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Letter from Donald Menzel to Robert Low, 15 March 1968

March 15, 1968

Mr. Robert J. Low
Dept. of Physics amd Astrophysics
202 Woodbury Hall
University of Colorado
Boulder, Colorado 80302

Dear Bob;

I have your radar-visual report sent in by Forrest D. Perkins. Not much one can do about it, since he doesn't even remember the month, let alone the exact date. However, the report is similar to many others that I have seen. Without specific information of times and directions, it is difficult to analyze or confirm what seems a reasonable explanation.

First of all, I have to point out again the impossibility of making a definite identification between radar and visual sightings, especially, under conditions such as those assumed. Radar tells you the direction in which you sent off the signal and it measures the distance to whatever is reflecting the radar pulse. And sometimes it doesn't even do that, in view of the possibility of getting reflections at very long range. You thus have three perimeters, altitude, bearing and distance. As for a visual sighting, you have at best only two, altitude and bearing. You have no estimate of distance and hence, if you happen to see some illumination -- even if it's only a bright star near the horizon -- in the same general direction, an inexperienced radar operator will often claim the simultaneous radar-visual sighting, whether the conclusion is justified or not.

Well, 4000 mph is pretty high speed. I strongly suspect the trapping was involved, that turbulence changed the picture for successive scans, to give the appearance of motion, even though none actually existed. In brief, MTI will not discriminate between a return from a moving target or a ground return from a stationary target, if the radar beam is reflected from a moving layer. This could account also, for the existence of the stationary target. As for the blurred light viewed from the tower, we have no assurance whatever that this was not an after image effect, a visual impression resulting from the operator's having inadvertently seen a bright light at close range.

This is about all I can do without further information.

Looking forward to seeing you soon, I am,

Cordially yours

Donald H. Menzel

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