satellite communication
TRANSCRIPT
Satellite Communication
Franz Gio M. MangiJ-Glenn R. PalmeroAris N. Sadiwa
What is a Satellite?
Satellite• The word satellite originated from the Latin word
“Satellit”- meaning an attendant, one who is constantly hovering around & attending to a “master” or big man.
• an object that moves around a larger object.• Can be NATURAL SATELLITE (moon,Earth) or
ARTIFICIAL SATELLITE (man-made satellite)
Artificial Satellite• an artificial body placed in orbit around the
earth or moon or another planet in order to collect information or for communication.
• Examples : space station, space capsule, spacecraft, communications satellite, weather satellite
Communication Satellite• an artificial earth satellite• facilitates communications, as radio,
television, and telephone transmissions, by means of the reflection or the amplification and retransmission of signals between stations on earth or in space.
Communication Satellite• relays and amplifies certain
telecommunications signals, acting as a middleman between a source and a receiver.
Passive and Active Satellite• Passive Satellite merely reflects or scatters incident radiation from
the Earth, a portion of the radiation being reflected or scattered back in the direction of the Earth.
require surface transmitters of much greater power than would active relays (unless the passive reflectors are extremely large )
consist of an omnidirectional scatterer such as a spherical body, like a balloon satellite, or a directive scatterer
Works as a “reflector”
• Active Satellite one which has transmitting equipment aboard,
such as a transponder, a device which receives a signal from Earth, amplifies it, and retransmits the same signal back to Earth (either immediately or after a delay).
must carry aboard receiving and transmitting equipment and the necessary power sources.
Works as “re-transmitter”
Parts of Communication Satellite• Antennas and transceivers send and
receive radio signals to and from the Earth or another satellite;
• Rocket motors move the satellite in space;
• Fuel tanks store the fuel for the rocket;
Parts of Communication Satellite• Solar panels use solar cells to turn the
sun's energy into electricity;• Batteries store the electricity generated
by the solar panels; and.• On-board processors provide the “brain”
of the satellite and tell the satellite to do what humans want it to do.
In order for a satellite to work effectively, several different subsystems must work together:
• propulsion system used to move the satellite in space. consists of a large rocket motor that is used to
move the satellite into the desired location, as well as smaller thruster rockets that keep the satellite at that location.
• attitude control systemkeeps the satellite pointed towards the
desired location on the Earth. • power system generates electricity from solar cells
placed on panels outside of the satellite.
• communications systemconsists of antennas used to transmit and
receive radio signals to and from the Earth, as well as electronic devices to strengthen or “amplify” these signals.
These electronic devices are called “transponders.”
• thermal control system necessary to keep the electronics on board the
satellite cool enough to work properly.• telemetry and command system consists of antennas and computers on board
that satellite that allow people on the Earth to tell the station what to do and to monitor the health of the satellite.
Satellite Communication• signal transferring between the sender and
receiver is done with the help of satellite.• the signal which is basically a beam of modulated
microwaves is sent towards the satellite.• the satellite amplifies the signal and sent it back
to the receiver’s antenna present on the earth’s surface.
• all the signal transferring is happening in space.
• Also known as space communication.
Why Use Satellite Communication?• To permit communication over large
distances• overcome the problems involved in radio
signal to span oceans and continents with sub-marine cables, landlines and terrestrial relay stations, for long-distance transmission of radio, telephone and television signals.
• To overcome the disadvantage of line-of-sight communication which is only 45-55km
• Replaces the use of relay stations (terrestrial communication) to overcome the curvature of earth as a main obstruction in line-of-sight communication.
Satellite communication able to provide:
• Entertainment - Broadcasting via satellite offers a variety of programming to the avid viewer including local and foreign programs.
• do serve civilian in rural area where terrestrial communication network does not exist by providing telephony service.
• In military sector, providing robust and sophisticated secure communications network
• to provide communication when the terrestrial systems fail due to disaster such as earthquake, volcanic eruption floods, drought, cyclones, landslides and epidemics.
• Basically, satellite communications are nearly based on terrestrial communication, using the line-of-sight propagation via repeaters.
Some History………………….• The concept of using object in space to reflect signals for
communication was proved by Naval Research Lab in Washington D.C. when it use the Moon to establish a very low data rate link between Washington and Hawaii in late 1940’s.
• Russian started the Space age by successfully launching SPUTNIK the first artificial spacecraft to orbit the earth, which transmitted telemetry information for 21 days in Oct. 1957.
• 1962 Telstar I (AT&T) – operated for only a few weeks due to radiation damage
• 1963 Telstar II (AT&T) – first transatlantic video information
• 1963 Syncom II – (Syncom I was lost)• 1964 Syncom III – used to transmit 1964
Olympics from Tokyo• 1965 Early Bird I – first Intelsat satellite
• 1969 Intelsat satellites – placed over the Atlantic, Pacific and Indian oceans, allowing worldwide satellite communication
• 1972 Anik A-1 (Canada) – first commercial domestic communications satellite.
• 1972 Westar 1 & 2 (Western Union) – first commercial United States domestic communications satellite
Telstar Syncom 2
Components of Satellite Communication
• Uplink – a ground station where information is transmitted to the satellite.
• Communication satellite - must have a receiver, a transmitter, an amplifier and prime electrical power to run all of the electronics.
• Downlink – ground station that receives transmitted signal converted in different frequencies
Satellite Orbits• All satellites are held in orbit are held in orbit
between to the inertia (centrifugal force) due to their motion and the centripetal force of gravity
Classification of Satellite Orbit
• All orbits are elliptical.• Elliptical orbits can be used to give quasi
stationary behavior viewed from earth using 3 or 4 satellites.
• Inclined orbits are useful for coverage of equatorial regions.
In an elliptical orbit,…….
• Perigee – distance of closest to the earth.• Apogee - farthest distance from earth
• A circular orbit is a special case of an elliptical orbit in which the apogee and perigee are equal.
Orbital Calculations• Any satellite orbiting the earth must satisfy this
equation:
Where: v = velocity of satellite (m/s) d = distance above earth’s surface (km)
Problem 1:• Find the velocity and the orbital period of a
satellite in a circular orbit (a)500 km above the earth’s surface (b) 36, 000 above the earth’s surface
Geosynchronous Orbit• The choice of orbit is an important consideration in
satellite system design.• Most satellite communication use the geosynchronous
orbit• This is a circular orbit• It is 35,784 km above the earth’s surface• Satellite’s orbital period is equal to the time the earth
rotate once(24 hours)
Geostationary Orbit• Also called as geostationary Earth
orbit or geosynchronous equatorial orbit• is a circular orbit 35,786 km (22,236 mi)• above the Earth's equator and following the
direction of the Earth's rotation.• has an orbital period equal to the Earth's
rotational period (one sidereal day)
• A geostationary orbit is a particular type of geosynchronous orbit, the distinction being that while an object in geosynchronous orbit returns to the same point in the sky at the same time each day, an object in geostationary orbit never leaves that position.
• Question:
Why must a geostationary satellite also be equatorial?
Geostationary Satellite• an earth-orbiting satellite, placed at an altitude
of approximately 35,800 kilometers (22,300 miles) directly over the equator
• revolves in the same direction the earth rotates (west to east)
• At this altitude, one orbit takes 24 hours, the same length of time as the earth requires to rotate once on its axis.
• A single geostationary satellite is on a line of sight with about 40 percent of the earth's surface.
• Three such satellites, each separated by 120 degrees of longitude, can provide coverage of the entire planet, with the exception of small circular regions centered at the north and south geographic poles.
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.
Disadvantages: 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.
Antenna Siting and Adjustment in Geostationary Satellites
• An antenna is aimed at a satellite by:Adjusting it’s azimuth(horizontal position)Adjusting it’s elevation(vertical angle with
the ground)• These angles are called “look angles”
Azimuth Angle Elevation Angle
Three pieces of information that are needed to determine the look angles for the geostationary orbit :
a. Earth station latitudeb. Earth station longitudec. Satellite orbital position
• If an antenna is to be used with more than one geostationary satellite, it need to be rotated along only one axis.
• Polar mount a movable mount for satellite dishes that allows the dish
to be pointed at many geostationary satellites by slewing around one axis.
It works by having its slewing axis parallel, or almost parallel, to the Earth's polar axis so that the attached dish can follow, approximately, the geostationary orbit which lies in the plane of the Earth's equator.
Adjustments for Polar Mounts• Declination angle is a "tipping down" of the dish on the
mount to allow it to observe geostationary satellites.
Amount by which the antenna axis is offset from the earth’s axis.
Declination calculation
Where:θ – declination angle R – radius of the earth (6400 km) H – height of the satellite above the earth (36000 km) L – earth station latitude
Problem 2:• Calculate the angle of declination for an
antenna using a polar mount at a latitude of 45 degrees.
Actual Path Length• If the antenna is at the equator, its path
length is 36,000 km from the geostationary satellite.
• At the same longitude, it can extend up to 41,700 km from an earth station close to either the North or South pole.
• Actual Path Length can be found from the equation:
Where: d – distance to the satellite in km r – radius of the earth in km h – height of the satellite above the equator θ – angle of elevation to satellite at antenna site
Note: 90 degrees is the angle of elevation for antenna at the equator, and at the same longitude as the satellite, decreasing to zero at about 80 degrees latitude
Problem 3:• Calculate the length of the path to a
geostationary satellite from an earth station where the angle of elevation is 30 degrees.
Propagation Time in Geostationary Satellite
• One disadvantage of this system is the considerable amount of time it takes for a signal to make a round trip within the communication system.
• Such delay will revealed in the following problem
Problem 4:• Telephone communication takes place between two
earth station via satellite that is 40,000 km from each station. Suppose Bill, at station 1, asks a question and Sharon, at station 2, answers immediately, as soon as she hears the question. How much time elapses between the end of Bill’s question and the beginning of Sharon’s reply, as heard of Bill?
Commonly Used Geostationary Frequency
Band Uplink(GHz) Downlink(GHz) Use
S 1.6265 – 1.6605 1.53 – 1.559 Marine(Inmarsat)
C 5.925 – 6.425 3.7 – 4.2 Commercial
X 7.9 - 8.4 7.25 – 7.75 Military
Ku 14 – 14.5 11.7 – 12.2 Commercial
K 27.5 – 30.5 17.7 – 20.2 Commercial
Ka 30 - 31 20.2 – 21.2 Military
Power Requirement in Geostationary Satellite System
• For transmitter- several kilowatts• For satellite -10w
• Note: This are used in commercial ground stations. Frequency Bands used are C and Ku.
Applications of Geostationary Satellite• Television and Radio Broadcasting via Satellite• Telephony via satellite• Frequency Division Multiplexing – Frequency
Modulation• Time Division Multiple Access• Data via Satellite• Mobile Telephone System
Beam Switching
Cross Links• A crosslink is when satellites communicate directly
with each other, instead of communicating with a ground station which, in turn, communicates with other satellites.
Cross Links• The frequency range of 58 -62 GHz has
been assigned to this satellite links.• This are common in military satellite
system
Other Types Of Orbital 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.
Question:• Why is the number of satellites needed for
continuous communication greater at lower satellite altitudes?
The Iridium system has 66 satellites in six LEO orbits, each at an altitude of 750 km.
• Iridium is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale
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.
Disadvantages:A network of LEO satellites is needed,
which can be costlyLEO satellites have to compensate for
Doppler shifts cause by their relative movement.
Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.
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.
• The GPS constellation calls for 24 satellites to be distributed equally among six circular orbital planes Glonass (Russian)
AdvantageA 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.
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.
Molniya Orbit/Satellite
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
Communications satellite advantages & disadvantages
Advantages: Flexibility Mobility Speedy deployment Provides coverage over the globe
Disadvantages: Cost Propagation delay Specialised satellite terminals required
Communications satellite applications Telephony
- Fixed points, earth station, Satellite, earth station, fixed points.
Television & Radio - e.g. Direct broadcast satellite (DBS) & Fixed service satellite (FFS)
Mobile satellite technology - Special antenna called mobile satellite antenna. - No matter where or how this antenna is mounted on.
Amateur radio - Access to OSCAR satellite. - Low earth orbits.
Internet - High Speed. - Useful for far away places.
Military - Uses geostationary satellites. - Example: The Defense Satellite Communications System (DSCS).
Advantages of satellite over terrestrial communication
The coverage area of a satellite greatly exceeds that of a terrestrial system.
Transmission cost of a satellite is independent of the distance from the center of the coverage area.
Satellite to Satellite communication is very precise.
Higher Bandwidths are available for use.
Disadvantages of satellites:Launching satellites into orbit is costly.Satellite bandwidth is gradually
becoming used up.There is a larger propagation delay in
satellite communication than in terrestrial communication.
First Launching Of Satellites By Country
