Characterization of the DC-3 In-Flight GPS Antenna Performance

T.A. Skidmore, F. van Graas, L. Marti

Abstract:	It is well-recognized that GPS multipath is the dominant non-correctable error source in local-area differential GNSS systems such as the ground-based augmentation system (GBAS) for aircraft precision approach. Errors due to multipath also contribute to the overall system performance degradation of space-based augmentation systems (SBAS) such as the Wide Area Augmentation System (WAAS). To control, reduce, and limit multipath from GPS receivers employed at the ground reference station, the Federal Aviation Administration's GBAS, the Local Area Augmentation System (LAAS), employs a network of multi-element integrated multipath limiting antennas (IMLA). These ground-based antennas perform remarkably well, with their performance being studied extensively and well documented. Unfortunately, due to the construction and large physical size of the IMLA, they cannot be employed on the aircraft. Thus, standard patch-type airborne antennas must continue to be used on the aircraft for the reception of the GPS/GNSS signals. To fully understand the impact of multipath on key system performance parameters such as accuracy, integrity, and continuity (and the resulting availability of these functions), the properties and origin of airborne multipath and other errors/anomalies must be investigated. This paper presents results from a series of flight tests conducted during March 2003 and July 2003 on the Ohio University DC-3 flying laboratory. The primary goal of the flight tests was to investigate and characterize the nature of the biases induced on the code-phase measurements as the aircraft performed a series of tactical maneuvers. The relationship between the satellite azimuth and elevation relative to the aircraft is investigated. Signal distortions originating from different aircraft features such as the wings, fuselage, and tail are investigated. Code-minus-carrier processing techniques are employed to isolate the ranging biases on a per satellite basis. For the airborne sensor, NovAtel OEM4 GPS receivers were used, representing the state-of-art in receiver design. Repeatability of the airborne satellite tracking errors is also considered. Finally, the results of the investigation are interpreted in light of their implications on the vertical protection level (VPL) equations used to insure system integrity for operations such as Category III aircraft precision approach.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	897 - 907
Cite this article:	Skidmore, T.A., van Graas, F., Marti, L., "Characterization of the DC-3 In-Flight GPS Antenna Performance," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 897-907.
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