With regard to the issue of a greater F10.7 value leading to a shorter
lifetime, Ted's explanation is basically correct. However, the SatEvo
algorithm uses an old technique initially developed by Desmond King-Hele
which does not explicitly derive or use a ballistic coefficient.
The program computes an expected lifetime using the simplest of
formulae, namely
Lifetime (in days) = Q / ndot
Q is a function that depends on the mean motion (n), the object's
eccentricity (e) and the scale height of the atmosphere (H). It is the
latter that is a function the object's height and the F10.7 flux. For an
object in a circular orbit (ie e=0) with n=16 revs per day, Q works out
at about 0.153 at F10.7 = 150 and about 0.13 at F10.7 = 80. So, yes, a
lower prevailing F10.7 flux leads to a shorter lifetime (and an earlier
decay) than if the F10.7 flux were higher. Note my use of "prevailing".
If the flux were to increase, then the atmosphere would expand and the
drag (ndot) term would increase too. However, the proportional increase
in ndot would likely be significantly greater than the increase in Q, so
the effect would be to shorten the lifetime. Note that you'd need to use
the new increased value of ndot with the new F10.7 flux to get a shorter
lifetime estimate.
In short, SatEvo only handles steady ("prevailing") F10.7 fluxes through
elset to decay. It was not written to cope with F10.7 values that
fluctuate during the remaining lifetime of the object - specifically, it
takes no account of the resulting changes in ndot due to changes in
F10.7 flux between the elset and the decay. Rather, it evolves the elset
using a constant F10.7 flux to bring about decay at the predicted time.
As some have you have noticed, I've placed Windows and Linux version of
the latest SatEvo (v0.51) on my website.
http://www.wingar.demon.co.uk/satevo/
Each comes with (the same) readme. Check the latter if you run into a
problem about a missing CC3260MT.DLL under Windows.
Alan
--
Alan Pickup /
Edinburgh /
Scotland / COSPAR 2707: 55.8968N 3.1989W +208m (WGS84 datum)
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