Satellites

Satellite communications Introduction

Before the lecture Try to find out more by reading: http://ctd.grc.nasa.gov/rleonard/regcontents.html http://www.aticourses.com/iridium.htm http://www.aticourses.com/global_positioning_system.htm http://www.mlesat.com/Article9.html http://www.mlesat.com/tutorial.html

Satellites Several types LEOs - Low earth orbit MEOs - Medium earth orbit GEOs - Geostationary earth orbit

GEOs Originally proposed by Arthur C. Clarke Circular orbits above the equator Angular separation about 2 degrees - allows 180 satellites Orbital height above the earth about 23000 miles/35000km Round trip time to satellite about 0.24 seconds

GEOs (2) GEO satellites require more power for communications The signal to noise ratio for GEOs is worse because of the distances involved A few GEOs can cover most of the surface of the earth Note that polar regions cannot be “seen” by GEOs

GEOs (3) Since they appear stationary, GEOs do not require tracking GEOs are good for broadcasting to wide areas

Early experiments US Navy bounced messages off the moon ECHO 1 “balloon” satellite - passive ECHO 2 - 2nd passive satellite All subsequent satellites used active communications

Early satellites Relay 4000 miles orbit Telstar Allowed live transmission across the Atlantic Syncom 2 First Geosynchronous satellite

Major problems for satellites Positioning in orbit Stability Power Communications Harsh environment

Positioning This can be achieved by several methods One method is to use small rocket motors These use fuel - over half of the weight of most satellites is made up of fuel Often it is the fuel availability which determines the lifetime of a satellite Commercial life of a satellite typically 10-15 years

Stability It is vital that satellites are stabilised to ensure that solar panels are aligned properly to ensure that communications antennae are aligned properly Early satellites used spin stabilisation Either this required an inefficient omni-directional aerial Or antennae were precisely counter-rotated in order to provide stable communications

Stability (2) Modern satellites use reaction wheel stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible including: eddy currrent stabilisation (forces act on the satellite as it moves through the earth’s magnetic field)

Reaction wheel stabilisation Heavy wheels which rotate at high speed - often in groups of 4. 3 are orthogonal, and the 4th (spare) is a backup at an angle to the others Driven by electric motors - as they speed up or slow down the satellite rotates If the speed of the wheels is inappropriate, rocket motors must be used to stabilise the satellite - which uses fuel

Power Modern satellites use a variety of power means Solar panels are now quite efficient, so solar power is used to generate electricity Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite Nuclear power has been used - but not recommended

Harsh Environment Satellite components need to be specially “hardened” Circuits which work on the ground will fail very rapidly in space Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully

Alignment There are a number of components which need alignment Solar panels Antennae These have to point at different parts of the sky at different times, so the problem is not trivial

Antennae alignment A parabolic dish can be used which is pointing in the correct general direction Different feeder “horns” can be used to direct outgoing beams more precisely Similarly for incoming beams A modern satellite should be capable of at least 50 differently directed beams

Satellite - satellite communication It is also possible for satellites to communicate with other satellites Communication can be by microwave or by optical laser

LEOs Low earth orbit satellites - say between 100 - 1500 miles Signal to noise should be better with LEOs Shorter delays - between 1 - 10 ms typical Because LEOs move relative to the earth, they require tracking

Orbits Circular orbits are simplest Inclined orbits are useful for coverage of equatorial regions Elliptical orbits can be used to give quasi stationary behaviour viewed from earth using 3 or 4 satellites Orbit changes can be used to extend the life of satellites

Communication frequencies Microwave band terminology L band 800 MHz - 2 GHz S band 2-3 GHz C band 3-6 GHz X band 7-9 GHz Ku band 10-17 GHz Ka band 18-22 GHz

Early satellite communications Used C band in the range 3.7-4.2 GHz Could interfere with terrestrial communications Beamwidth is narrower with higher frequencies

More recent communications Greater use made of Ku band Use is now being made of Ka band

Rain fade Above 10 GHz rain and other disturbances can have a severe effect on reception This can be countered by using larger receiver dishes so moderate rain will have less effect In severe rainstorms reception can be lost In some countries sandstorms can also be a problem

Satellite management Satellites do not just “stay” in their orbits They are pushed around by various forces They require active management

Systems of satellites Example - Iridium Deploy many satellites to give world wide coverage - including polar regions So far have not proved commercially viable Other systems “coming along” - Teldesic

The future Because Iridium has not been a commercial success the future of satellites is uncertain Satellites still have major advantages for wide area distribution of data

Chronology 1945 Arthur C. Clarke Article: "Extra-Terrestrial Relays" 1955 John R. Pierce Article: "Orbital Radio Relays" 1956 First Trans-Atlantic Telephone Cable: TAT-1 1957 Sputnik: Russia launches the first earth satellite. 1962 TELSTAR and RELAY launched 1962 Communications Satellite Act (U.S.) 1963 SYNCOM launched 1965 COMSAT's EARLY BIRD: 1st commercial communications satellite 1969 INTELSAT-III series provides global coverage

Chronology (2) 1972 ANIK: 1st Domestic Communications Satellite (Canada) 1974 WESTAR: 1st U.S. Domestic Communications Satellite 1975 RCA SATCOM: 1st operational body-stabilized comm. satellite 1976 MARISAT: 1st mobile communications satellite 1988 TAT-8: 1st Fiber-Optic Trans-Atlantic telephone cable 1994 GPS system deployed by USAF 1998-2001 Iridium

Satellites

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Satellites