2. Satellite Communication

3. What is satellite? Satellite is a repeater in the sky.
Receives uplink frequencies and translates to downlink
frequencies and transmits back to the earth station after
amplification
The translation is done to avoid +ve feed back.Trying to
receive and transmit an amplified version of the same
uplink waveform at the same satellite will cause
unwanted feedback, or ring around, from the downlink
antenna back into the receiver

4. What is a communications satellite and how does it work?
A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.
In addition to communications satellites.
In satcom the satellite acts as a media for the data transfer between the source and destination.
Between the source and destination we use a pair of frequencies to communicate.
The Satellite acts as a frequency translator and amplifier.

5. What are the different kinds of orbits?
An orbit is the path that a satellite follows as it revolves around Earth. In terms of commercial satellites, there are three main categories of orbits:
LEO( Low Earth Orbit)
500-2,000 km above the earth
These orbits are much closer to the Earth, requiring satellites to travel at a very high speed in
order to avoid being pulled out of orbit by Earth's gravity
At LEO, a satellite can circle the Earth in approximately one and a half hours

6. MEO( Medium Earth Orbit)
8,000-20,000 km above the earth
These orbits are primarily reserved for communications satellites that cover the North and
South Pole
MEO's are placed in an elliptical (oval-shaped) orbit

7. GEO ( Geosynchronous Orbit)
35,786 km above the earth
Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite
travels in the same direction and at the same speed as the Earth's rotation on its axis,
taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is
positioned over the equator in an assigned orbital location, it will appear to be
"stationary" with respect to a specific location on the Earth.
A single geostationary satellite can view approximately one third of the Earth's surface.
If three satellites are placed at the proper longitude, the height of this orbit allows
almost all of the Earth's surface to be covered by the satellites.

8. Polar Orbiting Satellite
These satellites orbit the earth in such a way as to cover the north and south polar regions.
These satellites if in a low earth orbit have to travel at a very high speed.
These satellites can be kept in low earth orbit ( km) or at 36000km apart.

9. Geo-Synchronous Satellite
Orbit on the equatorial plane - appears stationary
Altitude of Kms.
Circular orbit around earth with period of 24 hours.
Coverage of about 1/3 of Earth.
2 deg. apart. Identified by Longitudinal position with ref. to Greenwich.
Advantage Of Geostationary Satellite
Simple ground station tracking requirements.
Removes Satellite hand-over problems.
Negligible Doppler shift

10. Inclined Orbit Satellite
A disadvantage of Geostationary satellites is that points on Earth
beyond about 80 deg latitude are not visible.
Inclined orbits, on the other hand can provide visibility to the higher
northern and southern latitudes, although they require earth stations
to continually track the satellite.

11. Technical Term Geosynchronous Transponder Footprint Frequency
Polarization
Path Propagation
Timing
Satellite Access
Link Performance

12. Geosynchronous Transponder Satellite Footprint
Geosynchronous means that the satellite is synchronized with the earth in time and direction. It means that is time taken by a satellite to complete its orbit around earth is equal to the time taken by to earth rotates around its own axis.
Transponder
Frequency band on the satellite is divided into several channels. Each channels are called transponder Each transponder have 40 MHz.
Satellite Footprint
Coverage of entire surface of earth that is visible by the satellite.

14. Advantages And Disadvantages of Different Frequency Band
C-Band
Adv. : Broad Footprint, little rain fade
Disadv. : Weak signals, interference, large antenna sizes
and amplifiers
ExC-Band
Adv.. : Broad Footprint, little rain fade, less interference
Disadv. : Week signals, large antenna sizes and amplifiers
Ku-Band
Adv. : Focused Foot prints, no terrestrial interference
small antenna and amplifier
Disadv. : Interference to rain.
Ka-Band
Adv. : Focused Foot prints, large unused
bandwidths

16. VSAT Technology in Satellite Communication
VSAT stands for Very Small Aperture Terminal
It is a two way satellite ground station.
VSATs are most commonly used to transmit narrowband data or broadband data.
VSATs access satellites in geosynchronous orbit to relay data from small remote
earth stations (terminals) to other terminals or master earth station "hubs“.

17. Satellite Link Satellite Uplink Downlink Remote VSAT 1 Remote VSAT 2
Earth Station (HUB)

18. VSAT System Architecture
Topologies
Star Topology
As the Hub is a powerful receiver and retransmitter , the remote VSAT sites are
comparatively small.
A central uplink site, such as a Network Operation Center (NOC), to transport
data back and forth to each VSAT terminal via satellite
VSATs communicate to Hub on Inroutes and the Hub Communicates to VSATs on
the Outroute

19. Mesh Topology
Each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site.
VSATs are comparatively larger as they have to communicate directly with each other
VSAT’s communicate directly with each other, Hub will be involved only for call
setup and then fade out.( Hub will not be involved in permanent Circuits)

20. Parts Of VSATs Antenna Block Up Converter (BUC)
Low Noise Block Converter (LNB)
Orthomode Transducer (OMT)
Interfacility Link Cable (IFL)
Indoor Unit (IDU)

21. Types Of Antenna Prime Focus Antenna Single Reflector Antenna.
Feed horn is placed at the Focal point of the Reflector.
Antenna Electronics are placed on Feed.
More susceptible to Interference from Low elevation sources.
More Blockage because feed.
Antenna Efficiency is in the range of 60%.
Low Cost Antenna.
Primarily Used for Receive only applications.

22. Cassagrain Antenna Main reflector is Paraboloid
Sub-Reflector is hyperboloid and placed at Prime Focus
Feed is Corrugated Horn and is placed at Center of the Main Reflectors.
The paraboloid converges towards the Sub Reflector ( prime focus), which is then reflected by Sub-Reflector to form a Spherical Wave converging on the Feed.

23. Gregarion Antenna Main reflector is Paraboloid
Sub-Reflector is Paraboloid and placed at Prime Focus
Feed is Corrugated Horn and is placed at Center of the Main Reflectors.
The paraboloid converges towards the Sub Reflector ( prime focus), which is then reflected by Sub-Reflector to form a Spherical Wave converging on the Feed.

24. Offset Fed Antenna Used for Smaller Earth Stations.
Main Reflector is a section of Parabolic, cutoff above the axis.
Feed is located below the axis giving a completely unblocked Aperture.
High Antenna efficiency.

25. Block Up-Converter (BUC)
A BUC (Block Up-Converter) takes an L-Band input and transmits it upstream to the satellite on Ka, Ku, or C band.
BUC's are rated according to their output power. A low power Ka-Band BUC can transmit with as little as 2 watts, while a high power C-Band BUC can transmit with as much as 200 watts.

26. Low Noise Block Converter(LNB)
It is typically mounted at the focal point of the
receiving parabolic dish.
It's primarily used to amplify and convert received
satellite signals into frequencies compatible with
the satellite receiver.

27. Orthomode Transducer (OMT)
OMT is 3-port microwave waveguide
system that split input power into two
parts, allow the simultaneous operation of
two outdoor radio units
OMT can be directly mounted on the
antennas that have circular waveguide
connector.

28. Interfacility Link Cable (IFL)
Interfacility Link (IFL) Distribution
Cable is an indoor rated, connectorized
cable assembly that is used within a
building to connect a fiber feeder splice
point to an optical connector
distribution panel.
The cable can be terminated with
connectors at one or both ends in
conjunction with various styles of fan-
out kits at any breakout length.

29. Indoor Unit (IDU) Its Satellite Modem consists of Modulator and
Demodulator.
The IDU also determines the access schemes
under which the VSAT would operate.
The IDU also interfaces with various end user
equipment, ranging from stand alone computers,
LAN's, routers, multiplexes, telephone
instruments, EPABX as per the requirement.