WAAS-Based Flight Inspection System

Maintaining the accuracy of an Instrument Landing System (ILS) is very important because it is the primary landing guidance system during bad weather in the U.S. Therefore, the FAA periodically checks the accuracy of an ILS and calibrates any deviation, a procedure called Flight Inspection (FI). In order to check the accuracy of an ILS, the FAA uses an Inertial-based Automated Flight Inspection System (AFIS). The Inertial-based AFIS is a self-contained system that has a navigation-grade inertial navigation system (INS), a barometric altimeter, a radar altimeter, GPS, and a TeleVision Positioning System (TVPS). Using these sensors and known runway coordinates, the Inertial-based AFIS is able to meet the ILS calibration accuracy requirement which is to measure the deviations of the ILS within 0.015 degree accuracy (as small as 30 cm in vertical). In fact, this accuracy requirement can also be achieved by using a commercial RTK DGPS system with much lesser cost than the Inertial-based AFIS. Those flight inspection systems that use RTK DGPS are called a DGPS-based AFIS. However, the relatively large set-up time of the DGPS-based AFIS is a serious limiting factor to the FAA because they have to check a large number of ILS’s. Previously, from an effort to replace the Inertial-based AFIS to a lower cost system while maintaining or improving its efficiency and accuracy, WAAS-aided Flight Inspection System (WAAS-aided FIS) has been proposed [1]. This system uses a low grade INS, WAAS, a radar altimeter and a TeleVision Positioning System (TVPS). The advantages of this system are cheaper cost, better efficiency and better accuracy than the Inertial-based AFIS. The WAASaided FIS can be easily implemented in the Inertial-based AFIS due to the similarity of these two systems. However, the drawback of this system is the vulnerability to a possible accuracy degrade in rare events, for example a sharp ionospheric gradient and severe multipath, because only position outputs from the WAAS receiver can be used for the easy system realization from the Inertialbased AFIS. In this paper, we introduce the “WAAS-based Flight Inspection System (WAAS-based FIS)” which overcomes the shortcomings of the other flight inspection systems. The WAAS-based FIS is a self-contained system, on the airplane, equipped with a single frequency WAAS receiver, a radar altimeter and a TeleVision Positioning System (TVPS). In this system, the estimated flight trajectory is the sum of an accuracy position fix over the runway threshold from the radar altimeter and the TVPS and the precise relative position with respect to the position fix. An advanced algorithm called “TimeDifferenced Precise Relative Positioning Method” is used to provide the relative position, which uses timedifferenced carrier phase measurements as ranging sources and removes ionospheric delay effects by using code minus carrier phase measurements. This positioning scheme takes advantage of known runway coordinates and near-real time processing allowed in flight inspection. The WAAS-based FIS has been tested with flight test data taken in collaboration with the FAA, and the results confirm that the algorithm is able to produce position outputs that sufficiently meet the ILS calibration accuracy requirements. The advantages of the WAAS-based FIS are lower cost, better efficiency and better accuracy than the Inertial-based AFIS and more robustness than the WAAS-aided FIS.