L - 4INTRODUCTION

TO

SATELLITE COMMUNICATION

Introduction

INTRODUCTION TO SATELLITE COMMUNICATIONS

1. Introduction

Long distance communications, particularly to remote locations, using

conventional terrestrial media is both uneconomic and unreliable. A geo-stationary

communication satellite which acts as a repeater hung in the sky can cover a very

large area and provide a reliable and cost effective alternative.

Although satellite communication would seem to be a straightforward

extension of terrestrial radio system, the use of satellites for communications

brings in new operational features not found in terrestrial systems. In this hand

out, some of the features of satellite communication are discussed. Basic

knowledge of terrestrial radio systems is assumed.

2. Structure of a Satellite Link

A satellite link is similar to a terrestrial microwave link with two hops, the

satellite playing the role of a repeater (Fig.1).

Fig. 1

Two distinctly separated frequency bands for

the uplink path from the earth station to the satellite and

the downlink path from the satellite to the earth station.

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EARTHSTATION

Introduction

are used to facilitate separation of the transmitted and received signals at the

common earth station antenna. The satellite, as a repeater

receives the uplink signals from the earth stations,

translates them to the down link frequency band,

amplifies them, and

transmits back to the earth stations.

The up link and down link signals are modulated carriers as in terrestrial

microwave links. The satellite, as a repeater, does not distinguish between the

type of modulation used in the carrier. It can be a frequency modulated or a QPSK

carrier.

3. Choice of Frequency of Operation

The majority of present day communication satellites use the following

frequencies in the C–band and other bands assigned by CCIR :

“C” Band 5.925 GHz to 6.425 GHz 3.7 GHz to 4.2 GHz

used for Up link.used for Down link.

Extended “C” Band

5.85 GHz to 5.9 GHz6.425 GHz to 7.025 GHz used for Up link.

3.4 GHz to 3.7 GHz4.5 GHz to 4.8 GHz used for Down link.

“Ku” Band 12.75 GHz to 13.250 GHz14.00 GHz to 14.5 GHz used for Up link.

10.7 GHz to 11.7 GHz used for Down link.

“Ka” Band 27.00 GHz to 30.00 GHz18.10 GHz to 20.20 GHz

used for Up link.used for Down link.

The available frequency band in each direction is 500 MHz wide. The

satellite translates the uplink carrier frequencies by 2.225 GHz before re-

transmission back to the earth. While being at higher frequency, these frequency

bands permit much smaller size of antenna, but there are other technological and

atmospheric attenuation problems, which must be overcome.

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Introduction

3.1 RF Channels

The 500 MHz frequency band is divided into 12 sub–bands each of 36 MHz

bandwidth and with 4 MHz guard band between the adjacent sub–bands (Fig.2).

Each sub–band is treated as one RF.

Fig. 2

In the downlink frequency band, the satellite inserts one or more beacon

signals, which are used by the earth stations to track the satellite.

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Introduction

4. Orbit of a Satellite

The orbit of an artificial satellite is an ellipse, the plane of which passes

through the centre of the earth. Parameters of the orbit are defined by three laws

of Kepler. The third law relates to the period of revolution of the satellite. Fig.3

shows the period of revolution of a satellite as a function of its altitude.

Fig. 3

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InclinedSatellite

Introduction

4.1 Geostationary Satellite

Satellite at low altitude are moving satellites, which shift rapidly with respect

to a point on the surface of the earth. On the other hand, a satellite which appears

to be immobile when seen from the earth, is called geostationary. Majority of the

communication satellites are geostationary for the very practical reason that an

earth station antenna can be easily pointed towards its. Else the antenna has to

continuously track the satellite as it moves relative to the earth station. For a

geostationary satellite, the following orbital conditions must be satisfied :

Period of revolution should be same as period of the earth’s rotation

which is 23 hours 56 minutes and 4 seconds.

The altitude of the satellite should be 35786 km as per the Kepler’s

third law (Fig.3).

The orbit should be in equatorial plane of the earth (Fig.4).

The satellite should move in the easterly direction same as the

direction of rotation of the earth.

Fig. 4

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Introduction

5. Level Diagram

The geostationary communication satellite is at about 36000 km from the

earth. Over this long distance, the uplink and downlink carriers are attenuated to a

large extent. The received signals at the satellite and at the earth stations are very

weak necessitating critical design of the receive equipment. Fig.5 shows the order

of signal levels encountered in satellite communications.

Fig. 5

6. Satellite Delay

Transmission path of a communication link through a satellite is about

72000 km long (36000 km uplink path and 36000 km downlink path).

Electromagnetic waves travelling at 3 x 105 km/sec take about 240 ms. from one

end to the other. Such large propagation delay is at the limit of psychologically

tolerable values in telephony. The propagation delay results in “echo” in a

telephony channel. A special equipment called echo suppressors is incorporated

in the earth station to counteract the echo. Propagation delay also restricts the

number of satellite hops for building up a telephony circuit to one as the resulting

propagation delay will be much beyond acceptable value for more than one hop. 6

Introduction

7. Equipment in a Communication Satellite

The equipment carried aboard a satellite essentially consists of

Payload

Support subsystems

Payload refers to the equipment used to provide the service for which the

satellite has been launched. In communication satellites, payload consists of

transponders which carry out the repeater function and the transmit and receive

antennas. Support subsystems include altitude and orbit control equipment, power

subsystem, telemetry and tele-command subsystem, etc.

7.1 Transponders

A transponder is a series of interconnected units which form a single chain

for processing an RF channel between the receive and transmit antennas. Some

of the equipment in the RF chain is common for all the channels as shown in fig.

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Introduction

Fig. 7

The first stage is a 500 MHz bandpass filter followed by a low noise wide

band receiver in 1+1 redundant configuration. The wide band receiver is common

for all RF channels. It consists of a low noise amplifier and a mixer stage which

translates the entire frequency band 5925–6425 MHz received from the earth

stations to 3700–4200 MHz frequency band (Fig.7). The frequency translation is

carried out using a local oscillator at 2225 MHz. The input demultiplexer separates

the broadband input into 12 RF channels using filters. A separate Travelling Wave

Tube (TWT) amplifier for each channel provides sufficient gain and output power

for transmission back to earth. The RF channels are combined using an output

multiplexer and sent to the transmit antenna.

6. Satellite Communication in India

In India, satellite communication has been in use for overseas

communication by Videsh Sanchar Nigam Ltd. for many years. This service is

provided through INTELSAT satellite. For domestic communications, INSAT

program was formulated during mid seventies. Two locations in the geostationary

orbit were reserved for Indian satellites. These locations were 74o east and 93.5o

east. Series of four satellites (INSAT–I series) was planned to provide services in

the following areas :

Telecommunications

Radio and Television Boradcasting

Meteorology

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Introduction

First satellite of this series, INSAT–1A, was launched in 1982. The last

satellite of this series, INSAT–1D was launched in 1989. These satellites were

designed and fabricated by Ford Aerospace, a US firm. These satellites had the

following payload configuration :

Telecom Pay load– Number of transponders 12– Uplink frequency band 5.925–6.425 GHz– Downlink frequency band 3.700–4.200 GHz– Beacon frequencies 4.031, 4.038 GHz

Direct TV Broadcast Payload– Number of transponders 2– Transponder Bandwidth 36 MHz– Uplink frequency band 5.855–5.935 GHz– Downlink frequency band 2.555–2.635 GHz

“S” band

Meteorological Payload– Number of transponders 2– Transponder Bandwidth 200 KHz– Uplink frequency band 402.65–402.85 GHz

“L” band– Downlink frequency band Centred at 4038 MHz– Very High Resolution 1

Radiometer (VHRR)– Frequency of VHRR carrier 4034.55 MHz.

9. INSAT :o Indian National Satellite Systemo INSAT—1 in April 1982 o 5 Satellites in INSAT-2 and INSAT-3o 5 Communication Satellite planned in INSAT-4 Series.

SATELLITEs AND TRANSPONDER CAPACITIESS. No.

NAME OF SATELLITE

Transponder capacity Orbit location(Longitude)C -

BandExt-C band

Ku- band

S-band

Mobile (MSS0 (CxS &SxC)

1 INSAT-1D 12 - - 2 - 83 deg E2 INSAT-2A 12 6 - 2 - 74 deg E3 INSAT-2B 12 6 - 2 - 93.5 deg E4 INSAT-2C 12 6 3 1 1 93.5 deg E5 INSAT2E 12 5 - - - 83 deg E6 INSAT-2DT 25 - - 1 - 55 deg E7 INSAT-3B - 12 3 - 1 83 deg E

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Introduction

INSAT Satellite :

Satellite Location C-Band Xc-Band Ku-band Life

2E 83 Deg E 12 5 - 2011

3A 93 Deg E 12 6 6 2016

3B 83 Deg E - 12 3 2010

3C 74 Deg E 24 6 - 2015

3E 55DegE 24 12 - 2016

INSAT Satellite (Planned)

Satellite Location C-band Xc-band Ku-band Planned

4A 83 Deg E 12 - 12 9,054

4B 93 Deg E 12 - 12 9.06

4C 74Deg E - - 12 3.06

4D 93 Deg E 12 - - 9.07

4E 83 Deg E - - - 9.08

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Introduction

10. Advantages of Satellite Communications

Wide coverage

Almost one third of the earth except the polar regions is visible from a

geostationary satellite. It is, therefore, possible to cover wide geographical area

irrespective of intervening terrain using a single satellite. Satellite media is the

only alternative for remote areas inaccessible through terrestrial routes.

By suitable design and configuration of earth station equipment, satellite

links can be used for thin and heavy traffic routes in a cost effective manner.

Suitable for both Digital and Analog Transmission

Same satellite can be used for both digital and analog communication links.

Satellite is transparent to the type of service being provided.

High Quality

Satellite links are designed high quality of performance. The links are free

from atmospheric disturbances and fading. As only one repeater is involved, the

reliability is very high.

Flexibility

In terrestrial links, the topology of the network gets tied down to the

installed equipment. On the other hand, a satellite can be accessed from any

point on the earth from where it is visible. The earth stations can be relocated and

reconfigured providing complete flexibility of operation and utilisation of the

satellite capacity.

Quick Provision of Services

Compared to the terrestrial links, earth stations can be installed in much

shorter period and, therefore, services can become available faster.

Mobile and Emergency Communication

An earth station can be mounted on a vehicle to provide mobile

communication services. Using small air liftable earth station terminals,

telecommunication services can be extended to any location in emergency.

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Introduction

Types of Satellites

Anti-Satellite weapons, sometimes called "Killer satellites" are satellites designed to destroy "enemy" satellites, other orbital weapons and targets. Some are armed with kinetic rounds, while others use energy and/or particle weapons to destroy satellites, ICBMs, MIRVs. Both the U.S. and the USSR had these satellites. Links discussing "Killer satellites", ASATS (Anti-Satellite satellite) include USSR Tests ASAT weapon and ASAT Test. See also IMINT

Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.

Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.

Communications satellites are an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or low Earth orbits.

Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).

Navigation satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.

ORBITSOrbit is a path traversed by a satellite, orbit may be Equilateral, inclined

and Polar. The time taken to complete one orbit is known as Orbit Period. In GEO stationary orbit, it is equal to the Sidereal Day.

Altitude Classifications:Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 0 -

2,000 km (0 - 1,240 miles).

Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles. Low Earth Orbit satellites must

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Introduction

travel very quickly to resist the pull of gravity -- approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellites can orbit the planet in as little as 90 minutes.

Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet. Low Earth Orbit satellites typically operate in polar orbits. Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS). Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

Medium Earth Orbit (MEO) - Geocentric orbits ranging in altitude from 2,000 km (1,240 miles) - to just below geosynchronous orbit at 35,786 km (22,240 miles). Also known as an intermediate circular orbit.

Medium Earth Orbit (MEO) refers to a satellite which orbits the earth at an altitude below 22,300 miles (geostationary orbit) and above the altitude of Low Earth Orbit (LEO) satellites. Medium Earth Orbit represents a series of tradeoffs between geostationary orbit (GEO) and Low Earth Orbit (LEO). Medium Earth Orbit enables a satellite provider to cover the earth with fewer satellites than Low Earth Orbit, but requires more satellites to do so that geostationary orbit.

Medium Earth Orbit terrestrial terminals can be of lower power and use smaller antennas than the terrestrial terminals of geostationary orbit satellite systems. However, they cannot be as low power or have as small antennas as Low Earth Orbit terrestrial terminals. Medium Earth Orbit satellite systems offer better Round Trip Time (RTT) than geosynchronous orbit systems, but not as low as Low Earth Orbit systems.

High Earth Orbit (HEO) - Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,240 miles).

Geosynchronous Orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,240 miles). Such a satellite would trace an analemma in the sky. Geostationary orbits are often referred to as geosynchronous or just GEO.

A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky. A geostationary satellite is any satellite which is placed in a geostationary orbit. Satellites in geostationary orbit maintain a constant position relative to the surface of the earth. Geostationary satellites do this by orbiting the earth approximately 22,300 miles above the equator. This orbital path is called the Clarke Belt, in honor of Arthur C. Clarke.In other words, if a satellite in a geostationary orbit is in a certain place above the earth, it will stay in that same spot above the earth. Its latitude stays at zero and its longitude remains constant. A single geostationary satellite will provide coverage over about 40 percent of the planet. Geostationary

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Introduction

satellites are commonly used for communications and weather-observation. The typical service life expectancy of a geostationary satellite is ten to fifteen years. Because geostationary satellites circle the earth at the equator, they are not able to provide coverage at the Northernmost and Southernmost latitudes.

Choice of Transponder One-Way Communication: Data Relay Transponder Two-Way Communication: MSS Transponder

SATELLITEs AND TRANSPONDER CAPACITIES

S. No.

NAME OF SATELLITE

Transponder capacity Orbit location(Longitude)C -

BandExt-C band

Ku- band

S-band

Mobile (MSS0 (CxS &SxC)

1 INSAT-1D 12 - - 2 - 830 E

2 INSAT-2A 12 6 - 2 - 740 E

3 INSAT-2B 12 6 - 2 - 93.50 E

4 INSAT-2C 12 6 3 1 1 93.50 E

5 INSAT2E 12 5 - - - 830 E

6 INSAT-2DT 25 - - 1 - 550 E

7 INSAT-3B - 12 3 - 1 830 E

INSAT Satellite :

Satellite Location C-Band Xc-Band Ku-band Life

2E 830 E 12 5 - 2011

3A 930 E 12 6 6 2016

3B 830 E - 12 3 2010

3C 740 E 24 6 - 2015

3E 550 E 24 12 - 2016

INSAT Satellite (Planned)

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Introduction

SATELLITES IN OPERATION AS UPDATED ON 15-11-2006:

Satellite End of Life (EOL) Frequency Band/ No. of Transponders

INSAT-2E 2010 C/14, Ext. C/05, Ku/00, MSS/00

INSAT-3A 2015 C/12, Ext. C/06, Ku/06, MSS/00

INSAT-3B 2010 C/00, Ext. C/12, Ku/06, MSS/00

INSAT-3C 2011 C/24, Ext. C/06, Ku/00, MSS/01

INSAT-3E 2015 C/24, Ext. C/12, Ku/00, MSS/00

GSAT-2 2009 C/04, Ext. C/00, Ku/04, MSS/01

GSAT-3 2011 C/00, Ext. C/06, Ku/06, MSS/00

INSAT-4A 2017 C/12, Ext. C/00, Ku/12, MSS/00

INSAT-HIGH POWER (GE-1A)

FUTURE PLAN

Satellite Orbit Launch Date Band/Transponders

INSAT-4C 74 E Mission failed C/00, Ext. C/00, Ku/12

INSAT-4B 93.5 /Q1 2007 Arian-5 C/12, Ext. C/00, Ku/12

GSAT-5/INSAT-4D

82 E / Q3 2007 GSLV-MK-2

C/12, L-Ext. C/06, Ku/00

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Introduction

INSAT Satellite (Planned)

Satellite Location C-band Xc-band Ku-band Planned

4A 830 E 12 - 12 9-05

4B 930 E 12 - 12 9-06

4C 740 E - - 12 3-06

4D 930 E 12 - - 9-07

4E 830 E - - - 9-08

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C- Band : U/L : 5.925 – 6.425 GHz. D/L : 3.7 – 4.2 G Hz. Total 500 M Hz BW.

Frequency Bands For Satellite Communication

Extended C- Band :U/L : 6.725 – 7.025 GHz.D/L : 4.5 – 4.8 G Hz. Additional 300 MHz BW.

Ku band :U/L : 14.0 - 14.5 G Hz.D/L : 10.95 – 11.2 and 11.45 - 11.7 GHz. A total of 500 MHz BW in Ku band.