Re: Do TLEs get old?

From: michael.waterman@gecm.com
Date: Tue Apr 11 2000 - 02:50:51 PDT

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    The main reason why TLEs get old is that the NDOT2 term in the 
    elements varies with atmosphere density, which depends on solar
    activity, which is not very predictable.
    
    The value of quoted value of NDOT2 is usually calculated from
    values of NN from the current and previous orbits, but on the
    first few issued elements is often a guess.
    
    If the elements are D days old, then the effect of the NDOT2
    term is C=D*D*NDOT2/NN days.
    
    For example, my current element file has Zarya:
    1 25544U 98067A   00094.17409444 +.00089428 +00000-0 +66613-3 0 05134
    2 25544 051.5876 163.9534 0008992 011.1335 030.6436 15.71977101078137
    
    Tonight Apr 11 = day 102.9   
    D = 102.9-94.17 = 8.73
    C = 8.73*8.73*.00089428/15.719 = .0043 days = 6.24 mins
    
    Actual error from more recent elements (epoch 102.12878) = 0.23min,
    so in this case the NDOT2 was quite accurate.
    
    Usually after a week the likely error in the time when the satellite
    is seen is very approximately D/4, after a month maybe D/2. NDOT2 is
    more reliable for low orbits than high orbits.
    
    My prediction program displays C, so I immediately get an idea
    of the likely error, and whether I need new elements.
    
    Mike Waterman      michael.waterman@gecm.com
    Site Yateley = COSPAR 2115 =  51.3286N  0.7950W  75m.
    
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