[geocentrism] Re: Geosynchronous/Geostationary satellites
- From: "Neil Robertson" <nroberts@xxxxxxxxxxxxxxxxxx>
- To: <geocentrism@xxxxxxxxxxxxx>
- Date: Mon, 25 Oct 2004 15:45:03 +1000
> Neil, are you a BA'er also? If so, who are you there?
Yes Gary I have lurked over there for some time although I rarely post. I go
by my initials NGR.
> I think I am beginning to see this figure eight now, though it is a tough
> one to visualize.
>
I am glad you are begining to understand. Perhaps if you tried a few models
or pen and paper to help you better visualise what actually happens?
> I assume the whole figure eight is visible in the northern hemisphere due
> to
> the distance involved to the satellite?
Remember that the satellites orbit over the equator and as such any pattern
will be visible equally in the Northern and Southern hemispheres. The bottom
line is that if the figure of eight was large enough you would observe it
from both hemispheres. If it was small enough you would not observe the
change from the earth surface but it would exist nonetheless.
> If the orbits of geosynchronous satellites are not 100% stable, as you
> said,
> are they 100% stable for geostationary satellites? Why wouldn't the same
> earth density forces, as well as the moon attraction you mentioned, affect
> geostationary satellites? What is so special about them that they can
> withstand these forces?
As I said in my first post a geostationary satellite is a special case of
geosynchronous satellite and they are affected by the same forces. When the
technicians send up the satellite they aim to have it stationary in the sky
to meet its communication or weather gathering purpose. The whole idea is to
have it hover over one spot on the earths surface but due to the various
forces I mentioned previously there is a tendancy for the satellite to
drift. The satellite controllers use the small jets on the satellite to
nudge it back into position when it drifts outside of acceptable parameters.
That drift distance is decided upon by the technicians and could be a small
distance or a larger distance depending on the operation parameters of the
satellite.
You have to remember that a communication satellite has a proscribed
footprint that it covers on the earths surface and this is continent sized.
A small deviation in the spot the satellite occupies in the sky is not a
problem and the technicians allow the satellite to drift around a box in the
sky. If they did not allow this leeway they would run out of fuel for the
adjusting thrusters very quickly which would not be a practical outcome.
One thing that you should understand is that my comment on geostationary and
geosychronous satellites was a technical argument. The reason that the names
are used interchangeably is that for all practical purposes they mean the
same thing. To be technically accurate a geostationary satellite would need
to be fixed in the sky and the only way that can occur is if it is directly
over the equator in a perfectly circular orbit and at zero inclination with
the equator. In practise for the reasons I have discussed previously there
is a slight drift in the orbital elements over time and as a result that
figure of eight path arises. If the drift was large enough the satellite
would be seen to not hold a set position in the sky but move around a set
position in the sky. In a technical sense the satellite is not stationary
but is geosynchronous, but in a practical sense it is stationary for the
purpose it is used.
I should also point out that the satellite controllers at times deliberately
allow the satellite to drift around a box area in the sky because it
enlarges the footprint on earth that the satellite covers. One classic
example would be to look at the poles. From the earths poles a geostationary
staellite is on the horizon and not in a good position to act as a
communcation device. If you allow drift into the equation you will get the
satellite to include the pole area. It is my understanding that internet
connectivity to the antarctic research bases was arranged using this means.
>Also, if a geosynchronous satellite is doing a
> figure eight out there, is it not requisite upon a receiving instrument to
> be moving in a like manner? (This no doubt touches on what Dr. Jones and
> Philip were discussing.)
The communication satellite sends its radio waves in a cone directed at the
earths surface. In view of the distance the satellites orbits above the
earth that cone is quite large when it reaches the earths surface, continent
sized in fact as I have previously mentioned. To receive satellite TV
broadcasts you only need a small antenna placed on your roof(thats they way
they do it around my area at least) which is directed to that sweet spot in
the sky. That fact that the satellite moves slightly in practise and is not
in exactly the same position at all times is irrevalent to the receiver as
the transmission cone covers a wide area.
> What different purposes would exist for the two different types of
> satellites? Why would we launch a "figure eight-ing" satellite instead of
> an LEO one? Isn't the tracking problematic in the same kind of way, only
> the former is much slower and the LEO much quicker?
Its a horses for courses situation. The geostationary satellites provide an
excellent vantage point above the earth and a few can cover most of the
earths surface. From the viewpoint of say some communication they are ideal
becuase of the simplification of the receiving equipment. The satellite is
fixed(relatively) in the sky and you can use a fixed antenna. The
geostationary orbit is used by weather satellites as well and I'm sure you
have seen the evening weather forcast where the cloud patterns for the last
24 hours are projected in movie form over a map of your area. These movies
are captured by geostationary weather satellites stragegically placed around
the globe.
There are drawbacks to the geostationary satellite and one is that because
of their height above the earth there is a round trip delay in the
communication signal of some 550 to 650 milliseconds which despite its
seemimly small size can be noticed by satellitte phone uses and can be
distracting. The beauty of it is though that you have communications in say
the Australian outback where the placing of mobile phone towers is
impractical. Of course you dont have any trouble with satellite TV reception
because it is one way and any slight delay is irrelevant.
LEO satellites have there place to and are on the increase in uses. As you
mentioned the fact that they are moving considerably faster that their
Geostationary brothers the receiving equipment is more complicated. Advances
are being made all the time in terms of communciation equipment and the cost
of orbiting the satellites is less than for geostationary orbits so that is
a plus too. I understand in fact that one of Bill Gates's plans is to have a
group of LEO satellites in orbit to set up a global braodband empire. I
believe the company is called Teledesic but I don't know if he is still
persuing this idea. LEO satellites are used to take photos of the surface
for defense, mapping, locating mineral deposits and environmental
monitoring. Landsat is a typical example.
> I guess in my suspicious moments I think things like the government is
> placing these satellites doing figure eights just so they can't be
> followed
> as easily. Like, perhaps the whole figure eight is to scramble Howard
> Stern?
>
> uh....ha.
Steady Gary conspiracy theories are usually a dead end in critical thinking.
> Referring back to my first question, is there ever a time when a
> geosynchronous satellite would disappear from view down low, or up too
> high?
>
Well sure. If the orbit inclination is large enough the satellite will
disappear from view at times in its orbit. The poster child for this is
where a satellite has a polar orbit which is inclined 90 degrees to the
equator. Such a satellite may orbit in 24 hours but its orbit would be
perpendicular to the horizon. Such satellites do exist and are used for
mapping purposes and to carry out surveillance. The beauty of this is that
as the earth rotates the satellite has access to virtually every point on
the earths surface. I suspect that the US millitary are very familiar with
this sort of orbit.
> Also, do these geosynchronous satellites (or geostationary ones for that
> matter) "slip" in their orbits about the earth? That is, would we see it
> at
> the top of the eight at, say, noon one day and then 1 pm the next? Is
> this
> type of orbit EVER observed?
Obviously if you were going to set up a geostationary satellite the whole
idea is for it to not move very far from a fixed position but there is no
reason you could not set up an orbit for a satellite used for another
purpose that would have such an orbit. In fact such orbits are called
walking orbits or precessing orbits. As I mentioned previously the earth is
not a perfect sphere or of uniform density and these fluctuations effect the
satellites orbit trajectory. In addition the gravitation from the moon the
sun and other large bodies in the solar system have a gravitational
influence on any orbiting satellite. The technicians are able with proper
planning to design an orbit which takes advantage of these influences to
induce a precession in the satellites orbital plane.
> The satellite would still follow the figure eight but just not in the same
> time as the earth's rotation.
>
> Thank you for your aid,
>
> Gary Shelton
You welcome,
Neil.
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