An Investigation of Tropospheric Errors On Differential GNSS Accuracy and Integrity

Trent A. Skidmore, Frank van Graas

Abstract:	This paper provides an in-depth analysis of the effects of the troposphere on satellite-based differential augmentations for aircraft precision approach and landing applications. The results of this effort are applicable to both the Space and Ground Based Augmentations Systems (GBAS/SBAS), including the Wide and Local Area Augmentation Systems (WAAS/LAAS) using GPS and/or Galileo. It is widely recognized that the dominant, non-common error sources in differential GNSS for aviation are noise, multipath, the ionosphere, and the troposphere. Of these errors, the troposphere is perhaps the least studied and least quantified error source within the aviation community. To fill this information void, the current models used to estimate the tropospheric corrections in the WAAS and LAAS Category I aircraft precision system are analyzed and compared. The potential for erroneous tropo corrections is illustrated by comparing models over a range of input parameters, including temperature, pressure, relative humidity, refractivity index (and uncertainty), as well as the tropospheric scale height. While the LAAS troposphere model primarily address the differential vertical tropospheric gradient between the LAAS ground station and approaching aircraft as a function of the altitude difference, it is also important to account for the tropo errors due to lateral separation. For operations at large airports, the LAAS ground station may be separated by distances up to three miles from the approach-end of the runway. At this distance during clear, stable weather conditions, errors due to lateral separation are often considered negligible. However, under severe weather conditions, localized weather behavior may cause the differential tropospheric corrections to exceed their budgeted values, especially for low-elevation satellites. Furthermore, the LAAS tropo corrections must be valid over the entire 23-nautical mile LAAS service volume to ensure safe terminal-area operations. Clearly, over this distance, lateral tropo gradients may become significant. Analogous to the strict error budgets developed for multipath and ionospheric errors, tropospheric errors must also be carefully considered and properly accounted for. To address this issue, several case studies are shown using weather information obtained from the Ohio University Scalia Laboratory for Atmospheric Analysis. The results of this research are of particular importance to WAAS and LAAS development because, if and when the aviation community adopts a multi-frequency, multiconstellation architecture to meet the most stringent requirements of Category II/III aircraft precision approach and landing operations, tropospheric anomalies will remain as a key error source that must be well understood and modeled in this aviation safety-of-life application.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	2752 - 2760
Cite this article:	Skidmore, Trent A., van Graas, Frank, "An Investigation of Tropospheric Errors On Differential GNSS Accuracy and Integrity," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 2752-2760.
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