New navigation system helps keep pilots on course

New navigation system helps keep pilots on course

Without WAAS, GPS data is accurate to 100 meters; with it, plane stays within four meters of the path

By Tony Lee Orr

GCN Staff

Darren J. Mollot, legislative fellow for Sen. John D. Rockefeller IV of W.Va., watches a computer depicting how WAAS is helping pilots during final approach and landing of a demonstration flight.

The flight was bumpy, but the Wide Area Augmentation System worked smoothly during a recent demonstration of the navigation system, which supplements global positioning system information to keep pilots on course.

WAAS is a significant component in the Federal Aviation Administration's modernization plans because it eventually will let pilots choose more direct routes and is potentially much safer than existing navigation methods, said officials of Raytheon Co., which developed the system.

WAAS expands the number of satellite ranging signals, increasing navigation system availability, said John D. Britigan, manager of navigation and landing systems for Raytheon. In addition, the system monitors GPS performance, warning users when GPS information is unreliable, he said.

During a flight from Washington to Atlantic City, N.J., this month, the program exceeded FAA requirements of a 7.6-meter margin of error, keeping the plane within four meters of the plotted path. Without WAAS, GPS has an accuracy of 100 meters.

Even during the flight's approach over Donald Trump's Taj Mahal, when turbulence caused stomach-churning anxiety, the system appeared to help keep the airplane on course.

WAAS begins working when antennas on rooftops receive signals from GPS satellites, Britigan said. The receivers process the signals and determine a position. Reference station processors compare the GPS-derived position against a surveyed, known location, then process the information and send it to WAAS stations over a fiber-optic network, he said.

The master station receives a gigabit of data per second from all of the stations, Britigan said. The information is sent through correction processors, which generate correction values for each satellite in view and construct ionospheric models with correlating correction values. All that data is sent through verification processors that monitor the calculations, he said.

It's a match

A comparator board ensures that the correction and verification outputs agree, Britigan said. The master station then sends messages at a transfer rate of 250 bits/sec to ground uplink subsystems, he said.

There are two uplink systems for each GPS satellite, each of which receive the information. The stations then broadcast the WAAS signal to geostationary satellites, which relay the signal to pilots, Britigan said.

The next step is certification of the system, he said.

FAA's specifications for WAAS allow for only one second of unalarmed bad data in 47.5 years'a statistic that is monumentally difficult to prove, Britigan said.

'Observation of the signal alone cannot prove the required level of integrity is being met,' he said. 'Statistical, physical and mathematical analysis must also be used.'

FAA certification authorities determined earlier this year that WAAS algorithms needed refinement to ensure the analysis can prove the system's safety and dependability, Britigan said.

FAA is pondering whether to roll out part of WAAS early or to wait until certification issues are resolved and then implement the entire program at once, he said.

Cost-benefit studies indicate that even a small savings in flying time can amount to a large savings in fuel costs. In addition, WAAS is cheaper to maintain than the current system, Raytheon officials said.

WAAS reference stations comprise a number of IBM RS/6000 computers running AIX and linked by TCP/IP through an MCI WorldCom Inc. fiber-optic link. A Motorola Inc. board running a VRTX real-time operating system from Mentor Graphics Corp. of Wilsonville, Ore., resides in the RS/6000 as a safety processor to monitor the bigger computer, officials said.

Reference stations are tied to the WAAS master station, which has the same setup connected to an uplink station running the same equipment, officials said.


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