Transcript Satellite Communication

Satellite Communication
1
Basic Satellite System
 A satellite is basically any object that revolves around a planet in a circular or
elliptical path.
 A communication satellite is like a big microwave repeater in the sky.
 A communications satellite (sometimes abbreviated to comsat) is an artificial
satellite stationed in space for the purposes of telecommunications. Modern
communications satellites use a variety of orbits including geostationary
orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar)
Earth orbits.
 For fixed (point-to-point) services, communications satellites provide a
microwave radio relay technology complementary to that of fiber optic
submarine communication cables. They are also used for mobile applications
such as communications to ships, vehicles, planes and hand-held terminals,
and for TV and radio broadcasting, for which application of other
technologies, such as cable, is impractical or impossible.
2
Basic Satellite System
3
Satellite for Communications
•
Economical Long Distance Communications
•
Broadcast Capability
•
Wideband Capability
•
Broad Coverage
•
Freedom from Natural Barriers
– The cost of transmitting information via satellite is essentially independent of distance. It is
the same for two points 5000 or 100 km apart
– Satellites can be used as broadcast transmitters, relaying signals from one transmit earth
station to multiple receive earth stations within the coverage area. Conversely, satellites can
receive signals from multiple transmitting stations for relay to a central station.
– Satellite repeaters are usually wideband devices that can transmit large amounts of
information. Tens of megahertz of bandwidth are available in each repeater. Each repeater
may be used between any two points within the coverage of the satellite.
– the satellite can serve any station within view of its antenna. about 42% of the earth's
surface is within view of a geostationary satellite.
– Satellites overcome natural barriers such as mountains, cities, deserts, or oceans.
4
Satellite for Communications
•
Better Coverage for Rural and Undeveloped Areas
•
New Markets
•
New Common carriers
•
New Services
•
Customer's Premises Services
– A satellite can provide the same type of service to both large cities and rural areas. Satellite
communications accelerate the flow of industry and data processing facilities to rural areas.
The satellite can serve both developed and undeveloped areas.
– Satellites have encouraged the development of specialized television networks for cable
television, payTV, ethnic and language groups, religious groups, sports, and news gathering,
for example.
– The availability of satellites has led to the emergence of new common carriers. Other
common carriers may lease one or more repeaters and sublease individual circuits or
television channels. The resulting competition leads to increased market expansion.
– Communications satellites can pick up data from a network of sensors, and relay it to one
point for data analysis. The same satellite can then send the results (such as earthquake
warnings) back to the source areas.
– Small earth stations with small antennas can provide access to databases, branch offices, and
management information systems. These stations are known as very small aperture
terminals (VSATs). These terminals are usually located on the customer's premises.
5
Disadvantages of Satellites
• Launching satellites into orbit is costly.
• Satellite bandwidth is gradually becoming
used up.
• There is a larger propagation delay in satellite
communication than in terrestrial
communication.
6
Technology Comparison
Satellite communication
Terrestrial communication
1. Star topology.
1. Mesh topology.
2. Satellite is a critical component;
2. Node failures do not affect the
its failure results in total failure
entire network. The network is
of the network.
fault tolerant.
3. Broadcast in nature.
3. Point-to-point in nature.
4. Large distances and hence large
4. Distances are relatively small and
time delays are involved, typically
hence the time delay is small,
270 ms for one way communication.
typically 40 ms for intercontinental
calls.
5. Microwave communication.
5. Microwave, cable or optical fibre
communication.
6. Susceptible to weather conditions
at higher bands.
6. Cable or optical fibre communication
is insensitive to weather
conditions.
7. Satellite life span is typically 7 years
7. Life span is large, typically 30 years
7
Service Types
•
Fixed Service Satellites (FSS)
– a radio communication service between specified fixed points on the Earth's surface when
one or more satellites are used
– The main signals transmitted via the links of FSS are telephony, telegraphy, facsimile, data
transmission, television and sound programs.
• Example: Point to Point Communication
•
Broadcast Service Satellites (BSS)
– Television and sound programs are routed directly in the down links to the general public
reception
• Example: Satellite Television/Radio
– Also called Direct Broadcast Service (DBS).
•
Mobile Service Satellites (MSS)
– Telecommunications satellites were first used to set up links over very long distances, as the
financial penalty of using satellites for short distances when compared to conventional
methods, was too large. The first fixed-point links were therefore intercontinental.
• Example: Satellite Phones
8
Satellite Services
Fixed
Broadcasting
Mobile
Land Mobile
Maritime Mobile
Aeronautical Mobile
Intersatellite
Radiodetermination
Earth exploration
Space Research
Meteorological
Amateur
Radionavigation
Aeronautical Navigation
Standard Frequency and time Signal
9
Frequency Bands
•
•
•
•
•
•
•
In any communication satellite, one or more transponders are used to receive, amplify and
retransmit the incoming signals.
The upward and downward transmission frequencies are chosen to be different to avoid
interference.
Three frequency bands are allocated for commercial satellite communication:
Band
Down link frequency
Up link frequency
Bandwidth
C
3.7-4.2 GHz
5.925 - 6.425 GHz
500 MHz
Ku
10.9-11.7 GHz
14.00-14.5 GHz
500 MHz
Ka
17.7 - 21.2 GHz
27.5 — 31.0 GHz
3.5 GHz
The C band is widely used. A satellite commonly contains 12 transponders having BW of
36MHz each. The whole satellite has a BW of 500MHz.
Recent satellites are designed to operate in the Ku band.
Heavy demand for bandwidth in the C and Ku bands is pushing up the frequency of
operation to Ka band and above.
At higher frequency attenuation is more. Hence more power will be required for signal
transmission to ensure that it reaches the destination with the required minimum power.
Higher power requirements will involve use of high power amplifiers with high ratings and
heat sinks. This will increase weight and power supply ratings of the equipments. This does
not make any difference to the ground station. But for the satellite this will result in higher
power consumption, which will reduce its efficiency. Hence the ground station uses high
power equipments to generate high frequency which can reach the satellite.
10
Functional block diagram of a transponder
Downlink signal at 4GHz
Diplexer
Uplink signal at 6GHz
Diplexer
Low Noise Amplifier
Band pass
filter
Down
Converter
Band pass
filter
TWT Amplifier
Local Oscillator
TWT=Travelling-wave Tube
11
Functional block diagram of an earth station
12
Factors in satellite communication
• Other impairments to satellite communication
– The distance between an earth station and a satellite (free
space loss).
– Satellite Footprint:
• The satellite transmission’s strength is strongest in the center of
the transmission, and decreases farther from the center as free
space loss increases.
– Atmospheric Attenuation caused by air and water can
impair the transmission. It is particularly bad during rain
and fog.
13
Footprint
• Satellites process microwaves with bidirectional antennas
(line-of-sight).
• Therefore, the signal from a satellite is normally aimed at a
specific area called the footprint.
• The signal power at the center of the footprint is maximum.
• The power decreases as we move from the footprint center.
• The boundary of the footprint is the location where the power
reaches a predefined threshold.
14
Satellite Footprint
15
16
17
* Van Allen Radiation Belt: It is one of at least
two layers of energetic charged particles (plasma)
that is held in place around the planet Earth by the
planet’s magnetic field. The belts extend from an
altitude of about 1,000 to 60,000 km above the
surface in which radiation levels vary.
18
19
20
21
22
23
24
(GEO)
25
26
28
29
30
31
LEO
• Advantages
– A LEO satellite’s proximity to earth compared to a GEO satellite gives it a
better signal strength and less of a time delay, which makes it better for
point to point communication.
– A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.
• Disadvantages
– A network of LEO satellites is needed, which might be costly
– LEO satellites have to compensate for Doppler shifts cause by their
relative movement.
– Atmospheric drag effects LEO satellites, causing gradual orbital
deterioration.
32
33
VSAT Network
•
•
A very small aperture terminal (VSAT), is a two-way satellite ground station with a
dish antenna that is smaller than 3 meters. The majority of VSAT antennas range
from 75 cm to 1.2 m. Data rates typically range from 56 kbit/s up to 4 Mbit/s.
VSATs access satellite(s) in geosynchronous orbit to relay data from small remote
earth stations (terminals) to other terminals (in mesh topology) or master earth
station "hubs" (in star topology).
At the Very Small Aperture Terminal a lower performance microwave transceiver
and lower gain dish antenna (smaller size) is used.
•
VSAT networks are arranged in a star based topology.
•
Ideal for centralized networks with a central host (Banking institutions with
branches all over the country).
34
VSAT Network
35
END
36