by Paul Schreyer
Journal of 9/11 Studies Volume 37
April 2013

The radar coverage of the United States airspace is nearly complete. In particular the northeastern area, where all four hijackings took place on 9/11, has no “gaps” whatsoever in radar coverage. Nonetheless there was radar loss on 9/11 with respect to the third hijacked plane, American Airlines Flight 77, which was reported to have hit the Pentagon.

American 77 took off at 8:20 a.m. EST and was hijacked more than half an hour later. It began to change its course at 8:54 and, while slowly turning to the left, its transponder was switched off at 8:56.1 Until then it had been displayed on the radar scopes of air traffic control via the Higby radar site. This was a “beacon-only” site, meaning a site that could only display transponder signals. When American 77´s transponder was turned off, the plane was no longer visible to Higby radar.2

Nonetheless the area was covered by additional radar sites.3 Several sites that were not “beacononly” tracked American 77 after its transponder had been turned off. However the plane was lost to controllers because of the way computers processed the radar data – and because of an unexplained wide-ranging radar failure.

For the computer managing the incoming radar data, the airspace is divided into “radar sort boxes” (illustrated by the red grid in the map above). Each sort box is 16 nautical miles wide and each is assigned to a single radar site. Additionally each sort box is assigned to a second “supplemental” radar site for safety. If the first assigned site is declared to not work properly, for example because it is taken down for maintenance, the computer starts to display the data from the supplemental site on the controller´s scope. The data from all other radar sites covering the area of the sort box is rejected and not visible at all to air traffic controllers. This computer process is called “selective rejection”.4

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