Source: 9/11 Blogger
May 31, 2011

As the aircraft attacks of September 11, 2001 unfolded at the World Trade Center (WTC) and Pentagon buildings, maximum and near maximum augmented GPS positioning quality for the entire daylight period was provided to the geographic coordinates for the World Trade Center in New York City and the Pentagon in Arlington, VA. Based on output graphs generated by Trimble’s free and highly regarded downloadable GPS planning software [1], which utilizes GPS “almanac” data transmitted by the GPS constellation on September 11, 2001 and now archived at the website for the United States Coast Guard [2], conditions such as GPS satellite visibility and geometric “dilution of precision” are both shown to be at maximum or within minutes of maximum during the aircraft impacts at the World Trade Center and Pentagon. Such augmented GPS service is now in routine use by the U.S. commercial aviation industry to allow aircraft flight management computers to utilize precise aircraft positional information, accurate to within just several meters. This GPS service was activated on a conditional basis by the FAA just one year before the September 11, 2001 attacks [3]. This service provided virtual aerial guidance corridors only 243 feet wide and a 95% confidence that an aircraft’s true position will fall within any such designed corridor. The augmented GPS service was also utilized to precisely survey the Ground Zero site immediately after September 11, 2001. [4] Such corridors can be navigated entirely by autopilot and flight management systems scheduled in 1996 and 1998 to be contained by United and American airlines Boeing 757 and 767 aircraft like those used during the terrorist attacks of September 11, 2001.

Graphs generated by the Trimble software, reveal that during the impact UA 175 with World Trade Center 2, eleven out of eleven GPS/WAAS satellites were visible (or maximum satellite visibility) from the latitude/longitude coordinates of the WTC (40° 42′ 42″ N; 74° 0′ 45″ W)[5]. During the impact of AA 11 with World Trade Center 1, ten out of eleven GPS/WAAS satellites were visible from the same WTC coordinates. The impact of AA 11 occurred just ten minutes prior to the largest segment of maximum GPS satellite visibility (eleven out of eleven satellites) from the WTC coordinates for the daylight period on September 11, 2001. The total period of maximum satellite visibility occupied only twelve percent (or less than one and one half hours) of the daylight period. The impact of AA 77 occurred just ten minutes after the expiration of the largest segment of maximum GPS satellite visibility (twelve out of twelve satellites) from the Pentagon coordinates for the daylight period on September 11, 2001. This period of maximum satellite visibility occupied only six percent (or approximately 45 minutes) of the daylight period.

GPS satellite visibility from the WTC – 6am to 6pm (aircraft attack period noted by red band)

GPS satellite visibility from the WTC – 8:30am to 9:30am (aircraft impacts noted by red bands)

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