Strange drag terms in STS-97 TLEs

From: Edward S Light (light@argoscomp.com)
Date: Mon Dec 04 2000 - 08:32:32 PST

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Something has puzzled me ever since the space shuttles have been
docking with ISS (or Mir). When the two objects are docked, one would
expect their individual orbital elements to be very similar, and
usually they are ... except,  on occasion, for the drag terms.

As examples, these two recent ISS element sets ...

ISS
1 25544U 98067A   00339.28681853  .00181550  00000-0  18601-2 0  3471
2 25544  51.5776 355.6267 0003161 224.0512 136.0230 15.63847880116580
ISS
1 25544U 98067A   00339.35071896  .00164528  00000-0  16885-2 0  3488
2 25544  51.5759 355.3033 0003731 216.8846 143.1911 15.63865858116592

imply an "average" drag term of:
    (15.63865858 - 15.63847880) / 2*(339.35071896 - 339.28681853) =
    ( 0.00017978 ) / 2*(0.06390043) = 0.00140672
which is reasonably consistent with the values given in each element
set (0.00181550 and 0.00164528).

Yet when we examine two recent STS-97 elements sets ...

STS-97 DOCK
1 26630U 00078A   00339.33683744  .00003359  00000-0  35031-4 0   161
2 26630  51.5765 355.3707 0004042 216.2067  65.6687 15.63832397   513
STS-97 DOCK
1 26630U 00078A   00339.47852265  .00003295  00000-0  34307-4 0   177
2 26630  51.5753 354.6571 0003663 214.3302 145.7455 15.63872724   544

we obtain an "average" acceleration term os:
    (15.63872724 - 15.63832397) / 2*(339.47852265 - 339.33683744) =
    ( 0.00040327 ) / 2*(0.14168521) = 0.00142312
which is consistent with the ISS results but vastly different from the
values given in each element set (0.00003359 and 0.00003295).

Why are the STS TLE's acceleration terms so unrealistically low?  I
guess to make one's own predictions for such a docked pair, it is wiser
to use the ISS TLEs rather than those of STS-97.

Clear and dark skies!
   Ed Light

Lakewood, NJ, USA
N 40.1075, W 074.2312, Hgt +24 m (80 ft)

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