RE: How inaccurate are predictions made months in advance?

Ling,Alister [Edm] (Alister.Ling@EC.gc.ca)
Mon, 27 Apr 1998 11:30:07 -0600

Regarding reliability of Iridium flare predictions:

I've done a simple bit of note taking for the last month, and the
results are listed below. They're probably fairly representative from
month to month.

As you know, the critical things about the Iridiums are the brightness
of the flares, and how narrow the ground tracks are. Slight changes in
orbit make big differences in the output.

I took two elsets, the first April 4, the second April 16. By April 25th
(3 weeks after the first elset) I noted:

1. Times of maximum flares varied up to +/- 40 seconds  (no great deal)
2. More importantly, some mag -6 flares from the 1st elset did not break
past mag 0 (the cutoff I used)
3. There were mag -6 flares predicted from the 2nd elset that did not
make it past mag 0  from the 1st elset.
4. Roughly 60% of the flares had different predicted magnitudes, OFTEN 3
mags different (not a surprise really)
5. Roughly 30% of the predictions remained about the same, but given the
notes above, you'd never know in advance which were going to change
compared to those staying the same.

I'd expect that some of the predictability may be higher closer to the
summer and winter solstices since the Sun's declination will hardly be
changing. Remember that the Sun moves its own diameter in the sky in 2
minutes, and if in 3 weeks the timing error is already as much as 40
seconds...it's easy to see why the predictions can change so quickly due
to the "high precision alignment" required with the Iridium flares.

Good observing,

Alister Ling.
N54 30' W113 30'.