Decay rate of USA 129 r, #24681

Bjoern Gimle (b_gimle@algonet.se)
Sun, 19 Jan 1997 17:26:43 +0100 (MET)

In the last three elsets posted for USA 129 r, #24681,
ndot2 has changed from .01518 to .02490 to .01330

I have checked ndot2 and MM of all the elsets I have saved, and
searched for an approximate best fit, using my decay program
and its log function and graphs.

The conclusions are highly speculative, and I have not investigated
the effect on agreement with elset epochs (or observation fits)

Except for the 1996 elsets, there is no indication of significant errors
in MM, but the decay element, ndot2, ( which does not appear to deviate
more than 10% from my smoothed function, averaged over a day or so) 
shows much larger variations in the elsets posted.

Using SatEvo on the published elsets naturally shows a similar large
variation, decay Jan.25-Feb.05. Substituting my smoothed values in
the elsets, I get a smooth set of decay predictions, but there is a
gradual progression from Feb.03 to Jan.29. The SatEvo evolution of the
97001.5 elset shows a decay rate more constant than my smoothed function
(and the average of the posted elsets), with decay on Feb.03. This
also leads to a Mean Motion on Jan.17 of 15.518, much less than the
given 15.560. 

This looks like the behaviour of a large eccentricity orbit, whereas
the posted (and smoothed) elsets behave like a less eccentric orbit,
increasing the decay rate more rapidly.

There could be many explanations to this:

1. SatEvo's atmosphere model is generally incorrect.
2. The atmosphere is currently much different from average
3. The density at perigee has increased considerably last two weeks.
4. The perigee is moving over the equator, making it lower.
5. The eccentricity of the rocket's orbit is lower than computed.

I see no evidence for the first four points. Argument Of Perigee
has moved from 138 down to 69 degres, according to elsets, still at
quite high latitudes. Reducing e by 0.01 and using the smoothed
97001.5 elset, SatEvo gives ndot2=0.0150 and MM=15.5380 on 97017,
closer to posted, and smoothed, elsets.

If it is possible to fit an orbit with smaller eccentricity to
available observations, the higher perigee would also explain
the strangely low decay rate, which has puzzled analysts from
the discovery, leading to speculations of a very large mass.

Elsets posted    ndot2     ndotdot6 MM         e :

96365.52702280  .01097930  00000-0 15.12918679 0533247 
96365.52701960  .01098464  00000-0 15.12923644 0537085 
96366.58430690  .01220474  00000-0 15.15596164 0542171 
97 01.50791800  .01244224  00000-0 15.17919842 0531505 
97 02.56192110  .01020686  00000-0 15.20047638 0528127 
97 04.53436490  .00959924  00000-0 15.23839338 0506368 
97 06.56741040  .00954858  00000-0 15.27717527 0498408 
97 07.54926320  .00964146  00000-0 15.29637502 0492390 
97 08.52975180  .01097522  00000-0 15.31910590 0476076 
97 09.50886640  .01078668  00000-0 15.33990437 0479853 
97 11.78648514  .01625000  00000-0 15.40570033 0435000 
97 11.85243630  .01446399  00000-0 15.40455303 0442942 
97 13.47262092  .01510000  00000-0 15.45505325 0435000 
97 13.53835740  .01510046  00000-0 15.45676063 0419000 
97 15.79859908  .01435000  00000-0 15.52064688 0417000 
97 15.86402340  .01433330  00000-0 15.52210943 0394003 
97 16.82909254  .01518000  00000-0 15.55387590 0361000 
97 17.15133102  .02490000  00000-0 15.56077282 0367600 
97 17.15133102  .01330000  00000-0 15.56077282 0367600 

Smoothed ndot2 and MM :
96999.5270	.0088              15.1419 	       
97000.5843	.0090              15.1607 	       
97001.5079	.0093              15.1776 	       
97002.5619	.0096              15.1975 	       
97004.5344	.0102              15.2364 	       
97006.5674	.0109              15.2791 	       
97007.5493	.0112              15.3008 	       
97008.5298	.0116	           15.3233
97009.5089	.0121	           15.3465
97011.7865	.0132	           15.4040
97011.8524	.0132	           15.4057
97013.4726	.0142	           15.4501
97013.5384	.0142	           15.4520
97015.7986	.0158	           15.5197
97015.8640	.0159	           15.5218
97016.8291	.0167	           15.5532
97017.1513	.0169	           15.5640

Modified ndot2, SatEvo :

97001.50791800  .00930000  45818-4 15.17760000 0531505
97002.56211304  .00944832  48026-4 15.19736316 0523287
97004.53498376  .00974889  53259-4 15.23522713 0507553
97006.56837831  .01011840  67695-4 15.27558736 0490796
97007.55032268  .01032343  71594-4 15.29565822 0482468
97008.53096375  .01054022  75874-4 15.31611592 0473984
97009.51027969  .01076992  80590-4 15.33698302 0465333
97011.79006428  .01136375  93627-4 15.38740921 0444446
97011.85508996  .01138206  94016-4 15.38888827 0443833
97013.47867365  .01186434  10472-3 15.42661701 0428221
97013.54353366  .01188476  10519-3 15.42815738 0427583
97015.80947713  .01266641  12581-3 15.48373622 0404608
97015.87409737  .01269086  12645-3 15.48537481 0403931
97016.84257291  .01307388  13746-3 15.51032217 0393627
97016.90708221  .01310055  13819-3 15.51201067 0392930
97017.16504894  .01320878  14144-3 15.51879749 0390128

97024.78353371  .01823859  34880-3 15.75265625 0293826
97031.45609024  .03440123  19963-2 16.07188873 0165581

Modified ndot2 and e, SatEvo :

97001.50774305  .00929997  62649-4 15.17759675 0431506 
97002.56192820  .00950420  66599-4 15.19741761 0423178 
97004.53475331  .00992667  75933-4 15.23573386 0407087 
97006.56800778  .01042263  87120-4 15.27708615 0389737 
97007.54983073  .01068836  93491-4 15.29781033 0381048 
97008.53030564  .01097376  10079-3 15.31904598 0372153 
97009.50940250  .01128170  10913-3 15.34083225 0363053 
97011.78841543  .01210652  13359-3 15.39407103 0340930 
97011.85341268  .01213264  13435-3 15.39564650 0340278 
97013.47614457  .01283590  15646-3 15.43613401 0323574 
97013.54096433  .01286644  15761-3 15.43780003 0322889 
97015.80513140  .01408355  20401-3 15.49870065 0297967 
97015.86968900  .01412323  20579-3 15.50052161 0297226 
97016.83713060  .01475882  23309-3 15.52845045 0285888 
97017.15921977  .01498921  24387-3 15.53803142 0282012 

97024.56742993  .02819396  15002-2 15.82717261 0170017 
97028.75740449  .83712563  45316+1 16.41344923 0016512 

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