# Re: How good ARE TSS predict

Philip Chien (kc4yer@amsat.org)
Sun, 17 Mar 1996 00:49:21 -0400

```Bill Moore asks:

>I'd like to pose the question:  with such a large extended object like
>TSS, just how good (and for how long) are predictions made from elsets
>using SGP4-based programs?

I'd suspect that they're going to be less and less accurate as the tether's
altitude decreases.  The SGP4 model, and most satellite tracking programs
make several assumptions and approximations when making their calculations.
The Earth's atmospheric model is approximated, without taking in to
account variations through the 11 year solar cycle.  More importantly for
this case the approximations never took in to consideration a satellite
dangling a 19 km. long string!

The lower portions of the tether are by now in the highest portions of the
practical atmosphere, and certainly at a higher drag level than the rest of
the system.  I'd suspect that it will 'skip' behind the satellite for a
while, until the satellite reaches a lower altitude, but that's just a
guess.

Reentry, whenever it occurs, will be rather incredible to watch for the
lucky observers (if any).

"Bill Bard" <wtba@qmgate.eci-esyst.com> said:

>Regarding Bill Moore's comments, I wonder if you plot TSS's path across
>the sky (using as current elements as possible) and compare it with the
>observed path across the sky, which part of the tether agrees with the
>predicted path? The satellite or partway down the tether?

In theory the NORAD elements should be based on the mathametical center of
gravity for the TSS with the tether.

The satellite mass is about 518 kgs.  The tether mass is about 161 kg
(assuming a mass of 8.2 g/m according to the spects).  This results in the
center of gravity about 2.5 to 3 km. below the satellite.

Rob McNaught rmn@aaocbn1.aao.gov.au commented:

>I'm a bit rusty on how the orbits are updated.  If radar is used, what sort of
>signal is produced from a sphere plus tether, and if only the sphere is
>visible, it will be above the true center of mass and thus will produce
>discrepant orbits depending on the difference between the two.

Most of the space command tracking sites are C-Band radar.  There are some
visual tracking sites also from what I understand.  While a single radar
site would get its greatest signal from the sphere, the combined values
from all of the sites around the world would have to be integrated with the
proper center of gravity for the orbits to make any sense or have any
meaning.  If you just use the radar calculations, based on the sphere's
'orbit' you'd quickly find out that it's not in a stable orbit around the
Earth since it's travelling about 3 km. higher than it's supposed to.

---------------

On a separate and related topic.  I've plotted the TSS satellite's
altitude, perigee, and apogee vs. time based on all of the two line
elements I could collect.

I've sent it to Neil Clifford (instead of commiting the grevious sin of
E-mailing it to the entire group) and I'm assuming that he'll make it
available somewhere at the FTP site.  After 'splashdown' I'll update the
graph with the final information.

Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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```