From: ke4zv!gary@gatech.edu (Gary Coffman)
Subject: Re: Anti-Patriot media is at it again
Keywords: Patriot, effectiveness, media
Organization: Gannett Technologies Group
Date: Sat, 10 Oct 1992 13:32:24 GMT


From ke4zv!gary@gatech.edu (Gary Coffman)

In article <Bvq3vH.C9s@law7.DaytonOH.NCR.COM> ppl@athena.mit.edu (Paul Podvig) writes:
>
>Whether or not you see a motion blurred image depends not only on speed
>of the object, but also on how far from the camera the object is. If
>the missiles are, say, 10 km from the camera (and magnification of the
>camera is not very large, which is true since the field of view is
>large), you will see the missiles as if they are not moving during
>the frame at all.

Oh dear, there are two issues here. One is optical resolution and the
other is an artifact of the NTSC scanning process. Let's address the
optical issue first. If the distance is so great and the focal length
so short that different pixels aren't exposed by the target during a
field, yet we know from closing rate that the target has moved 100
meters during the field, then the *optical* spatial resolution is
inadequate to determine if a hit or a miss of 100 meters occurs
during the field. In this case, Raytheon's claim is valid and we can
stop here.

On the other hand, let's assume that the image is magnified sufficiently
by the optics that more than one pixel is sequentially illuminated by
the motion of the objects. The way a Sony BVP-5 CCD imager works under
NTSC scanning, each individual pixel integrates charge received over a 
1/30th of a second interval and is then destructively read out by the 
clocking scan. The scan is interlaced so that half of the array is
read out each field, each alternate line, two fields per frame, and 
each pixel is read once each frame. So during the pixel charging time, 
the object will move 200 meters. Depending on the optical zoom ratio, 
the charge on the pixel will be the average of several points on the 
missile, or of illumination from the missile and from the empty sky as 
the missile moves out of the focal point of the pixel. This is called 
motion blurring. 

Now adjacent pixels in the horizontal plane are read out either 150
nanoseconds earlier or later than the central pixel. So their accumulated
charge integration is essentially the *same* 1/30th of a second as the
central pixel. However, adjacent pixels in the vertical direction are
in a *different* field. Their charge integration is also 1/30th of a
second, but it starts 1/60th of a second earlier or later since it's
in a different scan field. That means that motion along a vertical
or diagonal is going to be stepped by 1/60th of a second, 100 meters
of target motion. This is an inevitable result of the interlaced
scanning done in NTSC. This is called temporal displacement.

The only way around this is to use a mechanical shutter to expose
all the pixels for a short and fixed interval, say 1/1000th of a
second, that repeats at a 1/30th second interval synchronized with 
the frame scan retrace. That gives *all* the pixels the exact same 
charge integration duration over the exact same interval while allowing 
readout to proceed at normal NTSC rates and interlace. The drawback to 
this method, aside from the introduction of moving parts, is that the
brightness of the image is decreased by a ratio of 30,000:1. That
means it can't be used under ordinary lighting. So field news cameras
don't use this mechanical shutter and suffer from motion blurring and
temporal scan displacement artifacts. Those artifacts mean that rapid
motion, such as missile intercepts, can't be imaged truthfully over
time intervals that represent at least 100 meters of target motion.

Gary


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