Date: Sat, 25 Jul 2009 05:30:33 -0500
From: Francis Ridge <>
Subject: Operation Interloper Case - Track Follows sweep



Source:  Lt. Thomas A. Featherstone, USN, Class IX, Naval Intelligence School

In early May, 1952, Lt. Featherstone was operating the GCA equipment at NAS, Willow Grove, Pa. Weather conditions were very bad; celling 250 feet; visibility less than one mile.

While awaiting an inbound plane, this officer, in the first director's position, noticed a target near the center of the scope.    By the next sweep of the antenna, the target had advanced almost five miles.    It was seen to move as the trace swept past it.   By the fourth sweep of the gear, the object has sped out to the 10-mile limit of the scope.

Between the second and forth sweeps, he checked the second director's scope, thinking what he had  been an illusion, or malfunction of his set. The contact was visible on both scopes.

The gear operates at 32 rpm.   The speed of the object, therefore, was approximately 3,600 mph.

The outgoing target followed precisely the northeast leg of the NAS range, disappearing on course 060°.

The following morning, the weather conditions were the same.   A second fast-moving target, was picked up by radar, on a bearing of 120°, heading toward the range station.    When it intercepted the southwest leg of ithe range, the object changed course to 060° and followed the range to the :limit of the. scope.

The GGA equipment at NAS Willow Grove has two radar channels, each one independent of the other.   The first sighting was made on channel A; the second, on channel B.    The search antenna is constructed so that no reception is obtained over 6,000 feet.   However, during inclement weather, the beam has been known to reach heights of 8,000 feet,

[See drawings on original document above]