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Old July 30th 17, 03:39 PM posted to uk.telecom.broadband,uk.tech.digital-tv,alt.satellite.tv.europe
NY
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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.


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.