Re: Decay rate of USA 129 r, #24681

Alan Pickup (alan@wingar.demon.co.uk)
Sun, 19 Jan 1997 19:26:22 +0000

One factor which must be remembered is the apparent effect of the
geomagnetic storm on January 10. Very many satellites show an increase
in drag over the following few days, well above any expected from the
normal progress of a decay. The perturbation is probably beyond anything
which might be accommodated  by any smoothly varying curve. It is also
apparent in the calculated ndot2s for USA 129 r, as in the following
list:

In message <199701191626.RAA09306@angel.algonet.se>, Bjoern Gimle
<b_gimle@algonet.se> writes
>...
>
>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 

Notice how ndot2 shows a sharp increase in the first elset after the
storm (97010.25-97010.50). It then declines, though the ndot2 of 0.0249
on the 17th appears anomalous.

I have taken a closer look at recent OIG-published elsets for 
#24702 = Bion 11 SL-4 r, comparing the published ndot2s with ndot2s
predicted by SatEvo for the corresponding MM (mean motion). The SatEvo
evolution derives from the published elset for 97009.036, prior to the
storm. The following table lists just the elset epoch (rounded) and the
ratio of the published to the predicted ndot2. When the ratio is more
than 1.0, we may take the atmospheric density to be greater than
expected by that amount. Where there were duplicate elsets for the same
epoch, they are listed in elset-number, so the latter is (presumably)
the more accurate.

     Epoch   ndot2 ratio

   97009.036    1.00 (fixed)
       9.411    0.99
       9.473    0.98
      10.160    1.07
      10.160    1.05
      10.285    1.03 (at the beginning of the geomagnetic storm)
      10.597    1.21
      10.659    1.86
      11.159    1.62
      11.595    1.55
      12.156    1.44
      12.593    1.36
      13.216    1.27
      13.465    1.17
      13.776    1.16
      13.901    1.15
      14.212    1.11
      14.586    0.96
      14.897    1.09
      15.146    1.01
      15.146    1.02
      15.892    0.99
      16.265    1.03
      16.452    1.00
      16.762    1.04
      17.196    1.01
      17.819    0.95
      18.502    0.93
      18.874    0.94
      19.246    0.99
      19.495    0.94

The effect of the storm is apparent until the 13th. During this period,
the perigee of #24702 fell from 211 km to 199 km, while the apogee fell
from 332 km to 283 km. Admittedly, the perigee is rather higher than
that of USA 129 r (around 150 km), but it is hard to find lower-perigee
objects which have plenty of published elsets covering the period and
which are not also much closer to decay.

A check of many other objects shows a similar pattern, though with
evidence that the peak ndot2 is reached later for higher perigees. At
400 km, for example, a peak of 2.0 times "normal" seems to have occurred
on the 14th. One caveat, however, is that there may be a "time constant"
in the orbital derivation method which may be longer for more slowly
decaying (higher-perigee) objects. Perhaps the atmospheric density
increases in phase at all altitudes, but it takes longer for USSPACECOM
to register the effect when the decay rate is lower.

Alan
-- 
 Alan Pickup | COSPAR site 2707:  55d53m48.7s N   3d11m51.2s W  156m asl
 Edinburgh   | Home:    alan@wingar.demon.co.uk      +44 (0)131 477 9144
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