Zigzagging flare coordinate

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
12 May 1998 12:02:48 -0800

Hi All,

Jean Monseur posed a question about IRIDFLAR which I'm sure has
puzzled other users as well:

> This morning I found for Iridium 12 and 15 some intriguing data:

> Latitude:  49.23333  Longitude: -0.11767  Altitude:     5.0 m
> -----------------------
> IR  15
> 49.2539    0.8207
> 49.4913    0.7491  69.0 E
> 48.4580    1.0577
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> IR 12
> 49.0920   -0.8032
> 49.2451   -0.8006  49.6 W
> 49.0089   -0.8047
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> What could be the explanation of these zigzaging trails ??
> Regular strait lines were found at my other locations.

The source of the confusion is that the center line in each triplet
combines mismatched temporal and spatial information.  As
mentioned in the documentation file (FLARE161.TXT), the coordinates
given on the middle line are the coordinates of the point on the flare
central line that is closest to the observer.  In most cases, the flare
at this point occurs at a time close to the peak flare time at the
observer's location (which is given on the middle line).  This is because
the flare center lines usual run pretty close to north-south.

However, for low elevation passes, sites at high latitude, or a =
combination
of both, the flare center lines may be considerably tilted from a =
north-south orientation, and can have a slight curve to them.  In Jean's =
Iridium 15
example, the track is oriented NNW (az 349) to SSE (az 169), and he
is located 43 miles west-southwest of the track.  Because of this, the
peak flare time at Jean's location and the peak time at the track =
near-point
differ quite a bit.  In fact, the flare at the track near-point occurs =
before
even the START of his flare.  Thus the apparent zigzag in coordinates.
The true temporal order of events for this case is:

1.  peak flare at point on flare centerline closest to Jean's location
2.  beginning of flare at Jean's location
3.  peak of flare at Jean's location
4.  end of flare at Jean's location

The problem is that line 2 in the triplet gives the time of #3 above, but
the flare coordinates of #1.  Perhaps I should go to a 4-line notation
that includes all four times and points?  Bjoern Gimle has suggested
this.  If enough others concur, I'll work on the necessary modifications.
Thanks, Jean, for your question.

Best wishes,

Rob