Re: Which is correct?

From: Mike McCants (mmccants@io.com)
Date: Thu Mar 23 2006 - 12:35:03 EST

  • Next message: Lutz Schindler: "Observations 23.03.06"

    Bjorn posted:
    
    >> What interests me is this: Has anybody actually observed
    >> *Unknown 041206* and found out which prediction is correct?
    
    >The elsets are there because someone (Mike?) has computed
    >them based on observations by someone (Greg?)
    
    Yes.  The elset was generated by Fitelem using Greg's observations
    near the time of the epoch.
    
    >So the question comes down to "What model and possible
    >corrections/improvements is Mike's prorgam (FitElem?) using?"
    
    Fitelem uses SGP4 - even for an object in a 12 hour orbit like this one.
    
    I do not believe that there has ever been a problem with the fundamental
    definition of the SGP4 algorithm.  To first order, SGP and the "precessing
    ellipse" models match SGP4 within a few seconds of time of prediction
    for time within a few weeks of the epoch of the elset.
    
    >Does one of the models use Epoch-of-date and the other a fixed date Equator?
    
    I assume that SGP4 and SDP4 use epoch-of-date since tles are always
    supposed to be epoch-of-date.  At any rate, I do not think that kind
    of error would cause a 50 seconds time of prediction difference.
    In looking at the SkyMap output on my screen, I have an alt/azi display.
    
    >> epoch of the elset.  The 50 second difference between the
    >> two models remained the same.  So it appears to be a fundamental
    >> discrepancy between the two models.
    
    Since an object in a 12 hour orbit can have a non-zero "drag" term
    due to the longitude-dependent terms in the Earth's gravitational
    field, there was the possibility that SGP4 could differ from SDP4
    after 8 days because SGP4 uses the drag term or the BStar term
    and SDP4 computes this effect independently.  So if there was a
    difference in the computation of this effect, that difference
    should be larger or smaller depending on the time from epoch.
    
    The actual "drag" term in this case was 0.000011 and the effect
    of that value over 8 days would be about 30 seconds.
    
    The fact that the difference remains 50 seconds regardless of the
    time from the epoch of the elset indicates that the computation
    of that effect (30 seconds) does match for SGP4 and SDP4 and that
    the difference is fundamental to the algorithm.
    
    I have now run SkyMap 6.6 to generate predictions for 4/13/06.
    This is 25 days from the epoch of the elset.  The difference
    between the SGP and the SDP4 prediction remains about 50 seconds.
    
    I moved the observer's lat/long from -33/18 up to -3/30 and the
    difference did not change.
    
    The elset in question:
    
    1 90027U 04841A   06071.55035598 0.00001113  00000-0  11045+1 0    09
    2 90027  62.3773 158.6750 6940454 268.4454  91.5512  2.00728666    05
    
    does not have an epoch at an equator crossing.  A Spacecom elset
    would have an earlier epoch and a mean anomaly of about 15 degrees.
    
    This elset also fits the observations:
    
    1 90027U 04841A   06071.44518823 0.00001113  00000-0  11045+1 0    04
    2 90027  62.3773 158.7150 6940454 268.4454  15.5512  2.00728666    06
    
    But it also gives a 50 second difference.
    
    I then changed the observer to lat/long 50/-1 and the time to
    3/20 at 23:00 and the alt/azi is now 64 at 135 and the time
    difference remains 50 seconds.
    
    --------
    
    Now I have noticed that Greg has an observation of object 28938:
    
    3/21 1:01:53.2  13 31.10 -16.61 2000 MTSat 2 H2A Rk 28938 06 004B
    
    There is a Spacecom elset with epoch 3/20.
    
    SeeSat (SGP4) predicts:
    
    HrMn Sc   Alt    Azi     R  A    Dec
    0101:48 70.82 331.11  13 31.01 -16.75
    0101:49 70.83 331.23  13 31.19 -16.74
    0101:50 70.84 331.34  13 31.37 -16.74
    
    Fitelem (SGP4) says that the residuals are 4 seconds of time
    (1:01:53 versus 1:01:49) and 0.13 degrees cross-track.
    
    SkyMap (SDP4) says (from my PC screen):
    
    01:01:50 Alt 70.83 Azi 331.30
    
    So SkyMap SDP4 agrees with these predictions.
    So there is no difference between SGP4 and SDP4 in this case.
    
    Mike McCants
    
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