Re: time difference

From: Ed Davies (edavies@nildram.co.uk)
Date: Fri Oct 11 2002 - 12:15:34 EDT

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    Marcus wrote:
    
    > Considering that STS-112 is docked to the ISS, why H-A gives two times with
    > a difference of 3 seconds each other?
    
    I asked almost the same question a week ago:
    
    > Though other comments in this group indicate that Progress M1-9
    > has docked with the ISS, HA is saying it is two seconds behind
    > in orbit.  Could HA be still using old elements for it as 
    > presumably new ones will not have been issued since docking?
    
    Mike McCants replied with a simple explanation which I guess will
    also apply here:
    
    > The "one hour ago" elements from OIG are:
    > 
    > ISS (ZARYA)
    > 1 25544U 98067A   02276.16030093  .00055685  00000-0  65283-3 0  2145
    > 2 25544  51.6347 230.8205 0017186 328.6962  81.8736 15.60780477220874
    > SOYUZ TM-34
    > 1 27416U 02020A   02275.47199074  .00055222  00000-0  64944-3 0  1517
    > 2 27416  51.6352 234.2875 0017116 326.2129 174.3876 15.60703815 25005
    > PROGRESS M1-9
    > 1 27531U 02045A   02275.47199074  .00055222  00000-0  64944-3 0   320
    > 2 27531  51.6352 234.2875 0017116 326.2129 174.3876 15.60703815  1063
    > 
    > So the Soyuz and Progress do have identical elsets, but they are both
    > about 12 hours older than the ISS elset.  So depending on the accuracy
    > of the mean motion and drag, a two second discrepancy between elsets
    > that are 12 hours apart is not unreasonable.
    
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