Possibly "silly" questions

From: Stuart Eves (stuart@evesreading.demon.co.uk)
Date: Thu Jun 14 2001 - 13:59:38 PDT

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

    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
    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.
    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
    >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
    Stuart Eves
    Unsubscribe from SeeSat-L by sending a message with 'unsubscribe'
    in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org

    This archive was generated by hypermail 2b29 : Thu Jun 14 2001 - 14:35:38 PDT