by: dr. n. ioannides (feb. 2010)ct0004n - l.05 - satellite communications - pp 1/28 satellite...

By: Dr. N. Ioannides (Feb. 2010)

CT0004N - L.05 - Satellite Communications - pp 1/28

Satellite CommunicationsSatellite Communications

Saroj RegmiSaroj Regmi

Lecture 05Lecture 05

CT0004NCT0004NPrinciples of Comms SystemsPrinciples of Comms Systems



• Introduction to Wireless Communications.

• Radio Wave Frequency Bands.

• Modes of Propagation of Signals.

• Mobile Radio Systems.

Last Lecture: 04Last Lecture: 04Wireless Communications & Mobile TelephonyWireless Communications & Mobile Telephony



Today’s Lecture: 05Today’s Lecture: 05Satellite CommunicationsSatellite Communications

• Introduction to Satellites,

• Components of a Human-Made Satellite,

• Launching a Satellite,

• Orbital Altitudes,

• Satellites in Orbit,

• Satellite Systems, GSO, MEO and LEO Satellites,

• Satellite Payload.



Satellite CommunicationsSatellite Communications

• A satellite is an object that goes around, or orbits, a larger object, such as a planet.

While there are natural satellites, like the moon, hundreds of man-made satellites also orbit the Earth.

• Communications via satellite is a natural outgrowth of modern technology and of the continuing demand for greater capacity and higher quality in communications.

Satellite communications are reliable, survivable, secure, and a cost effective method of telecommunications.

• Today, there are more than 150 communications satellites in orbit, with over 100 in geosynchronous orbit.



• 1957: Sputnik, the first artificial satellite placed in orbit by the Russians signalling the beginning of a new era.

• 1958 Score, 1960 Echo and 1962 Telstar: Communications satellites launch by NASA orbiting the earth every 90 minutes providing communication to a specific region for a few minutes per orbit.

• 1963 Syncom 2: The first communications satellite in geostationary or geosynchronous orbit was launched by NASA.

Syncom 2 could provide coverage 100% of the time, 24 hours a day to a fixed area.

Could view approximately 42% of the earth.

A system of three such satellites, with the ability to relay messages from one to the other could interconnect virtually all of the earth except the polar regions.

Historical DevelopmentsHistorical Developments



Components of a Human-Made Satellite SystemComponents of a Human-Made Satellite System

UPLINK DOWNLINK



Launching a SatelliteLaunching a Satellite

• A satellite must be launched at 27,500 kmph if its forward momentum is to balance the earth’s gravity thus causing it to circle the earth.

If launched at more than 38,000 kmph it will leave the gravitational pull of the Earth.

If launched at less that 27,000 kmph it will fall back to earth.



• Communications antennae, radio receivers and transmitters enable the satellite to communicate with one or more ground stations, called command centres.

• Messages sent to the satellite from a ground station are uplinked; messages transmitted from the satellite to Earth are downlinked.

Satellite Communication CapabilitiesSatellite Communication Capabilities



1 GHz 10 GHz 20 GHz 30 GHz

L S C Ku Ka

FSS Uplinks

FSS Downlinks

Mobiles

RainfallFading

Negligible

RainfallFading

Significant

RainfallFadingSevere

• The most commonly used satellite frequency carrier bands are:

C band (4-8 GHz),

Ku band (10-18 GHz),

Ka band (18-31 GHz).

Frequency Spectrum for SatelliteFrequency Spectrum for Satellite



• Many satellites are powered by rechargeable batteries. Solar panels recharge many satellite batteries.

• Other satellites have fuel cells that convert chemical energy to electrical energy, while a few rely on nuclear energy.

Satellite Power SourceSatellite Power Source



• Small thrusters provide attitude, altitude, and propulsion control to modify and stabilize the satellite's position in space.

• Specialized systems such as sensors accomplish the tasks assigned to the satellite.

Satellite Control SystemSatellite Control System



• Consist of Space Segments and Ground Segments.

The Space Segment is composed of satellites, classified as:

Geostationary Orbit (GSO) satellites,

Non-geostationary Orbit (NGSO) satellites, including Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) satellite, according to the orbit altitude above the Earth's surface.

Satellite SystemSatellite System



The Ground Segment consists of:

Gateway Stations (GSs):

Acting as network interfaces between various external networks and the satellite network.

Also perform protocol, address, and format conversions.

A Network Control Center (NCC) and Operation Control Centers (OCCs):

Handle overall network resource management, satellite operation, and orbiting control.

Satellite System (…2)Satellite System (…2)



Satellites in Orbit (Space Segment)Satellites in Orbit (Space Segment)



LEO

MEO

GEO

First Van Allen Belt Second Van Allen Belt

Orbital AltitudesOrbital Altitudes



Circular Orbits Highly Elliptical Orbits (HEO)

PolarInclined

Equatorial

PolarInclined

Equatorial

Satellite Orbits for TelecommunicationsSatellite Orbits for Telecommunications

• Identify how satellites orbit the earth.



Global

Hemispheric

Regional or Zone

Spot

Satellite FootprintsSatellite Footprints

• The footprint identifies the ground coverage of a satellites transmission.



• The GSO satellite is 35,786 km above the equator, and its revolution around the Earth is synchronized with the Earth's rotation.

• It appears fixed to an observer on the Earth's surface, and may serve as a repeater in the sky.

• Its high altitude allows each GSO satellite to cover approximately one third of the Earth's surface, excluding the high latitude areas. The area of coverage of a satellite is called its footprint. Three GSO satellites are sufficient for global coverage.

• The majority of satellites in operation nowadays are placed in GSO.

Geostationary Orbit (GSO) SatellitesGeostationary Orbit (GSO) Satellites



• The cost of launching GSO satellites is high:

Their high altitude and the inherent signal degradation calls for large antennas and large transmission power for both the GSO satellite and ground terminals.

• The most significant problem is the large propagation delay for GSO satellite links (typical round-trip delay is 250–280 ms, which is undesirable for real-time traffic).

Geostationary Orbit (GSO) Satellites (…2)Geostationary Orbit (GSO) Satellites (…2)



Non-Geostationary Orbit (NGSO) SatellitesNon-Geostationary Orbit (NGSO) Satellites

• NGSO Satellites are split into two different groups according to the orbit altitude above the Earth's surface:

Medium Earth Orbit (MEO) satellites.

Low Earth Orbit (LEO) satellites.



• MEOs distance from the Earth's surface is from 3000 km up to the GSO orbit with a typical round-trip propagation delay of110–130 ms.

• LEOs are located 200–3000 km above the Earth's surface.

For a LEO satellite the round-trip delay is 20–25 ms, which is comparable to that of a terrestrial link.

• LEO and MEO satellites are closer to the Earth's surface thus requiring much smaller antennae sizes and transmission power levels.

MEO & LEO SatellitesMEO & LEO Satellites



• Their footprints are much smaller than those of the GSO thus a constellation of a large number of satellites is necessary for global coverage.

The lower the orbit altitude, the greater the number of satellites required.

• Also, since satellites travel at high speeds relative to the Earth's surface, a user may need to be handed off from satellite to satellite as they pass rapidly overhead.

Therefore, steerable antennas are crucial to maintain continuous service.

MEO & LEO Satellites (…2)MEO & LEO Satellites (…2)



• The satellite payload is responsible for the satellite communication functions.

Traditional satellites, especially GSOs, act as repeaters between two communication points on the ground, i.e. there is no onboard processing (OBP) making them simple and easy to implement.

Some satellite systems allow OBP, including demodulation and remodulation, decoding and recoding, transponder and beam switching, and routing to provide more efficient channel utilization.

OBP can support high-capacity intersatellite links (ISLs) connecting two satellites within line of sight. By using a sophisticated constellation with ISLs, connectivity in space without any terrestrial resource is possible.

Satellite PayloadSatellite Payload



• Too expensive!

• The satellite payload must be simple and robust since once the satellite is launched, it is very expensive and almost impossible to upgrade or repair due to the fact that the space environment, with radiation, rain, and space debris, is very harsh for satellites.

Satellite MaintainabilitySatellite Maintainability



Comparing GSO, MEO and LEO SatellitesComparing GSO, MEO and LEO Satellites

ParameterParameter GSOGSO MEOMEO LEOLEO

Distance from Earth (km) 36,0003,000 – 36,000

200 – 3,000

Antenna Size Large Medium Small

Transmitter Power Large Medium Small

Round Trip Propagation Delay (ms)

250 – 280 110 – 130 20 – 25



SummarySummary

• Introduction to Satellites,

• Components of a Human-Made Satellite,

• Launching a Satellite,

• Orbital Altitudes,

• Satellites in Orbit,

• Satellite Systems, GSO, MEO and LEO Satellites,

• Satellite Payload.



Tutorial QuestionsTutorial Questions

1. Information is transmitted from the UK to the USA via a geostationary satellite. How long would it take for the signal to be received if the satellite is positioned above both ground stations at a distance of:

a. 33,000 km, b. 38,000 km, c. 42,000 km, d. 55,000 km.

2. How long would it take a wave to propagate up to and down from a communications satellite if the satellite is positioned above the earth’s ground stations at a distance of:

a. 30,000 km, b. 35,000 km, c. 40,000 km, d. 50,000 km.

3. What is the wavelength of a satellite microwave transmission if the uplink frequency is:

a. 7 GHz b. 12 GHz, c. 14 GHz, d. 18 GHz, e. 30 GHz.

4. What is the wavelength of a satellite microwave transmission if the downlink frequency is:

a. 9 GHz b. 15 GHz, c. 19 GHz, d. 25 GHz, e. 28 GHz.

5. What is the velocity of a satellite transmission if the satellite is:

a. GSO, b. MEO, c. LEO

by: dr. n. ioannides (feb. 2010)ct0004n - l.05 - satellite communications - pp 1/28 satellite...

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