Satellite Communications

TOPICS • • • • • • • • •

Introduction to Satellite Communications Orbital Aspects of Earth Satellites Types of Satellites Satellites Communication Systems Satellite Subsystems Earth Stations Satellite Routing Satellite Handover Applications of Satellites

Introduction to Satellite Communications

1. Definition:  Satellite is a physical object that orbits or revolves around some celestial body.  In general Satellite is an artificial satellite stationed in space for the purposes of telecommunications, military, surveillance, etc

2. History:  The first artificial satellite was the Soviet Sputnik-1, launched on October 4, 1957, and equipped with an on-board transmitter that worked on two frequencies, 20.005 and 40.002 MHz .  The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to  store and forward voice messages.  Telstar was the first active, direct communications satellite. Belonging to AT &T.

relay

Orbital Aspects of Earth Satellites

Here we deal with the following concepts:

1. Orbit Fundamentals 2. Geosynchronous Satellites 3. Station Keeping 4. Attitude Control 5. Satellite Position 6. Satellite Launching

1. Orbit Fundamentals:  Satellite keeps moving around the Earth in some orbital pattern .  Orbit Fundamentals is based on a. Orbit Shape b. Direction of satellite’s revolution c. Satellite Speed and Period d. Satellite Angles e. Satellite Repeaters

a. Orbit Shape  Satellite keeps moving around the Earth in some orbital pattern called “Orbit Shape”.  Orbit Shape can be either a. Circular Orbit b. Elliptical Orbit

b. Direction of satellite’s revolution 1. Posigrade Orbit i.e. satellites revolution=direction of Earth’s rotation 2. Elliptical Orbit i.e. satellites revolution=against the direction of Earth’s rotation

c. Satellite Speed and Period  The speed of the satellite is measured in miles per hour, kilometer per hour , or knots.  Speed varies depending upon the distance of the satellite from Earth.  Two types of Periods ----- 1. Sideral Period 2. Synodic Period

d. Satellite Angles 1. Angle of Inclination Is the angle formed between the equatorial plane and the satellite’s orbital plane as the satellite enters the northern hemisphere.

2. Angle of Elevation Is the angle that appears between the line from the Earth station’s antenna to the satellite and the line between the Earth station’s antenna and the Earth’s horizon.

3. Polar Orbit 4. Equatorial Orbit

e. Satellite Repeaters  To use a satellite for communications relay or repeater purposes ground station antenna must track or follow the satellite as it passes overhead.  Height and speed only determines how long the satellite can stay connected with the ground station.  Some time the satellite may disappear around the other side of the Earth.  To solve this its be launched in a very long elliptical orbit.

2. Geosynchronous Satellites:  A geostationary satellite revolves around the earth at a constant speed once per day over the equator.  It appears to be in a fixed position to an earth-based observer.  Usually geosynchronous satellites are placed at a distance of 22,300 miles or 35,860 km above the Equator.  The satellite at that distance revolves around the Earth in exact 24 hours.  Speed of the satellite=7000 miles/hour

Advantages of Geosynchronous Satellites:  Since the satellite remains apparently fixed, no special earth station tracking antennas are needed  The antenna can simply be pointed at the satellite and remain fixed.  Continuous communications are possible.  Most communication satellites used today are geosynchronous satellites.

Disadvantages of Geosynchronous Satellites:  During an eclipse the Earth or moon gets between the satellite and the Sun, is causes the sunlight to be blocked from the solar panel.  So an eclipse shuts off all power to the satellite.  To avoid this backup batteries are used.

3. Station Keeping:  Even with a very good launch the satellite can drift some-what from its orbit. This is called “Orbital Drift”.  It is caused by a variety of forces like sun’s, moon’s gravitational pull, etc.  The process of firing the rockets under ground control to maintain or adjust the orbit is referred to as “Station Keeping”

4. Altitude Control:  Satellites have to be placed in some altitude for optimal performances. This is called as Altitude Control.  Stabilizing the satellite is also called as Altitude Control.  Two types stabilization are there:  

Spin Stabilization Three axis Stabilization

 Most common is the Spin Stabilization, where the satellite spins around using the thrusters attached to it on its primary axis.

5. Satellite Positioning:  In order to use a satellite, it has to be positioned in space properly, usually it a predetermined by design of the satellite and is achieved during launch.  Once the position is known, the earth station antennas have to pointed at the satellite for optimal transmission and reception.  The location of a satellite is generally specified in terms of latitudes and longitudes.

6. Satellite Launching:  Satellites are placed into their orbits by mounting them on top of rockets which literally shoot them into space.  Occasionally, the rocket will contain more than one satellite. Here the main satellite is called as “Initial Payload” and others as “Secondary payload”.  The satellite is first put into what is called a “transfer orbit”, a highly elliptical orbit that permits adjustments to the satellite to be made prior to its being placed into final position.

Types of Satellites

Satellites

Natural Satellites

E.g.: Moon

Natural Satellites

Based on Orbiting the Earth

Geostationary Satellites Medium Earth Orbiting Satellites Low Earth Orbiting Satellites Highly Elliptical Orbiting Satellites Polar Satellites

Based on Application Remote Sensing Satellites Meteorological Satellites Communication Satellites Navigation Satellites Scientific and Military Satellites

A. Natural Satellites:  A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy.  E.g.: Moon

Fig : Jupiter's Moon

B. Based on Orbiting the Earth:  There are five types. 1. Geostationary Satellites Satellites are placed above the equator at a distance of about 36000 km. Almost today all satellite orbiting the Earth are of this type 2. Medium Earth Orbiting Satellites Operate at a distance of about 5,000-12,000 km. Up to now there has not been many satellites in this class. -

3. Low Earth Orbiting Satellites -

Are placed at an altitude of 5,00-1,500 km. Typical duration of them are 95-120 minutes. They try to ensure a high elevation for every spot on Earth to provide high quality communication link. Uses advanced compression schemes, transfer rate of 2,400 bits/sec can be enough for voice communication.

4. Highly Elliptical Orbiting Satellites -

Comprises of all satellites with a relatively low-altitude perigee and an extremely high-altitude apogee. It has the advantage of long dwell times at a point in the sky during the approach to and descent from apogee.  E.g.: US’s Sirius Satellite

5. Polar Satellites -

These satellites orbit from Northern Hemisphere to Southern hemisphere. E.g.: PSLV, Polar Wind(USA) They follow highly elliptical orbit, inclined about 86 o with an orbital period of 18 hours It gathers multi-wavelength imaging of the aurora, and measures the entry of plasma into the polar magnetosphere, etc..

C. Based on Applications :  There are five types. 1. Remote Sensing Satellites Are a series of Earth Observation satellites, which observes weather, landscapes, atmosphere, oceanic surface, climate changes, urban planning, etc.. Two types of remote sensing --- 1. Active 2. Passive -

2. Meteorological Satellites a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, or geostationary, etc.. It sees clouds and cloud systems, City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc., are other types of environmental information collected using weather satellites.

3. Communication Satellites They aid telecommunications, as by reflecting or relaying a radio. -

have been a significant part of domestic and global communications since the 1970s.

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Uses --- Telephony, Satellite TVs, Satellite Satellite Radio, Aircraft communications, etc..

Internet,

4. Navigation Satellites -

Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage.

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allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-ofsight by radio from satellites.

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Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

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As of 2009, the United States NAVSTAR Global Positioning System (GPS).) is the only fully operational GNSS.

A handheld GPS Receiver

5. Military and Scientific Satellites -

A military satellite is an artificial satellite used for a military purpose, often for gathering intelligence, as a communications satellite used for military purposes, or as a military weapon.

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Many cryptographic algorithms are used to encode the signals, use special frequency ranges, advanced transmitting and receiving equipments .

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Scientific satellites gather data for scientific analysis. This includes observations of the atmosphere of our planet, the stars, the sun and other parts of space.

Military Satellite

Satellite Communica tion Systems

 Communication Satellites are originators of information.  They instead relay stations for other sources.

Here we deal with following concepts:

the

1. Transponders 2. Satellite Frequency Allocations 3. Satellite Bandwidth 4. Increasing Channel Capacity

1. Transponders -

Satellite contains a receiver which picks up the transmitted signal, amplifies it, and translates it into another frequency. The transmitter-receiver combination in the satellite is known as “Transponder”. Uplink – Upto 6GHz Downlink – Upto 4GHz Typical transponder has a wide bandwidth. But use only a single signal to minimize interference and to improve communication reliability.

2. Satellite Frequency Allocations -

Most satellites operate in microwave frequency spectrum.

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It is divided up into frequency bands which have been allocated into satellite as well as other communications services such as radar.

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The most widely used satellite communications band is the C band.

FREQUENCY

BAND

225-390 MHz

P

350-530 MHz

J

1530-2700 MHz

L

2500-2700 MHz

S

3400-6425 MHz

C

7250-8400 MHz

X

10.95-14.5 GHz

Ku

17.7-21.2 GHz

Kc

27.5-31 GHz

K

36-46 GHz

Q

46-56 GHz

V

56-100 GHz

W

3. Increasing Channel Capacity Although the transponders are quite capable, they nevertheless rapidly become overloaded with traffic. For these reasons, numerous techniques have been developed to effectively increase the band-width and signal carrying capacity of the satellite. Two of these techniques are: 1. Frequency Reuse 2. Spatial Isolation -

Satellite Subsystems

Regulators, protection and conditioning

DC/DC Converters, DC/AC Inverters

DC to all subsystem

DC and AC to special subsystem

Charger and Batteries

Solar Panel

Communication Subsystem Receiver

Frequency Translator

Transmitter

Power Sub System

Altitude Control Subsystem

Transponder Other Transponders

Antenna Subsystem

Communications Antennas

I/Ps from onboard sensors

Telemetry, Tracking, and Control Subsystem

Ctrl Sgls to all Telemetry Antenna subsystems

Propulsion Subsystem

AKM

Jet Thrusters

 Generally satellites have many subsystems which join together for the fully operation of the satellite.  The various subsystems in a general communication satellites are: 1. Power Subsystem 2. Communication Subsystem 3. Antenna Subsystem 4. Telemetry, Tracking, and Control Subsystem 5. Propulsion Subsystem 6. Altitude Control Subsystem

Earth Stations

LNA

Demodula tor

BPF

BPF LO

Base Band O/P

Down Converter

Receive Subsystem

Carrier Oscillator

Antenna Subsystem

UP Converter

Driver

Demodula tor

BPF HPA

LO

GCE-Transmit

Transmit Subsystem

Fig: General Block Diagram of

Power Subsystem

Base Band I/P

GCE-Receive

Diplexe r

 The earth station on the ground is the terrestrial base of the system.  The earth station communicates with the satellite to carry out designated mission.  It may be located at the end user’s facilities or may be located with ground-based intercommunication links between the earth station and the end user.  Many earth stations are now located on top of tall buildings or in other urban areas directly where the end user resides.

 The various subsystems in an earth station are: 1. Antenna Subsystem 2. Receive Subsystem 3. Transmit Subsystem 4. Ground Communication Equipment (GCE) Subsystem 1. GCE Transmit Subsystem 2. GCE Receive Subsystem

5. Power Subsystem

Satellite Routing

Satellite Routing Mo bi le U (M se UL r L ink )

Gateway Link (GWL)

GWL

Li er Us L) le bi (MU Mo

Inter Satellite Link (ISL)

nk

Spot Beam

Foo tP

Base Station Or Gateway ri n t

ISDN

PSTN User Data

GSM

Satellite Handover

 There are four types of satellite handovers. They are: 1. Intra-satellite Handover 2. Inter-satellite handover 3. Gateway Handover 4. Inter-system Handover

Applications Of Satellite

Various applications of satellites are: 1. Remote Sensing Satellites 2. Meteorological / Weather Satellites 3. Communication Satellites 4. Navigation Satellites 5. Military Satellites 6. Space Exploration Satellites

1. Remote Sensing Satellites

Indian Remote Sensing satellite’s image

2. (a) Meteorological Satellites

2. (b) Weather Satellites

Image of a Weather Satellite Report

Various weather satellites orbiting the Earth

3. Communication Satellites

4. Navigation Satellites

5. Military Satellites

6. Space Exploration Satellites

E.g. Martian Communication