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