Re: weird Omid orbit

From: Tony Beresford (
Date: Sat Feb 28 2009 - 06:04:34 UTC

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    At 03:37 PM 28/02/2009, Darwin Teague wrote:
    >I did some predictions of Iran's Omid satellite and noticed 
    >something that I  thought was odd.
    >It's closest approach is one minute later for 4 consecutive nights. 
    >Is this just a coincidence,
    >  a common occurence  or is it in some special orbit that allows it 
    > to do that?
    >3-22 19:55 132-26
    >3-23 19:56  135-57
    >3-24  19:57  319-63
    >3-25  19:58   323-31
    The orbit height is deceasing through air drag and hence the period 
    is decreasing gradually.
    TLEs show the  revoulotions/day relative to the satellite perigee, 
    which unless the
    inclination is close to 63.6 or its complement 116.4 is a moving 
    target. This is technically called
    the anomalistic period.
    The nodal period or the time it takes between  equator crossing is 
    the one that describes
    best the time between closest approaches. The nodal period of OMID is 
    close to 90 minutes at the moment
    so its close approaches are nearly at same time of day. In OMID;s 
    case its accidental.
    Because the shape of the Earth causes the node of the orbit to 
    precess, the passes seen
    on different days are further to the west unless the inclation is 90 degrees.
    Tony Beresford
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