Decay watch: May 16 (Delta 2 post-mortem)

Alan Pickup (alan@wingar.demon.co.uk)
Sun, 16 May 1999 11:15:56 +0100

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Object: #25347 = 98- 32 F = Iridium 70 Delta 2 r

Decay predictions:
Source   Prediction made    Predicted decay at      Latitude Longitude
               UTC                UTC                  deg      deg

SpaceCom   May 11 17:34     May 15 18:01 +-2d        18.5 S   124.9 E
SpaceCom   May 12 19:49     May 15 18:13 +-2d        60.8 S   163.8 E
SpaceCom   May 13 13:36     May 15 13:25 +-9h        19.8 S    69.2 E
SpaceCom   May 14 15:01     May 15 15:25 +-15h       26.7 S   146.2 W
SpaceCom   May 15 05:28     May 15 15:48 +-7h        63.7 N   136.7 W
SpaceCom   May 15 09:26     May 15 15:47 +-5h        61.4 N   137.8 W
SpaceCom   May 15 13:25     May 15 15:38 +-2h        23.6 N   143.8 W

SatEvo     May  9 14:25     May 15 18:00 +-1.5d
SatEvo     May 12 18:20     May 15 17:42 +-15h
SatEvo     May 13 19:20     May 15 11:41 +-9h        30.0 N    88.7 E
SatEvo     May 14 18:50     May 15 11:49 +-5h         0.0      90.1 E
SatEvo     May 15 10:30     May 15 14:05 +-2h        16.1 N   122.4 W
SatEvo     May 16 08:00     May 15 14:24 +-30m       82.1 N     3.5 E

Final elsets:
Iridm 70 Delta r 5.9  2.4  0.0  5.6 d            156 x 147 km
1 25347U 98032F   99135.46280069  .11948106 -16080-5  31496-3 0  4739
2 25347  84.0798 320.0917 0007234  52.7192 308.0497 16.45374817 56269
Iridm 70 Delta r 5.9  2.4  0.0  5.6 d            151 x 139 km
1 25347U 98032F   99135.52344543  .15514774 -16886-5  24458-3 0  4743
2 25347  84.1073 320.0661 0009110 356.6314   3.4627 16.48001220 56279

Note: The final elsets show the rocket running only 0.8 second early 
and 0.2 second late respectively against the SatEvo prediction in my 
previous posting. SpaceCom has yet to release a post-decay report 
for this and it is possible that there may be a further elset.

While the drag (ndot2) terms above may be consistent with SpaceCom's 
decay time, they are certainly too low and *in*consistent with the 
changes in mean motion implied by the same elsets. For example, the
increase in mean motion between the two elsets implies that the drag
over that interval averaged 0.2165 and, since it would have been 
increasing during that period, should have been even higher at the 
epoch of the final elset and not the 0.155 reported. This behaviour
(SpaceCom underestimating the drag near decay) is par for the course.

A further SatEvo analysis, satisfying exactly the equator crossing 
times implied by the final three elsets, leads to the following elset 
for the start of final orbit:
Iridm 70 Delta r 5.9  2.4  0.0  5.6 d            125 x 122 km
1 25347U 98032F   99135.58401075 1.18765927  19592+2  60760-3 0 94720
2 25347  84.0918 319.9876 0001984  25.2507 334.7590 16.56008306 56286

The track took it northbound just E of San Francisco at 14:10 UTC
towards decay near northern apex. It might have decayed near to
the beginning of the orbit, or it might have survived as far as the 
following southbound equator crossing near 46 deg E longitude at 
14:44 UTC.
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Alan
-- 
 Alan Pickup | COSPAR 2707:   55d53m48.7s N   3d11m51.2s W   156m asl
 Edinburgh   | Home:   alan@wingar.demon.co.uk    +44 (0)131 477 9144
 Scotland    | SatEvo page:     http://www.wingar.demon.co.uk/satevo/