WAAS-Aided Flight Inspection Truth System

The FAA currently uses its Automated Flight Inspection System (AFIS) to check the accuracy of Instrument Landing Systems (ILS) and other navaids. It is desirable to measure the deviations of the ILS to within 0.015 degree accuracy. Therefore, a flight inspection system requires a high level of accuracy in determining position which is computed by a Flight Inspection Truth System (FITS). The AFIS has a navigation grade inertial navigation system (INS), a barometric altimeter, a radar altimeter, GPS, and a TeleVision Positioning System (TVPS). The AFIS is self-contained in that it does not require any facilities on the ground. The primary sensor in the AFIS is the INS whose error characteristics are limited by drifts. These drifts are measured by using a TVPS and a radar altimeter, then the measurements from each sensor are fused with proprietary filtering techniques to result in the best possible accuracy in position. The error characteristics of the navigation grade INS causes the position computed from the current AFIS to have errors at distances far from the runway that are larger than desirable. This paper discusses techniques that will improve the accuracy and allow for better efficiency in the flight inspection procedures. The FAA has recently commissioned the Wide Area Augmentation System (WAAS), which provides corrections to GPS through a network of 25 reference stations throughout the U.S. WAAS accuracy (95%) over the U.S. is better than 1 meter in horizontal and 2 meters in vertical. The current standalone WAAS does not meet the ILS calibration accuracy requirements by itself, but WAAS can be used as a complementary sensor to the INS and radar altimeter for the flight inspection system because the position error in WAAS for the duration of an approach is approximately constant. This research focuses on the developments of the enhanced flight inspection system aided by WAAS, which does not require significant changes in the current AFIS. It has been found that the fusion of WAAS, radar altimeter, and INS makes it possible to obtain the required position accuracy. Another advantage is that an inspecting aircraft no longer has to fly over both ends of the runway in order to calibrate the INS drifts. The TVPS may also be eliminated for the inspection of CAT1 or CAT2 ILS. Preliminary results using the data from flight tests assure the WAAS-aided flight inspection system provides drift-free accurate positions.