FW: Orbital Drift - Info Request

From: Brierley David (DMBRIERLEY@dera.gov.uk)
Date: Tue Mar 21 2000 - 04:00:46 PST

  • Next message: John Gardner: "Off topic. IRIDIUM--2573 objects in harms way"

    Here is some feedback from an amateur tracker as requested by Don
    Gardner in his E-mail (see below).
    My experience of generating elsets for TiPS, 96-29F, from my own and
    other visual observations is that TiPS is pretty predictable.  When I
    generate an elset made from 10 or 12 observations spread over a month, I
    expect predictions made from it to be good to less than 20 secs along
    track after a *month* or so.  Similar accuracy can be achieved by
    fitting an orbit to a number of NRL elsets spanning two or three weeks.
    TiPS is decaying pretty slowly at the moment, but since its orbit is so
    near the "critical" inclination of 63.4 degrees, its eccentricity is
    increasing continuously, and it might be a threat to ISS in a couple of
    decades from now.
    As far as observing TiPS is concerned, I find the tether is a lot harder
    to observe now than it was at launch, and I can't see it in 20x80
    binoculars unless it's well up in a nice dark sky.
    Different observers have used different techniques on this unique
    object.  I take positions on the centre of the tether which is the
    centre of drag, whereas Russell Eberst takes them on the centre of
    gravity (about 4/5ths of the way down the tether - correct me if I'm
    wrong Russell).  Others might use Ralph or Norton which are visible from
    time to time. But far as orbital determination is concerned, it doesn't
    seem to make any difference.
    David M Brierley
    Malvern, Worcestershire, UK
    Station 2675, 52.1358N 2.3264W 70m
    > By the way, could you confirm something for me?  I was down at
    > NASA-Marshall 
    > this week trying to help the meteoroid/debris people understand why
    > TiPS hasn't been 
    > severed yet.  Of more importance to me was trying to warn them about
    > how quickly the el-sets 
    > degrade, which is going to be a big concern when TiPS comes down
    > through the ISS altitude.  
    > I have been trying to warn people about the fact that we do not
    > predict the orbital motion of 
    > tether systems particularly well.
    > My first cut at it is that the US Space Command GP el-sets drift by
    > ~150 
    > km/day which translates to 20 sec/day alongtrack (falling behind the
    > actual position).  
    > Have you received any feedback from the amateur trackers which support
    > this?
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