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Radar Echoes from Meteors and Fireballs
Dr. James E. McDonald

(from Dr. MacDonald's private papers - dictated 12/31/69)

  1. In the Bluebook file on Lakenheath, Gregory and also Hynek toy with the idead that some of the observed radar targets might have been meteors. They speak loosely of the ionized wake.

  2. As noted in my AAAS MS, meteor-radar work is customarily done at frequencies two orders of magnitude lower than typical search radar frequencies (10-100 mcs). However, what one could not rule out were very rare instances in which the search radar might see the higher electron densities associated with what McKinley terms the head echo.

  3. Relevant references:
    McKinley, Meteor Science and Engineering
    Skolnik, Introduction to Radar Systems
    Lovell, Meteor Astronomy
    Hawkins, Meteors, Comets and Meteorites
    Blackmer, et al. (Condon Report, p. 682-3)

  4. Skolnik and SRI both state that the relation of a meteor echo is proportional to the square of the radar wavelength, and that the power returned is proportional to the cube of the wavelength.

    See Hawkins, p.41, for cube-law for power returned.

    These two laws strongly bias the probability of meteor-detection to long wavelengths and low frequencies down in the VHF, rather than in the UHF and SHF characteristic of most radar work.

  5. As is pointed out by SRI, most meteor echoes have durations under a second, although rarely one might encounter durations of a number of seconds.

    This short-duration, together with the scanning rates of most search radar, ensure that the meteor echoes will appear on radar screens as essentially point targets.

  6. For example, an S-band or L-band search set with a scan rate of, say, 6 rpm and looking at a meteor echo that might span 20 degrees of arc, would have to be oriented somewhere towards that arc, in order to fortuitously get a return. If the scan rate were 6 rpm, it sweeps 36 degrees each second, and in the very rare case where one might imagine ionization lasting several seconds, it is remotely conceivable that an entire streak of train would show up on the scope, with all of these very favorable circumstances.

    However, if only the head echo had electron densities high enough to show on the search set, and if the decay times were a small fraction of a second, then nothing but a single blip would show at the particular azimuth where the beam swept the moving fireball.

  7. Thus, in extremely rare circumstances, very bright fireballs with high residual ionization might show as short streaks across the PPI, but more commonly they would at best yield point-target blips.

  8. I know of no references in the literature to reports of meteor-track sightings on search radars. Check with Atlas.

  9. Note, also, that maximum return is obtained when the radar beam is perpendicular to a meteor trail.

  10. See Britannica Xerox for definitions of "underdense" trails (equivalent to optically thin) v. "overdense" trails, (equivalent of optically thick). See also McKinley.

  11. Note that under no circumstances would a search radar be likely to see more than a single blip (or streak) from even a bright fireball, since the recombination times would always be short compared to the sweep, for search radars.

    For very fast-scanning airport surveillance radars (30 rpm) it might be possible to see an intense echo on more than one scan, but then the sdtreak would appear in the same geographic location on successive scans, and would not conceivably fool an operator into construing it as a moving target.

    In all, meteors are incapable of simulating UFOs.

  12. Lovell (page 45-6) gives equations confirming Skolnik, to the effect that, for both underdense and overdense trails, power returned varies as wavelength cubed, while duration varies as wavelength squared.

    Lovell gives 1012 electrons/cm as the dividing line between overdense and underdense trails.

    He also indicates that that limiting line-density is typical of meteors of about the fifth magnitude. Hence the majority of meteors in the range of visual magnitudes are in the overdense category and behave like metallic cylinders, yielding skin reflection off the ionized wake.

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