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)

GPS satellite visibility from the Pentagon - 6am to 6pm (aircraft impact noted by red band)

GPS satellite visibility from the Pentagon - 9am to 10am (aircraft impact noted by red band)

GPS position quality is strongly affected by the number of GPS satellites visible to a GPS receiver and the orbital geometry of GPS satellites with respect to a GPS receiver. Augmented GPS signal receivers and Flight Management Systems that utilize augmented GPS signals for navigation and positioning purposes were scheduled to be contained by American and United airlines Boeing 757s and 767s by the late 1990s. [6][7] GPS almanac data contains course orbital parameters from all global navigation satellites for a given brief period. Such almanac data would also allow one to predict days in advance the type of GPS satellite visibility and GDOP conditions occurring during the WTC and Pentagon aircraft attacks, through the utilization of GPS almanac data generated on dates prior to September 11, 2001.

"Factors that can degrade the GPS signal and thus affect accuracy include the following: Number of satellites visible - The more satellites a GPS receiver can "see," the better the accuracy; Satellite geometry/shading - This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping." [8]

"Dilution of precision (DOP) or geometric dilution of precision (GDOP) is a GPS term used in geomatics engineering to describe the geometric strength of satellite configuration on GPS accuracy. DOP Value Rating Description:1: Ideal This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times. 1-2: Excellent At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications. 2-5: Good Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user. 5-10: Moderate Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended. 10-20: Fair Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location. >20: Poor At this level, measurements are inaccurate by as much as 300 meters with a 6 meter accurate device (50 DOP × 6 meters) and should be discarded." [9]

GDOP value of approximately “2” (or approximately “excellent”) during the WTC aircraft impacts (noted by red bands), 9/11/2001:

GDOP value of approximately “2.4” (or approximately “excellent”) during the Pentagon aircraft impact (noted by red band), 9/11/2001:

The following is an illustrative summary of the positioning capability provided by the augmented GPS service (WAAS), activated 13 months prior to September 11, 2001.

"WAAS also supports required navigation performance (RNP) operations, says Raytheon, providing a precision navigation capability down to RNP 0.02 (an accuracy of 0.02nm)."[10]

RNP .02 / Boeing 767-200 / World Trade Center Tower:

1 nautical mile = 6,076 feet

RNP 0.02 = RNP (0.02 nautical mile radius) x 2 = RNP (121.5 foot radius) x 2 = a 243 foot wide corridor.

WAAS supported RNP corridor:

GPS supported RNP aircraft containment radius:

"Accuracy and integrity are expressed in terms of nautical miles and represent a containment radius of a circle centered around the computed FMC position where there is a defined containment probability level of the actual aircraft being inside the containment radius. For accuracy the containment probability level is 95%." [11]

By 1996 and 1998, United and American airlines Boeing 757 and 767 aircraft, like those used during the terrorist attacks of September 11, 2001, were scheduled to contain flight management systems and signal receivers capable of utilizing the new GPS service.

Rockwell's Collins Landing System Picked for Both Airbus and Boeing Planes

“CEDAR RAPIDS, Iowa, Sept. 6, 1996 /PRNewswire/ -- Rockwell's Collins Commercial Avionics, based in Cedar Rapids, has made major announcements of the selection of its Multi-Mode Receiver (MMR) landing system by two of the world's leading aircraft manufacturers, Airbus and Boeing ... Subsequent certifications are planned for Boeing's ... 757 and 767 ... The Multi-Mode Receiver ... expands capabilities required by the air transport industry as the Global Navigation Satellite System (GNSS) is phased into operation.”[12]

HONEYWELL ANNOUNCES ORDERS FOR NEW-GENERATION "PEGASUS" FLIGHT MANAGEMENT SYSTEM

“FARNBOROUGH, ENGLAND, SEPT. 7, 1998 - The Honeywell "Pegasus" flight management system earned its first FAA certifications March on the Boeing 757, 767 and MD-90 aircraft types ... Airlines get FANS-1/FANS-A capability ... FANS-1 ... allows operators to obtain more economical routings and to utilize satellite navigation.” [13]

Boeing 757s and 767s containing such Flight Management Computers and Multi-mode Receivers, can precisely self-navigate entirely under augmented GPS-guided autopilot control, to destinations still out of sight and along routes that reportedly never vary more than 18 meters.

"Guided entirely by autopilot, an Air China Boeing 757 jet last month snaked along a narrow river valley between towering Himalayan peaks ... the airplane automatically followed the twists of the valley, descending on a precisely plotted highway in the sky toward a runway still out of sight ... Using global-positioning satellites and on-board instruments, Naverus' navigation technology pinpoints the location of a fast-moving jet to within yards ... "You're watching the whole thing unfold. The airplane is turning, going where it's supposed to go ... it's all automatic.""[14]

"For this RNP approach in Tibet, an Air China Boeing 757 was relying on dual GPS receivers, flight path computers and inertial reference systems ... the aircraft we are on is equipped with Honeywell Pegasus flight management systems and Rockwell Collins multi-mode receivers."[15]

References:

[1] Trimble's Planning Software

http://www.trimble.com/abouttrimble.shtml

[2] GPS ALMANACS/YUMA FOR YEAR 2001

http://www.navcen.uscg.gov/?Do=gpsArchives&path=ALMANACS/YUMA&year=2001

[3] AMENDED VERSION: Wide Area Augmentation System Signal Now Available

http://www.faa.gov/news/press_releases/news_story.cfm?newsId=5249

[4] WAAS: Back in Step, Avionics Magazine, February 1, 2002

http://www.aviationtoday.com/av/categories/commercial/12571.html

[5] Coordinates for the World Trade Center

http://en.wikipedia.org/wiki/World_Trade_Center

[6] Rockwell's Collins Landing System Picked for Both Airbus and Boeing Planes

http://www.highbeam.com/doc/1G1-18652301.html

[7] HONEYWELL ANNOUNCES ORDERS FOR NEW-GENERATION "PEGASUS" FLIGHT MANAGEMENT SYSTEM

http://www.aviationnow.com/shownews/farnday1/pressr15.htm

[8] Sources of GPS signal errors

http://www8.garmin.com/aboutGPS/

[9] Dilution of precision (GPS)

http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)

[10] UPS wins FAA certification for wide-area GPS receiver

http://www.flightglobal.com/articles/2003/01/07/159964/ups-wins-faa-certification-for-wide-area-gps-receiver.html

[11] RNP Capability of FANS 1 FMCS Equipped 757/767

http://www.boeing.com/commercial/caft/reference/documents/RNP757767.pdf

[12] Rockwell's Collins Landing System Picked for Both Airbus and Boeing Planes

http://www.highbeam.com/doc/1G1-18652301.html

[13] HONEYWELL ANNOUNCES ORDERS FOR NEW-GENERATION "PEGASUS" FLIGHT MANAGEMENT SYSTEM

http://www.aviationnow.com/shownews/farnday1/pressr15.htm

[14] Kent company bringing a navigation revolution

http://seattletimes.nwsource.com/html/businesstechnology/2003316294_naverus22.html

[15] Air China's First RNP Approach Into Linzhi Airport, Tibet

http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=comm&id...