Re: OGO 1

From: Mike McCants (
Date: Wed Jan 16 2008 - 15:31:44 UTC

  • Next message: Brad Young: "BY Obs Jan 15"

    Curtis Haase posted:
    >I found that the two methods give a similar path among the
    >stars, but with a large time difference (over 11 minutes!).
    For a pass over BCRC last Sunday evening using the current tle:
    Using SGP4, seesat gives:
    OGO 1   2008 JAN 14
     time    alt   azi   range   hgt
    0320:00 37.2 337.3   12336 11097
    0330:00 34.0 346.3   11421 10058
    Highfly gives a very similar prediction.
    However, seesat using SDP4 gives:
     time    alt   azi   range   hgt
    0310:00 38.8 335.8   12344 11175
    0320:00 35.3 344.8   11420 10119
    0330:00 31.2 354.9   10584  9113
    So, yes, the offset in this case is about 11 minutes early for SDP4.
    Based on an eccentric object that I observed recently with a
    time difference of about 30 seconds, I found that the SDP4
    prediction was correct.
    However, an object like this is a very special case.  If you look
    at the actual epochs of the tles, they are even multiples of hours.
    The current tle is Jan 14 at 0400 UT.  In the past, tles for objects
    like this have sometimes had epochs "in the future".  So I believe
    that Spacecom is using some kind of numerical integration program
    to track objects like this one.  Then the output of that program is
    converted to a tle.  So it is certainly possible that the SGP4
    prediction is correct.
    >Has anyone actually observed OGO 1 to see which approach is correct?
    Not that I know of.  However, the "4 day" objects (Astron and its rocket)
    have been observed in the distant past.  They also had epochs at even hours
    and SGP4 and Highfly seemed to work ok or I am mis-remembering or the SGP4
    versus SDP4 difference happened to be much smaller at the time of observation.
    Mike McCants
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