Practical No. 2
SATELLITE COMMUNICATIONS
INTRODUCTION:
A satellite network is a combination of modes that provide communication from one point on
the earth to other. A node in the network can be a satellite an earth station, or any end user terminal
or a telephone. Although a real satellite such as the moon can be used as a relaying node in the
network, the use of artificial satellites is preferred because we can install electronic equipment on the
satellite to regenerate the signal that has lost its energy during travel.
Another restriction of using a natural satellite is their distances from the earth which create a
long delay in communication. Satellite networks are like cellular networks in that they divide the
planet into large cells. Satellites can provide transmission capability to and from any location on
earth no matter how remote it is. This advantage makes high quality communication available to
undeveloped parts of the world without requiring a huge investment in ground based infrastructure.
An artificial satellite needs to have an orbit, the path in which it travels around the earth. The
orbit can be equatorial, inclined or polar. The period of a satellite that is the time required for a
satellite to make a complete trip around the earth is determined by Kepler’s law which defines the
period as a function of the distance of the satellite from the centre of the earth. Satellites process
microwaves with bi directional antennas that is line of sight. Therefore the signal from a satellite is
normally aimed at a specific area called the footprint. The signal power at the centre of the footprint
is highest. The power decrease as we move from the foot print centre. The boundary of the foot print
is the location where the power reaches a pre defined threshold.
Based on the location of orbits satellites can be divided into three categories that is
geosynchronous earth orbit, low earth orbit and medium earth orbit. Location of orbits is the distance
of orbits from earth’s surface. One reason for having different reasons is due to the existence of two
Van Allen belts. A Van Allen is a layer that contains charged particles. A satellite orbiting in one of
these two belts would be totally destroyed by the energetic charged particles. The medium earth
orbits are located between these two belts. The frequencies reserved for satellite microwave
communication are in gigahertz range. Each satellite sends and receives over two different bands.
Transmission from the earth to the satellite is called uplink and transmission from the satellite to the
earth is called downlink.
Compared to the fiber optical communication, satellite communication has the advantage that
the quality of transmitted signal and location of sending and receiving stations are independent of
distances.
HISTORY:
The first artificial satellite was placed in orbit by the Russians in 1957. That satellite, called
Sputnik, signaled the beginning of an era.
The United States, who was behind the Russians, made an all-out effort to catch up, and
launched Score in 1958. That was the first satellite with the primary purpose of communications.
The first regular satellite communications service was used by the Navy in 1960. The moon
was used to bounce teletypewriter signals between Hawaii and Washington, D.C. During the early
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1960s, the Navy used the moon as a medium for passing messages between ships at sea and shore
stations. This method of communications proved reliable when other methods failed.
Military satellite communications technology was at a low level until 1965. At that time high
quality voice transmissions were conducted between a satellite and two earth stations. That was the
stepping stone to the Initial Defense Communications Satellite Program (IDCSP), which will be
covered later in this chapter.
Experience with satellite communications has demonstrated that satellite systems can satisfy
many military requirements. They are reliable, survivable, secure, and a cost effective method of
telecommunications. You can easily see that satellites are the ideal, if not often the only, solution to
problems of communicating with highly mobile forces. Satellites, if properly used, provide much
needed options to large, fixed-ground installations.
For the past fifty years, the Navy has used high-frequency (hf) transmissions as the principal
method of sending messages. In the 1970s, the hf spectrum was overcrowded and "free" frequencies
were at a premium. Hf jamming and electronic countermeasures (ECM) techniques became highly
sophisticated during that period. As a result the need for new and advanced long-range transmission
methods became apparent.
Communications via satellite is a natural outgrowth of modern technology and of the
continuing demand for greater capacity and higher quality in communications.
In the past, the various military branches have had the resources to support their
communications needs. Predicted usage indicates that large-scale improvements will have to be
made to satisfy future needs of the Department of Defense. These needs will require greater capacity
for long-haul communications to previously inaccessible areas. Satellite communications has the
most promise for satisfying these future requirements.
Satellite communications basics
When used for communications, a satellite acts as a repeater. Its height above the Earth
means that signals can be transmitted over distances that are very much greater than the line of sight.
An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on
one frequency. The satellite receives the signal and retransmits it on what is termed the down link
which is on another frequency.
Using a satellite for long distance communications
The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is
called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting
a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make
sure that any out of band signals such as the transponder output are reduced to acceptable levels so
that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure
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that spurious signals are reduced to acceptable levels. Figures used in here are the same as those
mentioned earlier, and are only given as an example. The signal is received and amplified to a
suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a
superheterodyne radio receiver. As a result the communications satellite receives in one band of
frequencies and transmits in another.
In view of the fact that the receiver and transmitter are operating at the same time and in close
proximity, care has to be taken in the design of the satellite that the transmitter does not interfere
with the receiver. This might result from spurious signals arising from the transmitter, or the receiver
may become de-sensitised by the strong signal being received from the transmitter. The filters
already mentioned are used to reduce these effects. Signals transmitted to satellites usually consist of
a large number of signals multiplexed onto a main transmission. In this way one transmission from
the ground can carry a large number of telephone circuits or even a number of television signals.
This approach is operationally far more effective than having a large number of individual
transmitters.
Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further
capacity can be achieved using several satellites on different bands, or by physically separating them
apart from one another. In this way the beamwidth of the antenna can be used to distinguish between
different satellites. Normally antennas with very high gains are used, and these have very narrow
beamwidths, allowing satellites to be separated by just a few degrees.
Separating satellites by position
Telecommunications satellite system
Communications satellites are ideally placed to provide telecommunications links between
different places across the globe. Traditional telecommunications links used direct "cables" linking
different areas. As a result of the cost of installation and maintenance of these cables, satellites were
seen as an ideal alternative. While still expensive to put in place, they provided a high bandwidth and
were able to operate for many years.
In recent years the bandwidth that can be offered by cables has increased considerably, and
this has negated some of the gains of satellites. Additionally the geostationary satellites used for
telecommunications links introduce a significant time delay in view of the very large distances
involved. This can be a problem for normal telephone calls.
Mobile satellite communications systems
There are many instances where communications need to be maintained over wide areas of
the globe. Ships, aircraft and the like, need to be able to communicate from points all around the
world. Traditionally HF radio communications ahs been used, but this is unreliable. Satellite
communications provide an ideal solution to this problem as satellite communications are much
more reliable and they are able to provide interference free stable communications links. As a result,
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Satellite communications is now fitted as standard to all maritime vessels, and it is becoming
increasingly used by aircraft, although it is not yet adopted for Air Traffic management (ATM).
In addition to these users, these services can be sued by many land mobile or land portable
radio users. Satellite terminals provide are able to access the satellite and the users is able to achieve
communications from almost anywhere on the globe. As these communications satellites are in
geostationary orbits, communications is not possible towards the poles as in these regions it is not
possible to see the satellites.
Direct broadcast communications satellites
Another variant of communications satellites is those used for direct broadcasting. This form
of broadcasting has become very popular as it provides very high levels of bandwidth because of the
high frequencies used. This means that large numbers of channels can be carried. It also enables
large areas of the globe to be covered by one delivery system. For terrestrial broadcasting a large
number of high power transmitters are required that are located around the country. Even then
coverage may not be good in outlying areas.
These DBS satellites are very similar to ordinary communications satellites in concept.
Naturally they require high levels of transmitted power because domestic users do not want very
large antennas on their houses to be able to receive the signals. This means that very large arrays of
solar cells are required along with large batteries to support the broadcasting in periods of darkness.
They also have a number of antenna systems accurately directing the transmitted power to the
required areas. Different antennas on the same satellite may have totally different footprints.
Satellite phone systems
Satellites have also been used for cellular style communications. They have not been nearly
as successful as initially anticipated because of the enormously rapid growth of terrestrial cellular
telecommunications, and its spread into far more countries and areas than predicted when the ideas
for satellite personal communications was originally envisaged. Nevertheless these satellite phone
systems are now well established and have established a specific market. Accordingly these satellite
phone systems are now widely available for mobile communications over wide areas of the globe.
The satellite phone systems that are available have varying degrees of coverage. Some
provide true global coverage, although others are restricted to the more densely populated areas of
the globe.
The systems that were set up used low earth orbiting satellites, typically with a constellation
of around 66 satellites. Handheld phones then communicated directly with the satellites which would
then process and relay the signals as required.
Other satellite phone systems use a number of geostationary satellites, although these satellite
phone systems generally require the use of a directional antenna in view of the larger distances that
need to be covered to and from the satellite. Additionally the levels of latency are higher (i.e. time
delay for the signal to travel to and from the satellite) in view of the much higher orbit required.
However as the satellites are geostationary, satellite or beam handover is less of a problem.
The main advantage of the satellite system is that it is truly global and communications can
be made from ships, in remote locations where there would be no possibility of there being a
communications network. However against this the network is expensive to run because of the cost
of building and maintaining the satellite network, as well as the more sophisticated and higher power
handsets required to operate with the satellite. As a result calls are more expensive than those made
over terrestrial mobile phone networks.
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Antenna’s used in satellite communication
Although the basics of satellite communications are fairly straightforward, there is a huge
investment required in building the satellite and launching it into orbit. Nevertheless many
communications satellites exist in orbit around the globe and they are widely used for a variety of
applications from providing satellite telecommunications links to direct broadcasting and the use of
satellite phone and individual satellite communication links.
Parabolic antenna
A parabolic antenna for Erdfunkstelle Raisting the biggest facility for satellite
communication in the world, based in Raisting, Bavaria, Germany. A parabolic antenna is a highgain reflector antenna used for radio, television and data communications, and also for radiolocation
(radar), on the UHF and SHF parts of the electromagnetic spectrum. The relatively short wavelength
of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the
desired highly directional response for both receiving and transmitting.
With the advent of TVRO and DBS satellite television, the parabolic antenna became a ubiquitous
feature, not only in rural locales where CATV and terrestrial signals were limited or non-existent, but
also in urban and suburban regions, where the aforementioned services compete with CATV and
broadcast media. Extensive terrestrial microwave links, such as those between cellphone base
stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier
applications included ground-based and airborne radar and radio astronomy.
Satellite Transponder
A transponder is a broadband RF channel used to amplify one or more carriers on the
downlink side of a geostationary communications satellite. It is part of the microwave repeater and
antenna system that is housed onboard the operating satellite. Examples of these satellites include
AMC 4 and Telstar 5, located at 101 and 97 degrees west longitude, respectively. These satellites
and most of their cohorts in the geostationary orbit have bent-pipe repeaters using C and Ku bands; a
bent pipe repeater is simply one that receives all signals in the uplink beam, block translates them to
the downlink band, and separates them into individual transponders of a fixed bandwidth. Figure 1
shows the basic concept. Each transponder is amplified by either a traveling wave tube amplifier
(TWTA) or a solid state power amplifier (SSPA). Satellites of this type are very popular for
transmitting TV channels to broadcast stations, cable TV systems, and directly to the home. Other
applications include very small aperture terminal (VSAT) data communications networks,
international high bit rate pipes, and rural telephony. Integration of these information types is
becoming popular as satellite transponders can deliver data rates in the range of 50 to 150 Mbps.
Achieving these high data rates requires careful consideration of the design and performance of the
repeater.
Block diagram of a basic satellite transponder
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Figure 1. Elements of a basic Satellite link showing ground stations and the satellite
transponder.
The nature and location of the various system impairments are also shown.The most
significant impairments to digital transmission come about in the filtering, which constrains
bandwidth and introduces delay distortion, and the power amplification, which produces AM/AM
and AM/PM conversion. These effects will be discussed in detail later in this article. For maximum
power output with the highest efficiency (e.g., to minimize solar panel DC supply), this amplifier
should be operated at its saturation point. However, many services are sensitive and susceptible to
AM/AM and AM/PM conversion, for which backoff is necessary. With such an operating point,
intermodulation distortion can be held to an acceptable level; however, backoff also reduces
downlink power.
The transponder itself is simply a repeater. It takes in the signal from the uplink at a
frequency f1, amplifies it and sends it back on a second frequency f2. Figure 2 shows a typical
frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz, and the downlink
frequency is at 4 GHz. The 24 channels are separated by 40 MHz and have a 36 MHz useful
bandwidth. The guard band of 4 MHz assures that the transponders do not interact with each other.
Advantages of Satellite communication
Availability
Internet connection types. Satellite Internet access is a way for those who do not have access to
terrestrial broadband connections such as cable or DSL to have access to high-speed Internet access.
Satellite also is one of the only ways to receive Internet service in areas where telephone lines are not
available.
Speed
-up, with entry-level service tiers typically
providing approximately 1 mbps download speeds--nearly 18 times faster than a dial-up modem.
Faster speeds are generally available at higher service tiers. In general, the highest speeds available
to home satellite Internet customers are slightly slower than the highest speeds offered by cable and
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DSL providers. Additionally, many satellite providers limit the amount of data that can be
downloaded during short time periods to curb frequent large file transfers.
Latency
-latency, meaning that a great deal of time is
required for packets of information to travel to the satellite and back. The total delay can amount to
about one second from the time that you send a request to the Internet to the time that a reply is
received. Satellite Internet providers use various technologies to make this delay less noticeable to
the end user and create an acceptable experience for browsing the Web. However, the latency makes
a satellite Internet connection unsuitable for high-speed gaming.
Reliability
-based satellite Internet connections are generally no less reliable than terrestrial
broadband. However, all satellite communication is subject to interruption during periods of heavy
snow or rainfall. Talk to other customers about their experiences if you live in an area where either
of these are common. The likelihood of weather-related interruptions is lessened with a larger
satellite dish, which some providers offer.
Cost
he equipment costs
several hundred dollars to purchase, and some types of installations incur additional fees.
Additionally, the monthly cost for satellite Internet tends to be slightly higher than the cost of cable
or DSL. There are ways of reducing the up-front cost. The equipment can be leased rather than
purchased, and discounts or rebates may be available. Sometimes, installation fees are included in
the lease price.
Disadvantages of Satellite communication
List of satellites launched by ISRO
1. Aryabhata 19.04.1975 First Indian satellite. Provided technological experience in building and
operating a satellite system. Launched by Russian launch vehicle Intercosmos.
2.Bhaskara-I 07.06.1979 First experimental remote sensing satellite. Carried TV and microwave
cameras.
Launched
by
Russian
launch
vehicle
Intercosmos.
3.Bhaskara-II 20.11.1981 Second experimental remote sensing satellite similar to Bhaskara-1.
Provided experience in building and operating a remote sensing satellite system on an end-to-end
basis.
Launched
by
Russian
launch
vehicle
Intercosmos.
4.Ariane Passenger Payload Experiment (APPLE) 19.06.1981 First experimental communication
satellite. Provided experience in building and operating a three-axis stabilised communication
satellite.
Launched
by
the
European
Ariane.
5.Rohini Technology Payload (RTP) 10.08.1979 Intended for measuring in-flight performance of
first experimental flight of SLV-3, the first Indian launch vehicle. Could not be placed in orbit.
6.Rohini (RS-1) 18.07.1980 Used for measuring in-flight performance of second experimental
launch of SLV-3.
7.Rohini (RS-D1) 31.05.1981 Used for conducting some remote sensing technology studies using a
landmark sensor payload. Launched by the first developmental launch of SLV-3
8.Rohini (RS-D2) 17.04.1983 Identical to RS-D1. Launched by the second developmental launch of
SLV-3.
9.Stretched Rohini Satellite Series (SROSS-1)24.03.1987 Carried payload for launch vehicle
performance monitoring and for Gamma Ray astronomy. Could not be placed in orbit.
10.Stretched Rohini Satellite Series (SROSS-2)13.07.1988 Carried remote sensing payload of
German space agency in addition to Gamma Ray astronomy payload. Could not be placed in orbit.
11.Stretched Rohini Satellite Series (SROSS-C)20.05.1992 Launched by third developmental
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flight
of
ASLV.
Carried
Gamma
Ray
astronomy
and
aeronomy
payload.
12.Stretched Rohini Satellite Series (SROSS-C2)04.05.1994 Launched by fourth developmental
flight
of
ASLV.
Identical
to
SROSS-C.
Still
in
service.
Indian
National
Satellite
System
(INSAT)
13.INSAT-1A 10.04.1982 First operational multi-purpose communication and meteorology satellite
procured from USA. Worked only for six months. Launched by US Delta launch vehicle.
14.INSAT-1B 30.08.1983 Identical to INSAT-1A. Served for more than design life of seven years.
Launched
by
US
Space
Shuttle.
15.INSAT-1C 21.07.1988 Same as INSAT-1A. Served for only one and a half years. Launched by
European
Ariane
launch
vehicle.
16.INSAT-1D12.06.1990 Identical to INSAT-1A. Launched by US Delta launch vehicle. Still in
service.
17.INSAT-2A 10.07.1992 First satellite in the second-generation Indian-built INSAT-2 series. Has
enhanced capability than INSAT-1 series. Launched by European Ariane launch vehicle. Still in
service.
18.INSAT-2B 23.07.1993 Second satellite in INSAT-2 series. Identical to INSAT-2A. Launched by
European
Ariane
launch
vehicle.
Still
in
service.
19.INSAT-2C 07.12.1995 Has additional capabilities such as mobile satellite service, business
communication and television outreach beyond Indian boundaries. Launched by European launch
vehicle.
20.INSAT-2D 04.06.1997 Same as INSAT-2C. Launched by European launch vehicle Ariane.
Inoperable
since
Oct
4,
97
due
to
power
bus
anomaly.
21.INSAT-2DT
January
1998
Procured
in
orbit
from
ARABSAT
22.INSAT-2E 03.04.1999 Multipurpose communication & meteorological satellite launched by
Ariane.
23.INSAT-3B 22.03.2000 Multipurpose communication - business communication, developmental
communication
and
mobile
communication
purpose.
24.GSAT-1 18.04.2001 Experimental Satellite for the first developmental flight of Geo-synchronous
Satellite
Launch
Vehicle,
GSLV-D1.
25.INSAT-3C 24.01.2002 To augment the existing INSAT capacity for communication and
broadcasting,
besides
providing
continuity
of
the
services
of
INSAT-2C.
26.KALPANA-1 12.09.2002 METSAT was the first exclusive meteorological satellite built by
ISRO
named
after
Kalpana
Chawla.
27.INSAT-3A 10.04.2003 Multipurpose Satellite for communication and broadcasting, besides
providing
meteorological
services
along
with
INSAT-2E
and
KALPANA-1.
28.GSAT-2 08.05.2003 Experimental Satellite for the second developmental test flight of India's
Geosynchronous
Satellite
Launch
Vehicle,
GSLV
29.INSAT-3E 28.09.2003 Exclusive communication satellite to augment the existing INSAT
System.
30.EDUSAT
20.09.2004
India's
first
exclusive
educational
satellite.
31.HAMSAT 05.05.2005 Microsatellite for providing satellite based Amateur Radio Services to the
national
as
well
as
the
international
community
(HAMs).
32.INSAT-4A 22.12.2005 The most advanced satellite for Direct-to-Home television broadcasting
services.
33.INSAT-4C 10.07.2006 State-of-the-art communication satellite - could not be placed in orbit.
34.INSAT-4B 12.03.2007 An identical satellite to INSAT-4A further augment the INSAT capacity
for
Direct-To-Home
(DTH)
television
services
and
other
communications.
35.INSAT-4CR 02.09.2007 Designed to provide Direct-To-home (DTH) television services, Video
Picture Transmission (VPT) and Digital Satellite News Gathering (DSNG), identical to INSAT- 4C .
Indian
Remote
Sensing
Satellite
(IRS)
36.IRS-1A 17.03.1988 First operational remote sensing satellite. Launched by a Russian Vostok.
37.IRS-1B 29.08.1991 Same as IRS-1A. Launched by a Russian Launch vehicle, Vostok. Still in
service.
38.IRS-1E 20.09.1993 Carried remote sensing payloads. Could not be placed in orbit.
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39.IRS-P2 15.10.1994 Carried remote sensing payload. Launched by second developmental flight of
PSLV.
40.IRS-1C 28.12.1995 Carries advanced remote sensing cameras. Launched by Russian Molniya
launch
vehicle.
Still
in
service.
41.IRS-P3 21.03.1996 Carries remote sensing payload and an X-ray astronomy payload. Launched
by
third
developmental
flight
of
PSLV.
Still
in
service.
42.IRS-1D 29.09.1997 Same as IRS-1C. Launched by India's PSLV service. In service.
43.IRS-P4 Oceansat 26.05.1999 Carries an Ocean Colour Monitor (OCM) and a Multi-frequency
Scanning
Microwave
Radiometer
(MSMR),
Launched
by
India's
PSLV-C2,
44.Technology Experiment Satellite (TES) 22.10.2001 Technology Experiment Satellite Launched
by
PSLV-C3
.
45.IRS-P6 Resourcesat-1 17.10.2003 Launched by PSLV - C5, carries three camera, names, LISS4,
LISS-3
and
AwiFS
46.CARTOSAT -1 05.05.2005 Launched by PSLV-C6, carries two panchromatic cameras - PAN
(fore) and PAN (aft) - with 2.5 meter resolution. The cam mounted with a tilt of +26 deg and -5 deg
along
the
track
to
provide
stereo
images.
47.CARTOSAT – 2 10.01.2007 Launched by PSLV-C7, it is an advanced remote sensing satellite
carrying a panchromatic camera capable of providing scene specific spot imageries.
48.SRE – 1 10.01.2007 Launched by PSLV-C7, Space capsule Recovery Experiment (SRE-1),
intended to demonstrate the technology of an orbiting platform for performing experiments in
microgravity conditions. SRE-1 was recovered successfully after 12 days over Bay of Bengal.
49.CARTOSAT-2A 28.04.2008 Identical to CARTOSAT - 2, launched by PSLV-C9
50.IMS-1 28.04.2008 Launched by PSLV-C9 along with CARTOSAT-2A and other Eight
51.CHANDRAYAAN- 1 22.10.2008 Launched by PSLV-C11. Launched at Shriharikota, Andhra
Pradesh.
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