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A geostationairy satellite is a satellite that, because of it's orbital position, rotates around the earth. It also does not fall into space and does not fall to the earth. It seems to be in the same position in the sky in all time. This delicate balance depends on the speed of the satellite.
If you want a satellite to rotate around the earth once every day, so you can 'see' it constantly, it has to be in an orbital position 35,786 km above the earth's surface. At this position, the satellite is 'geosynchronous'. It has the same rotation speed as the earth, but viewed from the earth it can go up and down. If a geosynchronous satellite hangs above the earth and seems to be hanging 'still', it is in a geostationary orbit. This means the angle of the orbitflight is 0 degrees in comparison to the equator. A geostationary satellite therefore is a geosynchronous satellite that has a trajectoryangle of 0 degrees.
Geostationary satellites are the most common satellites there are because they are easy to use.
You only have to aim a dish once, then you can fix it. Until the satellite operator moves the satellite, you can always receive it's signal. Almost every television station nowadays has a channel at some geostationary satellite. It used to be possible to view the earth and all of its surrounding satellites in 3D at NASA, but they took the page offline. This is what it looked like:
You can clearly see the 'Clarke Belt', the circle of satellites that are in geostationary orbit. It was named 'Clarke Belt' after science ficion writer Arthur C. Clarke, who had the first idea for in orbit relay antennas in 1945.
There is a visualization in Google Earth by Analytic Graphics Inc. which shows the real-time position of more than 13 thousand satellites around the Globe. It also shows Space debris and Rocket Bodies. Here is a link to the official KML file, if you have Google Earth it is a beautiful plugin!
Satellite News Gathering is very popular at the moment. It is possible to get television images from anywhere in the world to just about everywhere. Because there are so many geostationary satellites, the price of a satellite connection is relatively cheap. Much cheaper than most terrestrial connections. It often happens that a SNG car is only a few kilometers away from the studio and still they use satellites to deliver the images.
Transponders are units of the satellite which receive the signal, amplify it, and then return it to the earth. A satellite is like a mirror: what you send to it, it amplifies and then returns. Because of this amplification, it is not possible to receive and send back at the same frequency. The transponder would oscillate and no signal would be amplified. It's like putting a microphone in front of a speaker and turning op the volume. An average Ku satellite today (for example Eutelsat W3) has about 24 transponders, and it can broadcast from 34 analog TV channels up to hundreds of digital TV channels. Any satellite can return electrical signals and the satellite doesn't care if these are telephone calls, TV signals, internet backbone connections or anything in development.
So the frequency someone uses to send a signal to a satellite, is not the same as the frequency it returns the signal on. Usually, the difference is about 4 GHz, the uplink frequency is higher than the downlink. For example :
Uplink : 14,480 GHz
Downlink : 11,080 GHz
These values can be different, it depends on the satellite builder, and the type of transponder. Some transponders have a different bandwidth. There are bandwidths of 36 MHz, which are often used, others have a bandwidth of 72 MHz. There are more possibilities though, it's just what the buyers ordered from the manufacturer. Only receiving satellite signals ? Then you are concerned with the downlink frequency.
In order to use the maximum of the bandwidth of the satellite, the builders soon came with the idea to divide the transponders into two types : horizontal and vertical. If we broadcast half the signals in a vertical way (the waves are vertically broadcast down) and the other half horizontally, the bandwidth has just about doubled ! This is a huge form of compression, invented in the sixties. It is called linear polarization. There is also circular polarization, where the electromagnetic waves rotate.
LNB, Feedhorn & Dish
The dish on which the signals are received picks up everything from space and then centers it in one small point. This way, a huge amplification of the weak waves from 36000 kilometers away is obtained. Also, the bigger the dish, the greater the amplification. The second amplification is done in the LNB, the Low Noise Block. People also call this an LNA (Amplifier) and LNC (Converter). This unit, which amplifies the weak signals, has a huge influence on the signal quality. A good LNB now has a noise ratio of 0.7 dB.
A feedhorn is placed in front of the LNB and makes sure signals which would have just missed the center of the LNB are still reflected into it. It also increases field strength and compensates irregularities in the shape of the dish. A feedhorn is a cone-shaped tube, with the narrow end towards the LNB. Today most feedhorns are integrated in the housing of t'e LNB so often people speak of the 'LNB and feedhorn' or LNBF.
There are generally four kinds of dishes :
The Offset dish is often used in smaller setups because of its better efficiency at smaller sizes. The Gregorian has even greater efficiency, but is much more expensive and very sensitive to weather influences. At larger sizes, the Primefocus and Cassegrain dishes are used more often because of their greater amplification and mechanical stability. In small sizes, the 'shadow' their LNB has on the dish makes them inefficient.
Ku & C Band
A LNB not only switches between horizontal and vertical, it also amplifies the signal and converts is down. If we would not convert the signals a satellite uses (10.7 - 12.75 GHz), they would not go further than 1 meter through the coaxial cable ! The frequency range of the signal that goes through the cable is from 920 to 2150 MHz. This bandwidth is called the L-band. The Universal LNB, which is most common, divides the satellite's bandwidth into two parts, the lower part (10.700 - 11.700 GHz) and the upper part (11.700 - 12.750). The entire bandwidth simply is too large to go through one coaxial cable. The bandwidth we're talking about is called the Ku band, and therefore the lower and upper parts are called Ku low and Ku high. There are lots of LNB's though, all with different parameters.
In the USA Ku band is only used from 11.7 - 12.7 GHz, whilst in europe it goes from 10.7 - 12.75 GHz. In the USA there is both linear and circular polarisation in the Ku band, in most of the world it is linear.
There are more frequency bands used by satellites, one of them is the C band. Its bandwidth ranges from 3.700 - 4.200 GHz and the signals are very weak. Extended C band goes from 3.400 - 4.200 GHz. Only dishes 2 meters wide or more are suitable for C band reception. It is used more frequently in the United States because there is more space and the C band signals cover a larger area. With C band it is possible to cover an entire continent !
The signals we can receive from a satellite are in analog TV transmissions much like their terrestrial equivalents. Analog terrestrial broadcasting uses Amplitude Modulation, while satellites use Frequency Modulation. This method requires much more bandwidth but the signals are less sensitive for amplitude variations, which can occur often in satellite receiving because of the weather.
Analog TV channels have their audio on subcarriers, which are usually FM modulated signals. The subcarriers are often 6 to 7 MHz from the TV carrier, and therefore are not interfering with the video which is limited at 5 MHz.
If you want to use an analog receiver, it is necessary to set the the frequency of the subcarrier in order to get an audio signal out of it. The bandwidth of audio signals can also vary, but generally they are wide (280 - 400 kHz) or small (110 - 130 kHz). Because of the noise in small audio signals, preëmphasis and deëmphasis are used (much like in terrestrial FM radio). Common are 50 µs, 75 µs and J17. Wrong settings produce horrible sounds. Noise reduction is often used, the most common noise reduction for analog satellite receivers is Panda 1. The Panda logo is printed on the box if the genuine parts are used during production.
Digital Video Broadcasting
Digital TV transmissions usually tend to use QPSK. Quadrature Phase shift Keying is a way to send over two bits per modulation key. A digital DVB (MPEG-2) receiver has to know what the FEC (Forward Error Correction) is, what the bit rate or symbol rate is, and it needs PID codes when more channels are in a multiplex, MCPC. Multiple Channel Per Carrier is often used for Direct to home Broadcasting, but when only one channel is in a multiplex, it is called SCPC (Single Channel Per Carrier). PID codes are Program Identification Data, the receiver has to know which data is for which channel and what audio data and what is video is, or for example TV-text. Modern DVB receivers can figure out all the necessary information just mentioned automatically. The early models could not, and it was hard to get the right settings.
Symbol rate and Bit rate are related, most receivers use Symbol rate for manually setting a value. The formula for calculating from Bit rate to Symbol rate is :
Symbol Rate = Bit Rate / ( 2 × FEC × (188/204)) or the other way around : Bit Rate = Symbol Rate × 2 × FEC × (188/204)
Most people call any DVB receiver an IRD, Integrated Receiver Decoder, because it decodes the datastream. I don't, because when you think about it, every form of modulation/demodulation is a form of coding/decoding and you should then call every radio a decoder. For me an IRD is a DVB receiver with a built in Conditional Access Module. For example Viaccess, Mediaguard or Irdeto. For more information on DVB, check http://www.dvb.org/.
DiSEqC en USALS
Digital Satellite Equipment Control, or DiSEqC is a special communication protocol for use between a satellite receiver and a device such as a LNB switch or a small dish antenna rotor. It uses the 22 kHz signal to send data messages back and forth through the existing coaxial cable.
There are 4 versions of DiSEqC in use:
- DiSEqC 1.0, which allows switching between up to 4 LNB's
- DiSEqC 1.1, which allows switching between up to 16 LNB's
- DiSEqC 1.2, which allows switching between up to 16 LNB's and control of a simple satellite rotor
- DiSEqC 2.0, which adds bi-directional communications to DiSEqC 1.2
All four variations were standardised by February 1998, prior to general use of digital satellite television. They are all back compatible - a DiSEqC 2.0 receiver can control a 1.0 switch; but a 1.0 receiver cannot control motorised features.
The terms DiSEqC 1.3 and 2.3 are often used by manufacturers and retailers to refer to other protocols (1.3 usually refers to USALS receivers), but these uses are not authorised by Eutelsat, the developer of the system who now acts as the protocol standards agency. Many DVB satellite-receivers are equipped with some DiSEqC version, also outside Europe where it originated.
USALS stands for Universal Satellites Automatic Location System and is developed by Stab. It is an unofficial extension of the DiSEqC protocol. With USALS it is no longer necessary to manually search and store every known satellite position. In a USALS capable satellite receiver the geographic earth coordinates of the satellite dish are entered. The USALS system can then calculate all rotor angles for every satellite receivable on that location.
Because of licenses for video material and fear of competition, broadcasters often scramble their channels and make sure conditional access modules are built in the receivers. With DVB it is not always like with analog satellite transmissions, where the decoder was connected onto a scart port in the receiver. The decoder is embedded in the receiver, except of course in the receivers that can only receive Free To Air channels. A good development is Common Interface, with which you can switch conditional access module. A common interface module is shaped like a PCMCIA module ofted seen in laptops. Common interface allows people to subscribe to more than one provider. The only problem is getting a card. Because of fear of competition and the video licenses that are organized per country, the broadcasters only let you subscribe to their channels if you live in the country they broadcast for. For example: I can't legally get a Sky Digital card (UK) when I live in Holland. It is even illegal to export cards out of the country. There are many systems for DVB encryption, here are a few:
- Betacrypt (by Comvenient GmbH / Beta Technik) 0x1700 - 0x17FF
- AccessGate (by Telemann) 0x4800 - 0x48FF
- BISS (Basic Interoperable Scrambling System) (by European Broadcasting Union) 0x2600 - 0x26FF
mode 0 = Free To Air
mode 1 = Session Word 12 digits hexadecimal, example: A13DBC42908F
mode E = Encrypted Session Word 16 digits hexadecimal, example: F76EE249BE0145BB
- Codicrypt (by Scopus) 0x2200 - 0x22FF
- Conax (by Conax SA) 0x0B00 - 0x0BFF
- Cryptoworks (by Philips) 0x0D00 - 0x0DFF
- Digicipher (by Motorola) 0x0700 - 0x07FF
- Irdeto (by Irdeto Access BV) 0x0600 - 0x6FF
- KeyFly (by SIDSA) 0x4AA0 - 0x4AAF
- MDS (by Mentor Data Systems, Inc.) 0x2500 - 0x25FF
- Nagravision (by Kudelski) 0x1800 - 0x18FF
- PowerVU (by Scientific Atlanta) 0x0E00 - 0x0EFF
- RAS (Remote Authorisation System) (by Tandberg) 0x1000 - 0x10FF
mode 1 = Free To Air
mode 2 = Key encrypted 7 digits decimal, example: 3845622
- Seca Mediaguard (by Canal+ Technologies) 0x0100 - 0x01FF
- Viaccess (by France Télécom) 0x0500 - 0x05FF
- VideoGuard (by NDS) 0x0900 - 0x09FF
- Wegener Compel (by Wegener Communications)
High Definition Television (HDTV)
High Definition televisie is a new TV standard. As the name implies, high definition so much sharper images and as usual satellite plays a big roll in new developments. The high definition demands huge bandwidth. Uncompressed standard television in studio quality already needs +/- 270 Mbit per second, with HD this is much more. An uncompressed HD signal in studio quality can occupy as much as 1.485 Gbit per second! However, there are many HD standards and they don't all need this much bandwidth but 5 times standard TV is fairly common. Even in a MPEG2 compressed DVB standard, HD still needs a lot of bandwidth. That is why for HD an Advanced Video Codec is being used more and more, H264. This is a part of MPEG4. It is a newer and more efficient standard than MPEG2, but it needs more computer power on both encoding and decoding.
The Advanced Video Codec's were not supported by DVB, that is why DVB-S2 was introduced. To furthermore increase capacity, HD is often transmitted in 8PSK modulation. This form of modulation transmits three bits per modulation key, and this increases the datarate by 50% compared to standard QPSK.
All these new developments are not easy for consumers. An HD signal can be transmitted in DVB or DVB-S2. Modulation can be QPSK or 8PSK. It's also possible that the compression standard is H264, or MPEG2. All these options make for a lot of combinations. Now try to find a receiver that is capable to receive and decode all of them!
Furthermore, some television feeds or backhauls have 4:2:2 sampling. The signals for consumers are all in 4:2:0. In HD this is the same, 4:2:2 and 4:2:0. So the demanding user will add this to the long list of options a receiver must be able to do.
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