Re: Something fun moving across TASS

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Mon, 5 Jan 1998 20:19:27 -0500, "Pittinger, Martin J." <PittiMJ1@central.SSD.JHUAPL.edu> wrote:
*>Something fun to try,
*>You could determine a rough distance by measuring the separation between
*>outer streaks (Boeing 737-300 is 94' 9" or 737-900 is 112' 7" wing tip
*>to wing tip). Then there's the strobe on the bottom, flashing at a 1/sec
*>rate (most do) so you could almost calculate speed (737-300 approx.
*>speed is 495 mph & 737-900 approx. speed is 530 mph) w/ no head wind.
*>Finally with the angle of the streaks in a TASS image you could get a
*>rough direction. OK, it's not exact but it's still fun to try.

Check my arithmetic and logic on the following line of thought:

Speed vs. length of streak gives rough altitude; altitude vs. width
of streak gives rough size. Let's see: presuming 30,000 feet a wingspan
of 100 feet is:  arctan(100/30000) ~= .19 degrees = 684 arc seconds.
TASS pixel size is 13.75 arc sec/pixel, so that trail would be *huge*,
50 pixels wide: It would manifest as a PAIR of trails.

My casual observing experience suggests that commercial aircraft in the
distance might be percieved as one dot (especially naked eye) but are
in general percieved as two. Private aircraft would be
closer (brighter) but shorter: again, experience suggests closer wins
out and again there is some visible distance between the strobes.
So one criterion for recognition is a PAIR of trails. Except, of course,
when these considerations do not occur and you have a small, distant aircraft.
(Solar reflection off a distant aircraft is a case in point. Satellites
can be comparable in terms of brightness and apparent motion.)

Of course, comparing data across different cameras or days or sites will
eliminate the effects of these particular objects. Also, any information
on the "shape" of the PSF's gathered along the trail - namely all the
"sausages" pointing in the same direction" is evidence of a trail and not
a point image. Helpful constants in this regard is the pixel exposure
time and pixel rate of shift: 470 seconds of exposure, and
0.91416 seconds/row. If you know the duration of the event, say five seconds,
then you know it will be exposed over about five pixels. If the event is
prolonged, the total exposure time comes into play. If moving, the pixel shift
comes into play.

With this in mind, let's see what an airplane strobe will do.
Note that the airplane strobe rate is 1 per second: that means, depending
on angle (with preference to right angles to the drift, i.e. north-south)
you may well get an apparent string of pixels of varying intensity as the
strobe falls in and out of phase of the drift scan rate. So, some pixels
may fall below the detection threshold - remember one bright second of
strobe is measured against 469 seconds of darkness - but some may not.
Consequently, the "streak" may become detected as a "string of pearls".

This should be written up so we don't have to rediscover it again and again.

HErb JOhnson



  **** ------------------------------------------------------ ****

Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
hjohnson@pluto.njcc.com                 Ewing, in central New Jersey, USA

                 amateur astronomer and astro-tour guide
            supporter of classic S-100 computers as "Dr. S-100"
        rebuilder of Mac Plus computers for your computing pleasure
     and senior engineer and asteroid spotter at Astro Imaging Systems