An object was tracked by radar and sighted visually by radar.

Condon Report: Kincheloe AFB, Sault Saint Marie, Mich., 11-12 September 1967, 2200-2330 LST. Weather: clear, ceiling unlimited, visibility unlimited (over 20 mi.), no thunderstorms in area, wind at surface 140°/4 knots, aloft 240°-27O°/15-35 knots. The radio refractivity profile from Sault Saint Marie for the most applicable time is shown in Fig. 21.

This is a good example of moving radar targets that cannot be seen visually, where there is a "forbidden cone" over the radar site. Some of the returns were even seen to approach within 5-15 mi. of the radar and disappear, apparently subsequently reappearing on the other side of the radar scope at about the same range that they disappeared. This sort of behavior is symptomatic of AP-echoes.

The meteorological data tend to confirm this interpretation. The refractivity profile shown in Fig. 21 displays three peculiarities: a strong subrefractive layer at the surface, a strong elevated duct at 325-520 m. (about 1100-1700 ft.) and a super-refractive layer at 1070-1360 m. (about 3,500-4,500 ft.). A ray-tracing is shown for this profile in Fig. 20. The ray shows noticeable changes in curvature as it passes through the different layers, an indication that strong partial reflections would be expected. With this profile, moving AP-echoes, produced in the manner described by Borden and Vickers (1953), could be expected to appear at apparent heights of between 2,000-3,000 ft. and 7,000-9,000 ft. No height information was supplied with this report, so the calculation above cannot be verified.

In summary, it appears that this is a case of observations of moving AP-echoes produced by unusually well stratified atmospheric conditions.

Google map of this location.