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Introduction Satellite Orbits Overview of a satellite system Link Budget Digital Communication Technologies Coverage Satellite Systems & Applications (Examples)

Satellite CommunicationsSatellite Communications

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IntroductionIntroduction Satellite communications systems exist because Satellite communications systems exist because

earth is a sphere.earth is a sphere.– Radio waves travel in straight lines at the Radio waves travel in straight lines at the

microwave frequencies used for wideband microwave frequencies used for wideband communicationscommunications

– Repeater is needed to Repeater is needed to convey signals very long convey signals very long distancesdistances

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Communications satellites is an artificial station in Communications satellites is an artificial station in space which operates as a radio relayspace which operates as a radio relay

Introduction (cont.)Introduction (cont.)

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Introduction (cont.)Introduction (cont.) Satellites are important in: voice communications, video & radio

transmission, navigation (GPS), remote sensing (maps, weather satellites) etc.

A majority of communication satellites are in geostationary earth orbit an altitude of 35 786 km– Satellite in “fixed place”– Typical path length from earth station to a GEO satellite

is 38 500 km

Satellite systems operate in the microwave and millimeter wave frequency bands, using frequencies between 1 and 50 GHz – Above 10 GHz rain causes significant attenuation of the

signal

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History of Satellite Communications – History of Satellite Communications – Some MilestonesSome Milestones

Satellite communications began in October 1957 with the launch by the USSR a small satellite called Sputnik 1 (4.10.1957)– Beacon transmitter, no communications capability

3.11.1957 Sputnik 2

12.4.1961 Vostok 1

First true communication satellites (Telstar I & II) were launched in July 1962 & May 1963

10/1964 Syncom 2: First GEO satellite, 7.4/1.8 GHz (one TV-channel or several 2-way telephone connections

1987 TVSAT: First DBS-satellite (Direct Broadcast Satellite, Television-broadcasts directly to home)

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Satellite OrbitsSatellite Orbits

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Geostationary orbitsGeostationary orbits

A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. A geostationary satellite revolves around the earth at a constant speed once per day over the equator.

As a result, an antenna can point in a fixed direction and maintain a link with the satellite. The satellite orbits in the direction of the Earth's rotation, at an altitude of approximately 35,786 km (22,240 miles) above ground. This altitude is significant because it produces an orbital period equal to the Earth's period of rotation

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The geostationary orbit is useful for communications applications because ground based antennas, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite’s motion. Especially for applications that require a large number of ground antennas (such as direct TV distribution).

Geostationary OrbitsGeostationary Orbits

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Satellite Orbits [5]Satellite Orbits [5] Geostationary Orbit (GEO) satellites, i.e. satellites that are stationary

with respect to a fixed point on the earth

– Good coverage: Theoretically, only three GEO satellites are sufficient to serve all the earth.

– The simplest space configuration and simple space control system– No need for tracking system at the earth stations– No variation of propagation delay and elevation angle– Negligible Doppler effects

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Low-Earth-orbiting SatellitesLow-Earth-orbiting Satellites A Low Earth Orbit (LEO) typically is a circular orbit about 400 kilometers

above the earth’s surface.

It requires a period (time to revolve around the earth) of about 90 minutes.

Because of their low altitude, these satellites are only visible from within a radius of roughly 1000 kilometers from the sub-satellite point.

In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity.

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LEOLEO

Low earth orbiting satellites are less expensive to position in space than geostationary satellites and, because of their closer proximity to the ground ,

It require lower signal strength (Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic). So there is a trade off between the number of satellites and their cost.

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  MarkerMarkerDistance above Distance above earth (km)earth (km)

Low Earth Low Earth Orbit (LEO)Orbit (LEO)Cyan areaCyan area160160 to 2,000to 2,000

Medium Medium Earth Orbit Earth Orbit

(MEO)(MEO)Yellow areaYellow area2,0002,000 to 34,780to 34,780

International International Space Space

Station (ISS)Station (ISS)Red dotted Red dotted

lineline500500

Global Global Positioning Positioning

System System (GPS) (GPS)

satellitessatellitesGreen dash-Green dash-

dot linedot line20,23020,230

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Overview of a Satellite Overview of a Satellite SystemSystem

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The satellite system is composed of a space segment and a ground segment

The Space Segment contains one or several active satellites organizers.

The Control Segment consists of all ground facilities for the control and monitor the satellites (tracking, telemetry and command) and for management the traffic and the resources onboard the satellite

The Ground Segment consists of all traffic earth stations

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Satellites – Satellite SubsystemsSatellites – Satellite Subsystems Attitude and Orbit Control System

– Rocket motors to move satellite back to the correct orbit– Keep antennas point toward to earth

Telemetry, tracking, command and monitoring – Telemetry system monitor satellite health, tracking system is located

at the earth station and provides information about elevation & angles of the satellite

Power system – Electrical power from solar cells

Communication subsystem– Major component of communications satellites, one or more

antennas & a set of receivers and transmitters (transponders).

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May 2015 Satellite communication system & interfacing with terrestrial entities

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What Are Satellite PayloadsWhat Are Satellite Payloads ? ?• The payloads on communications The payloads on communications

satellites are effectively just satellites are effectively just repeaters. They receive the signals repeaters. They receive the signals that are transmitted to them and then that are transmitted to them and then retransmit them at a different retransmit them at a different frequency back to earth frequency back to earth

• Modern satellites do more than this. Modern satellites do more than this. They receive the signals and then They receive the signals and then demodulate them to access the data, demodulate them to access the data, the data can then be processed the data can then be processed before being modulated and before being modulated and retransmitted. The data can be retransmitted. The data can be stored for later retransmission or stored for later retransmission or modulated using a different method, modulated using a different method, even at a different data rateeven at a different data rate

A wireless repeater

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A typical satellite consists of a number of repeaters (transponders), each of A typical satellite consists of a number of repeaters (transponders), each of which provides a large-capacity communication channel. which provides a large-capacity communication channel.

Each transponder has a receiver tuned to a frequency range that has been Each transponder has a receiver tuned to a frequency range that has been allocated for uplink communication signals from Earth to the satellite. allocated for uplink communication signals from Earth to the satellite.

Following the receiver, each transponder consists of a frequency shifter to Following the receiver, each transponder consists of a frequency shifter to lower the received signals to a downlink frequency, a filter tuned to the lower the received signals to a downlink frequency, a filter tuned to the frequency of the transponder and a power amplifier to transmit signals frequency of the transponder and a power amplifier to transmit signals back to Earth. back to Earth.

Satellite

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The communication capacity of a satellite is determined by the number of The communication capacity of a satellite is determined by the number of transponder channels and the volume of communication that can be transponder channels and the volume of communication that can be transmitted on each channel. transmitted on each channel.

Although this varies from one type of satellite to another, the most Although this varies from one type of satellite to another, the most commonly used satellite in 1995 had 24 transponders. Each can carry a commonly used satellite in 1995 had 24 transponders. Each can carry a colour TV signal (or 6 digitally compressed TV signals) or at least 1200 colour TV signal (or 6 digitally compressed TV signals) or at least 1200 telephone voice signals in one direction. Each new generation of satellites telephone voice signals in one direction. Each new generation of satellites tends to have increased communication capability tends to have increased communication capability

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Ground SegmentGround Segment

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Control SegmentControl Segment

Communications satellite operations are monitored from control rooms such as this Communications satellite operations are monitored from control rooms such as this one, where minor orbit adjustments can be made and communications functions can one, where minor orbit adjustments can be made and communications functions can be regularly checked. If problems occur, technicians can attempt repairs or transfer be regularly checked. If problems occur, technicians can attempt repairs or transfer communications to a different satellite communications to a different satellite

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Common Frequency BandsCommon Frequency Bands

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Main ParametersMain Parameters Coverage Cost: Infrastructure and bandwidth DATA structure size Mobility vs Transportability Latency: Time critical applications Complexity: Size of network Regulatory Interface: GPS, Device telemetry Interoperability: Switchover to available network