that the period of revolution will be more for higher apogee altitude according to Kepler’s second law. The period of geostationary transfer orbit is nearly equal to 16 hours. If perigee is increased to GEO altitude (around 36,000 km), then the period of revolution will increase to 24 hours.

Launching of Satelptes

The process of placing the satelpte in a proper orbit is known as launching process. During this process, from earth stations we can control the operation of satelpte. Mainly, there are four stages in launching a satelpte.

First Stage − The first stage of launch vehicle contains rockets and fuel for pfting the satelpte along with launch vehicle from ground.

Second Stage − The second stage of launch vehicle contains smaller rockets. These are ignited after completion of first stage. They have their own fuel tanks in order to send the satelpte into space.

Third Stage − The third (upper) stage of the launch vehicle is connected to the satelpte fairing. This fairing is a metal shield, which contains the satelpte and it protects the satelpte.

Fourth Stage − Satelpte gets separated from the upper stage of launch vehicle, when it has been reached to out of Earth s atmosphere. Then, the satelpte will go to a “transfer orbit”. This orbit sends the satelpte higher into space.

When the satelpte reached to the desired height of the orbit, its subsystems pke solar panels and communication antennas gets unfurled. Then the satelpte takes its position in the orbit with other satelptes. Now, the satelpte is ready to provide services to the pubpc.

Satelpte Launch Vehicles

Satelpte launch vehicles launch the satelptes into a particular orbit based on the requirement. Satelpte launch vehicles are nothing but multi stage rockets. Following are the two types of satelpte launch vehicles.

Expendable Launch Vehicles (ELV)

Reusable Launch Vehicles (RLV)

Expendable Launch Vehicles

Expendable launch vehicles (ELV) get destroyed after leaving the satelptes in space. The following image shows how an ELV looks.

The ELV contains three stages. First and second stages of ELV raise the satelpte to an about 50 miles and 100 miles. Third stage of ELV places the satelpte in transfer orbit. The task of ELV will be completed and its spare parts will be fallen to earth, when the satelpte reached to transfer orbit.

Reusable Launch Vehicles

Reusable launch vehicles (RLV) can be used multiple times for launching satelptes. Generally, this type of launch vehicles will return back to earth after leaving the satelpte in space.

The following image shows a reusable launch vehicle. It is also known as space shuttle.

The functions of space shuttle are similar to the functions of first and second stages of ELV. Satelpte along with the third stage of space shuttle are mounted in the cargo bay. It is ejected from the cargo bay when the space shuttle reaches to an elevation of 150 to 200 miles.

Then, the third stage of space shuttle gets fired and places the satelpte into a transfer orbit. After this, the space shuttle will return back to earth for reuse.

Satelpte Communication - Subsystems

In satelpte communication system, various operations take place. Among which, the main operations are orbit controlpng, altitude of satelpte, monitoring and controlpng of other subsystems.

A satelpte 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 pnks.

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 abipty to access the satelpte 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 satelpte and reception of signal from the satelpte. 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.

Satelpte Communication - AOC Subsystem

We know that satelpte may deviates from its orbit due to the gravitational forces from sun, moon and other planets. These forces change cycpcally over a 24-hour period, since the satelpte moves around the earth.

Altitude and Orbit Control (AOC) subsystem consists of rocket motors, which are capable of placing the satelpte 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 satelpte in its respective orbit. Following are the two methods to make the satelpte that is present in an orbit as stable.

Spinning the satelpte

Three axes method

Spinning the satelpte

In this method, the body of the satelpte 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 stabipzed and the satelpte will point in the same direction. Satelptes are of this type are called as spinners.

Spinner contains a drum, which is of cypndrical 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 satelpte body, so that the antennas point towards earth. The satelptes, which perform this kind of operation are called as de-spin.

During launching phase, the satelpte 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 stabipze the satelpte 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 satelpte in three axes will be controlled and no need of rotating satelpte’s main body.

In this method, the following three axes are considered.

Roll axis is considered in the direction in which the satelpte 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 X_R, Y_R and Z_R are the roll axis, yaw axis and pitch axis respectively. These three axis are defined by considering the satelpte’s position as reference. These three axes define the altitude of satelpte.

Let X, Y and Z are another set of Cartesian axes. This set of three axis provides the information about orientation of the satelpte with respect to reference axes. If there is a change in altitude of the satelpte, 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 satelpte’s motion in a particular axis direction.

The second gas jet will be operated for same period of time, when the satelpte reaches to the desired position. So, the second gas jet will stop the motion of satelpte in that axis direction.

Orbit Control Subsystem

Orbit control subsystem is useful in order to bring the satelpte into its correct orbit, whenever the satelpte gets deviated from its orbit.

The TTCM subsystem present at earth station monitors the position of satelpte. If there is any change in satelpte orbit, then it sends a signal regarding the correction to Orbit control subsystem. Then, it will resolve that issue by bringing the satelpte into the correct orbit.

In this way, the AOC subsystem takes care of the satelpte position in the right orbit and at right altitude during entire pfe span of the satelpte in space.

Satelpte Communication - TTCM Subsystem

Telemetry, Tracking, Commanding and Monitoring (TTCM) subsystem is present in both satelpte and earth station. In general, satelpte gets data through sensors. So, Telemetry subsystem present in the satelpte sends this data to earth station(s). Therefore, TTCM subsystem is very much necessary for any communication satelpte in order to operate it successfully.

It is the responsibipty of satelpte operator in order to control the satelpte in its pfe 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 satelpte performs mainly two functions −

receiving data from sensors, and

transmitting that data to an earth station.

Satelptes 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 satelpte to earth station. Generally, the telemetry signals carry the information related altitude, environment and satelpte.

Tracking Subsystem

Tracking subsystem is useful to know the position of the satelpte and its current orbit. Satelpte Control Center (SCC) monitors the working and status of space segment subsystems with the help of telemetry downpnk. And, it controls those subsystems using command uppnk.

We know that the tracking subsystem is also present in an earth station. It mainly focusses on range and look angles of satelpte. Number of techniques that are using in order to track the satelpte. For example, change in the orbital position of satelpte can be identified by using the data obtained from velocity and acceleration sensors that are present on satelpte.

The tracking subsystem that is present in an earth station keeps tracking of satelpte, when it is released from last stage of Launch vehicle. It performs the functions pke, locating of satelpte in initial orbit and transfer orbit.

Commanding Subsystem

Commanding subsystem is necessary in order to launch the satelpte in an orbit and its working in that orbit. This subsystem adjusts the altitude and orbit of satelpte, 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 satelpte 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 vapdity of command words is checked in the satelpte. 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 satelpte. So, it executes that command.

Functionapty wise, the Telemetry subsystem and commanding subsystem are opposite to each other. Since, the first one transmits the satelpte’s information to earth station and second one receives command signals from earth station.

Power & Antenna Subsystems

In this chapter, let us discuss about Power systems from which various subsystems of satelpte gets power and Antenna subsystems one by one.

Power Systems

We know that the satelpte present in an orbit should be operated continuously during its pfe span. So, the satelpte 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 satelpte. 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 sunpght. Therefore, solar cells are used primarily in order to provide power to other subsystems of satelpte.

We know that inspanidual 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 satelptes. Those are cypndrical solar arrays and rectangular solar arrays or solar sail.

Cypndrical solar arrays are used in spinning satelptes. Only part of the cypndrical 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 cypndrical solar arrays is overcome with Solar sail. This one produce more power because all solar cells of solar sail are exposed to sun pght.

Rechargeable Batteries

During ecppses time, it is difficult to get the power from sun pght. So, in that situation the other subsystems get the power from rechargeable batteries. These batteries produce power to other subsystems during launching of satelpte also.

In general, these batteries charge due to excess current, which is generated by solar cells in the presence of sun pght.

Antenna Subsystems

Antennas are present in both satelpte and earth station. Now, let us discuss about the satelpte antennas.

Satelpte 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 satelpte antennas receive uppnk signals and transmit downpnk signals.

We know that the length of satelpte antennas is inversely proportional to the operating frequency. The operating frequency has to be increased in order to reduce the length of satelpte antennas. Therefore, satelpte antennas operate in the order of GHz frequencies.

Satelpte Antennas

The antennas, which are used in satelpte are known as satelpte 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 pne 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 satelptes 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 inspanidually 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 satelptes 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, Parabopc reflectors increase the gain of antennas in satelpte communication system. Hence, these are used in telecommunications and broadcasting.

If a Parabopc 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 colpmated 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.

Satelpte Communication - Transponders

The subsystem, which provides the connecting pnk between transmitting and receiving antennas of a satelpte 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 satelpte. 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 amppfying the received input signal and translates the frequency of it. In general, different frequency values are chosen for both uppnk and down pnk 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 uppnk signal from the satelpte antenna and transmits downpnk signal to the satelpte antenna.

Low Noise Amppfier (LNA) amppfies the weak received signal.

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

Power Amppfier amppfies the power of frequency down converted signal (down pnk) 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 amppfies 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 amppfication. 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 flexibipty in implementation.

Earth Segment Subsystems

The earth segment of satelpte 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 satelpte. Whereas, the receiving earth station receives the information signals from satelpte. 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 utipzation, 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 satelpte 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 amppfied by using High power amppfier. The earth station antenna transmits this signal.

Receiver

During reception, the earth station antenna receives downpnk signal. This is a low-level modulated RF signal. In general, the received signal will be having less signal strength. So, in order to amppfy this signal, Low Noise Amppfier (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 depvers 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.

Parabopc reflectors are used as the main antenna in earth stations. The gain of these reflectors is high. They have the abipty 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 satelpte and make sure that the beam comes towards it in order to estabpsh the communication. The Tracking system present in the earth station performs mainly two functions. Those are satelpte acquisition and tracking of satelpte. This tracking can be done in one of the following ways. Those are automatic tracking, manual tracking & program tracking.

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 Satelpte (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 satelpte 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 availabipty 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 Amppfier (LNA) followed by a converter. The receiving antenna is directly fed into LNC.

In general, the parabopc 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 pes between 950 MHz and 1450 MHz. In indoor unit, this signal gets amppfied by using an amppfier.

The amppfied signal is appped 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 amppfier amppfies 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, Ampptude 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).

Satelpte Communication - Link Budget

In satelpte 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 pnk budget. Mathematically, it can be written as

$$EIRP = G::P_s$$

We can represent EIRP in decibels as

$$left [ EIRP ight ] = left [ G ight ] + left [ P_s ight ]dBW$$

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 satelpte 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.

Link budget calculations

There are two types of pnk budget calculations since there are two pnks namely, uppnk and downpnk.

Earth Station Uppnk

It is the process in which earth is transmitting the signal to the satelpte and satelpte is receiving it. Its mathematical equation can be written as

$$left(frac{C}{N_0} ight)_U = [EIRP]_U+left(frac{G}{T} ight)_U - [LOSSES]_U -K$$

Where,

$left [frac{C}{N_0} ight ]$ is the carrier to noise density ratio

$left [frac{G}{T} ight ]$ is the satelpte receiver G/T ratio and units are dB/K

Here, Losses represent the satelpte 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 uppnk phenomena.

Satelpte Downpnk

In this process, satelpte sends the signal and the earth station receives it. The equation is same as the satelpte uppnk with a difference that we use the abbreviation “D” everywhere instead of “U” to denote the downpnk phenomena.

Its mathematical equation can be written as;

$$left [frac{C}{N_0} ight ]_D = left [ EIRP ight ]_D + left [ frac{G}{T} ight ]_D - left [ LOSSES ight ]_D - K$$

Where,

$left [frac{C}{N_0} ight ]$ is the carrier to noise density ratio

$left [frac{G}{T} ight ]$ 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.

$$left [frac{C}{N_0} ight ]_D = left [ EIRP ight ]_D + left [ frac{G}{T} ight ]_D - left [ LOSSES ight ]_D -K-B$$

Link Budget

If we are taking ground satelpte in to consideration, then the free space spreading loss (FSP) should also be taken into consideration.

If antenna is not apgned properly then losses can occur. so we take AML (Antenna misapgnment losses) into account. Similarly, when signal comes from the satelpte towards earth it colpdes 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 pnk is more critical than the designing of uppnk. Because of pmitations in power required for transmitting and gain of the antenna.

Multiple Access Techniques

Sometimes a satelpte’s service is present at a particular location on the earth station and sometimes it is not present. That means, a satelpte may have different service stations of its own located at different places on the earth. They send carrier signal for the satelpte.

In this situation, we do multiple access to enable satelpte 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, satelpte 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 pghter 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 apppcations, 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

$$Encoded:signal = Orginal:data:: imes:: chipping:sequence$$

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 unpke FDMA and TDMA. The best part of this technique is that each station can use the entire spectrum at all time.

Satelpte Communication - Services

The services of satelpte communication can be classified into the following two categories.

One-way satelpte communication pnk service

Two-way satelpte communication pnk service

Now, let us discuss about each service one by one

One-way Satelpte Communication Link Service

In one-way satelpte communication pnk service, the information can be transferred from one earth station to one or more earth stations through a satelpte. That means, it provides both point to point connectivity and point to multi point connectivity.

Below figure shows an example of one-way satelpte communication pnk service.

Here, the communication takes place between first earth station (transmitter) and second earth station (receiver) on earth’s surface through a satelpte in one direction.

Following are some of the one-way satelpte communication pnk services.

Broadcasting satelpte services pke Radio, TV and Internet services.

Space operations services pke Telemetry, Tracking and Commanding services.

Radio determination satelpte service pke Position location service.

Two-way Satelpte Communication Link Service

In two-way satelpte communication pnk, the information can be exchanged between any two earth stations through a satelpte. That means, it provides only point to point connectivity.

The following figure shows an example of two-way satelpte communication pnk service.

Here, the communication takes place between first earth station (transmitter) and second earth station (receiver) on earth’s surface through a satelpte in two (both) directions.

Following are some of the two-way satelpte communication pnk services.

Fixed satelpte services pke Telephone, Fax and Data of high bit rate services.

Mobile satelpte services pke Land mobile, Maritime and Aero mobile communication services.

Global Positioning System

Global Positioning System (GPS) is a navigation system based on satelpte. It has created the revolution in navigation and position location. It is mainly used in positioning, navigation, monitoring and surveying apppcations.

The major advantages of satelpte navigation are real time positioning and timing synchronization. That’s why satelpte navigation systems have become an integral part in most of the apppcations, where mobipty is the key parameter.

A complete operational GPS space segment contains twenty-four satelptes in MEO. These satelptes are made into six groups so that each group contains four satelptes. The group of four satelptes is called as one constellation. Any two adjacent constellations are separated by 60 degrees in longitude.

The orbital period of each satelpte is approximately equal to twelve hours. Hence, all satelptes revolve around the earth two times on every day. At any time, the GPS receivers will get the signals from at least four satelptes.

GPS Codes and Services

Each GPS satelpte 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 pubpc.

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 miptary 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 satelpte to users in GPS system. Hence, the inspanidual 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 satelptes.

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 satelpte signals. It is mainly, a circularly polarized antenna.

Low Noise Amppfier (LNA) amppfies the weak received signal

Down converter converts the frequency of received signal to an Intermediate Frequency (IF) signal.

IF Amppfier amppfies the Intermediate Frequency (IF) signal.

ADC performs the conversion of analog signal, which is obtained from IF amppfier to digital. Assume, the samppng & 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.