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Re: (meteorobs) Iridium Flares
Hi Pierre, hi all,
De : Pierre Martin <p.martin@cyberusdot ca>
À : meteorobs@latrade.com <meteorobs@latrade.com>
Date : dimanche 15 février 1998 17:35
Objet : Re: (meteorobs) Iridium Flares
>after. Satellite location was very accurate but intensity off by a couple
>of magnitudes to appear visually more like a -3. I need to enter more
>accurate positions for my exact site to correct this.
You need to enter positions accurate to one minute to get good forecast for
iridium passes. But there is another point that can make touble in accurate
magnitude forecast. Here is an exerpt (sp?) from the help of the iridflar
DOS program I use to have my own forecast off line:
Satellite Orientation Errors
----------------------------
Paul Maley of NASA Johnson Space Center has reported that once each
Iridium satellite reaches its parking orbit, its orientation relative
to its velocity vector is maintained to within +/- 0.5 degrees in pitch,
+/- 0.4 degrees in roll and +/- 0.5 degrees in yaw. If orientation
errors conspire in a worst-case fashion, the minimum flare angle
(a.k.a. mirror angle) for a particular pass can increase by as much
as 1.4 degrees (two times the rss of 0.5 and 0.5). This could change
the brightness of a best-case pass by over 6 visual magnitudes (chang-
ing flare angle from 0 to 1.4). The effect is not as severe for larger
flare angles; for example, a change from 1.0 deg to 2.4 decreases the
theoretical brightness by only 2.3 visual magnitudes.
Since the satellite orientation errors cannot be known apriori, it is
not possible to predict which flares will be absolute "show stoppers."
Of course, orientation errors can work for or against you. A predicted
flare angle of 1.5 degrees could conceivable be improved to 0.1 degrees,
transforming a medium brightness flare into a -8 magnitude searchlight!
another point is there are at least 3 anomalous Iridium satellites. Here is
another comment from the same text:
Anomalous Satellites
--------------------
At least three Iridium satellites have orientation errors which exceed
the above limits, causing unpredictable flare behavior. These are
Iridiums 21, 27 and 36. That is not to say that these satellites do
not produce flares; rather, their orientation errors prevent accurate
prediction of flare circumstances. They may flare when IRIDFLAR says
they will, and they may not. To improve your chances of seeing flares
from these three, I suggest relaxing the mirror angle requirement to
8 or 10 degrees. You will get more false alarms, but you may also be
pleasantly surprised to see a bright flare when a "dud" was predicted.
Sharp-eyed users will notice that predictions for Iridium 11 are unlike
those of all the other Iridium satellites. As of version 1.3, IRIDFLAR
models Iridium 11 with a different orientation. As of November 25,
1997, this orientation has not been confirmed by a flare observation.
If anyone should happen to observe a predicted flare from Iridium 11,
please contact me!
Hope this helps.
Jean-Christophe 'Papou' Millot - email : papou@canl.nc
Consultant en Technologies de l'Information - IT Consultant
B.P. 13581 98803 Noumea CEDEX
Nouvelle-Caledonie - New Caledonia (22d17S, 166d28E) UT+11
Tel/Fax/Rep : (687) 27 69 84 - Mobile : (687) 77 19 97