2. 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.
2
EARTH
STATION
3. 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 GHz
6.425 GHz to 7.025 GHz
used for Up link.
3.4 GHz to 3.7 GHz
4.5 GHz to 4.8 GHz
used for Down link.
“Ku” Band 12.75 GHz to 13.250 GHz
14.00 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 GHz
18.10 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.
3
4. 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.
4
5. 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
5
Inclined
Satellite
6. 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
6
7. 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 10
5
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.
7
8. 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.
8
9. 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 74
o
east and 93.5
o
east. Series of four satellites (INSAT–I series) was planned to provide services in
the following areas :
• Telecommunications
• Radio and Television Boradcasting
• Meteorology
9
10. 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 System
o INSAT—1 in April 1982
o 5 Satellites in INSAT-2 and INSAT-3
o 5 Communication Satellite planned in INSAT-4 Series.
SATELLITEs AND TRANSPONDER CAPACITIES
S.
No.
NAME OF
SATELLITE
Transponder capacity Orbit
location
(Longitude)C
-Band
Ext-C
band
Ku-
band
S-
band
Mobile
(MSS0
(CxS
&SxC)
1 INSAT-1D 12 - - 2 - 83 deg E
2 INSAT-2A 12 6 - 2 - 74 deg E
3 INSAT-2B 12 6 - 2 - 93.5 deg E
4 INSAT-2C 12 6 3 1 1 93.5 deg E
5 INSAT2E 12 5 - - - 83 deg E
6 INSAT-2DT 25 - - 1 - 55 deg E
7 INSAT-3B - 12 3 - 1 83 deg E
10
11. 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
11
12. 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.
12
13. 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.
ORBITS
Orbit 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.
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14. Introduction
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
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
14
15. Introduction
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
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
-Band
Ext-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 :
15
16. Introduction
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)
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
16
17. Introduction
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
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
17
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.
