Possibly "silly" questions

Date: Thu Jun 14 2001 - 13:59:38 PDT

• Next message: Steve Adams: "Possibly "silly" questions"

```Steve,

The period of a truly geostationary satellite would be 86164 seconds. In
reality, however, no satellites are really geostationary, becuse their
orbits are continuously perturbed, chiefly by irregularities in the
Earth's gravitational field, the gravitational fields of the Moon and
the Sun, and solar radiation pressure.

Simplistically, these forces result in the satellites drifting either
Eastwards or Westwards over time depending on where they're located on
the geostationary ring.  Satellites within roughly +/- 90 degrees of 76
degrees East tend to head towards that point if uncontrolled, since it's
a kind of stable point. (Imagine a child's swing - it will tend to
oscillate back and forth about the lowest point. Failed GEO satellites
do the same sort of thing, and in their case the "bottom of the swing"
is 76 East (or about 104 West for satellites on the other side of the
globe). In the case of GEO satellites, of course, there's virtually no
drag to bring them to rest, so they keep on swinging back and forth
indefinitely.

Most active Western satellites perform regular "East-West station-
keeping" manoeuvres to counteract this drift, and keep them within about
0.1 degrees of their assigned slots, although some, such as a few of the
older Russian satellites, can oscillate by a degreee or more.

Another effect caused by the gravitational perturbations is to change
the inclination of a geostationary satellite's orbit, i.e. instead of
lying flat in the plane of the equator, the orbit plane gets
progressively more tilted with time.  This causes the satellite to move
North and South of the equator during the day.  As was pointed out, this
means that the sub-satellite point often traces out a narrow figure of
eight on the surface of the Earth, (although you can get cases where
it's an elliptical shape too).  Again many satellites perform regular
manoeuvres to control this orbit plane change, keeping the inclination
to 0.1 degrees or less. This is, unsurprisingly, called "North-South
station keeping". However, because it takes a lot more fuel to
accomplish than East-West station keeping, some satellites don't bother
contolling their inclination.  These are referred to as "free-drift
missions", and their inclinations can get as high as 5 degrees or more
on occasion. (The inclination value typically changes by about a degree
per year, and is often "biased" at the start of the mission, so that the
perturbations initially reduce the inclination down close to zero, and
then gradually cause it to rise again. You can use this fact to estimate
the design life of a free-drift satellite mission.  The lifetime in
years is roughly twice the inclination value in degrees).  These free-
drift satellites are more accurately described as geosynchronous.

Regards,

Stuart

In message <0A7752CCE935D511B65800B0D07861A416E9E2@240sexc001.pdl.co.nz>
>Hi List
>
>I know these may be silly questions but the answers are important to me...
>
>Does the term "Geostationary" mean that the satellite remains fixed in orbit
>above a geographical position on Earth?
>(i.e. orbits at such a speed as to appear "still" in the sky).
>
>If this is not true, are there such satellites and what is the correct
>terminology?
>
>If such do exist can someone suggest any that might be visible from my
>location listed below.
>
>Thank you - Steve
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Location - Kirwee, Canterbury, New Zealand
>Lat 43.5000 S Lon 172.2170 E
>Elev 150m
>GMT +12:00
>
>Work Ph: +64 3 338 9059        Fax: +64 3 338 0445
>DDI: +64 3 339 1623            VPN: 8523
>Mobile: +64 25 370 467       VPN: 6219
>
>
>
>
>
>
>
>
>
>
>
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--
Stuart Eves

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