(no subject)

Bjoern Gimle (b_gimle@algonet.se)
Wed, 4 Mar 1998 22:36:30 +0100

Tony Beresford wrote in February (but I have problems accessing my
notebook where my data and programs are) :

>There is only 1 really bright Iridium flare predicted for
>my site ( Adelaide SA 34.97S, 138.63E) for rest of February.
>Is this a planetwide situation? I was using Rob Matson's
>IRIDFLAR as prediction program. The GSOC site agrees.
>Running IRIDFLAR for monthly periods using the latest
>elements from OIG shows a seasonal effect, with more
>glints in December.

It is definitely not planetwide.

As you say, there are seasonal variations caused by:

Long winter nights without any chance of illuminated Iridiums.
Long twilight periods in summertime, favouring night flares.
Long days in summertime, favouring daytime flares.

There are also latitude variations:

At higher latitudes, all three effects above are enhanced (nil at equator)
At higher latitudes, more passes are visible, since Earth rotates slower
   (by a factor of 1/cos(lat) in km W-E under tracks)
At very high latitudes, Iridiums can be seen on successive passes on both
   sides of apex.
One or two MMAs paint their flare tracks over a major part of their orbit,
but the tracks are displaced by extreme solar declinations.

If you find any systematic effects over longitude, i.e. not caused
by chance alone, you'll have to convince me, since :

A single satellite repeats its pass time after three days, but then
the orbit plane has precessed 4.23 degrees West.
In these three days, the Sun's declination has changed up to 1.6 degrees.
With 11 satellites spaced equally in one plane, each satellite passes
2.29 degrees W. (Two satellites before, after three days, is 0.15 deg E)
A single satellite has its best ground track repeat after 18 days, 0.16 deg.W.
Over a season, a single plane precesses 120 degrees, so differences between
several planes are also evened out.
Less-than-perfect Mean Motion, RAAN, inclination, and in-plane separation
add to the non-repeatability.

The factors above are like a Heisenberg probability distribution
(e.g. electron cloud with no determinable orbits)

I have generated track plots (with Feb.12 elsets) for you Feb.18-28
and Mar.04-14, and for me Feb.18-28.

With the very permissive filtering, I got 50 / 48 / 181 day+night flares.
With one exception, magnitudes along "your" tracks are -5 .. -9.
Within a 3 deg long, 2 deg lat "square"  40 / 40 / 89 tracks are drawn.

I am surprised at my latitude advantage - it should be counteracted by
a winter disadvantage.

But the main thing is your distribution - you'll see points near you
that have the benefit of many close tracks, and others (but at higher
and lower latitudes) with crossing tracks adding more.

The same - even more pronounced - occurs in my area.

But I doubt you can find a pattern, that will repeat in the near
future, or same period next year, on this scale.

( Three .gif and one output file sent to Tony, Rob ... )

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