Satellite Television IntroductionDocument Transcript
Satellite Television IntroductionTV broadcast satellites are placed in orbit directly above the equator in a height of around36000km. They travel through space in the same speed earths rotation - so to us on theground the satellite seems to be stationary. This is known as a geo-synchronous (or geo-stationary) orbit.Ground controllers fire on-board jet thrusters occassionally to help keep the satellite in thespecified position in space. This will be the major factor in determining the life of yoursatellite, as onboard fuel does eventually come to an end.In Europe the orbital location of the satellite on the Clark Belt emerges by the quantity ofdegrees to the East or West of due South (for instance: Astra 1 = 19.2°E, Hispasat = 30°W).This will be the Azimuth.The Clarke BeltThe concept of geo-stationary satellites was first suggested by Arthur C Clarke in an articlewritten for Wirless World magazine in October 1946.The region in space occupied by these satellites is commonly referred to as The Clarke BeltGetting signals both to and from the SatelliteTelevision and radio programme signals are sent from your various originating studios withan Earth up-link station. From there the gathered bouquet of programmes is transmitted intospace employing a dish aimed with the orbiting spacecraft. This is called the up-link. Thefrequency is around 14GHz (in Europe)The satellite receives these signals, amplifies them and transmits it well towards earth over adifferent frequency. This work is performed by a transponder. This down-link operates atabout 11GHz (in Europe), within the so-called Ku microwave band).A group of satellites within the same orbital position (for example Eutelsats Hotbird) willhave a combined total greater than 70 transponders. A transponder rebroadcasts 1 analogueTV channel or possibly 14 digital TV channels (this depends for the digital compressiontechniques used:- lower compression = less channels = better pictures).This form of broadcasting is known as DTH (Direct-to-Home broadcasting).Each satellite includes a massive variety of solar cells. Some satellites are cube-shaped andstill have huge wing-like solar cell arrays extending through the main body, and some are
cylindrical and also have their entire surface coated with solar cells. The cells convertsunlight into electricity to function the satellite, providing power to the transponders also tomaintain the charge about the standby batteries for the times when the satellite is in theshadow of the Earth.The satellites useful for radio and television broadcasting usually are not usually owned fromthe broadcasting companies.Broadcasters lease the technical facilities from your satellite owner. In Europe both majorsatellite providers are Eutelsat and SES-Astra.The earth stations necessary to send the programmes up to the satellite (known as the up-link) are provided by the variety of organisations in lots of many different countries. Some ofthese are commercial organisations (eg BT in UK) and some are government departments(eg national PTT authorities).The FootprintThis will be the actual signal strength map (or Footprint) with the Astra 1G and 1H satellites(as published through the owners in the satellites, SES-Astra). Both birds are located withinthe same orbital position in space at 19.2°E.The antenna on the satellite directs the signal as a result of a specific land area. Just as thebeam from your flashlight gets wider and weaker over distance the same is true the satelliteradio signal.Contour lines for the map show the mandatory dish size for satisfactory all-weatherreception.Tuning into Satellite TV and RadioTo hear satellite TV some specific receiving products are needed.The greatest item will be the dish. The dish will be the antenna (aerial). It works just like acurved mirror to collect, reflect while keeping focused the radio energy from the satellite andfocusses it in to the throat with the LNB (Low Noise Block).The LNB amplifies these very weak signals and converts them to a lower frequency (fromabout 11GHz as a result of less than 2GHz), the so-called intermediate frequency. Thissignal travels across the coaxial cable towards the Receiver unit inside home.The Receiver (often referred to as set-top box, STB, or Digibox etc) performs every one ofthe electronic signal processing that is necessary to recover the TV signal. In addition itsends power and control signals inside the cable for the LNB. The Receiver is connected tothe TV set either by an antenna (aerial / RF) lead or SCART cable. Operation with the
receiver and channel selection is normally done with an infra-red remote control unit.This basic principle of getting the satellite TV signal may be the same whether an analogueor digital product is being used. However, the electronic processing of digital and analoguesignals is quite different indeed.Digital signals comply with the DVB-S (Digital Video Broadcasting - Satellite) protocol. Lookfor the DVB logo. For an analogue signal, the signal strength is extremely important, but alsofor digital reception just as much care have to be paid to maximizing signal quality (the higherthe quality number the less bit-rate errors the receiver must cope with).More Dishes, More SatellitesFor a greater choice of programming youll be able to look at several satellites. This can bedone by utilizing a motorized dish or a combination of several fixed dishes and LNBs.To use multiple dishes a straightforward switching system called DiSEqC (Digital SatelliteEquipment Control™ - manufactured by Eutelsat) s all that is needed, though the receiverhave to be DiSEqC compatable. The DiSEqC switch is normally fitted nearby the dishes plusa single cable feeds to the Receiver. The Receiver box sends control signals across thecable towards the DiSEqC exchange signal of automatically pick the correct blend ofdish/LNB and satellite.Upgraded versions with the DiSEqC protocol allow for control of motorised dishes (DiSEqC1.2 and two.0).A different kind of motorized satellite system is the horizon-to-horizon (H-H) method. Here anelectrically operated screw-jack pushes and pulls the dish which pivots over a specialmounting bracket. This makes the dish accurately continue with the position of the satellitesover the Clark Belt. Receivers incorporating this sort of motor drive will usually havePositioner included within the name/description.For sustained flexibility LNBs can be purchased with single, dual, quad or octo (8) outputs.Each output is very independent in the others. This allows multiple receivers (lounge,bedroom, kitchen, etc) for connecting to just one dish/LNB.If looking at two closely spaced satellites (eg Hotbird [13°E] and Astra-1 [19E°]) you are ableto use one dish and a couple LNBs mounted with a special extension arm. The dish thenone LNB is focussed about the weaker of the two satellites. The second LNB is moved overthe extension arm until an adequate signal through the second, stronger satellite is received(this picture shows a single output LNB looking at Hotbird and a quad output LNB looking atAstra-1 - remember that one output through the quad LNB isnt in use).