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RE: AO40 orbit drifting south, then north?



What I think you're describing doesn't sound all that surprising.  However,
I'm curious as to the conditions of your observations.  Are you continually
updating to the most current Keplerian elements, or propagating one set
long-term using SGP4.

If you're using updated Keps (which I hope is the case), you'll see the
"truth" including all perturbational effects including those not modeled by
SGP4.  I can do a long-term prediction with a high-fidelity integrator to
validate your observations, but it may take a few days before I can get to
it.

73, Ken N2WWD





-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Estes Wayne-W10191
Sent: Monday, March 25, 2002 4:59 PM
To: amsat-bb@AMSAT.Org
Subject: [amsat-bb] AO40 orbit drifting south, then north?


I recently looked at long-term predictions of the AO40 orbit using SatPC32,
which uses the SGP4 orbital model.  I see that the SSP latitude at apogee is
currently drifting south, and will then drift back to the north.  Here is
what I see:

DATE        SSP LATITUDE AT APOGEE
2002 Mar    -4
2002 Apr    -5
2002 May    -6
2002 Jun    -7
2002 Jul    -7
2002 Aug    -8
2002 Sep    -8
2002 Oct    -8
2002 Nov    -8
2002 Dec    -7
2003 Jan    -7
2003 Feb    -6
2003 Mar    -5
2003 Apr    -5
2003 May    -2
2003 Jun     0
2003 Jul    +1
2003 Aug    +3
2003 Sep    +4
2003 Oct    +6
2003 Nov    +7
2003 Dec    +8

Does the apogee footprint really move north and south by a range of 16
degrees?  Or is the SSP movement exaggerated by long-term prediction error
in the SGP4 model?

If the apogee SSP really does move this much, you can see that coverage for
high northern latitudes becomes poor in the second half of 2002, but gets
much better in late 2003.  Late 2002 would be a good time for Antarctic AO40
DXpeditions.  Generally, the "DX range" for hams in the mid-to-upper
northern latitudes will be significantly smaller in October 2002 than in
December 2003.  Tree obstructions also become a bigger problem for northern
hams in late 2002 as the pass elevations get lower.

Wayne Estes W9AE
Mundelein, IL, USA
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