Re: Decays

From: Harro.Zimmer (Harro.Zimmer@t-online.de)
Date: Thu Aug 31 2000 - 09:43:30 PDT

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    Raoul Behrend asks some critical questions about decay times ands positions.
    Here some general informations.
    
    >Well, I always asked me how can the localisations of impact be given at 10 or 1
    > km (0.1, 0.01degree) for a satellite which travel at the speed of 30Mm/s...
    (snip) 
    It is state of the art for more then 2 decades for the ballistic missile
    people to calculate the impact point of a warhead (also for a space object) based
    on the initial conditions with an accuracy better then 1.5 km. The software
    that they and I use considers a lot of parameters like up- and crosstrack lift,
    changes of the drag coefficient with altitude, winds ect. I start my REENTRY
    program when the perigee is below 120 km. The quality of the decay and reentry 
    prediction depends strongly from the atmosphere density model. MSIS-90 is the
    most reliable and also used by SpaceCom.
    
    >... and in counterpart estimate the epoch of the scratch only at minutes or  
    hours levels ? I saw uncertainties of more than 12 hours on these datas
    > (corresponding to many times the earth's size), but with always the
    > localisation of the impact at less the 10km, according to these datas.
    (snip)
    What means "decay time" in programs used by SpaceCom and myself? It means the
    final point of the reentry trajectory. The output shows the seconds, but there
    is a lot of uncertainty what's going on below 70 km (disintegration ect.). Is is
    a matter on convention to give the decay time at the minute level and also the
    position of the "impact point" with one decimal. (Sometimes I show 2 decimals.)
    Large uncertainties - as Raoul mentioned above - are mostly related to 
    predictions in advance  4 -  2 days  before decay. If SpaceCom shows in its
    FINAL REPORT for example a  decay time  with  +/- 02 hours decay window reflects
    it the fact that there is no sensor contact in this time span. But the time and
    position could be accurate. 
    
    Harro
    
    Harro.Zimmer@t-online.de
    Berlin, Germany
    
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