Development of an APL Error Model for Precision Approaches and Validation by Flight Experiments

R.J. Biberger, A. Teuber, T. Pany, G.W. Hein

Abstract:	Airport Pseudolites (APL) have been introduced to accomplish the stringent performance requirements towards Category II and III approach systems. APL may serve as an augmentation with regard to availability and continuity. However, due to effects that have to be considered and modeled in a different way than for GNSS signals, a more precise positioning using APL measurements cannot be taken for granted. During flight experiments at the airport of Braunschweig (Germany) the investigations were focused on three major aspects causing errors which cannot be mitigated by differential processing and play a key role in improving the precision of positioning using a wideband APL: The first aspect was to develop an APL error model. This model consists of two main components. Each component yields a certain correction value. The observed pseudoranges and carrier phases have to be modified by these corrections. The first component is a code and phase error model for APL signal reception on top mounted aircraft antennas based on Maxwell's equations. On the basis of multiple flight tests it was clearly to see that a loss in the line of sight to the APL does not interrupt signal reception. Thus, scattering and diffraction phenomena are present and strongly affect APL measurement quality. Code and phase measurement errors originating locally around the receiver antenna and inside the receiver contribute to navigation errors. The second component is an advanced tropospheric correction. For processing using differencing techniques this correction is important due to the different tropospheric signal paths of the ground based APL to the reference and rover station. The second aspect of this paper is dealing with the near-far problem and the associated receiver saturation effect. Since avoiding of this effect is difficult, a suitable calibration technique for the reference receiver is proposed. The relation between duty cycle and signal power of the APL on the one hand and their effects on the measured pseudoranges on the other hand are investigated. Third and last aspect of this paper are considerations concerning a suitable site design. These considerations consist of distances and geometry of the ground based and airborne components in relation to the respective runway. It will be shown that the ground based geometry will again have influence on some parameters of the APL error model. The use of the APL error model in connection with a carefully selected ground and aircraft antenna site design results in an improvement of positioning precision. Carrier-smoothed code solutions were processed with and without application of the error model and were compared with a reference solution of solved carrier phases ambiguities. The solutions with the applied APL error model show significantly better results.
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:	2308 - 2317
Cite this article:	Biberger, R.J., Teuber, A., Pany, T., Hein, G.W., "Development of an APL Error Model for Precision Approaches and Validation by Flight Experiments," 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. 2308-2317.
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