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Old July 31st 17, 09:58 AM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
Brian Gaff
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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
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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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