satellite

TOPICS

TOPICS

• Introduction to Satellite Communications • Orbital Aspects of Earth Satellites

• Types of Satellites

• Satellites Communication Systems

S t llit S b t

• Satellite Subsystems • Earth Stations

• Satellite RoutingSatellite Routing

• Satellite Handover

Introduction

Introduction

to

to

Satellite

Satellite

Satellite

Satellite

Communications

Communications

Communications

Communications

1. Definition:

f

™ Satellite is a physical object that orbits or revolves around

p y

j

some celestial body.

™ Satellite transmissions are categorized as either bus or

g

payload. The bus includes control mechanisms that

support the payload operation.

™ The payload is the actual user information that is

conveyed through the system.

™ In general Satellite is an artificial satellite stationed in

space for the purposes of telecommunications, military,

surveillance etc

2 History:

2. History:

™ Th fir t artifi ial at llit wa th S vi t Sp t ik 1 la h d

™ 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.f , y Belonging to AT &T in 1962. Telstar II was successfully launched in 1963. It was used for Telephone, television, fascimile and data transmissions

Sputnik ‐ I

•

The first artificial satellite

•

October 4, 1957

• LEO

LEO

•

frequencies

•

20 005 MHz

20.005 MHz

•

40.002 MHz. 

SCORE

• First voice communication

First voice communication

established via satellite

(US)

• 1958

• LEO

ECHO I

• First passive

First passive

communication

satellite launched into

space

Telstar I

´

First non

´

First

non-government

active

active

communication

satellite launched

satellite launched

´

1962

´

MEO

SYNCOM 2

´

First Geo

´

First Geo

Synchronous

Satellite

Satellite

´

Feb 1963

INTELSAT I

1964: International

Telecomm. Satellite

Organization (INTELSAT)

created

created.

1965: First

1965: First

communications satellite

launched into

geostationary orbit for

commercial use

E l Bi d

Early satellites were both of the passive and active type.

Ea ly satellites we e both of the passive and active type.

‰ Passive satellite is one that reflects a signal back to earth:

there are no gain devices on board to amplify or repeat the

g

p fy

p

signal

‰ An active satellite is one that electronically repeats a signal

y p

g

back to earth.(i.e., receives, amplifies, and retransmits the

signal)

•

Syncom I, launched in February 1963, was the first attempt

to place a geosynchronous satellite into orbit. Syncom I was

p

g y

y

lost during orbit injection.

•

Syncom II and Syncom III were successfully launched in

y

y

f y

February 1963 and August 1964, respectively. The Syncom

III satellite was used to broadcast the 1964Olympic Games

f

T k

from Tokyo.

•

In 1964 a commercial global satellite network known as

I

l

(I

i

l

T l

i i

S lli

Intelsat

(International

Telecommunications

Satellite

Organization) was established..

•

The first Intelsat satellite was Early Bird 1, which was

l

h d i 1965

d

id d 480

i

h

l F

launched in 1965 and provided 480 voice channels. From

1966 to 1987, a series of satellites designated Intelsat II,

Ill IV V and VI were launched 1ntelsat VI has a capacity

Ill, IV, V, and VI were launched. 1ntelsat VI has a capacity

of 80,000 voice channels.

•

In 1988 the first satellite system for mobile phones and data

n 1988 the fi st satellite system fo mobile phones and data

communication INMARSAT-C was established.

•

During 1993 the first digital satellite telephone system was

introduced and also in 1998 the global satellite systems for

small mobile phones was launched.

Types of Satellite Orbits

yp

f

The orbital elements of a particular satellite depend upon its

f p

p

p

intended application. The satellite orbits can be classified

on the basis of :

f h

b l l

1. Orientation of the orbital plane

2. Eccentricity

3 Distance from Earth

3. Distance from Earth

1. Orientation of the orbital plane

•

The orbital plane of the satellite can have various

ri t ti

ith r

t t th q t ri l l

f E rth

orientations with respect to the equatorial plane of Earth.

The angle between the two planes is called the angle of

inclination of the satellite.

inclination of the satellite.

•

On this basis, the orbits can be classified as equatorial orbits,

polar orbits and inclined orbits.

p

•

In the case of an equatorial orbit, the angle of inclination is

In the case of an equatorial orbit, the angle of inclination is

zero, i.e. the orbital plane of the satellite coincides with the

Earth’s equatorial plane (Figure 1).

Figure 1. Equatorial orbit

• _{A satellite in the equatorial orbit has a latitude of 0◦. For an angle of}

i li i l 90 h lli i id b i h l bi

inclination equal to 90◦, the satellite is said to be in the polar orbit (Figure 2). For an angle of inclination between 0◦ and 180◦, the orbit is said to be an inclined orbit.

•

For inclinations between 0◦ and 90◦, the satellite travels in

the same direction as the direction of rotation of the Earth.

the same direction as the direction of rotation of the Earth.

The orbit in this case is referred to as a direct or prograde

orbit (

(

Figure 3

). For inclinations between 90◦ and 180◦, the

satellite orbits in a direction opposite to the direction of

rotation of the Earth and the orbit in this case is called a

d

bi (

Fi

4

)

retrograde orbit (

Figure 4

).

Figure 3.

Prograde orbit Figure 4.

Retrograde orbit Retrograde orbit

2. Eccentricity of the Orbit :

On the basis of eccentricity the orbits are classified as

On the basis of eccentricity, the orbits are classified as

elliptical (Figure (a)) and circular (Figure (b)) orbits.

Needless to say, when the orbit eccentricity lies between 0

N

y, w

y

w

and 1, the orbit is elliptical with the centre of the Earth

lying at one of the foci of the ellipse. When the eccentricity

is zero, the orbit becomes circular.

Molniya Orbit 

U d b

i f d

d

ƒ Used by Russia for decades.

ƒ Molniya Orbit is an elliptical orbit. The satellite remains in a

l fi d

iti l ti t th f i ht h

nearly fixed position relative to earth for eight hours.

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

U f l i

l

i

•

One of the 910re interesting orbital satellite systems is the

Soviet Molniya system This is also spelled Molnya and

Soviet Molniya system. This is also spelled Molnya and

Molnia, which means "lightning" in Russian (in colloquial

Russian it means "news flash").

f

)

•

The Molniya satellites are used for television broadcasting

and are presently the only nonsynchronous-orbit commercial

satellite system in use. Molniya uses a highly elliptical orbit

with apogee at about 40,000 km and perigee at about 1000

kkm

GEO 36,000 km

MEO 5,000 – 15,000 km

LEO 500 -1000 km

Satellite communication

• Satellite is a physical objectp y j that orbits a celestial body. • Communication satellite

containing electronics equipmentq p acts as a repeater or relay station between two earth station.

´ Transponder is the basic component of a communication

satellite satellite.

Satellite communication

• Satellites are located by earth coordinates expressed in terms of latitude and longitude.

• The two angles used to point a ground station antennas are azimuth and elevation angles .

The main power supplies of a satellite are Solar Panels.p pp During eclipse, the satellite is powered by Batteries.

The main used of satellite is for communications, Consumers uses satellite for TV reception

Why do satellites stay moving and in orbit?

F1

F2

• F1 ‐ gravitational force  • F2 ‐ centripetal force 

Geocenter

the center of gravity of the earth

Geocenter

- the center of gravity of the earth

Satellite Orbit

• Satellite orbits the earth from the height of 100 to 22,300 mi and travel at speeds of 6,800 to 17,500 mi/hr.p , ,

Geostationary satellite

A satellite that orbits directly A satellite that orbits directly over the equator 22,300 mi from earth.

SYNCOM1 in February 1963:

(comms. failed).

SYNCOM2 in February 1963 SYNCOM3 in August 1964

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

Satellite Orbit

P i A

Perigee Apogee

Orbits

• LEO:  500 to 1 500 km GE O 500 to 1,500 km Low Earth Orbit. Low Earth Orbit. LE O LE O ´ MEO: 8,000 km - 18,000 km M di E th O bit M di E th O bit _MEO Medium Earth Orbit Medium Earth Orbit ´ GEO:  35,863 km

Geostationary Earth Orbit Geostationary Earth Orbit Geostationary Earth Orbit Geostationary Earth Orbit

Frequency Bands

• Three common bands:

Band Up‐Link  (Ghz) Down‐link (Ghz)  ISSUES Interference C 4 6 Interference  with ground  links. Ku 11 14 Attenuation _{due to rain} Hi h Ka 20 30 High  Equipment  cost

Lunching Satellites

•Many satellites are put in orbit by launching them form NASA’s space shuttle

How Does a Satellite Work?

• One Earth Station

sends a transmission to the satellite. This is called a Uplink.

• The satellite

Transponder converts Transponder converts

the signal and sends it down to the second earth station This is earth station. This is called a Downlink.

End of life

option of de-orbiting the satellite

l i th t llit i it t

leaving the satellite in its current orbit moving the satellite to a graveyard g g y

Kepler’s first law

states that the path followed by a satellite around the primary will be an ellipse. y y An ellipse has two focal points shown as F1 and F2

The center of mass of the two-body system termed the barycenter is The center of mass of the two body system, termed the barycenter, is always centered on one of the foci

• In our specific case because of the enormous difference • In our specific case, because of the enormous difference 

between the masses of the earth and the satellite, the center  of mass coincides with the center of the earth, which is 

therefore always at one of the foci.

• The semimajor axis of the ellipse is denoted by a and the • The semimajor axis of the ellipse is denoted by a, and the 

semiminor axis, by b. The eccentricity e is given by

a b a e 2 2 − =

Kepler`s Second Law Kepler s Second Law

Kepler’s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter.

Thus the farther the satellite from earth, the longer it takes to travel a given distance

Kepler’s Third Law

• According to the Kepler’s third law also known as the law of periods the • According to the Kepler s third law, also known as the law of periods, the square of the time period of any satellite is proportional to the cube of the semi-major axis of its elliptical orbit

semi major axis of its elliptical orbit.

• The expression for the time period can be derived as follows.

• A circular orbit with radius r is assumed Remember that a circular orbit is • A circular orbit with radius r is assumed. Remember that a circular orbit is only a special case of an elliptical orbit with both the semi-major axis and semi-minor axis equal to the radius.

semi minor axis equal to the radius.

Solar day and Sidereal day

y

y

• A day is defined as the time that it takes the Earth to rotate on

i i

its axis.

• However, there is more than one way to define a day: A sidereal day is the time that it takes for the Earth – A sidereal day is the time that it takes for the Earth to rotate with respect to the distant stars.

– A solar day is the time that it takes to rotate with respect toy p the Sun.

• A solar day is measured using the passage of the Sun across th k it l t 24 h

the sky—it lasts 24 hours

• A sidereal day (from the Latin word meaning star) is measured with respect to fixed stars—it lasts a little less than measured with respect to fixed stars it lasts a little less than 24 hours.

Orbital Aspects

Orbital Aspects

of

of

Earth Satellites

Earth Satellites

Earth Satellites

Earth Satellites

Here we deal with the following concepts:

1. Orbit Fundamentals

2 Geosynchronous Satellites

2. Geosynchronous Satellites

3. Station Keeping

4 A i d C

l

4. Attitude Control

5. Satellite Position

ll

h

6. Satellite Launching

1. Orbit Fundamentals:

™ Satellite keeps moving around the Earth in some orbital

pattern

pattern .

™ Orbit Fundamentals is based on

a Orbit Shape

a. Orbit Shape

b. Direction of satellite’s revolution

S t llit S

d

d P i d

c. Satellite Speed and Period

d. Satellite Angles

lli

a. Orbit Shape

™ Satellite keeps moving around the Earth in some orbital

pattern called “Orbit Shape”

pattern called Orbit Shape .

™ Orbit Shape can be either

Ci

l

O bi

b Elli i l O bi

b. Direction of satellite’s revolution

b. Direction of satellite s revolution

1 Posigrade Orbit

1. Posigrade Orbit

i.e. satellites revolution=direction of Earth’s rotation

2 Elliptical Orbit

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

kilometer per hour , or knots.

™ Speed varies depending upon the distance of the satellite from

Earth.

Earth.

d. Satellite Angles

1 A l f I li ti

1. Angle of Inclination

Is the angle formed between the equatorial plane and the satellite’s 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

S t llit R

t

e. Satellite Repeaters

™ To use a satellite for communications relay or repeater

f

y

p

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.

can stay connected with the ground station.

™ Some time the satellite may disappear around the other

id f th E th

side of the Earth.

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.

™ Th

lli

h di

l

d h

h i

™ The satellite at that distance revolves around the Earth in

exact 24 hours.

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

h

lli

Disadvantages of Geosynchronous Satellites:

™ During an eclipse the Earth or moon gets between the

t llit

d th S

i

th

li ht t b bl k d

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:

S

K p g

™ E

ith

d l

h th

t llit

d ift

™ 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

j

f

“Station Keeping”

4. Altitude Control:

A

C

™ Satellites have to be placed in some altitude for optimal

f

Thi i

ll d

Altit d C t l

performances. This is called as Altitude Control.

™ Stabilizing the satellite is also called as Altitude Control.

g

™ Two types stabilization are there:

™ _{Spin Stabilization}

™ _{Spin Stabilization}

™ _{Three axis Stabilization}

™ M t

i th S i

St bili ti

h r th

™ 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

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.

g

6. Satellite Launching:

™ S t llit l d i t th i bit b ti th t f

™ Satellites are placed into their orbits by mounting them on top of

rockets which literally shoot them into space.

™ _{i ll h k ill i h lli H h}

™ Occasionally, the rocket will contain more than one satellite. Here the

main satellite is called as “Initial Payload” and others as “Secondary payload”.

p y

™ The satellite is first put into what is called a “transfer orbit”, a highly

elliptical orbit that permits adjustments to the satellite to be madep p j prior to its being placed into final position.

T

T

Types

Types

ff

of

of

S

ll

S

ll

Satellites

Satellites

Satellites Natural Satellites Natural Satellites Based on Application Based on Orbiting the Earth Satellites Satellites Geostationary Satellites Remote Sensing Satellites E.g.: Moon Sate tes g Medium Earth Orbiting Satellites L E th Meteorological Satellites Communication g Low Earth Orbiting Satellites Highly Elliptical Orbiting Satellites Satellites Navigation Satellites g Orbiting Satellites y Scientific and Military Satellites Polar 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

the primary. Technically, the term natural satellite could refer

to a planet orbiting a star, or a dwarf galaxy orbiting a major

galaxy.

galaxy.

™ E.g.: Moon

Fig : Jupiter's Moon

Fig : Jupiter s Moon

B Based on Orbiting the Earth:

B. Based on Orbiting the Earth:

™ There are five types.

1. Geostationary Satellites

S lli l d b h di f b - Satellites are placed above the equator at a distance of about

36000 km.

- Almost today all satellite orbiting the Earth are of this typeA y g f yp

2. Medium Earth Orbiting Satellites

- Operate at a distance of about 5,000-12,000 km.

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.

U d d i h t f t f 2 400 bit / - Uses advanced compression schemes, transfer rate of 2,400 bits/sec

can be enough for voice communication.

4 Hi hl Elli ti l O biti S t llit 4. Highly Elliptical Orbiting Satellites

- Comprises of all satellites with a relatively low-altitude perigee and t l hi h ltit d

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.pp f p g

5. Polar Satellites

Th lli bi f N h H i h h - These satellites orbit from Northern Hemisphere to Southern

hemisphere. E.g.: PSLV, Polar Wind(USA)

- They follow highly elliptical orbit, inclined about 86_{They follow highly elliptical orbit, inclined about 86 with an orbital}o _{with an orbital}

period of 18 hours

- It gathers multi-wavelength imaging of the aurora, and measures the f l h l h

C. Based on Applications :

™ Th

fi

™ 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..

urban planning, etc..

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 - 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 3. Communication Satellites

- They aid telecommunications, as by reflecting or relaying a radio.y g

- 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

autonomous geo spatial positioning with global coverage.

- allows small electronic receivers to determine their location (longitude latitude and altitude) to within a (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

5 Military and Scientific Satellites 5. Military and Scientific Satellites

- A military satellite is an artificial satellite used for a militaryA y f f y 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

equipments .

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

Satellite

Satellite

Communication

Communication

Systems

Systems

™ Communication Satellites are originators of

i f

ti

information.

™ They instead relay stations for other sources.

Here we deal with the following concepts:

f

g

p

1. Transponders

2 Satellite Frequency Allocations

2. Satellite Frequency Allocations

3. Satellite Bandwidth

4 I

i Ch

l C

it

1. Transponders

Satellite contains a receiver which picks up the transmitted - 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

T i l d h id b d id h B l i l - 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

radar.

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

Fig: Frequency bands used in satellite communications

FREQUENCY BAND 225-390 MHz P 350-530 MHz J 1530-2700 MHz L 2500 2700 MHz S 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 46 56 GHz V 56-100 GHz W

3. Increasing Channel Capacity

- Although the transponders are quite capable, they neverthelessAlthough the transponders are quite capable, they nevertheless rapidly become overloaded with traffic.

- For these reasons, numerous techniques have been developed to ff l h b d d h d l

effectively increase the band-width and signal carrying capacity of the satellite.

- Two of these techniques are:Two of these techniques are: 1. Frequency Reuse

Satellite

Satellite

Satellite

Satellite

Subsystems

Subsystems

Subsystems

Subsystems

Solar Panel Charger and Batteries Regulators, protection and conditioning DC/DC Converters, DC/AC Inverters Power Sub System DC to all subsystem DC and AC to

special subsystem Communication Subsystem Receiver Frequency Translator Transmitter Transponder Altitude Control Subsystem Other Transponders Transponder Antenna Subsystem Telemetry, Tracking, and Control S b Propulsion Subsystem I/Ps from on-board Subsystem AKM Jet Thrusters Ctrl Sgls to all subsystems Telemetry Antenna sensors Communications Antennas

™

_ll

_lli

_h

_b

_{hi h j i}

™ Generally satellites have many subsystems which join

together for the fully operation of the satellite.

™ Th i b t

i

l

i ti t llit

™ The various subsystems in a general communication satellites

are:

1 Power Subsystem

1. Power Subsystem

2. Communication Subsystem

3 A t

S b t

3. Antenna Subsystem

4. Telemetry, Tracking, and Control Subsystem

l

b

5. Propulsion Subsystem

E th

E th

Earth

Earth

St ti

St ti

Stations

Stations

LNA

vider BPF Demodulator _and O/P Down Converter BPF Po w e r  Di v LO DEMUX Base B a Di l GCE-Receive Receive Subsystem Carrier Oscillator d I/P Diplexer Antenna Subsystem Demodulator DEMUX Divider Driver UP Converter Base Ban LO Po w er   BPF HPA GCE-Transmit Power Subsystem Transmit Subsystem

™ The earth station on the ground is the terrestrial base of the

system

system.

™ Th

rth t ti

i t

ith th

t llit t

rr

™ 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

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.

y w

™ 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

Satellite

Satellite

S

S

Routing

Routing

outing

outing

Satellite Routing

Inter Satellite Link

(ISL) Spot Beam Base Station Or Gateway Gateway ISDN PSTN GSM User Data

Satellite

Satellite

S

S

Handover

Handover

andove

andove

™ There are four types of satellite handovers. They are:

f

yp

f

y

1. Intra-satellite Handover

1. Intra satellite Handover

2 Inter-satellite handover

2. Inter-satellite handover

3 Gateway Handover

3. Gateway Handover

4 I t

t H d

4. Inter-system Handover

Applications

Applications

Of

Of

Satellite

Satellite

Various applications of satellites are:

1. Remote Sensing Satellites

2. Meteorological / Weather Satellites

3. Communication Satellites

4. Navigation Satellites

5. Military Satellites

SPACE CRAFT ANTENNAS

• The function of the spacecraft antennas is to receive and transmit two distinct classes of signals: broadband microwaveg frequencies for communication service, such as television; and narrow-band VHF for beacon, command, and telemetry.

Th i t th t lt d f th h

• The requirements on these antennas resulted from a thorough systems analysis which led to many factors involving not only the spacecraft, but the ground station as well.

• These factors include such vital questions as: the modulation method, the location of frequencies, the choice of polarizations the degree of attitude stabilization etc

Spin stabilization

Spin stabilization

• Stabilization accomplished by rotating the spacecraft mass,p y g p , thus using gyroscopic action as the stabilizing mechanism. Thrusters are fired to make desired changes in the spin-stabilized attitude

stabilized attitude.

• Spin-stabilized spacecraft provide a continuous sweeping desirable for fields and particle instruments, but they may require complicated systems to de-spin antennas or optical instruments which must be pointed at targets.

•

Wire antennas : monopoles and dipoles

Wire antennas : monopoles and dipoles

•

Horn antennas

fl

•

_{Reflector antennas}

• Wire antenna are used primarily at VHF and UHF to providep y p communications for the TT&C systems. They are positioned with great care on the body of the spacecraft in an attempt to provide omni directional coverage

provide omni-directional coverage.

• Most spacecraft measure only a few wavelengths at VHF frequencies which makes it difficult to get the required antenna patterns.

• An antenna pattern is a plot of the field strength in the far field of the antenna when the antenna is driven by a transmitter It is of the antenna when the antenna is driven by a transmitter. It is usually measured in decibels below the maximum field strength.

• Horn antennas are used at microwave frequencies when

l i l id b i d f l b l A

relatively wide beams are required, as for global coverage. A horn is a flared section of waveguide that provides an aperture several wavelengths wide and a good match between theg g waveguide impedance and free space.

• Reflector antennas are usually illuminated by one or more horns and provide a larger aperture than can be achieved with horns and provide a larger aperture than can be achieved with a horn alone.

• For maximum gain, it is necessary to generate a plane wave ing , y g p the aperture of the reflector. This is achieved by choosing a reflector profile that has equal path lengths from the feed to the aperture so that all the energy radiated by the feed and aperture, so that all the energy radiated by the feed and reflected by the reflector reaches the aperture with the same phase angle and created a uniform phase front.