A New Tropospheric Propagation Delay Mapping Function for Elevation Angles Down to 2o

J. Guo, R.B. Langley

Abstract:	The performance of tropospheric propagation delay prediction models used for microwave radiometric systems such as GPS typically degrades at very low elevation angles. In part, this is due to design considerations. The mapping functions used in the models to map the zenith delay prediction to the signal slant path were designed to be used above a limiting elevation angle. In the case of the Black and Eisner (B&E) mapping function, for example, its authors recommended its use for elevation angles of 7° and above. The B&E mapping function is used in computing the tropospheric delay in receivers which obtain error corrections from the Wide-area Augmentation System (WAAS) and other compatible space-based augmentation systems. This function was selected in the interest of computation simplicity and the need to provide delay corrections only for elevation angles above 5°. Through comparisons with ray tracing of global radiosonde profiles, the mean accuracy of the B&E mapping function (multiplied by zenith delay) has been assessed to be about 12 cm at an elevation angle of 6° and about 240 cm at 2°. On the other hand, the more modern, albeit more complex, Niell mapping function, which was originally proposed for the WAAS algorithm, has only a centimetre-level mean error at an elevation angle of 2°. In this paper, we report on an investigation to determine the error of several mapping functions, including the B&E function, at elevation angles as low as 2° and present a new model, UNBabc, which has better low-elevation-angle performance than B&E and requires only slightly more computation time.
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:	368 - 376
Cite this article:	Guo, J., Langley, R.B., "A New Tropospheric Propagation Delay Mapping Function for Elevation Angles Down to 2o," 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. 368-376.
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