Steve, Further to the replies you've already received... 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> , Steve Adams <steve.adams@pdl.co.nz> writes >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 > >Steve Adams >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 >E-mail: steve.adams@pdl.co.nz > > > > > > > > > > > >The contents of this E-mail may contain information that is legally >privileged and/or confidential to the named recipient. This information is >not to be used by any other person and/or organisation. The views expressed >in this document do not necessarily reflect those of the company. > > >----------------------------------------------------------------- >Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' >in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org >http://www2.satellite.eu.org/seesat/seesatindex.html > -- Stuart Eves ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Thu Jun 14 2001 - 14:35:38 PDT