Re: Early Bird R/B decay not seen in L.A.

From: Alan Pickup (alan@wingar.demon.co.uk)
Date: Tue Dec 12 2000 - 15:05:26 PST

• Next message: Alan Pickup: "Decay watch: 2000 Dec 12"

```Rob Matson writes..

>Using my copy of Alan's SATEVO, I get a decay at 18:10 UT, or
>12:10pm CST when the rocket body was on a descending pass over
>Mexico and into the Pacific Ocean.  Alan:  do I have to use
>different parameters to get the decay a half-rev earlier?
>(I'm using the default settings.)  --Rob

Yes, Rob, I'm using different parameters. In fact, I always iterate a
number of parameters to obtain the best fit over a series of recent
elsets. I then extrapolate that evolution onwards to decay. Parameters
that are always involved in the iteration are the starting values of
mean motion and the ndot2 drag term. Others I vary include the solar
flux and the object's dimensions. For #25124, the Early Bird rocket, I
found I had to "force" the solar flux higher than expected to obtain a
very good fit over the past couple of days - the way it comes into
SatEvo is that a higher flux leads to a later decay since a higher flux
leads to a higher atmospheric scale height. Unfortunately, the version
of SatEvo on my WWW site has a fixed scale height function.

Even when I get a good evolution, there remains the question of just
where around the orbit the object will re-enter. SatEvo provides a
reasonable evolution of the mean orbit, but it is not good at picking
the re-entry point around the orbit. Here I use a number of techniques,
but it really boils down to my judgement - factors include the orbital
inclination, the position of perigee around the orbit and the
eccentricity. For example, with the final predicted rev for #25124...
Early Bird r     3.0  0.0  0.0  6.0 d  2.7       127 x 122 km
1 25124U 97085B   00347.66845226  .50600977  27997+1  27293-3 0 98336
2 25124  97.1703  71.4607 0003850 195.5108 164.4774 16.55528465167119
...SatEvo gave me a "raw" decay time of 00347.713228, or Dec 12 17:07
UTC. At that moment, the rocket would have been close to its southern
apex over Antarctica where the Earth's surface (and atmosphere) is
farthest below the satellite's height in its near circular orbit. I
judge that the decay was more (and most) likely as it ran northwards
towards the equator over the Indian Ocean and into the denser atmosphere
over the equatorial bulge. Mike MccCants's LATLONG program (with the
above elset) gave it falling below the 100 km height above ground at
14.4 S, 90.2 E, which is good enough for me. However, the length of time
since the latest elset means there must be uncertainty of one hour or
so.

Alan
--
Alan Pickup / COSPAR 2707:  55d53m48.7s N   3d11m51.2s W      156m asl
Edinburgh  / SatEvo & elsets:    http://www.wingar.demon.co.uk/satevo/
Scotland  / Decay Watch: http://www.wingar.demon.co.uk/satevo/dkwatch/
*

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