Radar Tracking
On September 11, the radar systems used by the FAA in the areas along the flight paths of the four hijacked aircraft were a combination of the latest Air Route Surveillance Radar-4 (ARSR-4, areas near the coast) and earlier ARSR versions (areas further inland). The ARSR-4 system was developed jointly for the FAA and the USAF to replace earlier search and height-finder radars. ARSR radar systems work by detecting primary target data and merging it with data from a secondary beacon system on board the aircraft, known as transponders, and then transmitted to FAA’s Air Route Traffic Control Centers (ARTCC's, usually referred to by pilots and air traffic controller as "Centers") and Air Force Air Defense Sectors (NEADS/Rome, a JSS Region Operations Control Centre (ROCC)). The ARTCC's tracking the flights on 9/11 included Boston, New York, Washington, Cleveland, and Indianapolis.
The hijackers of the four planes switched off the transponders or changed their codes upon taking control, making it difficult to track them on radar. Northeast Air Defense Sector/NORAD personnel stated they had difficulty identifying and tracking the aircraft, though they were at times able to locate them:
- NEADS technicians spotted American Airlines flight 11 twenty miles north of Manhattan, just two or three minutes before it crashed into the north tower of the World Trade Center.
- “Looking at the general capitol area, one of the tracker techs thinks he spots the plane on radar.”
- The 9/11 Commission Report: “Radar techs at NEADS/Rome are tracking Flight 77 near Washington, D.C.”
An Enhanced Traffic Management System (ETMS) hubsite, which generates and receives data to/from many remote sites, achieved full operation in September 1992. Designed to be ETMS-interactive, the Standard Terminal Automation Replacement System (STARS) began in the early 1990s as a jointly-procured parallel-developed program of FAA and the Dept. of Defense. STARS is a fully digital system capable of displaying all aircraft using FAA and DoD surveillance systems within their defined airspace.
Even with loss of transponder-based secondary radar returns, aircraft can usually be tracked with long range primary radar through the use of repetitive triangulation iterations via multiple radars at precisely known distances, as used in the Joint Surveillance System. Real-time locating via transponders requires less computation. Multilateration, the process of locating aircraft based on the time difference of arrival (TDOA) of its transponder signal to three or more strategically placed receiver stations, was developed decades ago for the military. Because multilateration data is updated every second, targets move at a much smoother and more accurate progression. However, as the transponders in three of the four hijacked aircraft were switched off, and the remaining aircraft had changed its transponder code twice, multilateration was of little use to the military for tracking the hijacked flights on 9/11, though the FAA, who had been monitoring the flights continuously since their departure, was able to track the planes at least some of the time after being hijacked. The FAA had difficulty relaying the current aircraft positions to NORAD/NEADS by phone, so even though they had access to the same radar data, NEADS was unable to locate most of the flights. This intermittent data was of some use to the US Secret Service. Barbara Riggs, then Deputy Director of the Secret Service stated, “Through monitoring radar and activating an open line with the FAA, the Secret Service was able to receive real time information about other hijacked aircraft. We were tracking two hijacked aircraft as they approached Washington, D.C.”
Read more about this topic: U.S. Military Response During The September 11 Attacks
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