Unit 3 : Satellite communication

Prepared by:Chandra ShekharTNTC

3.1 Introduction: Historical development of satellite frequency 1Hr  List Satellite frequency bands 1Hr

3.2 Satellite orbits, Principle of Satellite Communication 1Hr  Satellite orbits, Principle of Satellite Communication 1Hr

3.3 Basic Satellite Technology – uplink and Down link . 1Hr3.4 Explain the block diagram and types of transponder 1Hr

  Explain the block diagram and types of transponder 1Hr3.5 Draw and explain block diagram of earth station. 1Hr3.6 Satellite system - Active and passive satellite, 1Hr3.7 Station keeping of Geo synchronous satellites. 1Hr3.8 TDMA and DAMA, 1Hr3.9 GPS, Remote sensing – GIS 1Hr

  GPS, Remote sensing – GIS 1Hr

Syllabus

What is satellite.?

• Natural Satellite eg: Moon (for earth)• Artificial satellite eg : Communication satellite• There are about 2,271 satellites currently in space, most of

them are used for communication.

• A satellite is an object that orbits another large object like planet.

History of satellite• In 1687 theoretical possibility of establishing of artificial

satellite by Isaac Newton.

• Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.

• After so many recitative articles from Arthur C.Clarke & some other electronic officers , many scientist took this challenge & Later developments are given in chronology.

• India has launched 82 Indian satellites (as of 10 March 2016) of many types since its first in 1975.

• Recent launching of India( IRNSS-1F 10 March 2016) by launch vehicle PSLV-C32 .

Chronology of developments

Frequency Bands• Different kinds of satellites use different frequency bands.

L–Band: 1 to 2 GHz, used by MSS S-Band: 2 to 4 GHz, used by MSS, NASA, deep space

research C-Band: 4 to 8 GHz, used by FSS X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial

imaging, ex: military and meteorological satellites Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS) K-Band: 18 to 26.5 GHz: used by FSS and BSS Ka-Band: 26.5 to 40 GHz: used by FSS

Frequency Bands• Three common bands:

1) C-Band.

2) KU-Band.

3) KA-Band.• Most common are C-Band & KU-Band.• C-Band occupy 4 to 8 GHz frequency:

- Low frequency.- Large antenna (2-3 meters).

• KU-Band occupy 12.5 to 18 GHz:- Large frequency.- Small antenna (18-inches)

Basic Satellite system

Explanation • Earth station once the satellite is launched successfully , the earth or

ground station process the base band signal and then transmitted to satellite through dish antenna.

• Uplink the communication link that exist from earth station

to orbiting satellite through a set of RF carriers is called uplink.

• Down link The communication link that exists from orbiting satellite to

earth station though a set of carriers .

How do Satellites Work

• Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.

• The two stations can use a satellite as a relay station for their communication

• One Earth Station sends a transmission to the satellite. This is called a Uplink.

• The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

How Satellites are used

• Service Types Fixed Service Satellites (FSS)

• Example: Point to Point Communication Broadcast Service Satellites (BSS)

• Example: Satellite Television/Radio• Also called Direct Broadcast Service (DBS).

Mobile Service Satellites (MSS)• Example: Satellite

Ground Segment• Collection of facilities, Users and Applications

• Earth Station = Satellite Communication Station (Fixed or Mobile)

Basic Orbits:1. LEO: Low Earth Orbit.2. MEO: Medium Earth Orbit3. GEO: Geostationary Earth Orbit

Other orbit Molniya OrbitHAPs

Types of Satellites

Low Earth Orbit (LEO)

• LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface.

• LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.

• A network of LEO satellites is necessary for LEO satellites to be useful

• 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

LEO

• Disadvantages A network of LEO satellites is needed, which can

be costly LEO satellites have to compensate for Doppler

shifts cause by their relative movement. Atmospheric drag effects LEO satellites, causing

gradual orbital deterioration.

LEO

Medium Earth Orbit (MEO)• A MEO satellite is in orbit somewhere between

8,000 km and 18,000 km above the earth’s surface. • MEO satellites are similar to LEO satellites in

functionality.• MEO satellites are visible for much longer periods of

time than LEO satellites, usually between 2 to 8 hours.

• MEO satellites have a larger coverage area than LEO satellites

• Advantage A MEO satellite’s longer duration of visibility and

wider footprint means fewer satellites are needed in a MEO network than a LEO network.

• Disadvantage A MEO satellite’s distance gives it a longer time

delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite.

MEO

Geostationary Earth Orbit (GEO)• These satellites are in orbit 35,863 km above

the earth’s surface along the equator.• Objects in Geostationary orbit revolve around

the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth

• At the Geostationary orbit the satellite covers 42.2% of the earth’s surface.

• Theoretically 3 geostationary satellites provides 100% earth coverage

Satellite 2

Satellite 1

Satellite 3

• Advantages A GEO satellite’s distance from earth gives it a

large coverage area, almost a fourth of the earth’s surface.

GEO satellites have a 24 hour view of a particular area.

These factors make it ideal for satellite broadcast and other multipoint applications.

GEO

GEO • Disadvantages

A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication.

GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

Other Orbits

• Molniya Orbit Satellites Used by Russia for decades. Molniya Orbit is an elliptical orbit. The satellite

remains in a nearly fixed position relative to earth for eight hours.

A series of three Molniya satellites can act like a GEO satellite.

Useful in near polar regions.

One of the newest ideas in satellite communication.

A blimp or plane around 20 km above the earth’s surface is used as a satellite.

HAPs would have very small coverage area, but would have a comparatively strong signal.

Cheaper to put in position, but would require a lot of them in a network.

These are powered by batteries.

High Altitude Platform (HAP)

Transponder

• Transponder is an automatic electronic monitoring or control device that receives, cross-examines, amplifies and retransmits the arriving signal. It is primarily implemented in wireless communication. The word ‘Transponder’ itself is a combination of two words; transmitter and responder (occasionally abbreviated to TPDR, TR, XPNDR, and XPDR).

• Transponder is also principally used as a re-transmitter due to the fact that it receives a definite signal from a specific source, then it amplifies (magnifies) the signal before sending it to a predefined location.

• Transponders have an abnormally large number of applications in various fields;

satellite communication, marine, automotive, road, motorsport, underwater etc.

• They are also used in simple day-to-day tasks such as opening a car’s door wirelessly.

• Active Transponders These devices constantly emit radio signals which are tracked and monitored. These

can also be automatic devices which strengthen the received signals and relay them to another location. An active transponder includes its own power supply.

Passive Transponder

A passive transponder does not include its own power source. The passive transponder collects power from a close by electric or magnetic field offered by a reader. The reader cross-examines the neighboring field for transponders that may be in its proximity and stimulates enough power into the transponder’s electronic circuitry that the transponder becomes active and retransmits to the reader its identification ID as well as any added information required.

Types of transponder

Block diagram of Earth Station

Block diagram explanationAn earth station consists of the following five main subsystems:

1. Antenna subsystem2. Receive subsystem3. Transmit sub system4. Ground communications equipment (GCE) subsystem5. Power subsystem

1. Antenna subsystem:

It consists of Parabolic reflector, Horn antenna, related mounts and a polarized diplexer that permits both transmitter & receiver to use the same antenna.

It isolates the highly sensitive receiver from the high power transmitter.

2. Receive subsystem:

Block diagram explanation

4. Transmit subsystem: The IF from the GCE-Transmit is translated to the final

output frequency by way of an up-converter circuit. The up-converter is a mixer with its local oscillator tuned

to generate the uplink frequency. All the up-converted signals are then applied to a power

combiner. this wave guide like device equally combines all the RF signals into the final signal to be transmitted.

The output signal is applied to the antenna subsystem through a band pass filter.

5. Receive subsystem:

Block diagram explanation

5. Receive subsystem: Polarized diplexer feeds a BPF to ensure only the

received frequencies pass through receiving circuits. The output of BPF feeds a LNA with an extremely low

noise figure. The LNA drives a power divider, where it splits the

recived signal into smaller but equal power signals. Power divider feeds a down-converter, where it

translated to Intermediate frequency (IF), usually of 70 MHz.

Demodulator recovers back to original communication signals.

Block diagram explanation

5. Receive subsystem: Polarized diplexer feeds a BPF to ensure only the

received frequencies pass through receiving circuits. The output of BPF feeds a LNA with an extremely low

noise figure. The LNA drives a power divider, where it splits the

recived signal into smaller but equal power signals. Power divider feeds a down-converter, where it

translated to Intermediate frequency (IF), usually of 70 MHz.

Demodulator recovers back to original communication signals.

Block diagram explanation

Satellite system – Active & Passive satellitesBased on satellite process i,e, whether satellite may just reflect the EM energy or it Process the EM energy & Retransmit back .These two approaches have been identified as Passive satellite & Active satellite

Passive satellites: are the satellites which simply reflect back EM energy towards earth in a similar manner of RADAR system.

Active satellites: are the satellites which will receive signals, translate them in frequency, amplify then re-transmits.

Active satellite Passive satellite Design of active satellite is complex

It’s simple

It has limited bandwidth hence supports only a finite number of users.

It has wide bandwidth hence accommodated and infinite number of users.

It supports longer distance space communication

It supports shorter distance space communication.

It uses two separate frequencies like uplink and downlink .

It uses only one frequency which is downlink.

Tracking of active satellite is easier.

Tracking of passive reflectors is difficult

Comparison of Active & Passive satellites

Station keeping of Geo synchronous satellites.Station keeping is the term used for maintaining a satellite in its correct orbital position.After the satellite launched & placed in orbit, it may drift.These drift are due to:

- Gravitational fields of Sun & Moon- Irregularities in Earth Gravitational force.

To avoid the above drifts we need to put a satellites into a proper positions such as for a GEO satellite :1. The latitude must be 0 degree2. Orbital height should be more than 36,000 Km from earth

surface3. Shape of the orbit should be circular

4.Satellite motion should be from west to east5. Longitude 162 degree and 348 degree6. Inclination should be around 55 degree.

When ever if there is any variations in the above stated points , it will alter the process of GEO.

Hence to avoid these variations it has to be periodically monitored and to be triggered from the Rockets to maintain the position of the satellite in orbit and this all are referred as Station keeping of Satellite.

DAMA & TDMA

FDMA

• Satellite frequency is already broken into bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA).

• Overall bandwidth within a frequency band is increased due to frequency reuse.

FDMA (cont.)

• The number of sub-channels is limited by three factors: Thermal noise (too weak a signal will be effected

by background noise). Intermodulation noise (too strong a signal will

cause noise). Crosstalk (cause by excessive frequency reusing).

FDMA (cont.)

• FDMA can be performed in two ways: Fixed-assignment multiple access (FAMA): The

sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.

Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication. It is the technology used to assign a channel to users based on the request issued from the users terminals to a network control system and priority.

GPS, REMOTE SENSING & GIS

GPS, REMOTE SENSING & GISGPS:GPS stands for global positioning system. It is a space based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the earth. At present there are 24 satellites which is revolving in a 6 orbital planes by splitting 4 satellite in each plane.It was invented by US army in 1993, by using this we can trace any information in the world and any landSEGMENTS/COMPONENTS OF GPS:1.Space segment 2.Control segment3.User segment

1.SPACE SEGMENT: It consist of satellites themselves, transmitting time codes and orbital position information to the users .The nominal GPS operational constellation ( group of similar stars or planets) consists of 24 satellites that orbit that the earth in 12hrs.The six orbits have approximately 55 degree inclination and separated by 60 degree each.2.Control segment: It ensures the overall system performance and accuracy it basically tracks the GPS satellites, updates in their orbiting position and synchronous their clocks. The master control facility is by US army and same other authorized units of US army .

3.User Segment: It consists of the actual GPS receivers which receives signals from the 4 satellites, computer time differences and determine the position. There is no limitations for no. of receivers but it should be capable of tuning to GPS transmission.

APPLICATION OF GPS:1.Survaving and mapping.2.Timing.3.Roads and high ways.4.Aviatation:used for safety and efficiency of flight.5.Marine.

REMOTE SENSING: It is the process of gathering and recording of information by a sensor on board of a satellite orbiting in space without any physical contact with the object or area been investigated. Data collection’s can be through precession instruments such as camera, sensors.

APPLICATIONS OF REMOTE SENSING:1.Geology ,soil testing, weather forecasting ,identifying water and ice surfaces.

GIS:(GEOGRAPHIC INFORMATION SYSTEM) It is a system which deals with information related to the spatial distribution of features on the earth surfaces. basically it is a computer based tool including software, hardware ,people and geographic information. This system allows to create, edit, enquire, analyze and display.

Data presentation: In GIS includes either of those forms1.Raster (2D)2.vector(3D)3.Reality

DAMA(demand assigned multiple access)

1.Demand assigned multiple access A user need a satellite service depending on its requirement capacity based on this. There are 2 types of capacity allocation techniques are there 1.TDMA2.DAMA

DAMA:1.Demand assigned multiple access2.Demand assigned multiple access is a technology used to assign a channel to clients that don’t need to use it consantly.

DAMA(CONT)

3.DAMA systems assign communication channels based on requests issued from user terminals to a network control system.4.When the circuit is no longer in use, the channels are then returned to the central pool for reassignment to other users.

TDMA

• TDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter.

• TDMA is increasingly becoming more widespread in satellite communication.

• TDMA uses the same techniques (FAMA and DAMA) as FDMA does.

Block diagram of TDMA

USER 1

SWITCHING CENTRE

USER 2

USER 3

SWITHCING CENTRE

USER 1

USER 2

USER 3

TDMA (cont.)

• Advantages of TDMA over DAMA. Digital equipment used in time division

multiplexing is increasingly becoming cheaper. There are advantages in digital transmission

techniques. Ex: error correction. Lack of inter modulation noise means increased

efficiency.

Thank You

Updated infos

There are about 2,271 satellites currently in space, most of them are used for communication.

India has launched 82 Indian satellites (as of 10 March 2016) of many types since its first in 1975

As of February 2016, 72 Global Positioning System navigation satellites have been launched;

not all remain operational. The minimum number of satellites for a full constellation is 24 operational 95% of the time