STUDY MATERIAL

SUBJECT:- PHYSICS

CLASS:- SEMESTER VI

Paper:-. DSE 3T(Communication Electronics)

Unit:- Satellite Communication - Introduction, need, geosynchronous satellite orbits

geostationary satellite advantages of geostationary satellites. Satellite visibility, transponders (C -

Band), path loss, ground station, simplified block diagram of earth station. Uplink and downlink.

Assembled by- S. JANA, B.M.M.

Dated:- 7.04.2020

*Origin of satellite:-

The concept of using object in space to reflect signals for communication was proved by Naval

Research Lab in communication when it use the Moon to establish a very low data rate link

between Washington and Hawaii very low data rate link between Washington and Hawaii in late

1940’s. Russian started the Space age by successfully launching SPUTNIK the first artificial

spacecraft to orbit the earth, This transmitted telemetry information for 21 days in Oct. 1957.The

American followed by launching an experimental satellite EXPLORER In 1958. In 1960 two

satellite were deployed “Echo” & “Courier ”In 1963 first GSO “Syncom “ The first commercial

GSO (Intelsat & Molnya) in 1965 these provides video (Television) and voice (Telephone) For

their audience

*Introduction:-

In 1962, the American telecommunications giant AT&T launched the world's first true

communications satellite, called Telstar. Since then, countless communications satellites have

been placed into earth orbit, and the technology being applied to them is forever growing in

sophistication.

Satellites are specifically made for telecommunication purpose. They are used for mobile

applications such as Communications to ships, vehicles, planes, hand-held terminals and for TV

and radio broadcasting. They are responsible for providing these services to an assigned region

(area) on the earth. The power and bandwidth of these satellites depend upon the preferred size of

the footprint, complexity of the traffic control protocol schemes and the cost of ground stations. A

satellite works most efficiently when the transmissions are Focused with a desired area. When the

area is focused, then the emissions don’t go outside that designated area and thus minimizing the

interference to the other systems. This leads more efficient spectrum usage.

*Basic Elements:-

Satellite communications are comprised of 2 main components:

• The Satellite:

The satellite itself is also known as the space segment, and is composed of three separate units,

namely the fuel system, the satellite and telemetry controls, and the transponder. The transponder

includes the receiving antenna to pick-up signals from the ground station, a broad band receiver,

an input multiplexer, and a frequency converter which is used to reroute the received signals

through a high powered amplifier for downlink. The primary role of a satellite is to reflect

electronic signals. In the case of a telecom satellite, the primary task is to receive signals from a

ground station and send them down to another ground station located a considerable distance away

from the first. This relay action can be two-way, as in the case of a long distance phone call.

Another use of the satellite is when, as is the case with television broadcasts, the ground station's

uplink is then downlinked over a wide region, so that it may be received by many different

customers possessing compatible equipment. Still another use for satellites is observation, wherein

the satellite is equipped with cameras or various sensors, and it merely downlinks any information

it picks up from its vantage point.

• The Ground Station :

This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or

transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband

processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to

an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion

as the uplink, ultimately converting signals received through the parabolic antenna to base band

signal.

*Various Uses of Satellite Communications:-

1. Traditional Telecommunications:

Since the beginnings of the long distance telephone network, there has been a need to connect the

telecommunications networks of one country to another. This has been accomplished in several

ways. Submarine cables have been used most frequently. However, there are many occasions

where a large long distance carrier will choose to establish a satellite based link to connect to

transoceanic points, geographically remote areas or poor countries that have little communications

infrastructure. Groups like the international satellite consortium Intelsat have fulfilled much of the

world's need for this type of service.

2. Cellular:

Various schemes have been devised to allow satellites to increase the bandwidth available to

ground based cellular networks. Every cell in a cellular network divides up a fixed range of

channels which consist of either frequencies, as in the case of FDMA systems, or time slots, as in

the case of TDMA. SinceSince a particular cell can only operate within those channels allocated

to it, overloading can occur. By using satellites which operate at a frequency outside those of the

cell, we can provide extra satellite channels on demand to an overloaded cell. These extra channels

can just as easily be, once free, used by any other overloaded cell in the network, and are not bound

by bandwidth restrictions like those used by the cell. In other words, a satellite that provides service

for a network of cells can allow its own bandwidth to be used by any cell that needs it without

being bound by terrestrial bandwidth and llocation restrictions.

3. Weather Forecasting:

Certain satellites are specifically designed to monitor the climatic conditions of earth. They

continuously monitor the assigned areas of earth and predict the weather conditions of that region.

This is done by taking images of earth from the satellite. These images are transferred using

assigned radio frequency to the earth station. (Earth Station: it is a radio station located on the earth

and used for relaying signals from satellites.) These satellites are exceptionally useful in predicting

disasters like hurricanes, and 4monitor the changes in the Earth's vegetation, sea state, ocean

colour, and ice fields.

4. Television Signals:

Satellites have been used for since the 1960's to transmit broadcast television signals between the

network hubs of television companies and their network affiliates. In some cases, an entire series

of programming is transmitted at once and recorded at the affiliate, with each segment then being

broadcast at appropriate times to the local viewing populace. In the 1970's, it became possible for

private individuals to download the same signal that the networks and cable companies were

transmitting, using c-band reception dishes. This free viewing of corporate content by individuals

led to scrambling and subsequent resale of the descrambling codes to individual customers, which

started the direct-to-home industry. The direct-to-home industry has gathered even greater

momentum since the introduction of digital direct broadcast service.

• C-band:

C-Band (3.7 - 4.2 GHz) - Satellites operating in this band can be spaced as close as two degrees

apart in space, and normally carry 24 transponders operating at 10 to 17 watts each. Typical receive

antennas are 6 to 7.5 feet in diameter. More than 250 channels of video and 75 audio services are

available today from more than 20 C-Band satellites over North America. Virtually every cable

programming service is delivered via C-Band.

• Ku-Band:

I) Fixed Satellite Service (FSS):

Ku Band (11.7 - 12.2 GHz) - Satellites operating in this band can be

spaced as closely as two degrees apart in space, and carry from 12 to 24 transponders that operate

at a wide range of powers from 20 to 120 watts each. Typical receive antennas are three to six feet

in diameter. More than 20 FSS Ku-Band satellites are in operation over North America today,

including several "hybrid" satellites which carry both C-Band and Ku-Band transponders.

PrimeStar currently operates off Satcom K-2, an FSS or so-called "medium-power" Ku-Band

satellite. AlphaStar also uses an FSS-Ku Band satellite, Telestar 402-R.

II) Broadcasting Satellite Service (BSS):

Ku-Band (12.2 - 12.7 GHz) - Satellites operating in this band are spaced

nine degrees apart in space, and normally carry 16 transponders that operate at powers in excess

of 100 watts. Typical receive antennas are 18 inches in diameter. The United States has been

allocated eight BSS orbital positions, of which three (101, 110 and 119 degrees) are the so-called

prime "CONUS" slots from which a DBS provider can service the entire 48 contiguous states with

one satellite. A total of 32 DBS "channels" are available at each orbital position, which allows for

delivery of some 250 video signals when digital compression technology is employed.

• DBS:

DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct

to the end user. More than four million households in the United States enjoy C-Band

DBS.Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS

launched in 1994.

5. Marine Communications:

In the maritime community, satellite communication systems such as Inmarsat provide good

communication links to ships at sea. These links use a VSAT type device to connect to

geosynchronous satellites, which in turn link the ship to a land based point of presence to the

respective nations telecommunications system.

6. Spacebourne Land Mobile:

Along the same lines as the marine based service, there are VSAT devices which can be used to

establish communication links even from the world's most remote regions. These devices can be

hand-held, or fit into a briefcase. Digital data at 64K ISDN is available with some (Inmarsat).

7. Satellite Messaging for Commercial Jets:

Another service provided by geosynchronous satellites are the ability for a passenger on an

airbourne aircraft to connect directly to a land based telecom network.

8. Global Positioning Services:

Another VSAT oriented service, in which a small apparatus containing the ability to determine

navigational coordinates by calculating a triangulating of the signals from multiple

geosynchronous

*Technological Overview:-

I) Frequency Allocations for Satellite Services:

Allocating frequencies to satellite services is a complicated process which requires international

coordination and planning. This is carried out under the auspices of the International

Telecommunication Union. To facilitate frequency planning, the world is divided into three

regions:

Region 1: Europe, Africa, what was formerly the Soviet Union, and

Mongolia

Region 2: North and South America and Greenland

Region 3: Asia (excluding region 1 areas), Australia, and the south-

west Pacific

Within these regions, frequency bands are allocated to various satellite services, although a given

service may be allocated different frequency bands in different regions. Some of the services

provided by satellites are

Fixed satellite service (FSS)

Broadcasting satellite service (BSS)

Mobile satellite services

Navigational satellite services

Meteorological satellite services

There are many subdivisions within these broad classifications; for example, the fixed satellite

service provides links for existing telephone networks as well as for transmitting television signals

to cable companies for distribution over cable systems. Broadcasting satellite services are intended

mainly for direct broadcast to the home, sometimes referred to as direct broadcast satellite (DBS)

service [in Europe it may be known as direct-to-home (DTH) service]. Mobile satellite services

would include land mobile, maritime mobile, and aeronautical mobile. Navigational satellite

services include global positioning systems, and satellites intended for the meteorological services

often provide a search and rescue service. Table 1.1 lists the frequency band designations in

common use for satellite services. The Ku band signifies the band under the K band, and the Ka

band is the band above the K band. The Ku band is the one used at present for direct broadcast

satellites, and it is also used for certain fixed satellite services. The C band is used for fixed satellite

services, and no direct broadcast services are allowed in this band. The VHF band is used for

certain mobile and navigational services and for data transfer from weather satellites. The L band

is used for mobile satellite services and navigation systems. For the fixed satellite service in the C

band, the most widely used subrange is approximately 4 to 6 GHz. The higher frequency is nearly

always used for the uplink to the satellite, for reasons which will be explained later, and common

practice is to denote the C band by 6/4 GHz, giving the uplink frequency first. For the direct

broadcast service in the Ku band, the most widely used range is approximately 12 to 14 GHz,

which is denoted by 14/12 GHz. Although frequency assignments are made much more precisely,

and they may lie somewhat outside the values quoted here (an example of assigned frequencies in

the Ku band is 14,030 and 11, 730 MHz), the approximate values stated above are quite satisfactory

for use in calculations involving frequency, as will be shown later in the text. Care must be

exercised when using published references to frequency bands because the designations have

developed somewhat differently for radar and communications applications; in addition, not all

countries use the same designations. The official ITU frequency band designations are shown in

Table 1.2 for completeness. However, in this text the designations given in Table 1.1 will be used,

along with 6/4 GHz for the C band and 14/12 GHz for the Ku band.

II) Satellites for Data:

• Characteristics

Incorporating into terrestrial networks is often hindered by three characteristics possessed by

satellite communication.

❍ Latency (propagation delay): Due to the high altitudes of satellite orbits, the time required for

a transmission to navigate a satellite link (more than 2/10ths of a second from earth station to earth

station) could cause a variety of problems on a high speed terrestrial network that is waiting for

the packets.

❍ Poor Bandwidth: Due to radio spectrum limitations, there is a fixed amount of bandwidth

allocable to satellite transmission.

❍ Noise: A radio signals strength is in proportion to the square of the distance traveled. Due to the

distance between ground station and satellite, the signal ultimately gets very weak. This problem

can be solved by using appropriate error correction techniques, however. SULU

• Error Correction:

Due to the high noise present on a satellite link, numerous error correction techniques have been

tested in on such links. They fall into the two categories of forward-error-correction (FEC) and

automatic-repeat-request (ARQ):

❍ Forward-error-correction (FEC):

In this method a certain number of information symbols are mapped to new information symbols,

but in such a way as to get more symbols than were original had. When these new symbols are

checked on the receiving end, the redundant symbols are used to decipher the original symbols, as

well as to check for data integrity. The more redundant symbols that are included in the mapping,

the better the reliability of the error correction. However it should be noted that the more redundant

symbols that are used to achieve better integrity, the more bandwidth that is wasted. Since this

method uses relatively a large amount redundant data, it may not be the most efficient choice on a

clear channel. However when noise levels are high, FEC can more reliably ensure the integrity of

the data.

❍ Automatic-repeat-request (ARR):

In this method, data is broken into packets. Within each packet is included an error checking key.

This key is often of the cyclic redundancy check (CRC) sort. If the error code reflects a loss of

integrity in a packet, the receiver can request the sender to resend that packet. ARR is not very

good in a channel with high noise, since many retransmissions will be required, and the noise

levels that corrupted the initial packet will be likely to cause corruption in subsequent packets.

ARR is more suitable to relatively noise free channels. toptop and Wait (SW) With this form of

ARR, the sender must wait for an acknowledgement of each packet before it can send a new one.

This can take upwards of 4/10ths of a second per packet since it takes 2/10ths seconds for the

receiver to get the packet an another 2/10th seconds for the sender to receive the acknowledgement.

■ Go-back-N (GBN)

This method of ARR is an improvement over stop and wait in that it allows the sender to keep

sending packets until it gets a request for a resend. When the sender gets such a request, it sends

packets starting at the requested packet over again. It can again send packets until it receives

another retransmit request, and so on.

■ Selective-repeat (SR)

This ARR protocol is an improvement over GBN in that it allows the receiver to request a

retransmit of only that packet that it needs, instead of that packet and all that follows it. The

receiver, after receiving a bad packet and requesting a retransmit, can continue to accept any good

packets that are coming. This method is the most efficient method for satellite transmissions of the

three ARR methods discussed. ARRARR methods can be demonstrated to provide a usable error

correction scheme, but it is also the most expensive, in terms of hardware. This is in part due to

the buffering memory that is required, but more importantly to the cost of the receiver, which needs

to be able to transmit re-requests. Systems such as the Digital Broadcast Satellites used for

television signal distribution would become inordinately expensive if they had to make use of

ARR, since the home based receiver would now need to be a transmitter, and the 18 inch dish

would be inadequate for the requirements of transmitting back to a satellite.

■ Hybrid Networks

In today's global networking landscape, there are many ways to transmit data from one place to

another. It is desirable to be able to incorporate any type of data transmission media into a network,

especially in networks that encompass large areas. A hybrid network is one that allows data to

flow across a network, using many types of media, either satellite, wireless or terrestrial,

transparently. Since each type of media will have different characteristics, it is necessary to

implement a standard transmission protocol. One that is normally used in hybrid networks is

TCP/IP. In addition, much work is being done to use TCP/IP over ATM for the satellite segments

of hybrid networks, about which more will be discussed later. One way to get around the need in

ARR for the receiver to have to request retransmit via an expensive and slow satellite link is to use

a form of hybrid network. In one form of hybrid network, the receiver transmits its requests back

to the sender via a terrestrial link. Terrestrial link allows for quicker, more economical and less

error prone transmission from the receiver, and the costs associated with the receivers hardware

are greatly reduced when compared to the costs involved if it had to transmit back over the satellite

link. There are products on the market today that allow a home user to get internet access at around

400MB via digital satellite, while its retransmit signals are sent via an inexpensive modem or ISDN

line.

In fact, a product currently being marketed by Direct PC called Turbo Internet uses a form of

hybrid network. The system uses two network interfaces; one connects via a special ISA bus PC

adapter to a receive-only Very Small Aperture Terminal (VSAT), while the other is a modem

attached to a serial port. Inbound traffic comes down to the VSAT, while outbound traffic goes

through the modem link. The two interfaces are combined to appear as a single virtual interface to

upper layer TCP/IP protocol stacks by a special NDIS compliant driver. The Serial Line Internet

Protocol (SLIP) is used to connect the modem-based link with an internet service provider.

Packets, which are encapsulated by the terminal such that the desired IP address of the destination

host is embedded underneath the IP address of the Direct PC Gateway, to which all packets leaving

the terminal must go. Once at the gateway, the outer packet is stripped, and the gateway contacts

the destination address within. Upon the gateway's receiving the request from the host, it then

prepares the packet for satellite transmission, which is then used to send the packet back to the

terminal.

A diagrammatical overview of a satellite communication.

*DIFFERENT SATELLITE ORBITS:

Satellites may be categorised based on the altitude, inclination etc. One popular classification is

based on the altitude of the orbit. These categories are low Earth orbit (LEO), medium Earth orbit

(MEO) and geostationary orbit (GEO).

1. Low Earth orbit (LEO):

A low Earth orbit is generally defined as an orbit below an altitude of

approximately 2000km.The altitude of LEO is usually not less than 500 km

because of the orbit decay due to larger atmospheric drag. An LEO is an orbit

around the Earth between the atmosphere and below the inner Van Allen

radiation belt (see Annexure 3A). This region of the belt is not used for the

placement of satellite, as it is full of high energy particles that may harm the

satellite. Iridium, Orbocomm, Globalstar satellites (see Chapter 12) are LEO

satellites.

2. Medium Earth orbit (MEO):

Medium Earth orbit is the region of space around the Earth above the LEO

(altitude of 2000 km) and below the geostationary orbit (altitude of 35786

km). The MEO lies within outer Van Allen radiation belt (see Annexure 3A).

Global navigation satellites in GPS and GLONASS constellations (see

Chapter 8) are MEO satellites.

3. The Geostationary Orbit:

• Introduction

A satellite in a geostationary orbit appears to be stationary with respect to the earth, hence the

name geostationary. Three conditions are required for an orbit to be geostationary:

1. The satellite must travel eastward at the same rotational speed as the earth.

2. The orbit must be circular.

3. The inclination of the orbit must be zero.

The first condition is obvious. If the satellite is to appear stationary, it must rotate at the same

speed as the earth, which is constant. The second condition follows from this and from Kepler’s

second law. Constant speed means that equal areas must be swept out in equal times, and this can

only occur with a circular orbit. The third condition, that the inclination must be zero, follows from

the fact that any inclination would have the satellite moving north and south, and hence it would

not be geostationary. Movement north and south can be avoided only with zero inclination, which

means that the orbit lies in the earth’s equatorial plane. Kepler’s third law may be used to find the

radius of the orbit (for a circular orbit, the semi major axis is equal to the radius). Denoting the

radius by aGSO, then

The period P for the geostationary is 23 h, 56 min, 4 s mean solar time (ordinary clock time). This

is the time taken for the earth to complete one revolution about its N-S axis, measured relative to

the fixed stars . Substituting this value along with the value for u(Miu),

This value is often rounded up to 36,000 km for approximate calculations. In practice, a precise

geostationary orbit cannot be attained because of disturbance forces in space and the effects of the

earth’s equatorial bulge. The gravitational fields of the sun and the moon produce a shift of about

0.85°/year in inclination. Also, the earth’s equatorial ellipticity causes the satellite to drift eastward

along the orbit. In practice, station-keeping maneuvers have to be performed periodically to correct

for these shifts, An important point to grasp is that there is only one geostationary orbit because

there is only one value of a that satisfies Equation for a periodic time of 23 h, 56 min, 4 s.

Communications authorities throughout the world regard the geostationary orbit as a natural

resource, and its use is carefully regulated through national and international agreements.

• Earth Eclipse of Satellite:

If the earth’s equatorial plane coincided with the plane of the earth’s orbit around the sun (the

ecliptic plane), geostationary satellites would be eclipsed by the earth once each day. As it is, the

equatorial plane is tilted at an angle of 23.4° to the ecliptic plane, and this keeps the satellite in full

view of the sun for most days of the year, as illustrated by position A in Fig. 3.8. Around the spring

and autumnal equinoxes, when the sun is crossing the equator, the satellite does pass into the

earth’s shadow at certain periods, these being periods of eclipse as illustrated in Fig. 3.8. The

spring equinox is the first day of spring, and the autumnal equinox is the first day of autumn.

Eclipses begin 23 days before equinox and end 23 days after equinox. The eclipse lasts about 10

min at the beginning and end of the eclipse period and increases to a maximum duration of about

72 min at full eclipse (Spilker, 1977). During an eclipse, the solar cells do not function, and

operating power must be supplied from batteries. Where the satellite longitude is east of the earth

station, the satellite enters eclipse during daylight (and early evening) hours for the earth station,

as illustrated in Fig. 3.9. This can be undesirable if the satellite has to operate on reduced battery

power. Where the satellite longitude is west of the earth station, eclipse does not occur until the

earth station is in darkness, when usage is likely to be low. Thus satellite longitudes which are

west, rather than east, of the earth station are more desirable.

• Launching Orbits:

Satellites may be directly injected into low-altitude orbits, up to about 200 km altitude, from a

launch vehicle. Launch vehicles may be classified as expendable or reusable. Typical of the

expendable launchers are the U.S. Atlas-Centaur and Delta rockets and the European Space

Agency Ariane rocket. Japan, China, and Russia all have their own expendable launch vehicles,

and one may expect to see competition for commercial launches among the countries which have

these facilities.

Until the tragic mishap with the Space Shuttle in 1986, this was to bethe primary transportation

system for the United States. As a reusable launch vehicle, the shuttle, also referred to as the Space

Transportation System (STS), was planned to eventually replace expendable launch vehicles for

the United States (Mahon and Wild, 1984).

Where an orbital altitude greater than about 200 km is required, it is not economical in terms of

launch vehicle power to perform direct injection, and the satellite must be placed into transfer orbit

between the initial low earth orbit and the final high-altitude orbit. In most cases, the transfer orbit

is selected to minimize the energy required for transfer, and such an orbit is known as a Hohmann

transfer orbit. The time required for transfer is longer for this orbit than all other possible transfer

obits. Assume for the moment that all orbits are in the same plane and that transfer is required

between two circular orbits, as illustrated in Fig. 3.10. The Hohmann elliptical orbit is seen to be

tangent to the low-altitude orbit at perigee and to the high-altitude orbit at apogee. At the perigee,

in the case of rocket launch, the rocket injects the satellite with the required thrust into the transfer

orbit. With the STS, the satellite must carry a perigee kick motor which imparts the required thrust

at perigee. Details of the expendable vehicle launch are shown in Fig. 3.11 and of the STS launch,

in Fig. 3.12. At apogee, the apogee kick motor (AKM) changes the velocity of the satellite to place

it into a circular orbit in the same plane. As shown in Fig. 3.11, it takes 1 to 2 months for the

satellite to be fully operational (although not shown in Fig. 3.12, the same conditions apply).

Throughout the launch and acquisition phases, a network of ground stations, spread across the

earth, is required to perform the tracking, telemetry, and command (TT&C) functions.

Velocity changes in the same plane change the geometry of the orbit but not its inclination. In

order to change the inclination, a velocity change is required normal to the orbital plane. Changes

in inclination can be made at either one of the nodes, without affecting the other orbital parameters.

Since energy must be expended to make any orbital changes, a geostationary satellite should be

launched initially with as low an orbital inclination as possible. It will be shown shortly that the

smallest inclination obtainable at initial launch is equal to the latitude of the launch site. Thus the

farther away from the equator a launch site is, the less useful it is, since the satellite has to carry

extra fuel to effect a change in inclination. Russia does not have launch sites south of 45°N, which

makes the launching of geostationary satellites a much more expensive operation for Russia than

for other countries which have launch sites closer to the equator.

Prograde (direct) orbits have an easterly component of velocity, and these launches gain from the

earth’s rotational velocity. For a given launcher size, a significantly larger payload can be launched

in an easterly direction than is possible with a retrograde (westerly) launch. In particular, easterly

launches are used for the initial launch into the geostationary orbit.

The relationship between inclination, latitude, and azimuth may be seen as follows [this analysis

is based on that given in Bate et al. (1971)]. Figure 3.13a shows the geometry at the launch site A

at latitude Lambda(^)(the slight difference between geodetic and geocentric latitudes may be

ignored here). The dotted line shows the satellite earth track, the satellite having been launched

at some azimuth angle Az. Angle I is the resulting inclination.

4. @ Highly Ecentric Orbit(HEO)(Molniya orbit):

Highly eccentric, inclined and elliptical orbits are used to cover higher latitudes, which are

otherwise not covered by geostationary orbits. A practical example of this type of orbit is

the Molniya orbit. It is widely used by Russia and other countries of the former Soviet

Union to provide communication services. Molniya orbits are designed so that the

perturbations in argument of perigee are zero. So, typical eccentricity and inclination of the

Molniya orbits are 0.75 and 65° (or 116.6°), respectively (see Section 3.2). The apogee and

perigee points are about 40000 km and 400 km, respectively, from the surface of the Earth.

@ GEOSYNCHRONOUS EARTH SATELLITES:

A geosynchronous satellite is a satellite in geosynchronous orbit whose orbital period is exactly one sidereal day.This makes the satellite to rotate around the Earth with the same speed as the rotation of the Earth around its own axis. The Kepler’s laws dictate that the satellite would necessarily be at approximately 35768 km above the Earth. This orbit is in safe zone as it is outside the van Allen radiation belt. Normally, geosynchronous satellite has non-zero inclination and is also a circular orbit. IRNSS constellation has two geosynchronous satellites.

A special case of geosynchronous satellite is the geostationary satellite, where the inclination is zero. INSAT, GSAT (see Section 12.3) satellites and satellites used for VSATs are in the category of geostationary satellite

The orbit of a geosynchronous satellite is not exactly aligned with the Earth’s equator. In this condition, the orbit is known as an inclined orbit. In this case, satellite would not be stationary with respect to the Earth. Instead, the latitude and longitude deviation of the sub-satellite point (for geostationary circular orbit) would be governed by the following relations:

It will appear (when viewed by someone on the ground) that the sub-satellite point traces a sub-

satellite point reaches a maximum latitude of ±i. As inclination of the orbit decreases, the

magnitude of this oscillation becomes smaller. But, when the orbit lies entirely over the equator in

a circular orbit as the case of a geostationary satellite, the satellite remains stationary relative to

the Earth’s surface. Hence, there is permanent visibility of a geostationary satellite within its

coverage area. Geostationary satellites, thus, are widely used for communication.

• Limit of visibility:

It is the maximum value of b for geostationary satellite is 81.3°.This implies that the satellites

cover up to 81.3° (practically 70° excluding elevation lower than 10°) of latitude. In other words,

geostationary satellite does not have coverage in higher latitude near poles. This very fact

prompted Russia to design highly eccentric orbit (HEO) like Molniya orbit for better coverage of

higher latitude region of northern hemisphere. Along the longitude, the satellite covers the span of

around 160° degree. Thus, minimum three satellites are required to cover the globe (i.e., 360°).

The look angles for the ground station antenna are the azimuth and elevation angles required at the

antenna so that it points directly at the satellite. In Sec. 2.9.8 the look angles were determined in

the general case of an elliptical orbit, and there the angles had to change in order to track the

satellite. With the geostationary orbit, the situation is much simpler because the satellite is

stationary with respect to the earth. Although in general no tracking should be necessary, with the

large earth stations used for commercial communications, the antenna beamwidth is very narrow

and a tracking mechanism-

#NOTE:-

Perhaps the most important contribution of the world’s space efforts over the last half century has

been the advent of geosynchronous earth satellites. These devices have made global

communications possible connecting every point on earth with any other point. Although the use

of such satellites was mentioned as early as 1923 in Hermann Oberth’s book on space travel, it

was the science fiction writer Arthur C. Clark who popularized the idea of geo-satellites for radio

communication in 1945. The first near earth satellite (Sputnik) was launched in 1957 and the first

successful geosynchronous satellite (Syncom2) was placed into earth orbit in 1963. Since that

time hundreds of these satellites now exist in nearly circular orbits some 35,800 km above the

earth’s equator. It is our purpose here to discuss the basic mechanics behind a satellite in

geosynchronous orbit moving at speed rω at height r above the earth’s centre. Here ω=dθ/dt

matches the earth’s rotation rate so that the satellite will appear fixed above a given point along

the equator. A schematic of the set-up is as shown In this picture we are looking at the earth and

orbit from a point above the north pole so

that the satellite and the earth both rotate counter clockwise at the same angular speed ω.

The basic equation governing the satellite motion is Newton’s Second Law F=ma. In

polar coordinates this equation has the two components-

You may have wondered why home TV satellite receivers always point approximately south at a

fixed angle with respect to the horizon. This is because one is receiving the reflected signals from

a particular geosynchronous satellite sitting above the equator (probably above Ecuador for east

coast viewers). The angle above the horizon to which a parabolic receiver must be pointed is

approximately equal to π/2-LAT, where LAT is the local latitude. Use of several of such satellites

placed at equal distances around the equator makes possible almost instantaneous communication

between any two places on earth (arctic regions excluded). A slight delay in such communications

when they are two-way is noted because of the finite speed of light at c=3·108 m/s. For instance

in a two-way conversation with live pictures between Tahrir Square in Cairo and a home news

office in New York, one can expect a delay of about 4L/c where c is the speed of light and L the

distance from the sender or receiver to a geosynchronous satellite as shown in the figure

*The Satellite communication techniques:

The satellite communication can be divided into three sections:

1. Uplink Section (Ground Station)

2. Transponder (Airborne Satellite)

3. Downlink Section (Ground Station)

The uplink section consists of following units–

• Intermediate Frequency (IF) Modulator

• Band Pass Filter (BPF)

• Up Converter (Mixer, BPF & Uplink Frequency

Microwave Generator)

• High Power Amplifier (HPA)

• Transmitting (Tx) Antenna

The signal which user wants to send is called as Baseband Signal. It is feed to Intermediate

Frequency (IF) Modulator who converts baseband frequency to intermediate frequency through

different types of modulation process like ASK, FSK and, PSK according to the requirement. The

output IF is passed through a Band Pass Filter (BPF) for cutting off unnecessary frequency

components. The IF is sent to UP-Converter where IF range (MHz) is increased to Radio

Frequency (RF) range (GHz) with the help of Mixer & Uplink Frequency Microwave Generator.

Here, Radio Frequency (RF) = (LO + IF) and Intermediate Frequency (IF) = (LO – RF). Here, LO

is Local Oscillator Frequency. There is another BPF for the RF signal to become more accurate

uplink frequency. Finally, the RF is passed through a High Power Amplifier (HPA) for gaining

enough strength to travel a long distance before it is radiated through transmission (Tx) antenna.

Transponder consists of the following subsections–

• Receiving (Rx) Antenna

• Band Pass Filter (BPF)

• Low Noise Amplifier (LNA)

• Frequency Translator

• Low Power Amplifier (LPA)

• Transmitting (Tx) Antenna

Transponder implies to Transmitter + Responder. The transmitted frequency from the uplink

section and received frequency at the transponder by a receiving (Rx) antenna are the same. Noise

filtering is performed by a Band Pass Filter (BPF). Then the RF signal is sent to the Low Noise

Amplifier (LNA), a tunnel diode, which amplifies the signal but keeps noise level very low. To

get some specific advantages (e.g., small antenna size and less power consumption) downlink

frequency is usually kept 2 GHz less than uplink frequency. Frequency Translator performs the

frequency conversion with the help of Mixer & Microwave Shift Oscillator. Again BPF is used to

get more accurate downlink frequency. FinallyFinally, the downlink frequency is passed through

a Low Power Amplifier (LPA) for gaining strength to come back to earth before radiating through

transponder’s transmission(Tx) antenna.Downlink section consists of following units–

• Receiving (Rx) Antenna

• Band Pass Filter (BPF)

• Low Noise Amplifier (LNA)

• Down Converter (Mixer, BPF & Downlink Frequency

Microwave Generator)

• Intermediate Frequency (IF) Demodulator

The transmitted frequency from the transponder and received frequency at the receiving antenna

are the same. A Band Pass Filter (BPF) cuts off the unnecessary frequency components. Then the

RF signal is sent to the Low Noise Amplifier (LNA) for amplification. Then the RF signal is feed

to Down-Converter where RF range (GHz) is decreased to IF range (MHz) with the help of Mixer

& Downlink Frequency Microwave Generator. Intermediate Frequency (IF) Demodulator converts

intermediate frequency to baseband frequency. This baseband signal is the signal which was sent

via transponder by the user from the uplink section.

The complete block diagram of satellite communication system

*The Space Link:-

The design of a satellite link is very important, as it gives the estimate of the power that would be

received by the Earth station when the power is transmitted by the satellite transponder. Also, it

gives the estimate of the power that would be received by the transponder when the power is

transmitted by the Earth station. For the case of satellite link, it is quite reasonably assumed that a

signal propagates in line-of-sight (LoS) mode. It also takes into account the gains and losses that

are encountered due to various factors like propagation, antenna gains, feed line and miscellaneous

losses. Some of their contributions may be assessed through standard formula and some may be

evaluated through experiments. Link analysis is basically the estimation of power that is to be

transmitted from an Earth station to the satellite (uplink) and from the satellite to the receiver

(downlink), as seen in Figure ,so that optimum performance is achieved.

The characteristics of the radio terminals (like transmitter, receiver and antenna) and the

propagation medium should be known well for proper evaluation of link parameters.