# Extra flares are from solar arrays

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Wed Mar 07 2001 - 17:52:26 PST

• Next message: Greg Kail: "Use for Mir"

```Hi All,

After a bit of coding, I've determined the sun beta angles
for the Iridium double flares listed below, and they are
all in the range where the Iridium solar arrays do not
point at the sun:

> 2 Mar '01 IR36 19:40:30 m-3.4 az 129.7, el -35.9, beta 58.44
> 3 Mar '01 IR 7 19:34:10 m+3   az 127.1, el -36.0, beta 59.78
> 3 Mar '01 IR51 19:34:30 m-2   az 126.7, el -35.9, beta 59.82
> 4 Mar '01 IR61 19:37:05 m+1   az 128.4, el -38.5, beta 61.15
> 5 Mar '01 IR35 19:30:40 m+0   az 124.1, el -39.5, beta 62.52
5 Mar '01 IR35 19:30:50 m-3   az 123.3, el -38.6, beta 62.52

(Last data point corresponding to Michael Gill's observation
has been updated to reflect his corrected ground site
coordinates -- thanks Michael.)

In all of these cases, the solar panel gimbal "rule" in
effect is that the azimuth gimbals are locked down at +/-40
degrees, and the elevation gimbal swings 0-360 degrees
maximizing the solar incidence angle.  The combined action
of the azimuth/elevation gimbals is a bit messy to model
because it is not an alt-azimuth mount.  When the gimbal
azimuth is not zero, rotation about the elevation axis
causes the solar array normal to sweep out a cone rather
than a great circle.

The solar array gimbal azimuth can actually be computed
from the above derived az/el data using the following
equation:

gimbal_az = 90 - ACOS(SIN(Az)*COS(El))

For example, in the first case az=129.7, el=-35.9.  Plug
into the above and you get gimbal_az = 38.6.  The other
gimbal azimuths compute to 40.2, 40.5, 37.8, 39.7 and 40.8.
This seems to confirm the azimuth 40 lock-down rule.

Note that if the beta angle trend above continues, it will
soon reach 65 degrees, in which case the azimuth gimbals
will be locked at 90 degrees and the elevation gimbals
locked at 0 and 180 degrees.  In this configuration,
nighttime solar-array flares will no longer be possible.

I'll work on coding the solar arrays into IRIDFLAR, though
I suspect that the solar array pointing is not maintained
nearly as accurately as the spacecraft orientation since
it is not critical for the arrays to point precisely at
the sun.  This means that the solar array glint brightness
will not be very predictable, but I should at least be able
to flag ~when~ to look for it.

Cheers,
Rob

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