Re: Status and Identity of Iridium 30 and Iridium 31

From: Rod Sladen (satellite@rod.sladen.org.uk)
Date: Mon Jul 21 2003 - 04:52:02 EDT

  • Next message: Peter Wakelin: "SATOBS 2003 July 20"

    Iridium 30 and 31 do indeed have appeared to have exchanged places
    in the constellation on September 19-22, 2002.
    Many thanks, Walter, for highlighting this.  I had missed this
    swap entirely, but have now trawled back through saved elsets to
    work out what happened!
    Up to and including September 18, 2002, Iridium 30 was followed in
    the regular pattern by Iridium 31:
    IRIDIUM 30
    1 24949U 97051F   02261.14806094 -.00000231  00000-0 -89653-4 0  8872
    2 24949  86.3995  17.9087 0002209  56.9878 303.1529 14.34213466262410
    IRIDIUM 31
    1 24950U 97051G   02260.87532995 -.00000360  00000-0 -13554-3 0  8889
    2 24950  86.3993  18.0069 0002271  58.9272 301.2147 14.34212368262382
    
    IRIDIUM 30
    1 24949U 97051F   02261.84574731 -.00000193 +00000-0 -76017-4 0 08944
    2 24949 086.3998 017.6177 0002235 060.2566 299.8858 14.34213526262515
    IRIDIUM 31
    1 24950U 97051G   02261.85209058 -.00000211 +00000-0 -82404-4 0 08986
    2 24950 086.3990 017.5985 0002292 063.4936 296.6506 14.34213018262526
    
    A day later, Iridium 30 had been raised and Iridium 31 lowered:
    
    IRIDIUM 30
    1 24949U 97051F   02262.68297198  .00066488  00000-0  24949-1 0  8880
    2 24949  86.4041  17.2727 0002372  77.9375 276.0562 14.31204631262633
    IRIDIUM 31
    1 24950U 97051G   02262.68931529  .00036298  00000-0  12095-1 0  8891
    2 24950  86.4053  17.2485 0002117  83.7475 282.6644 14.37378306262644
    
    IRIDIUM 30
    1 24949U 97051F   02264.36216666 -.00003261  00000-0 -12451-2 0  8921
    2 24949  86.4020  16.5744 0002307  79.9658 280.1786 14.31170241262874
    IRIDIUM 31
    1 24950U 97051G   02264.42850689 -.00002106  00000-0 -71311-3 0  8927
    2 24950  86.4037  16.5201 0002285  89.2377 270.9082 14.37355916262897
    
    On this basis, Iridium 31 would have overtaken Iridium 30 at
    around 15:00 on September 20, 2002.
    Subsequent elsets show Iridium 31 preceding Iridium 30 in the
    regular pattern:
    IRIDIUM 30
    1 24949U 97051F   02266.59952037 -.00107328  00000-0 -38984-1 0  8935
    2 24949  86.4056  15.6428 0002675  71.8218 304.7160 14.34168198263199
    IRIDIUM 31
    1 24950U 97051G   02266.58661706 -.00070580  00000-0 -25474-1 0  8935
    2 24950  86.4059  15.6161 0003051  92.4409 250.2627 14.34220528263209
    
    Over the next few days, minor adjustments appear to have been made
    to get the satellites into precise new positions.
    I have no idea at all of the reason for this changeover!  One
    possibilty is that one or other or the satellites had a failed
    cross-link antenna and was repositioned to match with a
    corresponding failed cross-link antenna on a satellite in an
    adjacent orbital plane.  But that is pure guesswork!
    I have incorporated this change into my iridium web page
    http://www.rod.sladen.org.uk/iridium.htm
    
    I have rechecked the whole Iridium constellation formation as of
    yesterday, and there do not seem to have been any other changes
    that we have missed!
    -- 
    Rod Sladen
    Beeston, Nottinghamshire, UK
    52.923N, 1.219W
    
    
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