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 Introduction to to

Satellite Satellite CommunicationsCommunications

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 relay communications satellite. Belonging to AT &T.

Orbital Aspects Orbital Aspects ofof

Earth Satellites Earth Satellites

Here we deal with the following concepts:

1. Orbit Fundamentals2. Geosynchronous

Satellites3. Station Keeping4. Attitude Control5. Satellite Position6. Satellite Launching

1. Orbit Fundamentals:

Satellite keeps moving around the Earth in some orbital pattern .

Orbit Fundamentals is based ona. Orbit Shapeb. Direction of satellite’s revolutionc. Satellite Speed and Periodd. Satellite Anglese. Satellite Repeaters

a. Orbit Shape

Satellite keeps moving around the Earth in some orbital pattern called “Orbit Shape”.

Orbit Shape can be eithera. Circular Orbit b.

Elliptical Orbit

b. Direction of satellite’s revolution

1. Posigrade Orbiti.e. satellites revolution=direction of Earth’s rotation

2. Elliptical Orbiti.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 Angles1. 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 ElevationIs 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 Orbit4. 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 Types ofof

Satellites Satellites

Based on Application

Based on Orbiting

the Earth

Geostationary Satellites

Geostationary Satellites

Medium Earth Orbiting Satellites

Medium Earth Orbiting Satellites

Low Earth Orbiting Satellites

Low Earth Orbiting Satellites

Highly Elliptical Orbiting SatellitesHighly Elliptical

Orbiting Satellites

Remote Sensing Satellites

Remote Sensing Satellites

Meteorological Satellites

Meteorological Satellites

Communication Satellites

Communication Satellites

NavigationSatellites

NavigationSatellites

Scientific and Military Satellites

Scientific and Military Satellites

E.g.: MoonE.g.: Moon

PolarSatellites

PolarSatellites

Satellites

Natural SatellitesNatural

SatellitesNatural

SatellitesNatural

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.

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

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.

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

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

- 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.

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

- 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 the following concepts:

1. Transponders2. Satellite Frequency

Allocations3. Satellite Bandwidth4. 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.

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

- 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

Fig: Frequency bands used in satellite communications

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 Reuse2. Spatial Isolation

Satellite Satellite SubsystemsSubsystems

Solar PanelSolar Panel Charger and BatteriesCharger and Batteries

Regulators, protection and conditioning

Regulators, protection and conditioning

DC/DC Converters, DC/AC Inverters

DC/DC Converters, DC/AC Inverters

ReceiverFrequency Translator

Transmitter

Other Transponders

Transponder

Antenna SubsystemAntenna Subsystem

Telemetry, Tracking, and Control Subsystem

Altitude Control Subsystem

Propulsion Subsystem

DC to all subsystem DC and AC to special subsystem

AKMJet Thrusters

Ctrl Sgls to all subsystemsTelemetry Antenna

I/Ps from on-board sensors

Communication Subsystem

Communications Antennas

PowerSubSystem

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 Subsystem2. Communication Subsystem3. Antenna Subsystem4. Telemetry, Tracking, and Control Subsystem5. Propulsion Subsystem6. Altitude Control Subsystem

Earth Earth StationsStations

BPFBPF

LNA

LOLO

BPFBPF

Carrier Oscillator

DemodulatorDemodulator

Bas

e B

and

O/P

DemodulatorDemodulator

LO

BPF

HPA

Driver

UP Converter

Bas

e B

and

I/P

Diplexer

Down Converter

GCE-Receive

GCE-Transmit

Power Subsystem

Transmit Subsystem

Receive Subsystem

Antenna Subsystem

Fig: General Block Diagram of an Earth Station

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 Subsystem2. GCE Receive Subsystem

5. Power Subsystem

Satellite Satellite RoutingRouting

Base StationOr

Gateway

ISDN GSMPSTN

User Data

Spot Beam

Foot Print

GWL

Mob

ile U

ser

Link

(MU

L)

Mobile U

ser Link

(MU

L)

Gateway Link(GWL)

Inter Satellite Link (ISL)

Satellite Routing

Satellite Satellite HandoverHandover

There are four types of satellite handovers. They are:

1. Intra-satellite Handover

2. Inter-satellite handover

3. Gateway Handover

4. Inter-system Handover

ApplicationsApplicationsOfOf

Satellite Satellite

1. Remote Sensing Satellites

2. Meteorological / Weather

Satellites

3. Communication Satellites

4. Navigation Satellites

5. Military Satellites

6. Space Exploration Satellites

Various applications of satellites are:

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