RE: OE Astro observed with Nextsat

From: Ted Molczan (molczanssl@rogers.com)
Date: Wed May 16 2007 - 10:19:59 EDT

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    David Anderman wrote: 
    
    > Using the elements available from n2yo.com, the 2 satellites 
    > are reported to be 4 degrees of longitude and 1 degree of 
    > latitude apart right now, when flying over South America:
    > 
    > http://www.n2yo.com/?s=30774
    > 
    > http://www.n2yo.com/?s=30772
    > 
    > I don't know how this corresponds to the visual observations 
    > of 2 nights ago, whether the satellites are still as close as 
    > was reported, or if they are separating.
    
    As I write, Space Track's TLEs have not been updated since 2007 May 14 at
    20:00:51 UTC, 42 hours ago. To that time, only two TLEs of Nextsat (06006C /
    30774) relevant to its present free-flier status had been issued, the decay
    terms of which were about two orders of magnitude too high. By tonight,
    predictions made with those elements probably will be too early by more than 1
    minute of time.
    
    In reality, I expect that Astro will be manoeuvred as required to maintain about
    the same separation as observed on May 14 and 15 UTC, about 5 km, with Astro
    leading.
    
    To anyone wishing to observe the spacecraft in formation flight, I recommend
    using the latest epoch day 07134 elements of Astro (06006A / 30772), which
    agreed reasonably closely with my observations of early May 15 UTC. Allow at
    least several seconds prediction time uncertainty,
    
    When updated elements appear on Space Track, the choice of elements can be
    re-evaluated.
    
    Regarding the unrealistic decay terms of Nextsat, this is a common problem with
    elements derived from short observational arcs, due to the effects of small
    observational errors. I doubt this has any impact on mission operations, since
    they almost certainly do not rely on TLEs for navigation. 
    
    Ted Molczan
    
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