RE: Iridium spares

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Wed Feb 13 2002 - 13:58:24 EST

  • Next message: Sebastian Stabroth: "Re: Iridium spares"

    Hi All,
    
    Earlier I wrote (regarding Iridium nodal precession rates):
    
    > Not quite.  The spares are at a lower altitude.  This leads to a
    > slightly different nodal precession rate.  This differential
    > precession is pretty low (less than 1 degree per day) for
    > satellites with high inclinations, so it can take quite a
    > while depending on the target plane.
    
    Sebastian replied:
    
    > all the Iridium orbital planes drift at about -0.4 deg per day
    > in RA, but the differential rate because of the lower altitude
    > is only 0.01 deg per day. How long do you want to wait until
    > two planes are equal?
    
    Yikes -- that's what I get for trying to estimate the rates off
    a graph.  When I sit down and do the math, OmegaDot (the derivative
    of the RAAN) works out to 0.41124 deg/day for Iridium 83 (operational),
    and the rate is only about .00016 deg/day _slower_ for Iridium 86
    (spare).  The lower orbit of Iridium 86 would ordinarily have had
    a faster nodal precession rate; however, in 86's case the impact of
    the higher inclination (86.52 deg vs. 86.40 deg for Iridium 83)
    almost exactly cancels the effect of the lower orbit.  This says
    to me that this is not an accident -- that the inclination is
    deliberately higher for the spares in order to maintain the RAAN
    alignment.
    
    So I have to agree with Sebastian -- it's not practical to have
    satellite spares "jump" planes.  Even with an inclination difference
    of 0.5 degrees, the differential regression is only .057 deg/day
    at the operational altitude, meaning it would take nearly a year
    and a half just to move one plane over.
    
    Best,
    Rob
    
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