Re: Satellite period from transits.

From: pmorini@libero.it
Date: Mon Jan 27 2003 - 04:12:58 EST

  • Next message: David Brierley: "DMB some obs Jan 26-27"

    In my wandering in old publications, I got the sept/oct 1961 issue of 
    "Coelum" an astronomy magazine published in Italy by the Bologna 
    astronomical observatory.
    I found an article in which was suggested to time the eclipse of the 
    satellite caused by the shadow of the Earth.
    To take in account the movement of about 1 deg/day of the umbral cone of 
    the earth, for satellite with a period of about 2 hours and moving in 
    direct mode, the suggestion is to subtract 2 sec per evolution.
    The timing over 1 year gives for Echo sat the period decreasing from 
    118.28 min (31 aug 1960) to 116.9 min (15 aug 1961).
    What do you think about this kind of observation ?
    So crude as the one made with the dipleidoscope ?
    A big advantage is that you have not to build any instrument - need only 
    a binoculars.
     From observing satellite vanishing in the earth shadow, I guess you can 
    time the swicth-off-time with an approximantion of some seconds, the 
    observation program on a single satellite must be run for, say, 1 year 
    at least.
    I have found also a formula for the time of ascending node:
        T = D + AN + BN^2 + CN^3
    where D is the epoch date, A the period, N the number of revolutions, B 
    and C the drag terms.
    With the data obtained in 1 year of observation I could try to find B 
    and C with a min-square fit of the data with a 3rd order parabola ...
    Greetings
        Paolo Morini
    
    Michael Waterman wrote:
    
    >As has been pointed out, determining the orbit period of a
    >satellite from timing when it crosses the meridian (= the
    >line South to North) is not accurate. Over 1 orbit the error
    >is between 5 and 10%, over a day the error is about 1%.
    >
    >If however you time when a satellite crosses the East to 
    >West line the effect of the rotation of the earth is eliminated.
    >The error over 1 orbit is about 1%, and 0.1% over a day.
    >
    >Obviously this can only be done if the inclination of the
    >orbit is more (say at least 20 degrees) more than your latitude.
    >
    >Mike Waterman      mike.waterman@marconi.com
    >Site Yateley = COSPAR 2115 =  51.3286N  0.7950W  75m (reference OSGB36).
    >
    >
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