Re: Sat.program predictions

From: Mike McCants (mmccants@io.com)
Date: Wed Nov 01 2006 - 11:57:58 EST

  • Next message: Robert Holdsworth: "Another meteor near ISS path"

    Frank Reed posted:
    
    >Incidentally, can anyone point me to comparisons of the full models versus
    >simple precessing ellipses?
    >I understand I can expect ground track errors of a
    >couple of tenths of a degree in simple cases, but is it ever worse?
    
    As a simple test to compare a precessing ellipse against SGP4,
    I created an elset for a fictitious object in a plane many degrees
    east of ISS so that it will make a very favorable pass over me
    tonight at a high elevation and low height.
    
    1 90000U 06500A   06304.83777757  .00019838  00000-0  12002-3 0  9679
    2 90000 051.6346 293.1609 0012692 158.4739 291.3758 15.77504245454722
    
    My lat/long:  30.31N  97.86W  BCRC
    Tonight:  2006 Nov 2 Times are UT
    
    The Quicksat prediction (modified for greater displayed accuracy):
    
     H  M   S  Al Azi   Hgt  Rng    R A     Dec
     0 47 3.3  82 317   215  217  20 33.3  35.86
    
    The predictions from SeeSat using the original SGP4:
    
     H M   S   Alt   Azi      R A      Dec    Rng   Hgt
    0047:03.3 82.10 306.09  20 28.89  34.72   215   213
    0047:03.4 82.12 306.97  20 29.28  34.81   215   213
    0047:03.5 82.13 307.86  20 29.66  34.90   215   213
    0047:03.6 82.14 308.75  20 30.04  35.00   215   213
    0047:03.7 82.15 309.64  20 30.43  35.09   215   213
    0047:03.8 82.16 310.54  20 30.82  35.19   215   213
    0047:03.9 82.17 311.44  20 31.20  35.28   215   213
    0047:04.0 82.17 312.33  20 31.59  35.37   215   213
    0047:04.1 82.17 313.23  20 31.98  35.47   215   213
    0047:04.2 82.17 314.13  20 32.37  35.56   215   213
    0047:04.3 82.17 315.03  20 32.76  35.65   215   213
    0047:04.4 82.17 315.93  20 33.15  35.75   215   213
    0047:04.5 82.16 316.83  20 33.55  35.84   215   213
    0047:04.6 82.16 317.72  20 33.94  35.93   215   213
    
    If I take the Quicksat prediction (20 Hr, 33.3Mn, +35.86)
    and put into my Fitelem program, it computes residuals
    of 1.2 seconds and 0.09 degrees crosstrack.  The best fit
    is thus about RA 20 Hr 33.35 Mn +35.79 at time 00:47:04.44
    
    In general, all south-to-north predictions are off about
    1 to 1.5 seconds (the satellite is later than the prediction)
    and all north-to-south predictions are similarly off,
    but the satellite is earlier than the prediction.
    
    The crosstrack error is a maximum for lower heights and
    gets smaller with larger heights/ranges.  There are also
    small north/south crosstrack errors for objects in
    west-to-east orbits passing near the zenith.
    
    >Isn't element age a bigger issue than all these subtleties of models?
    
    This is quite possibly true for SGP4.  I believe that the SGP4 model
    has been stable for a long time and I have glanced at the new "combined"
    SGP4/SDP4 code and it appears to me that there were not any changes
    to the SGP4 model.  Of course if an object in low orbit is off
    30 seconds in time, the observed position will be different because
    the observer has moved (due to Earth rotation) during that time.
    
    The SDP4 model is an entirely different question.  I have found
    what I called "discrepancies" between the SGP4 model and the
    SDP4 model for one implementation of the SDP4 model for an orbit
    where the SDP4 model should be used.  The fixes in the new "combined"
    code appear to be only for the SDP4 model.
    
    Mike McCants
    
    -------------------------------------------------------------------------
    Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive:  
    http://www.satobs.org/seesat/seesatindex.html
    



    This archive was generated by hypermail 2b29 : Wed Nov 01 2006 - 12:02:41 EST