Satellite Communications

SUMMER TRAINING AT

Satellite CommunicationBy:Vandana ManralECE-B, 4th YearUniv. roll no.: 120070102121

To:Dr. Sandeep SharmaHODECE DEPT. DITU

OverviewAbout ONGC.Introduction.Advantages and Disadvantages.Orbit Types.Satellite Modules.How Satellites are used.Frequency Bands.Multiple Access Techniques.Earth Station.Specification of Earth Station at ONGC.

ONGCONGC is an Indian multinational oil and gas company headquartered in Dehradun. ONGC was set up under the visionary leadership of Pandit Jawaharlal Nehru.It is a public sector undertaking of government of India, under the administrative control of the ministry of petroleum and natural gas.It produces around 72% of Indias crude oil and around 48% of its natural gas.It is involved in exploring hydrocarbons in 26 sedimentary basins of India, and owns and operates over 1100km of pipelines in the country.

INTRODUCTION:A satellite is an object which has been placed into orbit by human endeavour.Satellites are used for a large number of purposes. Common types include military (spy) and Earth observation satellites, communication satellites, navigation satellites, weather satellites, and research satellites.Satellites are usually semi-independent computer controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, altitude control and orbit control.

Advantages and disadvantagesAdvantages: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.Higher Bandwidths are available for use.Disadvantages:Launching satellites into orbit is costly.There is a larger propagation delay in satellite communication than in terrestrial communication.

ORBIT TYPES

LEO: Low Earth Orbit.MEO: Medium Earth OrbitGEO: Geostationary Earth Orbit

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.A LEO satellites 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 network of LEO satellites is needed, which can be costly.

Medium Earth Orbit (MEO):

A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earths surface. MEO satellites have a larger coverage area than LEO satellites. A MEO satellites longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network.A MEO satellites distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite.

Geostationary Earth Orbit (GEO):

These satellites are in orbit 35,863 km above the earths surface along the equator.Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.A GEO satellites distance from earth gives it a large coverage area, almost a fourth of the earths surface.

GEO (contd.)A GEO satellites 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, centred above the equator, have difficulty broadcasting signals to near Polar Regions.At the Geostationary orbit the satellite covers 42.2% of the earths surface.Theoretically 3 geostationary satellites provide 100% earth coverage.

Satellite ModulesSpacecraft bus or service moduleThe Structural Subsystems: The structural subsystem provides the mechanical base structure, shields the satellite from extreme temperature changes and micro-meteorite damage, and controls the satellites spin functions.The Telemetry Subsystems :The telemetry subsystem monitors the on-board equipment operations, transmits equipment operation data to the earth control station, and receives the earth control stations commands to perform equipment operation adjustments.The Power Subsystems: The power subsystem consists of solar panels and backup batteries that generate power when the satellite passes into the earths shadow.

Satellite Module (contd.)The Thermal Control Subsystems: The thermal control subsystem helps protect electronic equipment from extreme temperatures due to intense sunlight or the lack of sun exposure on different sides of the satellites body (e.g. Optical Solar Reflector)The Orbit Controlled Control Subsystems: The orbit controlled subsystem consists of small rocket thrusters that keep the satellite in the correct orbital position and keep antennas positioning in the right directions.

Satellite Module (contd.)Communication payload: The second major module is the communication payload, which is made up of transponders. Transponders are capable of:Receiving uplinked radio signals from earth satellite transmission stations (antennas).Amplifying received radio signalsSorting the input signals and directing the output signals through input/output signal multiplexers to the proper downlink antennas for retransmission to earth satellite receiving stations (antennas).

How Satellites are usedService TypesFixed Service Satellites (FSS)Example: Point to Point CommunicationBroadcast Service Satellites (BSS)Example: Satellite Television/RadioAlso called Direct Broadcast Service (DBS).Mobile Service Satellites (MSS)Example: Satellite Phones

Frequency BandsDifferent kinds of satellites use different frequency bands.LBand: 1 to 2 GHz, used by MSSS-Band: 2 to 4 GHz, used by MSS, NASA, deep space researchC-Band: 4 to 8 GHz, used by FSSX-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellitesKu-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)K-Band: 18 to 26.5 GHz: used by FSS and BSSKa-Band: 26.5 to 40 GHz: used by FSS

Multiple Access TechniquesMultiple accesses is defined as the technique where in more than one pair of earth stations can simultaneously use a satellite transponder. It is a technique used to explore the satellites geometric advantages and is at the core of satellite networking.If the different transmissions are differentiated only for the frequency band, we have the Frequency Division Multiple Access (FDMA). If transmissions are distinguished on the basis of time, then they are considered as Time Division Multiple Access (TDMA). Finally, if a different code is adopted to separate simultaneous transmissions, we have the Code Division Multiple Access (CDMA).

Earth Station: Earth Stations are a vital element in any satellite communication network. The function of an earth station receive from and transmit information to the satellite network in the most cost efficient and reliable manner while retaining the desired signal quality. Depending on the earth application, an earth station may have both transmit and receive capabilities or may only be capable of either transmission or reception.

Components of Earth Station The transmitter is a ground station which sends information to the satellite. The transmission is done by means of the transmitting antenna. The uplink frequency ranges from 5.9-5 to 6.4-5 GHz or may also vary from 10.95-11.26GHz.From a transmitter, the link between satellite and receiver is known as downlink and frequency used is called as downlink frequency. The downlink frequency ranges from 3.70-4,2GHz or may also vary from 11.45-11.70GHz.Satellite is a device that performs two functions at the same time. One is that it receives the information originated from ground station (transmitter) and then secondly it sends this information to another ground station (receiver).

Specification of Earth Station at ONGCSatellite: In ONGC INSAT 3B was used earlier but now GSAT 10 is used.Location: 83 degreeFrequency: Uplink: 6.050MHz Downlink:3.825 MHzTransponder no.: 15Bandwidth: 36MHzAntenna: Cassegrain antennas ( in C-band frequency range which are generally 11m in diameter and are centre feed).

ReferencesWertz, James R. and Wiley J.Larson, Space Mission Analysis and Design, Microcosm Press, El Segundo CA 1999Peterson and Davie, on reserve in Barker Library http://www.isl.stanford.edu/people/gray/fundcom.pdfCenter for Satellite and Hybrid Communication Networks: http://www.isr.umd.edu/CSHCN// Lloyd's satellite constellations http://www.sat-net.com/L.Wood/constellations/ Feldman, Phillip M., An overview and comparison of demand assignment multiple access (DAMA) concepts for satellite communications networks Santa Monica, CA: RAND, 1996. , Student Project Report, 2000.F. Ananasso, F. Delli Pricoli, "The Role of Satellites in Personal Communications Services," IEEE Journal on Selected Areas in Communications," V 13, N 2, (February 1995), pp.180-195. A. Guntsch. Analysis of the ATDMA/PRMA++ Protocol in a Mobile Satellite Environment. In Proceedings 46th IEEE Vehicular Technology Conference '96, pp. 1225-1229, Atlanta, U.S.A., April 1996.

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