Re: Statistics on prediction accuracies

From: George Roberts (
Date: Fri Apr 08 2011 - 14:22:59 UTC

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    This is true but isn’t the ISS an outlier?  First of all it’s TLE is updated daily so it’s not an issue.  Secondly, I believe it is lower than the average LEO satellite and I’m guessing it has a high surface area to weight ratio.  High versus other satellites.  It’s drag coefficient is about 200X that of say Lacrosse 3.
    But I suppose if Frank is concerned about a particular prediction accuracy he should probably check both the epoch and the drag coefficient.  If he only cares about casual observing then 10 seconds error should be fine so any epoch older than a week he might want to check the drag coefficient also.  If he cares about sub second accuracy then he needs to get more sophisticated.
    On the other hand the brightness predictions for satellites is very difficult to predict.  So if heavens above says it is a magnitude 3.0 pass, it might be much dimmer and not seen at all.  When I show satellites to students (did one last night) I only show satellites predicted to be brighter than magnitude 3.  We are more likely to see them that way in our mag 4 limited skies.  The iridium flares are pretty reliable though.
    - George Roberts
    From: Björn Gimle 
    Sent: Friday, April 08, 2011 6:19 AM
    To: George Roberts 
    Cc: SeeSat-L 
    Subject: Re: Statistics on prediction accuracies
    Even moderate drag has a large effect over time.Consider orbit data (which you can view on H-A) e.g. for ISS, which may start this: 1 25544U 98067A   11097.75640206  .00060054 ...
    30 days after the Epoch (11097...) ie day 127 ie May 7 the cumulative drag is 30*30*.00060054 = 0.54 orbits or about 50 minutes.The error in this "average" is often 10-20%, 5-10 minutes in this case.
    Over a few days, the atmospheric density may vary a lot more than 20%,and this could cause elset data to be grossly misleading when extrapolated over longer time./Björn 
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