Satellite Communications

Satellite

A spacecraft placed in orbit around the earth which carries on board microwave receiving and transmitting equipment; repeater, capable of relaying signals from one point on earth to other points.

Significant Milestones in

Satellite Technology1940’s – 1950’s

- moon was the first passive satellite used

1945- Arthur C. Clarke proposed message transmissions via satellite

1954- the U.S. Navy conducted the Earth Moon Earth experiment

1957- Russia launched the SPUTNIK 1 the first active satellite

Significant Milestones in

Satellite Technology

1958- the U.S. launched the “SCORE”which rebroadcasted Pres. Eisenhower’s Christmas message.

Significant Milestones in

Satellite Technology

1960NASA, Bell Labs and Jet Propulsion Lab launched the “Echo”;accomplished the first transatlantic transmission

Significant Milestones in

Satellite Technology

1962- AT & T launched the TELSTAR 1 the first duplex satellite transmission

1963- AT & T launched TELSTAR 2 for telephone, TV, fax and data transmission

Significant Milestones in

Satellite Technology

1965- COMSAT launched the EARLY BIRD which transmitted messages to five earth stations with 66 telephone circuits using FDM techniques

Significant Milestones in

Satellite Technology

1966- The National Development Company and POTC established the DOMSAT ushering satellite technology to the Philippines

Significant Milestones in

Satellite Technology

1967- PHILCOMSAT; Philippines, installed the first earth station at the foot of Sierra Madre Mountains, Mount Pinugay, Baras, Rizal

Significant Milestones in

Satellite Technology

1970- China launched its first satellite

1972- USA launched LANDSAT 1 for remote sensing missions

Significant Milestones in

Satellite Technology

1974- Western Union launched the WESTAR for TV and data transmission

- Russia launched RESURS-0 for remote sensing mission

Significant Milestones in

Satellite Technology

1976- Indonesia launched its first satellite; PALAPA A-1

1986- France launched the

SPOT-1 for remote sensing service

Significant Milestones in

Satellite Technology

1988- India launched its first satellite; INSAT-C

- Russia launched the OKEAN for remote sensing

Significant Milestones in

Satellite Technology

1993- Thailand launched its first satellite; THAICOM 1.

- They also launched the Microsatellite Program through United NASA and GE

Significant Milestones in

Satellite Technology

1997- Mabuhay Philippines Satellite Corporation launched AGUILA II; the first Philippine Satellite

Significant Milestones in

Satellite Technology

Syncom I- was the first attempt to place geosynchronous satellite into orbit but was lost during orbit injection

Syncom III- used to broadcast the 1964 Olympic Games from Tokyo

Significant Milestones in

Satellite Technology

Satellite System A radio repeater in

the sky consisting of:

- a transponder

- a ground based station to control its operations

- a user network of earth stations that provide the facilities for transmission and reception of communication traffic through the satellite systems.

Satellite Link A communications path formed between

two or more ground stations transmitting and receiving radio communications traffic via satellite

a. Uplink- that portion of the satellite

communications link involving thetransmission of traffic from the groundstation up to the satellite

b. Downlink- that portion of a satellite

communications link involving thetransmission of traffic from the satellite toearth terminal

Satellite Link

Applications of Satellitea. Remote sensing

b. Weather Forecasting / Hazard Detection

Applications of Satellite

c. Agricultural Production Forecasting

Applications of Satellite

d. Intelligence / Surveillance Operations

Applications of Satellite

e. Navigation

Applications of Satellite

f. Search and Rescue

Applications of Satellite

g. Telecommunications / Telebroadcasting

Applications of Satellite

h. Interactive Education

Applications of Satellite

i. Space Research / Scientific Explorations

Applications of Satellite

Advantages of Satellite

Communications1. Wideband Capability

2. Wide Area of Coverage

3. Superior Reliability

4. Direct Services to Subscriber’s Premises

5. Point to Point and Point to Multipoint

Advantages of Satellite

Communications6. Readily compatible with new technology

7. Uniform access possibilities for all users

8. Allows flexibility / mobility of earth stations within the footprint

9. Reduced need for ground facilities (R/W, cable installation etc.

Disadvantages of Satellite

Communications1. High Initial Investment

2. Short Life Span; 7 – 10 years

3. Expensive Maintenance

4. Very Dependent on Reliability of Launch Vehicle

5. Require High Investments on Earth Stations

6. Subject to Interception and Jamming by Enemy Forces

7. Signal Propagation Delay / Delayed Response Time

8. Risky Launching / Failures in Attaining Proper Orbit

Types of Satellite Transmissions1. Payload – actual user information

conveyed through the system2. Bus Transmission – includes control

mechanisms that support the payloadoperation

Categories of SatellitesAccording to Function

1.Passive Satellite – a communicationssatellite not equipped with electronicdevices to retransmit communications.Such satellite acts only as a reflector ofradio waves.

2.Active Satellite – a satellite equipped withelectronic devices to receive, amplify,convert, and retransmit signals

Classifications of Satellite

Orbital PatternsSatellite Orbital Pattern

Orientation of Satellite Orbit with respectto the earth.

Angle of Inclinationangle formed by the Equatorial andorbital planes.

Classifications of Satellite

Orbital Patterns1. Equatorial Orbit – rotation about the

earth’s equator, usually for geosynchronouscommunications satellites. θi = 0˚;Coverage area: 42.5% (surface area)

2. Polar Orbit – rotation over the North and South Poles, used for non-communications mission such as weather and surveillance observation. θi = 90˚; Coverage area: 100% (surface area)

Classifications of Satellite

Orbital Patterns

3. Inclined Orbit – rotation creating as angle between the equatorial polar plane. They are easily launched and have higher payload capacity but are difficult to track. 0˚<θi<90˚; Coverage area: >42.5% (surface area)

Classifications of Satellite

Orbital Patterns

Based on Orbit

1.Non-Synchronous or Orbital – rotate around the earth in low altitude elliptical or circular pattern. (ωs ‡ ωe)

Categories of Satellites

a. Prograde Orbit – when a satellite orbitsin the same direction as that of earth’srotation (counterclockwise; ωs > ωe)

b. Retrograde Orbit – when a satellite orbitsin the opposite as the earth’srotation but as an angular velocity lessthan of the earth. (ωs < ωe; clockwise)

Categories of Satellites

2. Geostationary or Geosynchronous –orbit in a circular pattern with anangular velocity equivalent to that ofearth with an orbital time of about 24hours. (23 hours, 56 mins, 4.091 sec)

Categories of Satellites

2. Geostationary or Geosynchronousωs = ωe; orbits eastward, ccw;orbital pattern: equatorial, θi= 0˚;coverage area: 42.5% earth surface

Categories of Satellites

According to Elevation:

a. Low Earth Orbit (LEO)h: 700 – 1100 kmf: 1 – 2.5 GHzDelay: 10 msec

b. Medium Earth Orbit (MEO)h: 10,000 – 12,000 kmf: 1.2 – 1.66 GHzDelay: 60 msec

Categories of Satellites

According to Elevation:

c. Geostationary (GEO)h: 36,000 km (35,786 km

or 22,236 mi.)f: 2 - 18 GHzDelay: 250 msec

Categories of Satellites

A satellite remains in orbit because of a balance between two factors:

a. the satellite’s velocity; speed at which it would travel in a straight line

b. the gravitational force between the satellite and the earth

Satellite Attitude – refers to the satellite orientation with respect to earth.

Orbital TerminologyOrbit – the path of a celestial body or satellite

in unpowered flight.

Ascending Node – pointwhere the orbit crossesthe equatorial plane goingfrom South to North

Descending Node – pointwhere the orbit crossesthe equatorial plane goingfrom North to South

Apogee – the highest point of the orbit or itis the point in an earth satellite orbit,which is the farthest away from theearth; lowest velocity

Perigee – the point in an earth satelliteorbit, which is the closest in the earth;fastest velocity

Orbital Terminology

Eccentricity – a number between zero and one which describe the shape of an ellipse. A circle has an eccentricity of zero.

e=

where: e = eccentricityb = half of major axisa = half of minor axis

Orbital Terminology

Elliptical Orbits

Major Axis – the longer axis of the ellipse, the line adjoining the perigee and apogee thru the center of the earth, called the line of apsides

Minor Axis – the shorter axis of the ellipse, the line perpendicular to the major axis and halfway between the perigee and apogee

Orbital Terminology

Mean Distance – the average distance from the center of the earth

Orbital Terminology

NEWTON’S LAWS A body at rest tends to stay at rest and a body in

motion tends to stay in motion in the same direction and at the same speed unless acted upon by an outside force.

The change in the momentum of a body is proportional to the force acting on the body and is in the direction of the applied force.

For every action, there is always an equal and opposite reaction.

Kepler's LawKepler’s First Law – the orbit of any planet

is an ellipse with the sun at one focus

Kepler’s Second Law; Law of Areas – everyplanet moves so that its radius sectorsweeps over equal areas in equal intervalsof time.

Kepler's Law

Kepler’s Third Law; Law of Periods – thesquare of the periods of revolution of anytwo planets is to each other as the cubesof their respective mean distances fromthe sun.

(t/to)2 = (a/ao)3

Kepler's Law

Launching the Satellite Space shuttles carry some

satellites into space

Many satellites requireminor adjustments of theirorbit before they begin toperform their function.Built-in rockets calledthrusters, some as smallas a propelling pencil,make these adjustments.

Once a satellite is placedinto a stable orbit, it canremain there for a longtime without furtheradjustment.

Launching the SatelliteRocket Motors- Communications satellites are lifted intoorbit by space vehicles, which arelaunched vertically for structural andaerodynamic reasons. Rocket motorsprovide the power for these vehicles.Rocket motors do not depend on theatmosphere for oxidizing or burning fuel.They may classify as solid or liquidpropellant motors

Solid Propellant Rockets

- Works on the principlesimilar to small gunpowderrockets used for fireworksdisplays. It contains grainfuel, a combination ofcombustible material andoxidizer. Fuel is burned in arestricted combustion spacewith nozzles to direct andaccelerate the outgoing gasflow. Directional control isachieved by placingrestructures or deflectors tochange the direction ofthrust.

Launching the Satellite

They are usually used for upper stages of multiple stage vehicles such as the Polaris.

Liquid Propellant Rockets

- Carries two tanksoutside the combustionchamber, one for fueland the other for theoxidizer.

Launching the Satellite

Components of Satellite

Systems1. Earth Station and

Uplink Components

- Earth Station is that portion of satellite link which receives, processes and transmits communications between the earth and a satellite

Components of Satellite

Systems

Modulator

Base Band in FDM or PCM/TDM

BPF Mixer BPF

Microwave Generator

HPA

IF RF

UpConverter

Components of Earth Station

a. IF Modulator – converts input baseband signalto FM, QAM IF

b. IF-RF Microwave Up Converter – converts IF toRF carrier frequencies

c. High Power Amplifier (HPA) – providesadequate input sensitivity and output power topropagate signal transponder using a Klystron.

d. Output Band Pass Filter – limits bandwidth

Components of Satellite

Systems

2. Transponder Components

- microwave repeaters carried by communication satellites. Transponders have the following function: receiving, amplification, frequency conversion and retransmission.

Components of Satellite

Systems

Components of Satellite

Systems

BPF LNA Mixer BPF

MicrowaveShift Oscillator

2 GHz

LNA

FrequencyTranslator

Components of a Transponder

a. Input Band Limiting Device – limits total noise appliedto LNA input.

b. Input Low Noise Amplifier (LNA) – amplifies signals,usually a tunnel diode

c. Frequency Translator – shift oscillator, which convertshighband uplink frequency to lowband downlinkfrequency

d. Low-Level Power Amplifier – amplifies (TWT) RF signalfor transmission through the downlink to stationreceiver.

e. Output Bandpass Filter – limits output bandwidth

Components of Satellite

Systems

BPF LNA Mixer BPF

MicrowaveGenerator

Demodulator

DownConverter

Components of Satellite

Systems

3. Downlink Components

a. Bandpass Filter (BPF) – limits input noisepower to LNA

b. Low Noise Amplifier (LNA) – highlysensitive, low noise device usually a tunneldiode or parametric amplifier

c. RF-to-IF Down Converter – mixer /bandpass filter combination which convertsthe received RF signal to an IF frequency

Components of Satellite

Systems

- C band is the most commonly used band.

- Ku band is immune to terrestrial jamming but sensitive to atmospheric conditions such as transient noise

- The Ka-band has the strongest signal and higher data rate capability. It is highly directional and has a spot coverage

- X band – military

- L band

Spatial Separation

- Geosynchronous satellites share a limited space and frequency spectrum within a given area of geostationary orbit. Each is assigned a longitude spaced between 3 to 6 degrees in the arc approximately 22,300 miles above the equator

Frequency Allocations

Uplink and downlink frequency bands

- The uplink frequency is always higher thanthe downlink by 2225 MHz because higherfrequency offers greater spreading or FreeSpace Loss than lower frequencies.

Frequency Reuse Methods

- Frequency reuse is a technique employed inorder to conserve electromagneticfrequencies resources, they can beimplemented by the following methods:

Frequency Allocations

Multiple Coverage Areas- the use of the same frequency beamed towards different geographic areas on earth multiple spot beam antennas.

Dual polarization – sending different information signals using vertical or horizontal electromagnetic polarization. This is similar to Polarization Diversity in order to improve system stability

Frequency Allocations

- a geographic representationof a satellite’s radiationpattern, with contour linesrepresenting limits of equalreceive power density.

Satellite Footprint

1. Spot Coverage – beams concentrate the radiated power in a very small geographic area

2. Zonal Coverage – covers less than one-third of earth’s surface

3. Earth Coverage – covers approximately one-third of earth's surface (42.5 %) using a 17-degree beamwidth

Satellite Types as to Territorial

Coverage

1. Global – covers the whole world

2. Regional – covers selected regions suchas Pacific, Atlantic, etc.

3. National / Domestic – covers domestic ornational territory

Satellite TrackingLook Angle – the angle measure from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

Angle of Elevation – angle formed between the plane of a wave from an earth station antenna and the horizon, or the angle subtended at the earth station antenna between the satellite and the earth’s horizon

Azimuth – the horizontal pointing angle of an antenna measured in degrees in a clockwise direction from true North

Geostationary Satellite

Geostationary Satellite

Geostationary Satellite

CENTRIFUGAL FORCE

The rate of change of radial speed of a satellite with respect to its parent is proportional to the difference between the gravitational force and the centripetal force.

F = mv2/r

ORBIT TRANSVERSE VELOCITY

F = mv2/r

F = Gm1m2/r2

where v – transverse velocity

G – Universal gravitation constant(6.67 x 10-11 N-m2/kg2)

m1 – mass of earth

(5.98 x 1024 kg)

m2 – mass of the satellite

Satellite Velocity

4 x 1011

6400 + hkm

; m/sVs =

Siderial Period

T = 2π (6400 + hkm)/Vs

Ex. Determine the satellite’s velocity andsidereal period for an elevation equal to 1000km.

Length of the Path to a Geostationary Satellite from an Earth Station

d (r h)2 (rcos)2 rsin

Formula :

d = distance to the satellite in km

r = radius of the earth in km (6400 km)

h = height of satellite above equator ( 36 x 103 km)

= angle of elevation to satellite at antenna site

Ex. Calculate the length of the path to ageostationary satellite from an earth stationwhere the angle of elevation is 30.

Satellite Control

– involves actions necessary to position,track, monitor, and command the satelliteduring the course of its operational life.

It involves supervising and monitoring theestablishment, configuration and control ofeach required of link through satellitetransponder in accordance with desiredparameters. The two main aspects ofsatellite control are orientation and positioncontrol.

Satellite System Design

Parameters

Orientation – is required in a communicationssatellite for initial orientation and for all subsequentadjustment so the antenna will constantly view theearth and the solar cells will view the sun

Position Control – is required for initial satelliteemplacement and to compensate for forces whichtends to divert the satellite from its orbit. Suchforces include lunar and solar gravitational action.

Orientation Control - attitude control of a vehicleabout any or all of its axes (roll, pitch and yaw)projects for axial, radial axis and corrections.

Satellite System Design

Parameters

Satellite System Design

Parameters

Satellite Accessing TechniqueAccess Techniques- refers to the way a communications system uses a satellite transponder.

Multiple Access- in communications satellites, those satellites so equipped that they may function as a portion of a communications link between more than one pair of ground stations simultaneously.

Multiple Access Back-Off – refers to the satellite outputpower that is lost due to the necessity for backing offon earth station radiated power to avoid generatingexcessively high intermodulation products in thesatellite. It ranges from 1 to 2 dB

Satellite Multiple Access

Technique1. Frequency Division Multiple Access (FDMA)

– a satellite transponder is divided into smallerfrequencies hand segments where eachsegment is assigned to use for his uplink ordownlink frequency.

a. Pre-assigned – a given number ofavailable voice hand channels from eachearth station are assigned dedicateddestinations. This method is onlyeconomically feasible in situations wheresources / destination locations have verylow traffic density during the busy hour.

b. Demand Assigned (DAMA) – voice bandchannels are assigned on “as-needed”basis. It provides more versatility andmore efficient use of the frequencyspectrum.

Satellite Multiple Access

Technique

Three Methods of Controlling DAMA Systems

a. Polling – a master station “polls” otherstations sequentially. When a positivereply is received, a channel is assignedaccordingly. As the polling intervalbecomes longer and the system tends tobecome unwieldy because of excessivepost dial delay as a call attempt waits forthe pulling interval to run its course

Satellite Multiple Access

Technique

b. Random Access Central Control – acentral computer located at the masterstation coordinates the status of thechannels. Call requests are passed to thecentral computer and a channel isassigned if available. Once the call iscompleted and the subscriber goes onthe hook, the speech path is taken downand the channel used is returned to thedemand access pull of ideal channels.

Satellite Multiple Access

Technique

c. Random Access Distributed Control –uses a processor controller at each earthstation accessing the system. All earthstations in the network monitor thestatus information by means of thedigital order wire circuit. When an idlechannel is seized, all users are informedto the fact and the circuit is removedfrom the poll

Satellite Multiple Access

Technique

2. Time Division Multiple Access (TDMA)

- works in the time domain and is applicableto digital systems because informationstorage is required. In this method, stationsare restricted to precise time slot. It has norestrictions on frequency or bandwidth.Presently, it is the most dominant methodused of providing the most efficient methodof transmitting digitally modulated carries(PSK).

Satellite Multiple Access

Technique

- in TDMA, only one user appears on thetransponder at any given time. Each useris assigned a time slot to the satellite;transponder power and bandwidth areshared by several earth stations.

Satellite Multiple Access

Technique

Store and Forward – a technique in which amessage received from earth termed isheld in storage until the satellite is in viewof a second earth terminal, at which timethe message is transmitted.

Satellite Multiple Access

Technique

3. Code Division Multiple Access (CDMA) or Spread Spectrum Multiple Access (SSMA)

- each station may transmit whenever itwishes and can use any or all of thebandwidth allocated. Transmissions areseparated through envelope, encryption,and decryption techniques. It usesunique binary words called CHIP CODESto receive a particular earth station’stransmission.

Satellite Multiple Access

Technique

- a correlator checks or compares orthogonal codes with original chip codes to allow access to users

- the transmitted signal is spread over part or all of the available transponder hand width in a time-frequency relationship by a code transformation. Typically, the modulated carrier is ten to hundreds of times greater than the information bandwidth.

Satellite Multiple Access

Technique

Frequency Hopping – is a form of CDMAwhere a digital code is used to continuallychange the frequency of the carrier. Thetotal available bandwidth is partitioned tosmaller frequency bands and totaltransmission time is subdivided intosmaller time slots. Each station uses theentire RF spectrum but never occupiesmore than a small portion of thatspectrum at any one time.

Satellite Multiple Access

Technique

Satellite OrganizationsINMARSAT (International Maritime Satellite

Organization)

- recommended by the panel of expertscommissioned by the IntergovernmentalMaritime Consultative Organization(IMCO). They consider the introduction ofsatellite communications to the maritimesphere with the aim of improvingcommunications with ships, particularlyfor safety and distress purposes.

Satellite OrganizationsInternational Mobile Satellite

Organization (IMSO) (NEW)

- is the intergovernmental organizationthat oversees certain public satellitesafety and security communicationservices provided via the Inmarsatsatellites.

Satellite OrganizationsSome of these services concern:

Global Maritime Distress Safety System (GMDSS) established by the International Maritime Organization (IMO)

Search and rescue co-ordinating communications

Maritime safety information (MSI) broadcasts

Aeronautical mobile satellite (route) service, or AMS(R)S, through compliance with the Standards and Recommended Practices (SARPs) established by the International Civil Aviation Organization (ICAO)

General communications

- born in July 1979, very much along theINTELSAT lines, with COMSAT as thelargest shareholder.

Satellite Organizations

INTELSAT (International Telecommunications Satellite

Consortium)

- Founded in 1964 at Washington D.C. by COMSAT (Communication Satellite Corporation) of the United States, Overseas Telecommunications Commission of the Australia and nine other world communications agencies

Satellite Organizations

Satellite Organizations ITSO (International

Telecommunications Satellite Organization) – (NEW)

- is an intergovernmental organization withthe mission to ensure that Intelsat, Ltd.provides public telecommunicationsservices, including voice, data and video,on a global and non-discriminatory basis.Headquartered in Washington D.C., ITSOcurrently has 149 member countries.

MARISAT

- a ship-to-shore and shore-to-shipcommunications via a dedicatedgeostationary satellite system, providinghigh-quality telephony, data andtelex/telegraphy circuits

Satellite Organizations

IRIDIUM SATELLITE

The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface.

IRIDIUM SATELLITE

It was originally conceived by Bary Bertiger, Dr. Ray Leopold and Ken Peterson in late 1987

developed by Motorola on a fixed-price contract from July 29, 1993 to November 1, 1998 when the system became operational and commercially available.

IRIDIUM SATELLITE

consists of 66 active satellites in orbit, and additional spare satellites to serve in case of failure.

Satellites are in Low Earth Orbit

at a height of approximately 485 mi (781 km)

and inclination of 86.4°.

IRIDIUM SATELLITE

Orbital velocity of the satellites is approximately 17,000 mi/hr (27,000 km/h).

Satellites communicate with neighboring satellites via Ka band inter-satellite links.

IRIDIUM SATELLITE

Original concept - 77 satellites

Iridium

atomic number 77

the atomic number 66 is dysprosium

TELEDESIC

Teledesic was a company founded in the 1990s to build a commercial broadband satellite constellation for Internet services.

TELEDESIC

Using low-earth orbiting satellites small antennas could be used to provide uplinks of as much as 100 Mbit/second and downlinks of up to 720 Mbit/second.

TELEDESIC

The original 1995 proposal - 840 active satellites with in-orbit spares at an altitude of 700 km.

In 1997, the scheme was scaled back to 288 active satellites at 1400 km.

GLOBALSTAR

is a Low Earth Orbit (LEO) satellite constellation for satellite phone and low-speed data communications.

was launched in 1991 as a joint venture of Loral Corporation and Qualcomm.

GLOBALSTAR

The first satellites were launched in February 1998, but system deployment was delayed due to a launch failure in September 1998.

In February 2000, it launched the last of 52 satellites — 48 satellites and four in-orbit spares.

Another eight unlaunched satellites were maintained as ground spares.

ORBCOMM

ORBCOMM is a company that offers M2M global asset monitoring and messaging services from its constellation of 29 LEO communications satellites orbiting at 775 km.

Machine to machine (M2M) refers to technologies that allow both wireless and wired systems to communicate with other devices of the same type.

ORBCOMM

A total of 35 satellites were launched by ORBCOMM Global in the mid to late 1990s.

Of the original 35, a total of 29 remain operational today.

VSAT

A very small aperture terminal(VSAT), is a two-way satellite ground station or a stabilized maritime Vsat antenna with a dish antenna that is smaller than 3 meters.

The majority of VSAT antennas range from 75 cm to 1.2 m.

VSAT

VSAT

Data rates: 4 kbit/s up to 4 Mbit/s

(to a max downlink of up to 16 Mbit/s with some upgraded modules and enhancements)

VSAT

Applications: to transmit narrowband data

(point of sale transactions such as credit card, polling or RFID data; or SCADA),

To transmit broadband data

for the provision of satellite Internet access to remote locations, VoIP or video).

VSATs are also used for transportable, on-the-move (utilizing phased array antennas) or mobile maritime communications.

SCADA

SCADA (supervisory control and data acquisition) is a system operating with coded signals over communication channels so as to provide control of remote equipment (using typically one communication channel per remote station).

VSAT TOPOLOGIES A star topology, using a central uplink

site, such as a network operations center (NOC), to transport data back and forth to each VSAT terminal via satellite,

A mesh topology, where each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site,

A combination of both star and mesh topologies.

PARTS OF A VSAT

Antenna

Block upconverter (BUC)

Low-noise block downconverter (LNB)

Orthomode transducer (OMT)

Interfacility link cable (IFL)

Indoor unit (IDU)

PARTS OF A VSAT

A block upconverter (BUC) is used in the transmission (uplink) of satellite signals.

It converts a band of frequencies from a lower frequency to a higher frequency.

PARTS OF A VSAT

A low-noise block downconverter (or LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish.

Also called a low-noise block, LNC (for low-noise converter), or even LND (for low-noise downconverter), the device is sometimes wrongly called an LNA (low-noise amplifier).

PARTS OF A VSAT

The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and IF amplifier.

It receives the microwave signal from the satellite collected by the dish, amplifies it, and downconverts the block of frequencies to a lower block of intermediate frequencies (IF).

PARTS OF A VSAT

An orthomode transducer (OMT) is a waveguide component.

It is commonly referred to as a polarisation duplexer.

Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths.

PARTS OF A VSAT

All the outdoor parts on the dish are collectively called the ODU (Outdoor Unit), i.e. OMT to split signal between BUC and LNB.

The IDU is effectively a Modem, usually with ethernet port and 2 x F-connectors for the coax to BUC (Transmit) and from LNB (Receive).

GPS

> Global Positioning System

> a space-based satellite

navigation system that

provides location and time

information in all weather

conditions, anywhere on or

near the Earth where there

is an unobstructed line of

sight to four or more GPS

satellites

GPS

> NAVSTAR GPS

> the Navigation Satellite

Timing and Ranging Global

Positioning System

(NAVSTAR GPS) is a

worldwide navigation radio

aid which uses satellite

signals to provide accurate

navigation information

GPS

> GLONASS GPS

> Globalnaya

navigatsionnaya

sputnikovaya sistema or

Global Navigation Satellite

System, is a space-based

satellite navigation system

operated by the Russian

Aerospace Defence Forces.

GPS

> provides specially coded satellite signals that can be

processed in a GPS receiver, enabling the receiver to

compute position, velocity and time

> four GPS satellite signals are used to compute positions

in three dimensions and the time offset in the receiver

clock

> navigation in three dimensions is the primary function of

GPS

GPS

1. Space segment

> composed of a constellation of 24 satellites arranged in

six separate orbital planes of four satellites each on a

circular orbit and have the following characteristics:

a. 550 inclination to the equator

b. an altitude of approximately 20,200 km with an orbital

period of 12 sidereal hours

c. 11 hrs, 58 min orbital period

GPS

Satellites give:

a. satellite position

b. constellation data

c. atmospheric corrections

GPS

2. Control segment

Monitor stations locations

> Kwajalein

> Hawaii

> Ascension Island

> Diego Garcia

> Colorado Springs (master control station)

GPS

3. User segment

Main advantages

-anytime, anywhere, and in any weather

-highly accurate measurements

-GPS has almost endless applications

LATEST UPDATE

As of 2013, there are 1,071 operational satellites in orbit around the Earth.

2,600 satellites are no longer working

50 percent (536) were launched by US

Half of that 1071 (536) are in Low-Earth Orbit

About a twentieth (54) are in Medium-Earth Orbit, for GPS

The rest (431) are in geostationary orbit.