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-   -   BBC News Blunder (https://www.digitaltvbanter.co.uk/showthread.php?t=36238)

Java Jive[_2_] July 30th 17 10:28 AM

BBC News Blunder
 
"Powys man gets broadband via satellite over Africa"

http://www.bbc.co.uk/news/uk-wales-mid-wales-40745533

"[It is the same satellite companies like Sky use to broadcast their
TV signal and] they are positioned above the equator to maximise
coverage."

Oh dear! Actually, they are positioned over the equator because that
is the only place you can obtain a geo-stationary orbit, enabling
fixed dishes to work. Satellites that aren't in geo-stationary orbit
require movable dishes that can track an orbit to communicate with
them ...

http://www.macfh.co.uk/JavaJive/Audi...sisClarke.html
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NY July 30th 17 11:49 AM

BBC News Blunder
 
"Java Jive" wrote in message
...
"Powys man gets broadband via satellite over Africa"

http://www.bbc.co.uk/news/uk-wales-mid-wales-40745533

"[It is the same satellite companies like Sky use to broadcast their
TV signal and] they are positioned above the equator to maximise
coverage."

Oh dear! Actually, they are positioned over the equator because that
is the only place you can obtain a geo-stationary orbit, enabling
fixed dishes to work. Satellites that aren't in geo-stationary orbit
require movable dishes that can track an orbit to communicate with
them ...


Could you not have a satellite in a geostationary orbit above another line
of latitude than the equator, as long as it still rotates in a line parallel
to the equator and rotates around the earth at the same rate as the earth
spins? Or would there be a nett force dragging it towards the nearer of the
two poles - is the equator the only place for geostationary because there is
no nett force towards either pole?


AnthonyL July 30th 17 12:11 PM

BBC News Blunder
 
On Sun, 30 Jul 2017 11:28:58 +0100, Java Jive
wrote:

"Powys man gets broadband via satellite over Africa"

http://www.bbc.co.uk/news/uk-wales-mid-wales-40745533

"[It is the same satellite companies like Sky use to broadcast their
TV signal and] they are positioned above the equator to maximise
coverage."

Oh dear! Actually, they are positioned over the equator because that
is the only place you can obtain a geo-stationary orbit, enabling
fixed dishes to work. Satellites that aren't in geo-stationary orbit
require movable dishes that can track an orbit to communicate with
them ...

http://www.macfh.co.uk/JavaJive/Audi...sisClarke.html


What would the BBC know about programme transmission? It's like
expecting BT to understand the internet.



--
AnthonyL

Java Jive[_2_] July 30th 17 12:42 PM

BBC News Blunder
 
On Sun, 30 Jul 2017 12:49:34 +0100, "NY" wrote:

Could you not have a satellite in a geostationary orbit above another line
of latitude than the equator, as long as it still rotates in a line parallel
to the equator and rotates around the earth at the same rate as the earth
spins? Or would there be a nett force dragging it towards the nearer of the
two poles - is the equator the only place for geostationary because there is
no nett force towards either pole?


The gravitational attraction between the earth and the satellite can
be considered as acting between their respective centres of gravity,
that of Earth of course being at its centre. Therefore only an orbit
around the centre of Earth is stable. An orbit around a line of
latitude other than the equator would not be stable, and would require
a constant expenditure of fuel to counterbalance the gravitational
pull towards the centre of the earth. This constant expenditure of
fuel, as opposed to the occasional expenditure of fuel to make
corrections to maintain a stable orbit around the centre of Earth,
makes such a satellite uneconomic in the extreme.

I know less about GPS, but I believe even their satellites orbit
around Earth's centre. The system covers the whole globe by having
many satellites whose orbits are so arranged that wherever you are on
Earth, there will always be at least two or three satellites in the
sky above you.
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Java Jive[_2_] July 30th 17 12:43 PM

BBC News Blunder
 
On Sun, 30 Jul 2017 12:11:19 GMT, lid (AnthonyL)
wrote:

What would the BBC know about programme transmission? It's like
expecting BT to understand the internet.


They're a supposedly reputable news organisation, it's their job to
get their facts right!
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================================================== ======
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Peter Duncanson July 30th 17 01:22 PM

BBC News Blunder
 
On Sun, 30 Jul 2017 13:42:38 +0100, Java Jive
wrote:

On Sun, 30 Jul 2017 12:49:34 +0100, "NY" wrote:

Could you not have a satellite in a geostationary orbit above another line
of latitude than the equator, as long as it still rotates in a line parallel
to the equator and rotates around the earth at the same rate as the earth
spins? Or would there be a nett force dragging it towards the nearer of the
two poles - is the equator the only place for geostationary because there is
no nett force towards either pole?


The gravitational attraction between the earth and the satellite can
be considered as acting between their respective centres of gravity,
that of Earth of course being at its centre. Therefore only an orbit
around the centre of Earth is stable. An orbit around a line of
latitude other than the equator would not be stable, and would require
a constant expenditure of fuel to counterbalance the gravitational
pull towards the centre of the earth.


And I don't think that a circular path following a line of latitude is
technically an "orbit".

This constant expenditure of
fuel, as opposed to the occasional expenditure of fuel to make
corrections to maintain a stable orbit around the centre of Earth,
makes such a satellite uneconomic in the extreme.

I know less about GPS, but I believe even their satellites orbit
around Earth's centre.


They do.

The system covers the whole globe by having
many satellites whose orbits are so arranged that wherever you are on
Earth, there will always be at least two or three satellites in the
sky above you.


--
Peter Duncanson
(in uk.tech.digital-tv)

7 July 30th 17 01:33 PM

BBC News Blunder
 
NY wrote:

"Java Jive" wrote in message
...
"Powys man gets broadband via satellite over Africa"

http://www.bbc.co.uk/news/uk-wales-mid-wales-40745533

"[It is the same satellite companies like Sky use to broadcast their
TV signal and] they are positioned above the equator to maximise
coverage."

Oh dear! Actually, they are positioned over the equator because that
is the only place you can obtain a geo-stationary orbit, enabling
fixed dishes to work. Satellites that aren't in geo-stationary orbit
require movable dishes that can track an orbit to communicate with
them ...


Could you not have a satellite in a geostationary orbit above another line
of latitude than the equator


No because Earth is a slight doughnut shape with the bulge through
the equator, and the Moon is also orbiting around the equator
which means orbits other than equator are not stationary or stable.

Jim Lesurf[_2_] July 30th 17 02:02 PM

BBC News Blunder
 
In article , 7
wrote:

No because Earth is a slight doughnut shape with the bulge through the
equator, and the Moon is also orbiting around the equator which means
orbits other than equator are not stationary or stable.


IIUC In reality, none of the usual orbits are absolutely stable. Just that
some get closer than others. Even geostationary satellites tend to require
'stationkeeping' thrust burns and may drift away from their nominal
location when that is no longer possible. The usual circles and ellipses
are just a first approximation to real behaviour.

IIRC it is routine to 'park' geostationary sats elsewhere as they near the
end of the design life.

Jim

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Richard Tobin July 30th 17 02:35 PM

BBC News Blunder
 
In article ,
NY wrote:

Could you not have a satellite in a geostationary orbit above another line
of latitude than the equator, as long as it still rotates in a line parallel
to the equator and rotates around the earth at the same rate as the earth
spins? Or would there be a nett force dragging it towards the nearer of the
two poles - is the equator the only place for geostationary because there is
no nett force towards either pole?


The centre of a circular orbit must be the earth's centre of gravity,
so you can't have an orbit along a line of latitude.

You can have a 24 hour orbit at any orientation, so you could have a
tilted orbit that remained at the same longitude. But it would change
latitude during the day.

See https://en.wikipedia.org/wiki/Synchronous_orbit

-- Richard

NY July 30th 17 02:39 PM

BBC News Blunder
 
"Java Jive" wrote in message
...
On Sun, 30 Jul 2017 12:49:34 +0100, "NY" wrote:

Could you not have a satellite in a geostationary orbit above another
line
of latitude than the equator, as long as it still rotates in a line
parallel
to the equator and rotates around the earth at the same rate as the earth
spins? Or would there be a nett force dragging it towards the nearer of
the
two poles - is the equator the only place for geostationary because there
is
no nett force towards either pole?


The gravitational attraction between the earth and the satellite can
be considered as acting between their respective centres of gravity,
that of Earth of course being at its centre. Therefore only an orbit
around the centre of Earth is stable. An orbit around a line of
latitude other than the equator would not be stable, and would require
a constant expenditure of fuel to counterbalance the gravitational
pull towards the centre of the earth.


Ah, of course. The centre of an orbit about a latitude of (for example) 50
deg N is not at the centre of the earth. So I was right: there is a nett
force in a north/south direction.

I know less about GPS, but I believe even their satellites orbit
around Earth's centre. The system covers the whole globe by having
many satellites whose orbits are so arranged that wherever you are on
Earth, there will always be at least two or three satellites in the
sky above you.


Yes. I think GPS satellites are still in orbits about the centre of the
earth but at a lower altitude so they move relative to the earth's surface,
so a give point on the earth will see various satellites rising, travelling
across the sky and setting at different times of day. I'm not sure how long
a given satellite is above the horizon, or whether all satellites are at the
same altitude and so have the same orbit time. I imagine that GPS satellites
have orbits that are at various angles to the equator.



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