BBC News Blunder

In article , Java Jive
scribeth thus
"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...eAnalysisClark
e.html


And no mention of the laggy results either.

Bloody modern Jurno's know feck all!.
--
Tony Sayer

A more serious and puzzling question is: how did this utterly mundane story get into the BBC in the first place? Don't people in remote areas routinely use satellites to get Internet?

Some company's PR department must be laughing all the way to the bank.

On 30/07/2017 11:28, 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


--
Clive Page

"tony sayer" wrote in message
...
In article , Java Jive
scribeth thus
"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...eAnalysisClark
e.html


And no mention of the laggy results either.

Exactly. You get a wonderful throughput of data once you've sent your
request for a web page or a download, though I imagine the uplink/downlink
lag will cause problems with sending acknowledgements back: large packet
sizes and small ack-to-data ratio helps a lot.

Yes don't people actually understand orbits by now?
If there was just a slight inclination the sat would weave up and down as
viewed from the ground, after all.
The point of coverage depends very much on the aerials on the damn thing
and how they are aimed and focussed. Many sats have several places where
they target the signals from memory.
I often wonder aboutsat broadband though as surely it will be painfully
slow with every packet taking significant time to go up and down again.


Brian

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"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 ...

http://www.macfh.co.uk/JavaJive/Audi...sisClarke.html
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Yes you can position them anywhere, but the point is that it has to be at
the equator.
Obviously if the position was so low in the sky from the uk that it was
occulted by the land or buildings, its not going to work well and you would
be unlikely to be in its coverage area for any of the beams sent to earth or
receiving from it unless the programme material was uplinked from here.
Brian

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"NY" wrote in message
o.uk...
"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?

NY wrote:
"tony sayer" wrote in message
...
In article , Java Jive
scribeth thus
"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...eAnalysisClark

e.html


And no mention of the laggy results either.

Exactly. You get a wonderful throughput of data once you've sent your
request for a web page or a download, though I imagine the
uplink/downlink lag will cause problems with sending acknowledgements
back: large packet sizes and small ack-to-data ratio helps a lot.

I think most of the satellite devices at customer premises contain a
proxy. So the client devices (PC, Mac, whatever) see normal TCP
acknowledgement delays. The proxy then communicates via the satellite
with another proxy at the satellite's base station and between them they
handle the long response times. It may also trie to buffer and look
ahead. That proxy then communicates with the rest of the internet.

Web servers often think the client is in the country where the satallite
base station is located - which can give problems for region-specific
material.

So for downloading web pages the system works reasonably well. Anything
requiring user input will be a bit slow but may be no worse than the
ADSL alternative for a "difficult" location. Anything interactive
(Telnet, VOiP, VPN, games, etc.) is likely to be really treacly.

--
Graham J

"Brian Gaff" wrote in message
news
I often wonder aboutsat broadband though as surely it will be painfully
slow with every packet taking significant time to go up and down again.

The big problem with satellite broadband is flow control. When you request a
web page or a download, it will indeed take a significant time for the
request to reach the server and for the returned data to start arriving.
https://en.wikipedia.org/wiki/Geostationary_orbit says that the satellite is
about 36,000 km "above the earth's equator" (I presume that's distance from
the surface, not the centre, of the earth.

So the transit time for the data will be 2 x 36,000,000 / 3x10^8 = 0.25
seconds.

Once you've started the flow of data (the initial pause) it will arrive as
quickly as it would over landlines.

But the receiving end needs to be able to control the flow of data, so it
doesn't arrive faster than can be processed. And then TCP flow-control
packets have to travel back by the same route and so will take that same
time to arrive at the sending end.

I've forgotten what the maximum amount of data is that can be sent without
acknowledgement, but I seem to remember it's only a mater of a few 1500-byte
packets, so that means that every few kB of data needs a 0.25 second pause
while the sender says "OK. Ready for some more data" and another 0.25 sec
before the sender's data arrives. This is what slows down the overall data
transfer.

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.
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"Brian Gaff" wrote in message
news
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

A satellite can orbit the earth geosynchronously in an orbit about the
centre of the earth providing it's at the magic altitude of about 36,000 km
above the earth's surface. But as you say, unless the orbit is about the
equator, the satellite will change its elevation in the sky (north/south),
even though its azimuth (east/west) remains static.

An orbit parallel to the equator but at a latitude of (for example) 50
degrees (for northern Europe) will require constant burning of fuel to keep
it overhead, so it's not viable.

GPS satellites, as opposed to communications satellites, are a different
beast: they orbit lower (about 20,000 km) and they *do* move relative to the
ground: a GPS receiver will typically see several above the horizon, but not
always the same ones: the rotational period is about 12 hours. I presume
they again rotate about the centre of the earth, but at one of a variety of
angles with respect to the equator.

GPS sats are not in all sorts of inclinations.

Here's the post I sent yesterday:-

quote
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.
/quote

The GPS receiver calculates what is called the PVT, (Position, Velocity,
Time) solution based on the received signals from the satellites. The
velocity bit uses Doppler shift of the received frequency from those
satellites.

On Mon, 31 Jul 2017 10:58:30 +0100, "Brian Gaff"
wrote:

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.
--
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