More Superbird A

Rob Matson (Rob_Matson@cpqm.mail.saic.com)
3 Oct 1996 12:57:57 -0800

                      Subject:                              Time:  12:02
  OFFICE MEMO         More Superbird A                      Date:  96/10/03

Earlier I wrote:

> How fast would the sweeps drift south in this scenario?  Assuming a
> satellite elevation angle of 30 degrees from the ground (to simplify
> the math), and a range of 39,000 km, 2.3 deg/hour is the same as
> 0.038 deg/minute, and .038 deg at that range corresponds to a
> ground footprint of 52 km.  So Robert Sheaffer in northern CA and
> Ron Lee in CO should see their peak flashes several minutes
> before I do in southern CA.  This is testable.

To which Robert Sheaffer replied:

> ... it seems to me highly unlikely that the ground footprint of the
> reflection is merely 52 km. The time of the visibility of the flashing
> seems to overlap in Sacramento, Los Angeles, Colorado, and
> Tucson, even thought the peak times may possibly vary.  So
> this must be covering a rather large area.

Perhaps my choice of the words "ground footprint" misled you.  Reread what I
wrote -- what I was calculating was the slow-sweep velocity of the reflection.
 Think of it as a kind of raster-scan; I'm assuming the fast-sweep direction
is roughly east-west at over 20,000 km/sec.  But the slow-sweep direction is
north to south (at 3:30UTC, anyway) at only 52 km/minute.  The instantaneous
size of the solar reflection is many times larger than 52 km.  (If it wasn't,
you'd only see flashes for 1 minute!)  If the reflection hit the earth
perpendicularly at a range of 37,000 km, it would be a circle of diameter 323
km (201 miles).  However, for CA the projected circle becomes an ellipse over
400 miles long in the SW to NE direction.  (It's even longer than this in
northern CA and Colorado since Superbird A's elevation is even lower).  So for
me, I should see flashes for 12 to 13 minutes since that's how long it takes
to walk the 400-mile wide raster over my location.

Robert Sheaffer also wrote:

> What I suggest is that two observers with the greatest distance from
> each other (best, Ron and Rob, next, Ron and myself) be in contact
> with each other by telephone at the time of the observation, and
> tape the call on a speakerphone. Each indicates when he sees a
> flash.

Unfortunately this won't work.  We will all be seeing the same flashes within
a 20th of second of each other, probably less.  All we can compare are our
flash "windows".  Our windows will be shifted w.r.t. to one another, such that
are start, peak and end times will be different.  How our windows differ
should tell us the approximate direction of the fast-scan, which in turn
reveals the rotation axis of Superbird A.