Celestial TV

[hopewrites]

Terrestrial TV channel

Not to wander further off topic, but does that imply there are celestial TV channels out there? I mean, I know I've been off TV for a while, and the likelihood of advances while I'm not looking are large... but I didnt think there were off-world channels yet.

 

[Ray McCarthy]

off-world channels yet.

Yes, there are.
Proposed by A. C. Clarke 1946 or 1948.
TV from Geostationary Satellites. They even renamed that ribbon of space where an orbit takes one day, the Clarke Belt.
Naturally they would be in the plane of the ecliptic so you'd only have reception twice a day as the Earth is tilted. So they expend some fuel keeping along the space 22,500 Miles directly above the equator. Otherwise they would be in an inclined orbit.

Terrestrial channels come via a TV aerial from a mast usually on a hill. Big flat areas (Netherlands, parts of USA and Australia) simply have a taller mast. Due to spectrum limitations made worse by selling some of it to Mobile Phone operators it's not possible to have very many channels. Cable TV can have about ten times as many channels. Satellite TV can have very many more channels than Cable (Cable uses 110MHz to 862MHz, but needs a lot of that for Broadband, any 8MHz or 6MHz can be Broadband or TV). Satellite uses C band to feed remote towns' Cable Head Ends (3700 to 4200 MHz) which carries less channels than fibre fed cable systems. Regular TV satellite is Ku Band (9700 to 12600 MHz), much more than cable and newer systems use Ka Band (18500 22000 MHz).
Most people's dish only picks up one satellite. But some places it's worth adding a bar and more receivers (LNBF) on the end of the arm. As it's reflection, each position on the bar "points" at a different satellite. Or you can have a motorised polar mount, that gets about 22 satellites here.
1) You can't receive a Satellite that's below the horizon.
2) The satellites have a big dish (or sometimes four of them), so they only "illuminate" certain areas, if you are not in that footprint, you can't get it, though with a very much larger dish you can get reception just outside the footprint as the signal simply fades rather than sharply cut off.
3) Some channels need a subscription and are encrypted. Others are free.

A site about Terrestrial, Ku (Free UK TV) and Ka (Free Irish TV) in Ireland.
http://www.saortv.info/satellite-saorsat/saorsat-reception/


Best list for everywhere.
http://www.lyngsat.com/
Being in the Midwest of Ireland I can get 22 satellites from 42E to 0.8E in Europe list and 1W to 50W approx in Atlantic group
http://www.lyngsat.com/europe.html
http://www.lyngsat.com/atlantic.html

I'm at about 8W, 52N so a Satellite at 4W is almost due south, but because it's footprint is to Central Europe and Middle East even a 2m dish won't receive it.
Example Footprint of Satellite well above horizon that I can't receive:
http://www.lyngsat-maps.com/footprints/Astra-1G-Western-Europe.html

I have 28.2E, 19E, 13E, 9E from a 95cm dish that can feed 12 satellite set top boxes, so wife is watching free German Eurosport for French Open (British Eurosport is Pay TV only). I also have a 1.1m dish on a polar mount motor feeding one receiver for any Satellite that has a suitable footprint. Buildings, trees, hills, thermal noise from ground etc means you can't actually go from horizon to horizon.

Some more explanations
http://www.techtir.ie/saortv/saorsat-coverage
http://www.techtir.ie/howto/install-motorised-dish
http://www.techtir.ie/blogs/watty/rte-kasat-tests

 

[chrispenycate]

I was going to explain (although slightly less technically) what Ray was saying - the terrestrial- satellite labels are where the signal reaches you from, not where the program material originates. Otherwise 'celestial' I've not met much, certainly on religious broadcast networks. Maybe the Moon landing? On the other side, infernal broadcasting, I could believe in that…

Naturally they would be in the plane of the ecliptic so you'd only have reception twice a day as the Earth is tilted.

Why in the plane of the ecliptic? It should be perfectly stable - well, as stable as anything is - over the equator - what, from the rest of the solar system, would be perturbing that orbit? After all, ball of string orbits which actually go over the poles are stable, in total contradiction to the rest of the sun's family.

Sure, they do use reaction mass to absorb the little wobbles and imperfections in the Earth's gravitational field, but this is a tiny fraction of what would be expended bobbing twenty-three degrees across the equator all the time.

Besides, except in Scandinavia, would they ever go low enought to cut reception?

 

[Ray McCarthy]

Our equator is tilted WRT to plane of solar orbit. Gradually the sun and moon pull the Satellite from an orbit aligned with the equator, so the satellite appears visually to bob up and down twice a day. Once it's more than about 0.25 degree the signal suffers. This why 4.8m dishes on ground stations have motor bobbing the dish up and down by a small amount because to save fuel the Satellites are allowed to be slightly inclined orbit.

Edit: Looks like if you only corrected in bursts, a Direct to Home TV satellite needs correction about 4 or 5 times a year. They probably do smaller bursts more often. Newer satellites have Solar panel powered Xenon reaction mass Ion drives, which need less reaction mass than older hydrazine rockets for same time.

When fuel is low the satellite is removed from regular service and deliberately put in a fuel saving inclined orbit till only enough fuel to put it in a graveyard orbit is left.
3W and 14W are near end of life. Only used for specialised services where both ends can track the satellite.
http://www.lyngsat.com/atlantic.html
With a fixed dish, you only get a signal for about an hour twice a day.

A quick check reveals wikipedia says much the same thing, though not very clear either.
https://en.wikipedia.org/wiki/Inclined_orbit
ANYTHING in an equatorial orbit (at any height) will slowly be dragged by moon, sun (and probably Jupiter slightly) closer to the plane of ecliptic and thus to an Earth observer bob up and down. (Twice a day only if at geostationary / Geosynchronous altitude)


Smaller a dish is, the easier it is to point and more tolerant of slight inclination. A 45cm Sky Zone 1 dish is really easy to align compared to 1.1m which needs a professional meter. A 2m dish needs a calibrated mount. A 3.7m dish needs elevation and azimuth motors.

My Broadband is supposed to use a 30cm x 30cm panel for its terrestrial microwave link. It's actually 2 flat aerial arrays 13 x 26 approx. Because I'm too far away, I have a 44cm dish. It's much more critical and quite hard to find the signal less than 14km away.

After all, ball of string orbits which actually go over the poles are stable, in total contradiction to the rest of the sun's family.

They do drift. It doesn't matter as those need tracked anyway and purpose is to cover whole of the Earth's surface in overlapping strips.
One special trans polar orbit needs station keeping fuel occasionally.
Sun synchronous.
It can be in daylight so as to maximise solar power or on the dark side to study space with Earth blocking solar radiation and light.

See https://en.wikipedia.org/wiki/Geosynchronous_orbit
Geosynchronous is not naturally Geostationary
https://en.wikipedia.org/wiki/Geostationary_orbit

Sun Synchronous (head wrecking)
https://en.wikipedia.org/wiki/Sun-synchronous_orbit
It needs a bit more station keeping fuel than ordinary polar orbits but much less than Geostationary.

I did use to work in BBC and for an ISP that sold two way Satellite Internet links.

EDIT
This section explains it maybe a bit better

Orbital stability
A geostationary orbit can only be achieved at an altitude very close to 35,786 km (22,236 mi), and directly above the Equator. This equates to an orbital velocity of 3.07 km/s (1.91 mi/s) or an orbital period of 1,436 minutes, which equates to almost exactly one sidereal day or 23.934461223 hours. This ensures that the satellite will match the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.

A combination of lunar gravity, solar gravity, and the flattening of the Earth at its poles causes a precession motion of the orbital plane of any geostationary object, with an orbital period of about 53 years and an initial inclination gradient of about 0.85 degrees per year, achieving a maximum inclination of 15 degrees after 26.5 years. To correct for this orbital perturbation, regular orbital stationkeeping manoeuvres are necessary, amounting to a delta-v of approximately 50 m/s per year.

A second effect to be taken into account is the longitude drift, caused by the asymmetry of the Earth – the Equator is slightly elliptical. There are two stable (at 75.3°E, and at 104.7°W) and two unstable (at 165.3°E, and at 14.7°W) equilibrium points. Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation. The correction of this effect requires orbit control manoeuvres with a maximum delta-v of about 2 m/s per year, depending on the desired longitude.

Solar wind and radiation pressure also exert small forces on satellites which, over time, cause them to slowly drift away from their prescribed orbits.

In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite.

 

[Vertigo]

Fascinating, you might be able to answer a question that's been bugging me for sometime now. Do you know how far North (and South) you have to go before geostationary orbit goes below the horizon (my maths is just too rusty for me to figure it out ). I live pretty far North (Lat 57 36 44) and my dish is point almost horizontal. I think the guy that fitted it said we were about 2 degrees off horizontal here.

 

[Ray McCarthy]

57.5N isn't far north at all!

The modern dishes are offset feed, unless a centre fed, you can't see the real elevation. They look more horizontal pointing than they really are. It means snow doesn't collect. There are other reasons why smaller dishes are offset and larger ones are centred.

At 52N the Elevation for due south is about 30 degrees. (Me)*
At 57N the Elevation for due south is about 25 degrees.

Unless your dish is high up and ground clear, less than 10 degrees elevation a problem.
In a clear view, with large dish, perhaps 5 degrees is possible

So for Satellites to East or West of your due South, for 10 degrees elevation, 55 degrees either direction max (big dish and clear view)

71 North (or equivalent south of Equator) is 10 Degrees elevation for a Due South Satellite, if clear view and high up and big dish then 5 deg elevation allows only 33 degrees +/- of due south.
At 5 degrees elevation, for due south satellite: 76 N
80N is only 1 Degree elevation. On a tall mast / building / hill with enormous dish, probably with motor tracking. Zero elevation is about 81N.

I know they had to run fibre to an Antarctic base and Trondheim in Norway has difficulties. But you can run a the cable from a dish for many km and you can now get fibre connected LNBF (the receiver on end of arm).

You can calculate all this from Trig / Geometry, but I have a program. It also tells me other stuff.


[* As I'm Mid West of Ireland, my due south is Satellites about 8W, hence 42E to 50W is the limits for what I can receive. BUT beyond 42E and 50W, there are satellites above the horizon, but the Western ones point at USA, Mexico, South America and the Eastern ones at Central Europe, Africa and Asia, so no signal, too far out side footprint.]
If I had a 3.7m Mesh C band dish I could get this.
http://www.lyngsat.com/Intelsat-805-and-Galaxy-11-and-Amazonas-1.html
C band needs a dish four times bigger than normal DTH Ku Band.

The red line at left edge indicates the horizon cut off for SES 2
http://www.lyngsat-maps.com/footprints/SES-2-North-America-Ku.html
You can see it's impossible in Alaska, but a satellite due south of Alaska will work.

Anik F3 does work in Alaska and Canada
http://www.lyngsat-maps.com/footprints/Anik-F3-North-America-Ku.html
Note red line at top is horizon limits

 

[Ray McCarthy]

I think the guy that fitted it said we were about 2 degrees off horizontal here.

Most countries the Dish Installers are only "fitters" that are trained like dogs. They only know which end of drill is pointy. Your dish LNBF may be at 18 to 20 degree offset, so if your Satellite TV is from some place East or West of your local Due south, the elevation on bracket might be only 2 degrees, but real elevation 18 to 25, I can't tell without your longitude and the satellite name (you can pm the exact satellite or service provider and your location and I can be precise).

 

[Ray McCarthy]

A.C. Clarke wrote a Kid's SF to illustrate his TV satellite idea
USA (c) and pub. 1954 "Islands in the Sky"
I have the 1971 puffin reprint.
The first Geostationary TV Satellite was maybe mid 1960s., about 20 years after Clarkes paper. I'm sure various scientists in the 1930s knew of the possibility, but didn't write famous papers on it.

 

[Vertigo]

Most countries the Dish Installers are only "fitters" that are trained like dogs. They only know which end of drill is pointy. Your dish LNBF may be at 18 to 20 degree offset, so if your Satellite TV is from some place East or West of your local Due south, the elevation on bracket might be only 2 degrees, but real elevation 18 to 25, I can't tell without your longitude and the satellite name (you can pm the exact satellite or service provider and your location and I can be precise).

Well it certainly looks very flat. I certainly wouldn't know. But then I'm trying to visualise the focal point of a dish in my mind, which is a decidedly inaccurate process!

 

[Vertigo]

57.5N isn't far north at all!

The modern dishes are offset feed, unless a centre fed, you can't see the real elevation. They look more horizontal pointing than they really are. It means snow doesn't collect. There are other reasons why smaller dishes are offset and larger ones are centred.

At 52N the Elevation for due south is about 30 degrees. (Me)*
At 57N the Elevation for due south is about 25 degrees.

Unless your dish is high up and ground clear, less than 10 degrees elevation a problem.
In a clear view, with large dish, perhaps 5 degrees is possible

So for Satellites to East or West of your due South, for 10 degrees elevation, 55 degrees either direction max (big dish and clear view)

71 North (or equivalent south of Equator) is 10 Degrees elevation for a Due South Satellite, if clear view and high up and big dish then 5 deg elevation allows only 33 degrees +/- of due south.
At 5 degrees elevation, for due south satellite: 76 N
80N is only 1 Degree elevation. On a tall mast / building / hill with enormous dish, probably with motor tracking. Zero elevation is about 81N.

I know they had to run fibre to an Antarctic base and Trondheim in Norway has difficulties. But you can run a the cable from a dish for many km and you can now get fibre connected LNBF (the receiver on end of arm).

You can calculate all this from Trig / Geometry, but I have a program. It also tells me other stuff.


[* As I'm Mid West of Ireland, my due south is Satellites about 8W, hence 42E to 50W is the limits for what I can receive. BUT beyond 42E and 50W, there are satellites above the horizon, but the Western ones point at USA, Mexico, South America and the Eastern ones at Central Europe, Africa and Asia, so no signal, too far out side footprint.]
If I had a 3.7m Mesh C band dish I could get this.
http://www.lyngsat.com/Intelsat-805-and-Galaxy-11-and-Amazonas-1.html
C band needs a dish four times bigger than normal DTH Ku Band.

The red line at left edge indicates the horizon cut off for SES 2
http://www.lyngsat-maps.com/footprints/SES-2-North-America-Ku.html
You can see it's impossible in Alaska, but a satellite due south of Alaska will work.

Anik F3 does work in Alaska and Canada
http://www.lyngsat-maps.com/footprints/Anik-F3-North-America-Ku.html
Note red line at top is horizon limits

Again fascinating and thanks for the detailed reply. I always figured the cut off would probably be around "Lapland" somewhere.

 

[Ray McCarthy]

But then I'm trying to visualise the focal point of a dish

Don't try. it's at the horn of the LNBF on the end of the arm.

Note it is pointing up at an angle to the dish. Imagine a tangent to point on dish the LNB is pointing. The true elevation is the reflection of that angle to the tangent.
But if knew your location and which provider, then I'd know the true elevation anyway. When I'm receiving Turksat 42E on my 110cm motorised dish it looks like it's pointing at ground!
Azimuth 123.5
Elevation 15

The dish appears almost "vertical" when I select 28.2E (Freesat & Sky for UK & Ireland),
Azimuth 137
Elevation 22
Hence rule of thumb in UK & Ireland is to fold a square twice (22.5 degrees) and look along slope with base level pointing it southeast (135 degrees) and if you can see sky, you are not "blocked" by trees or buildings. My daughter's dish is on a pole on the ground at back of her garden to "see over" the house as mounting on front of house not possible for various reasons. The 30m of coax is no bother as half the receiver electronics is in the LNBF on the arm of the dish.

Highest Elevation here is 30.5 for due South, (8W approx) then the dish "falls" again as it moves west.
The highest elevation rises as you move to Equator, where a polar mount would be horizontal.

 

[Vertigo]

Expressed myself badly there; the focal point is, of course, where the receiver is. It's really the - I suppose - axis I should have said. I'm not unduly concerned with my set up, which works fine, I was just curious about the horizon. And latitudes somewhere in the neighbourhood of Trondheim look about where I would have expected it to be.

 

[Ray McCarthy]

focal point is, of course, where the receiver is. It's really the - I suppose - axis

Axis goes through the focal point. Where the LNBF is and may be outside the physical dish!
Imagine a symmetrical centre fed paraboloid dish about twice as tall as your dish. Then you only reproduce copies of part of one side of it. About a third to a fifth of the area (only a guess). That's what an offset dish is. If the arm is lost and you don't know angle or length of arm it's very hard to find the real focus. With a symmetrical centre fed dish you know focus is somewhere along the "normal" to centre of dish. It's possible easily to replace the missing LNBF support (usually a tripod rather than an arm).

For best results some offset dishes have an oval horn or oval fresnel type focus rings so the LNBF doesn't pick up noise past the rear of the small dish. Thus the slightly oval UK Sky perforated Ku band dishes need a different LNBF to better quality generic satellite dishes. A perforated dish is just to reduce weight (shipping cost) and manufacturing cost (cheaper press), they rust faster can same to slightly more wind load! Only C band and lower uses actual mesh, though often simply larger perforations and petals. Ka band dishes need to be solid.

 

[Hex]

Hi guys -- Ray asked us to move this bit of your discussion to Technology.

 

[Vertigo]

@Hex probably more appropriate!

@Ray McCarthy Hence my problem with looking at it and trying to figure what angle it is at. Mine is an ancient Sky one. I got Sky originally then switched to just using their free channels and then switched to Freesat but never changed the dish. Same physical satellite anyway I believe and it seems to work just fine!

 

[Ray McCarthy]

Same physical satellite anyway

Same group of satellites*. Only the EPG and Interactive transmissions are different. The actual free channels are the same physical transmission. BARB figures in UK and Nielsen in Ireland reveal that about 92% of viewing time of Sky Subscribers is actually of Free To Air TV.

For some reason I thought you were in USA!

* The beamwidth of a dish up to 90cm size means that a group, or constellation, of even seven satellites hundreds of kilometres apart seems like a single satellite. With a 110cm dish the satellites at the edge are a weaker signal, but if it's a group meant for your own country, it's not a problem. At 2.5m dish size you need a motorised positioner or you will lose channels as the sun transits "behind" the satellites** (it's actually the other way round) or a couple of times a day due to slight inclination of the orbit.

You can point at a different satellite by moving the LNBF rather than dish, as it's a mirror. But as then you are of the axis of the real focus the signal is much less. Thus a sky dish is too small. I have a 95cm dish with a curved and angled bar on the dish arm with four Quattro LNBFs, this then feeds a clever box with twelve outlets for satellite receivers. These simulate a four way switch and four regular LNBFs. Such technology can be expanded to 16 satellite positions (usually using multiple dishes) and over 2000 receivers for a large apartment block, small village in a valley, hospital, prison etc.
A motorised system, which I have also, can get as many satellites as you can receive, about 24 in East of England and 22 in West of Ireland, but can only feed one receiver. My motorised system has the larger 110cm dish as feeds are lower power and I'm outside of the normal footprints on most of the positions.
The Irish Channels work on a Sky Sub, but only if the viewing card issued to an Irish Address, so when people in Ireland cancel, the Sky box still gets the free UK channels, but they need an aerial (or Ka Sat which doesn't work on a Sky box) for Irish TV. All the Free To Air UK channels have "Broadcast Rights" that include Ireland as even in 1950s many people had UK TV via big masts. 75% of Ireland had some form of BBC & ITV before Satellite. But UK is 20x TV viewing public, so no Irish TV Station is FTA on normal Satellite. You can get some of the Irish Radio on Satellite.

** Twice a year reception is really poor for about 5 to 15 minutes for a few days. Think about apparent height of sun in sky in the morning when it's in the South East. It's a VERY strong wide band radio noise source!

 

[Vertigo]

For some reason I thought you were in USA!

Oh no Scottish Highlands me; half hour North of Inverness.

 

[Ray McCarthy]

half hour North of Inverness

Proper Nobility then. You'd get about 20 satellites there.

 

[Vertigo]

Ah well I watch very little TV these days so Free Sat gives me as much as I need.

 

[Ray McCarthy]

I mostly use it for the greater variety of radio. I use €4 legal FM transmitter sold as "mp3/phone adaptor to car radio" plugged into satellite box stereo out via SCART adaptor and actually powered off the sat box too. I have three such gadgets. So three extra FM stereo radio stations in the house, one is MP3s on computer, two are sat boxes.