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BBC News Blunder



 
 
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  #1  
Old July 30th 17, 10:28 AM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Java Jive[_2_]
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Posts: 1,774
Default 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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  #2  
Old July 30th 17, 11:49 AM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
NY
external usenet poster
 
Posts: 1,329
Default 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?

  #3  
Old July 30th 17, 12:42 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Java Jive[_2_]
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Posts: 1,774
Default 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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  #4  
Old July 30th 17, 01:22 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Peter Duncanson
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Posts: 4,272
Default 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)
  #5  
Old July 30th 17, 02:39 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
NY
external usenet poster
 
Posts: 1,329
Default 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.

  #6  
Old July 30th 17, 03:43 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Tony van der Hoff
external usenet poster
 
Posts: 1
Default BBC News Blunder

On 30/07/17 15:39, NY wrote:

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.


He
https://en.wikipedia.org/wiki/Global...4goldenSML.gif
  #7  
Old July 30th 17, 04:48 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Bill Wright[_3_]
external usenet poster
 
Posts: 2,448
Default BBC News Blunder

On 30/07/2017 15:39, NY wrote:

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.


Weather satellites orbit over the poles.

Bill
  #8  
Old July 31st 17, 04:16 PM posted to uk.tech.digital-tv
MR
external usenet poster
 
Posts: 31
Default BBC News Blunder

On Sunday, 30 July 2017 17:48:36 UTC+1, wrote:
On 30/07/2017 15:39, NY wrote:

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.


Weather satellites orbit over the poles.

Bill


Some high resolution ones do, but there is global coverage from geostationary satellites as well. This produces the famous squashing up of Scotland effect which so ****ed off the Scots when they introduced those evil brown weather charts a few years' back.

MR

  #9  
Old July 30th 17, 06:03 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Alan White[_3_]
external usenet poster
 
Posts: 95
Default BBC News Blunder

On Sun, 30 Jul 2017 15:39:17 +0100, "NY" wrote:

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.


The GPS satellites are in 55 degree, semi-sidereal orbits. This means
that their orbits are inclined at 55 degrees to the equator and that the
orbital period is 11h 58m. The last time I checked there were 27 active
satellites. Reception from three satellites is required for a 2D fix,
four satellites for a 3D fix. The horizontal position will have a
typical accuracy of better than 15 metres anywhere on the globe for 95%
of the time over a 30 day period. The 3D position altitude error is
roughly twice the horizontal error.

--
Alan White
Mozilla Firefox and Forte Agent.
By Loch Long, twenty-eight miles NW of Glasgow, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather
  #10  
Old July 31st 17, 09:58 AM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Brian Gaff
external usenet poster
 
Posts: 7,250
Default BBC News Blunder

No I don't think this is right. It is not a problem to orbit at any
inclination its just that even at a geosynchronous altitude the position
will appeared to drift up and down as the earth turns as the axis will not
be the same

GPS sats are in all sorts of inclinations and they all have very accurate
clocks on board and know where they are by this and their speed so that
means its simply a calculation for the gps. Indeed one of the bigger
problems with gps has been Doppler shift of the frequencies.
You will also find that the Iridium sat phone constellation are all over
the place, but once again like Gps the system knows where they are.

You have to be more or less dead on to the actual equator to be
geosynchronous without needing loads of fuel.
Bear in mind that inclination changes are expensive of energy. You are
changing several parameters at the same time.
If you speed up you get ovalisation of the orbit, if you slow down, the
same happens so you have to compensate for this, lower orbits are faster
and higher slower, which seems counter intuitive till you realise that all
orbits are in fact is a free fall toward the gravitational centre of what
you are orbiting and as things fall they speed up. they only stay the same
when in a circular orbit and they only match the rotation of the earth at
around 24000 miles out.

That is a long way out and why the footprints of coverage are large.
Brian

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"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. 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.
--
================================================== ======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html



 




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