As previously presented at the ION GPS 2002 meeting, the satisfactory ability to determine an aircraft attitude, computed by a GPS attitude determination algorithm that is under development, has been demonstrated using simulation and ground tests with real data. Now, for the final certification process of this development, it will be necessary to determine the true algorithm performance in terms of its accuracy, reliability and dynamic behavior. To accomplish that, a flight-test campaign is necessary to validate the attitude determination algorithm. In this phase, the measured aircraft attitude angle is compared with the true attitude reference angle, to allow the determination of the real errors. This work will present the flight test evaluation results of a GPS Attitude Determination System. This System is based on an algorithm that finds if the incoming signal arises from above or below the antenna array plane, thus being capable to be used in areas covered by pseudolites (GBAS). The System, was built with the use of a non-dedicated THALES Z-FX airborne GPS receivers and a Flight Tests Instrumentation (FTI) System that is composed by a set of transducers, an airborne PCM Data Acquisition System (DAS) PCU-800 from L-3 Communication, a Teac AB-80-F 8mm airborne tape recorder and a GPS/IRIG-B time base, to allow the time correlation of all measurements. The flight test campaign was conducted at the Brazilian's Flight Test Division T-25C 1956 Basic Trainer aircraft manufactured by EMBRAER. Measurements at the reference system are taken from a set of transducers, which are mounted on the aircraft's instrumentation bay to provide attitude data at 32Hz rate. Carrier phase measurements at 10Hz rate are also acquired by the DAS for attitude post-processing. An accurate alignment procedure is employed to correct the misalignment from FTI, as well from the GPS antenna reference to the aircraft reference frames to remove all possible systematic errors. The algorithm uses double-differences approach and detects if a given Satellite is above or under the antenna array plane. The performance and accuracy of the system is demonstrated under dynamics tests, which are fully compliant with the Federal Aviation Administration (FAA) Advisory Circular (AC) 25-7A, that requires the maneuvering range of ±30 degrees for the pitch and roll angles for aircraft certification. An appropriate flight path was selected and carried out several times to allow the proper determination of the System's characteristics. The results were within expected values.
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