How Does Satellite TV Work? All You Need To Know
If you’re a technician and wishes to know more, or if you’re sitting in your garden looking at your satellite dish and wondering, how does the satellite dish and satellite TV work? Read on for all you need to know. In this blog, we discuss what satellite TV is, how this is transmitted/ received and what the equipment does in between. It will leave you with a greater understanding and maybe even the confidence to attempt your own installation. Let’s begin!
The Satellites Are In Space!
I often get asked, where does a satellite dish get it’s signal from? Many believe it receives from land based TV transmitters like terrestrial TV services. This is not true, the clue is in the name, satellite dish! Sky TV, Freesat, Soarsat and so on. I sure most of you already know that, but the satellites that are used for TV services are geo-stationary satellites that rotate with the earth so that signals can be received with a fixed satellite dish. If the satellites didn’t remain in a fixed orbital position you would need a satellite dish that would be able to track it’s position so you could continue to receive your TV signal. Although systems such as this do exist, they are very expensive and not viable for the consumer market. Your Sky TV subscription certainly would be significantly more expensive.
As mentioned above, the satellites need to remain in a fixed position so that you can align your satellite dish and not have to worry about altering it again. Geo-stationary orbit, according to Wikipedia is at approximately 22,236 miles (35,786Km) away from the Earths sea-level and this is where the Direct to Home(DTH) satellites are situated. This is named the “Clarke Belt” after the scientist Arthur C Clarke who published an article in the UK Journal predicting inexpensive satellite communications at precise location above the Earths equator. This distance is typically much further than people usually expect, this also means there is not much concern with the satellites bashing into each-other as there are large distances in between them. A couple of degrees adjustment at your dish on the ground is very big by the time you’re up there in space.
In order for the signals to reach your antennas, they must pass up and down from space first. The uplink is the transmission to the satellites from land-based transmitters. The uplinks are transmitted from multiple positions and locations within a single frequency band at the same time.
The Downlink takes the uplink signals and re-transmits them back down to earth within a different frequency band. The signals from the downlink are received by an externally mounted satellite dish and then connected to your satellite TV receiver with connecting coaxial cables. Some services like Tooway or SES satellite broadband will allow two-way communication which can uplink and downlink at the same time.
Satellite TV Frequency Bands
Satellite TV primarily uses the Ku Band which is 10.7-12.75GHz, this is an amalgamation of the Fixed Satellite Service Band (FSS), the Direct Broadcasting by Satellite Band (DBS) and the Business Satellite Services Band (BSS). Other services such as satellite broadband services use the Ka Band(18.8-30Ghz) which with it’s higher frequencies allows for greater rate of data to be transmitted/ received.
As the downlink frequencies are way too high than that can be carried over any distance by a coaxial cable, at the dish LNB the signal is oscillated down to a more manageable frequency below 3Ghz. Depending on the service you have and the LNB in use this is usually oscillated down to the Intermediate Frequency(IF) band of 950-2150MHz. Sky Q and third-generation Freesat are exceptions to this as the Wideband LNB required goes as far down as 300MHz.
The word Transponder is derived from the words “transmitter” and “responder”. They are part of the uplink/ downlink process as they receive the uplink signal, amplify and rebroadcast them. The satellites for TV will have many transponders on each individual satellite each re-broadcasting different multiplexes on different frequencies. Thus, although different, transponders and multiplexes are considered the one and the same thing with regards to satellite TV reception. Each transponder multiplex will contain many TV services that appear as spikes when viewed on a TV spectrum analyser and should be thought of as groups of channels/ services.
Orbital Positions/ Locations
Depending on which satellite TV service you wish to receive,depends on which size satellite dish you need and where you need to point it. As all of the satellites rotate above the equator they are at 0 degrees latitude, this means that satellite dishes are aligned to degrees longitude east or west of due south. As I’m sure you will expect, there are lots of satellites up there in various orbital locations serving different countries and most of the planet. The most common orbital position we deal with in the UK is 28.2E, this is where you receive Sky/ Freesat TV services. There are actually at the time of writing this three satellites at 28.2E, these are Astra 2E, Astra 2F, Astra 2G, there is also Eurobird at 28.5E which can be received off the same fixed dish. These are all-together collectively referred to as "Astra 2". Other common orbital positions to note are the Hotbird satellites at 13E and Astra 1 satellites at 19.2E as these serve most of Europe and can be received with a relatively small satellite dish in the UK.
The Satellite Arc
As the Sat TV satellites orbit around the equator, when looking south in the northern hemisphere or north in the southern hemisphere these will form an invisible arc in the Sky with the highest point being due south(or north) and the further off the longitude adjustment the lower this will be on the horizon. It helps to familiarise yourself with the satellite arc as helps to find the satellites when aligning a satellite dish. The further you get away from the equator the lower on the horizon the arc will be, this can make it difficult to obtain line of sight and you may need to mount your dish higher up to get it. At the equator your satellite dish will literally point straight up into the Sky.
The terrestrial coverage of a particular satellite is called the Satellite Footprint. This is the area where the satellite downlink is directed to, this will be where the TV services on that particular satellite are required. For instance German TV stations transmitted to Germany, France to France and so on. It helps to familiarise yourself with the satellite footprint maps of the services that you wish to receive. This will allow you to find out whether you can receive that particular satellite can be received in your location and what the expected signal strength will be, this will enable you to identify the correct satellite dish size that you must install. You may find that you can receive some services that in your location not necessarily intended for your location, like foreign TV. This sometimes may also only bely possible with a large or very large satellite dish. When I say large, my interpretation of this is above 1m but they do get substantially bigger than this. An example of this is when trying to receive UK TV abroad, some of the satellite footprints are very focused spotbeams on the UK making it difficult to receive some UK satellite TV services beyond south of France, not without a very large dish that is.
Satellite Signal Polarisations
The are several polarisations that are used for telecommunications, these are horizontal, vertical, circular and elliptical polarisations. Nearly all satellite TV uses both horizontal and vertical polarisations for it’s transmissions. This allows it to maximise the available bandwidth for extra satellite TV services. You do not have to adjust the mounting of the antenna like you do most TV aerials to switch between the two polarisations as this is done at the LNB. The dish just needs to be aligned correctly with particular importance of setting the skew adjustment correct to differentiate between the horizontal/ vertical signals.
There are lots of different types of satellite dish, but they all work on the same or similar principle. The signal(down-link) is caught and reflected back to the satellite dish LNB, which is then in turn connected with coaxial cables that feed your satellite TV receiver. Owing to the frequencies used by satellite TV the satellite dish must be installed on an external wall with a clear uninterrupted view of the satellites in space. Satellite dishes are usually mounted direct onto walls but for a variety of reasons you can and may need to install the dish in an alternate dish position, like the chimney stack or on a pole in the garden.
The LNB is the part of the satellite dish that the cables connect into, this is usually sold separately to the satellite dish and the type that you install depends on the service that you wish to receive and the amount of TV positions that you wish to feed. Most LNB’s for satellite TV in the UK are the Universal Switching LNB type, these work but the satellite receiver varying the voltage and added a “tone” to request one of four satellite bands from the LNB that the channel requested is on. Over time this is starting to change with Sky Q and the new range of Freesat boxes requiring or being compatible with the Wideband LNB. These require two separate connections to your satellite TV RX each providing half of the TV channels/ frequencies.
The cables that connect to the satellite dish are called coaxial cables or just “coax” for short. There are lots of different types, some more suitable that others for satellite TV reception. When choosing one you should purchase one that is at least “double screened” so that it can handle the frequencies from the LNB and one that is “UV-stable” so that it is suitable for outdoor use. Traditionally, it has been the case with universal LNB’s that one cable would be required for a viewable-only Sat RX and two cable inputs for a model with an internal hard disc drive(HDD), commonly referred to as a PVR with separate cables feeding from your satellite dish to each other TV position that you wish to feed. This is starting to change with things like Sky Q that require two coax cables to a main box/ main TV and then all other boxes for other TV’s are connected wireless over a mesh network or via network cables.
Satellite TV Receivers
The satellite dish connects via coax cables to the satellite TV receiver, commonly referred to as a Sat RX, Satellite STB or something similar. This is usually an external piece of AV equipment that will connect to your TV via a HDMI cable nowadays, but more and more new TV’s are starting to build satellite tuners within which can often bypass the need for a separate box altogether.The satellite TV RX will usually be specific for the service that you wish to receive, for instance a Sky box for Sky, Freesat box for Freesat, TNTSat or Fransat for French satellite TV and so on. It is often possible to to use a generic satellite receiver for various satellite TV services but you may require a Conditional Access Module and Smart Card to be able to access encrypted TV services, while this may be perfectly fine for what you require it can be a bit burdensome as the box is not necessarily ideal for the service this wish to receive. For this reason, I prefer to get the specific box for the service you wish to receive.
The satellite STB sends a DC voltage via the coax cables which powers the LNB. This is usually 13-18V DC. Most satellite receivers will switch between 13V and 18V to switch between horizontal and vertically polarised signals and will provide a 22KHz tone to switch from the low-band to the high-band.
Most satellite TV providers are going down the encryption route, this is one of the benefits of switching to digital transmissions that allows a broadcaster to protect their content from people who are supposed to be able to view it or haven’t paid to watch it. The classic example is Sky TV, to view it you require a subscription, Sky box and Smart Card, this allows you to access Sky TV. If you were to pull the cancel the Sky subscription, replace with a different type or box remove the Smart Card, you will only be able to view the TV services that are unencrypted. You need to make sure you have the appropriate equipment for the service/ encryption which is one of the reasons that I suggest purchasing the specific equipment for the service.
Conditional Access Module(CAM)
If you satellite receiver doesn’t have an inbuilt CAM, to allow to “decrypt” the encryption, you can get plug-in ones that insert into a Common Interface(CI or CI+) slot on your Sat RX. You obviously need to make sure that the satellite box you purchase has at least one of these and the CAM you purchase is the right type for the service you wish to receive. If required a Smart Card can then be inserted into the CAM.
The process on encoding information onto a signal or carrier wave is called modulation. The process of extracting this off the signal/carrier wave for it’s intended purpose is called “demodulation”. With digital terrestrial and satellite TV this would be converting the binary information into a TV picture with sound and possibly some other interactive type services. There are many modulation techniques with TV services like COFDM & QAM but the most common for satellite TV is called Quadrature Phase Shift Keying(QPSK).
DVB-S vs DVB-S2
You may come across DVB-S or DVB-2 when searching for information on satellite TV. These are standards that have been developed that combine many things such as modulation, compression techniques and frequency ranges among others. The “DVB” part stands for Digital Video Broadcasting and the “S” stands for satellite. If there is a “T” instead of an “S” this refers to terrestrial, “C”refers to cable. The “2” stands for second generation.
As I’m sure you will expect DVB-S2 is an improvement on DVB-S which includes lower required transmission power and greater compression techniques. DVB-S supports MPEG-2 compression whereas DVB-S2 supports MPEG-4 compression. In short, this means that you can more information within the same bandwidth. Although by no means necessarily a rule, generally speaking at the time of writing this Standard Definition(SD) broadcasts use DVB-2 whereas High Definition(HD) broadcasts use DVB-S2 which allows the extra information required to provide better TV pictures. This must not be assumed to always be the case however. For example, a few years ago Fransat moved all of their TV transmissions to the DVB-S2 standard meaning that everyone who didn’t already have a compatible box had to go out and purchase a new one, otherwise they wouldn’t be able to continue to watch their French TV services.
Symbol Rate/ Forward Error Correction
When setting up your satellite TV RX or when researching information on transmissions like of Kingofsat, you will come across symbol rate and error correction usually you don’t need to configure such things but you may need to if you’re going to manual tune your equipment. Symbol Rate is the data rate of any particular transponder, for the DTH satellites this is usually 22,000, 27,500 or 30,000. Forward Error Correction(FEC) is the amount of error correcting bits that are built into the signal. Typically, FEC values would be 2/3, 3/4, 5/6 etc. Using 2/3 as an example would mean that every third bit is broadcast specifically for error correction. The higher ratio of the FEC the more robust and forgiving a TV signal will be meaning that it will be less likely to pixelate/ break-up. The downside is that with all of that data/ bandwidth being used specifically for error correction there will be less for the TV service itself making it less suitable for services such as HD content.
Sat/ Sky TV Questions- In Blog Comments Section Only Please
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Until next time,
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