Re: ISS Elsets
Björn Gimle (b_gimle@algonet.se)
Tue, 4 May 1999 17:53:47 +0200
> Here are the last 5 for the ISS:
>
...
> The drag term in the 2 most recent elsets are negative (-.00004731,
> -.00041328) . I associate this (in my own mind, anyway) as an orbit boost,
> yet the apogee and perigee remain essentially unchanged. What causes the
> drag term to go negative?
>
Theoretically, the drag term (and the Bstar for SGP4) may contain terms to
compensate for effects not included in the prediction models, like solar
radiation pressure and gravitational anomalies. However, they could hardly
be as large as these.
There have been various errors in published elsets, so I ran predictions
until 1999-05-10 for these five elsets + #541-#543 from OIG today, with
SkyMap's SGP and SGP4 to verify there was not a mismatch between the two
models.
The models were indeed equal to within four seconds (after 7-10 days).
Elsets #536-#538 were within 3 seconds of each other.
But #541 is 30 seconds behind, #540 70 seconds, #539 196 seconds !
#543 is 127 seconds ahead, #542 250 seconds !
One reason for this could be that USSPACECOM interprets a change in Mean Motion
as caused by drag. If the orbit determinations are close, this result in large
drag terms. The cause of the small MM difference could be a measurement error,
or more likely a side effect of attitude changes by the object (ISS)
However, it is valuable that these differences are observed. Users of old elset
can get very large errors, and when large changes in drag terms (or other
elements) it is wise to use both the latest elset, and one earlier, with
average or interpolated values, and the one which causes the earliest
predictions (note: #542 has a VERY large drag)
ISS 543 (ZARYA)
1 25544U 98067A 99124.20555087 .00070279 00000-0 83160-3 0 5438
2 25544 51.5938 59.4231 0005182 71.2922 288.8632 15.60603041 25736
ISS 541 (ZARYA)
1 25544U 98067A 99124.14151987 .00007912 00000-0 10000-3 0 5417
2 25544 51.5923 59.7436 0005403 69.9384 290.2341 15.60563410 25720
ISS 542 (ZARYA)
1 25544U 98067A 99124.14151717 .00107334 00000-0 12609-2 0 5424
2 25544 51.5942 59.7459 0005145 70.5986 289.5673 15.60606887 25720
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