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Re: AO40 orbit drifting south, then north?



Hi,

I am not an astronomer so am not qualified to speak about
precession, but I think that is what you are observing.
I loaded latest keps in IT1.5 yesterday, and saw ~8 degree
spinplane inclination. 2x8=16 -:)
Orbit time of ~19 hours, and I could visualise the cycle
as closer to 31 months, if you were to do the forecasting
with a relatively high accuracy.
The above does _not_ take into consideration any updated keps,
which would over time include the influence of Moon and Sun,
(and other gravitational influences).

Certainly, at some point in that cycle, extreme northern and
southern latitudes will encounter difficulties or benefits,
depending on point of view and time of the cycle.
One of the benefits, could be where for example a northern
hemisphere station accesses the satellite at low elevation, and
extends the southern coverage by ~16 degrees, and at the
opposite point in the cycle the same situation applies to
southern stations.
Swings and roundabouts, and eclipses -:( .

73 Jens    ZL2TJT

Estes Wayne-W10191 wrote:

> 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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