OrbitTrack not immune

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
1 Apr 1997 14:07:30 -0800

Hi Scott,

Was interested to see that OrbitTrack did not exhibit the SDP4 jump at either
of the times I indicated.  Unfortunately, this does not mean OrbitTrack is
immune to the problem; it just means that OrbitTrack's implementation of SDP4
is a variant I haven't considered.  Trust me -- it WILL make the jump
somewhere.  Without access to the OrbitTrack source code, though, I can't
predict where the jump will occur.

The reason I know OrbitTrack hasn't avoided the problem is that the positions
you reported during the second "jump window" are ALL off by about 5.4 degrees
in longitude.  Compare the following outputs:

Output from Curtis Haase software (which exhibits the jump):

Results using SDP4:
1997 JAN 17  LAT    WEST LONG
------------------------------
14 40 00    -6.08    14.06
14 40 30    -6.02    13.71
14 41 00    -5.95    13.36
14 41 30    -5.88    13.01
14 42 00    -5.82    12.68
14 42 30    -5.75    12.34  <---       Unexpected jump in subsat. longitude
14 43 00    -4.87     6.57  <---
14 43 30    -4.80     6.25
14 44 00    -4.73     5.93
14 44 30    -4.66     5.62
14 45 00    -4.60     5.31

 Case 2: 14:40 to 14:45:     <<SDP4>>
 OrbiTrack Position - 04/01/97 1331:59
 
 Pass - From: Fri 01/17/97 14:40:00 UTC
        To:   Fri 01/17/97 14:45:00 UTC
 
 Ariane V.22 Deb F
            Time UTC         Height  North   West      Satellite
  DOW   MM/DD/YY HH:MM:SS      km     Lat    Long
 
  Fri   01/17/97 14:40:00    8806.7  -6.07   14.06
  Fri   01/17/97 14:40:30    8896.2  -6.00   13.71
  Fri   01/17/97 14:41:00    8985.2  -5.93   13.36
  Fri   01/17/97 14:41:30    9073.7  -5.87   13.01
  Fri   01/17/97 14:42:00    9161.7  -5.80   12.68
  Fri   01/17/97 14:42:30    9249.3  -5.73   12.34
  Fri   01/17/97 14:43:00    9336.4  -5.67   12.02  -> no jump
  Fri   01/17/97 14:43:30    9423.0  -5.60   11.69
  Fri   01/17/97 14:44:00    9509.2  -5.54   11.38
  Fri   01/17/97 14:44:30    9594.9  -5.48   11.06
  Fri   01/17/97 14:45:00    9680.1  -5.41   10.76
 
I want you to concentrate on the longitudes.  Notice how Curtis's values and
OrbitTrack's agree prior to the jump.  They're both wrong at this point
because they're both using an argument of perigee that's off by more than 5
degrees.  After the jump, Curtis's values are correct.  Orbittrack's are still
wrong.  At some later point, Orbittrack will make "the jump" and its argument
of perigee will again be correct.

Note that the SGP values, although different from SDP4, are in closer
agreement with SDP4 AFTER the jump.  The along-track difference between the
two models is somewhere between 1 minute and 90 seconds.  Pre-jump, the
along-track difference is more than 9 minutes.

I have a fix for this problem for anyone who is interested.

--Rob