How satellite television works
A picture leaves a building in west London, travels 36,000 km straight up to a box that has been hanging over the equator since before you were born, and comes back down across a whole continent at once. Then a 45 cm mirror on your wall catches it.
It starts on the ground
Long before anything goes into space, a channel has to be assembled, squeezed, packed in with its neighbours and turned into a radio signal. Almost all of the clever engineering in satellite television happens in a building with a car park.
Compression is doing most of the work
Raw high-definition video is about 1.5 gigabits a second. A satellite transponder carries roughly 50 megabits in total, shared between everything on it. The encoder's job is to throw away around 99% of the data and leave something that still looks right — by describing what changed since the last frame rather than re-sending the picture, and by discarding detail the eye is poor at noticing.
This is why a fast-moving football crowd or falling confetti looks blocky while a newsreader looks perfect. Nothing is broken — those scenes genuinely contain more new information per second than the channel has room for.
Sharing the pipe
The multiplexer then interleaves several channels into a single stream. Cleverly, it does not give each one a fixed share: statistical multiplexing lets a channel showing a still caption give up its bandwidth to one showing a car chase, moment by moment. The total stays constant; the split moves constantly.
The takeaway By the end of this step the channel has stopped being a picture and become a stream of numbers. Everything after this is about moving those numbers 72,000 km without losing too many.
A satellite that never moves
Your dish is bolted to a wall. It does not track anything, has no motor, and has not been adjusted since the day it was fitted. That only works because of one specific altitude where an orbit takes exactly as long as a day.
Why the dish points at the horizon
The ring sits over the equator, so from the UK you are looking a long way south and not very far up — around 22 to 26 degrees of elevation, which feels surprisingly low the first time you notice it. Move to Spain and the dish tilts up; move to Scotland and it flattens further. Anywhere near the poles it would be pointing below the horizon, which is why satellite television simply does not work at high latitudes.
That southern arc across the sky has a name — the Clarke Belt, after Arthur C. Clarke, who described the idea in a magazine article in 1945, two decades before anyone could build one. He did not patent it. He later said he never imagined it would be practical in his lifetime.
Nothing stays put on its own
The Moon, the Sun and the fact that the Earth is not a perfect sphere all pull the satellite out of position. It carries fuel and thrusters, and fires them every couple of weeks to stay inside a box about 70 km across — close enough that your dish never notices. When that fuel runs out the satellite's working life ends, regardless of how healthy the electronics are, so operators plan a final burn to push it into a graveyard orbit a few hundred kilometres higher.
The delay you can hear
A quarter of a second is the physics — but the real gap is larger. Encoding, multiplexing and decoding all need buffers, so satellite is typically several seconds behind the same event on terrestrial or on a radio. If you have ever heard the neighbours cheer a goal before seeing it, this is why.
The takeaway The whole consumer satellite industry rests on one fact: at exactly this height, an orbit lasts one day. That is what makes a fixed dish on a wall possible.
What the satellite does
Less than you would think. A television satellite does not decode anything, does not store anything and does not know what a channel is. It catches a very faint signal, moves it to a different frequency, makes it enormously louder and points it back down. Engineers call it a bent pipe.
The numbers are absurd
By the time the uplink reaches the satellite it has spread out over 36,000 km and lost almost all of its strength — what arrives is measured in millionths of millionths of a watt. The satellite amplifies it by a factor of around a hundred billion and sends it back. On the ground, what your dish collects is still feeble enough that the first amplifier has to be cooled by nothing more sophisticated than the night air and yet add almost no noise of its own.
One satellite, many transponders
A modern broadcast satellite carries between thirty and sixty transponders, each handling a slice of spectrum around 26 to 36 MHz wide — enough for roughly 50 megabits a second, which is a dozen or so HD channels. Transponders are leased individually, which is why a broadcaster's channels tend to cluster together on the same frequency: they are renting the same pipe.
Living off sunlight, with an eclipse to plan for
Those solar panels produce ten to twenty kilowatts, most of which becomes heat and radio. Twice a year, around the equinoxes, the satellite passes through the Earth's shadow for up to seventy minutes a day and has to run on batteries — which is a significant part of why they are built the way they are. Everything about the design is a compromise with mass, because every extra kilogram had to be lifted to geostationary orbit.
Why it ends
A satellite's working life — typically fifteen years — is set by station-keeping fuel, not by electronics. When it is nearly out, the operator spends the last of it raising the satellite a few hundred kilometres into a graveyard orbit, because leaving a dead one drifting through the busiest ring in space is not an option.
The takeaway The satellite is a mirror with an amplifier. All the intelligence is at the two ends — which is exactly why a fifteen-year-old satellite can carry a video format that did not exist when it launched.
The footprint
The signal comes down in a shape — brightest where the satellite's antenna is aimed, fading towards the edges. That shape decides who can watch, how big a dish they need, and, increasingly, where a broadcaster is legally able to sell its programmes.
Wide beams and spot beams
A wide beam covers as much of the visible Earth as possible, which is convenient and imprecise. A spot beam concentrates the same power into a much smaller area, which makes the signal far stronger inside it — and almost absent outside. The same satellite usually carries both, on different transponders.
The famous example
For years, British expatriates across Spain and France watched UK television on a slightly larger dish, because the old satellites' beams spilled generously across Europe. In 2014 the UK channels moved to new satellites with a tightly shaped UK spot beam, and that overspill largely vanished. Whole businesses installing dishes on the Costa del Sol disappeared with it.
This was not an accident of engineering, it was the point. Programme rights are sold country by country, and a broadcaster that cannot technically contain its signal cannot honestly promise a rights holder it has. Beam shaping is as much a legal instrument as a technical one.
Why the edge is not a line
Coverage maps look like tidy contours, but the real edge is a gradient and it moves. Heavy rain, a slightly misaligned dish, a marginal LNB, a hot day — at the centre of a beam none of these matter, because there is signal to spare. At the edge, all of them do, on the same afternoon. This is why installers quietly fit larger dishes than the coverage map says are needed: the map describes clear-sky conditions, and the weather does not read maps.
The takeaway Whether you can receive a channel is not about your equipment first. It is about where you are standing inside a shape that was decided years ago on a drawing board.
Your dish and the LNB
The dish itself is the least clever object in this entire guide. It has no electronics, no moving parts and nothing to go wrong. It is a mirror, shaped so that everything hitting it arrives at one point — and it is what sits at that point that does the real work.
The block in low-noise block
The signal coming off the dish is at around 11 or 12 GHz. Sent down ordinary coaxial cable at that frequency it would be gone within a couple of metres. So the LNB — low-noise block downconverter — does two jobs at the focus: it amplifies while adding as little noise of its own as possible, and it shifts the whole block of frequencies down to something a cable can carry comfortably.
Four channels down one wire
The full satellite band is wider than the cable can carry, and each transponder is also transmitted in one of two polarisations — horizontal or vertical — which lets two transponders share the same frequency without interfering. So the LNB has four possible modes, and the box selects between them using the only two things it can send back up a single coaxial cable: the voltage it supplies, and a quiet tone.
| Box sends | LNB gives back | Oscillator |
|---|---|---|
| 13 V | Vertical, lower half of the band | 9,750 MHz |
| 18 V | Horizontal, lower half | 9,750 MHz |
| 13 V + a 22 kHz tone | Vertical, upper half | 10,600 MHz |
| 18 V + a 22 kHz tone | Horizontal, upper half | 10,600 MHz |
This is a genuinely elegant piece of design: no data protocol, no extra wires, just volts and a tone. It is also why a recorder wanting to watch one channel and record another needs two cables — it needs the LNB in two states at once. A quad LNB solves this by containing four independent receivers, and a Unicable system solves it differently by stacking several selected channels onto one cable at once.
Aiming it
Three adjustments matter, and all three interact: elevation (how far up), azimuth (how far round) and skew — rotating the LNB so its idea of "horizontal" matches the satellite's. Skew matters more the further east or west you are from the satellite's longitude; get it wrong and every horizontal transponder leaks into every vertical one. Given a 45 cm dish sees about 4° of sky, being 2° out already costs you half your signal, which is enough to be perfect in sunshine and unwatchable in rain.
The takeaway Nothing on your wall is smart. A curved sheet of steel and a sealed plastic box between them turn a continent-wide whisper into something a set-top box can read.
Tuning in
Everything the LNB selected is now travelling down one cable at once — dozens of transponders, side by side. The box has to pick exactly one of them, read the wobbles in it, and turn them back into the numbers that left London.
Why it fails all at once
Between the demodulator and the picture sits error correction, and it is astonishingly good. A stream arriving with thousands of wrong bits a second is repaired perfectly, with no visible effect whatsoever — so for most of the range between "perfect" and "broken" you see absolutely no warning that anything is wrong. Then the errors exceed what the correction can fix, and the picture blocks up and freezes within a second or two. There is no gradual fade, because all the graceful degradation was spent invisibly.
Strength and quality are different things
Every satellite box shows two bars, and confusing them causes most misdiagnoses. Strength is how loud the signal is — it will read high even if your dish is pointed at a completely different satellite, or at a brick wall, because the LNB is producing noise and noise is loud. Quality is how cleanly the constellation resolves, and it is the only number that matters. Strong and poor quality means aimed at the wrong thing; weak but good quality usually means a cable problem.
The details in the manual tuning screen
Adding a channel by hand asks for four things, and now they all mean something: the frequency picks the transponder, the polarisation tells the LNB which of its two receivers to use, the symbol rate says how fast positions are being sent, and the FEC rate says what fraction of the transmission is error-correction overhead rather than payload. Get one wrong and you get nothing — there is no partial match.
The takeaway DVB-S2 squeezes about 30% more into the same transponder than the original standard, purely through better maths at each end. The satellite did not change at all.
One stream, many channels
What comes out of the tuner is not a channel. It is a single conveyor belt of small packets, carrying a dozen channels' pictures, all their soundtracks, subtitles, programme guide data and a clock — shuffled together and labelled.
Channel numbers are a local invention
Nothing in the broadcast says “101”. The stream identifies services by number and name; your box holds a list mapping those to the positions you actually press. That list comes from the operator, which is why a Freesat box and a Sky box tuned to identical signals present completely different channel line-ups, and why a retune can shuffle everything around.
Why the guide takes a minute to fill in
Programme information is broadcast as tables carried in the stream itself, cycling round continuously — today's schedule frequently, next week's slowly. A box that has been unplugged has an empty guide and has to sit and collect it, which is why a freshly powered-on receiver shows "no information available" for a while and then quietly fills in. It is also why boxes ask to be left in standby rather than switched off at the wall.
Regional television, physically
Regional news variants are genuinely separate services in the multiplex, often on different transponders entirely. Your box is told which to use based on the postcode you typed in at installation. Get that wrong and you will be watching another region's news perfectly reliably — a configuration problem that looks nothing like a fault.
The takeaway A transponder is a shared pipe, not a channel. Changing channel within one multiplex is instant; changing to another transponder means retuning, which is why some channel changes are noticeably slower than others.
Paying for it
Broadcasting has an awkward commercial problem: the signal lands on everybody's roof whether they have paid or not, and there is no way to send anything back. Subscription television is the solution to that problem, and it is more ingenious than it looks.
Everything is sent to everyone
This is the part people find surprising. There is no individual stream for you. Your neighbour's box receives exactly the same bits, including the messages addressed to your card — it simply cannot do anything with them. Broadcast has no return path, so the whole system is built around sending everything everywhere and making the usefulness conditional.
It also explains the standing advice to leave a box in standby rather than unplugged. Entitlement messages for your card are broadcast on a cycle; a box that is switched off at the wall never hears them, and a subscription that should have renewed silently expires instead.
Why cards get replaced
The card is a small computer, and it is the only part of the chain an attacker gets to hold. When a card's security is broken, the broadcaster's answer is to change the algorithm and post everybody a new one — which is why subscription cards have been swapped out periodically for thirty years. Modern cards are also paired to a specific box, so a working card in the wrong receiver is no more use than no card at all.
Free-to-air is the same system, unlocked
Plenty of channels are broadcast with no scrambling at all. They travel through identical satellites, transponders and multiplexes; the only difference is that nobody applied a control word. That is why a basic receiver with no card picks up hundreds of channels, and why the same dish serves both.
And finally, a picture
Once descrambled, the video decoder rebuilds frames from the compressed stream, the audio decoder does the same for sound, and both are timed against a clock carried in the stream so they stay in step. The result goes out over HDMI — which performs its own little authentication handshake with your television, and which is its own occasional source of a black screen with perfectly good signal behind it.
The takeaway The signal was never protected from being received. It was protected from being understood — a completely different problem, and the only one that can be solved when you are shouting at a continent.
When it goes wrong
Satellite has a small, well-known set of failures, and most of them announce themselves by when they happen rather than what they look like. The timing is usually a better clue than the symptom.
What the timing tells you
| When it happens | Almost certainly |
|---|---|
| Only in heavy rain or snow, recovers after | Rain fade. Normal at the edge of a beam; a sign the dish has no margin if it happens in light rain. |
| Same few minutes each day, for about a week in March or September | Sun outage. The Sun passes directly behind the satellite and drowns it in noise. Nothing is wrong and nothing can be done. |
| Fine in winter, poor from May onwards | A tree. Bare branches pass the signal; leaves absorb it, and wet leaves absorb far more. |
| After a gale, and never recovered | Alignment. The dish or the bracket has moved, and the LNB is no longer at the focus. |
| Gradually worse over months, worst when wet | Water in the LNB or a corroded connector. Both are cheap; both are frequently misdiagnosed as a failing box. |
| Some channels only, permanently | Not reception at all — a subscription, a regional setting, or channels that have moved transponder. |
| Everything, suddenly, indoors | Power, cable or box. Check whether the LNB is still getting its voltage. |
The one measurement worth taking
Before anything else, look at quality, not strength, and look at it on a channel you know is on a different transponder. Good quality on one and nothing on another points at polarisation or band switching — a voltage or tone problem in the cable or LNB. Poor quality on everything, equally, points at the dish. Perfect quality with no picture points at the box, the card or the television.
Where this is all going
Satellite's great advantage is that it does not care how many people are watching: broadcasting to ten million costs precisely what broadcasting to ten does. Its disadvantage is that it cannot do anything else — no return path, no on-demand, no personalisation. That is why every satellite platform now ships a box with a broadband connection in the back, and why the long-term direction of travel is obvious even though the physics remains unbeatable for live events.
The takeaway Nine steps, 72,000 km, and a fifteen-year-old amplifier in space — and the part that actually fails is nearly always a plug on a wall, or a tree that has grown.